JP7747005B2 - Communication device, communication method, and communication system - Google Patents

Description

The technology disclosed in this specification (hereinafter referred to as "the present disclosure") relates to a communication device and communication method for receiving frames wirelessly transmitted from a terrestrial terminal, as well as a communication system consisting of a terminal and a receiving station.

In a wireless sensor network, attaching wireless sensor terminals to people and objects and periodically transmitting information acquired from the sensors makes it possible to create new services. For example, by attaching wireless sensor terminals with GPS (Global Positioning System) to elderly people or children and periodically transmitting location information, it becomes possible to provide a monitoring service.

Wireless communication systems for the Internet of Things (IoT) must enable communication with many wireless sensor terminals attached to various people and objects. Therefore, long-distance transmission, which enables wide-area communication, and multi-terminal accommodation, which allows a single base station to communicate with many terminals, are important. In recent years, Low Power Wide Area (LPWA), which has been considered a promising wireless communication method for IoT, has made long-distance transmission and multi-terminal accommodation possible, achieving long-distance transmissions of several kilometers to several hundred kilometers between terminals and ground-based receiving stations (see, for example, Patent Document 1). However, installing receiving stations requires securing land and laying a network for cloud connection, which is costly and time-consuming, hindering efforts to reduce the cost of communication services and expand communication areas.

Meanwhile, in recent years, private companies have been actively engaged in space development, and while launching a satellite previously required tens of billions of yen, development is underway with the aim of reducing costs to one-tenth to one-hundredth of that amount. Satellites are also becoming increasingly smaller, and as a result, it is being considered to launch multiple satellites at once to create constellations and provide services that cover the entire globe.

As satellite services become a reality, one possible way to expand the LPWA communication area is to install a receiver on a satellite and receive radio waves transmitted from a terrestrial terminal on the satellite. LPWA has achieved communication over distances of several hundred kilometers on the ground, and it is believed that low-earth orbit satellites would be sufficient for communication. Furthermore, since low-earth orbit satellites orbit the Earth, it is possible to cover the entire globe with a small number of satellites in use cases such as IoT, where data is transmitted once every few minutes to several days. Therefore, installing receivers on satellites is more cost-effective than installing many receiving stations on the ground, and this is expected to reduce the cost of communication services and rapidly expand the communication area. Furthermore, it would be possible to obtain information from the ocean, where it is difficult to install receiving stations on the ground, enabling the provision of new services.

Japanese Patent Application Laid-Open No. 2019-193306

3GPP TR 38.811 V15.1.0

The object of this disclosure is to provide a communication device and communication method that operates as a satellite receiving station that orbits the Earth in a predetermined orbit and receives frames from a terrestrial terminal, as well as a communication system consisting of a terrestrial terminal and a satellite receiving station.

A first aspect of the present disclosure provides a system for receiving frames from a ground terminal, the system comprising: a satellite receiving station that orbits the Earth in a predetermined orbit and operates as one of the satellite receiving stations;
a receiving unit that receives and processes frames from the terminal;
a determination unit that determines a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information when a frame is received from the terminal, a frame transmission interval of the terminal, and the positional relationship of surrounding satellite receiving stations;
The communication device is provided with:

The frame reception information includes a frequency error estimation result and a propagation delay estimation result at the time of frame reception. Alternatively, the frame reception information includes location information of the terminal.

Furthermore, the communication device according to the first aspect exchanges frame reception information with surrounding satellite receiving stations. In this case, the determination unit further determines, based on the frame reception information from the surrounding satellite receiving stations, a satellite receiving station that is suitable for receiving the next frame transmitted by the terminal.

A communication device according to a first aspect notifies a nearby satellite receiving station of information about a terminal added to the target terminal list based on frame reception information from the nearby satellite receiving station. Furthermore, based on the notification from the nearby satellite receiving station, the communication device according to the first aspect deletes the information about the corresponding terminal from its own target terminal list.

A communication device according to a first aspect notifies a nearby satellite receiving station that the determination unit has determined to be suitable for receiving the next frame transmitted from the terminal of the registration of the target terminal, and removes the terminal that has notified the nearby satellite receiving station of the registration of the target terminal from the reception targets of the own station. Furthermore, the communication device according to the first aspect sets the terminal that has been notified of the registration of the target terminal by the nearby satellite receiving station as a reception target of the own station.

If the frame reception information includes the frame transmission interval notified by the terminal, the determination unit estimates the expected reception time of the next frame transmitted from the terminal based on the frame transmission interval and the frequency error estimation result at the time of frame reception, and determines the satellite receiving station that is suitable for receiving the next frame transmitted by the terminal based on the expected reception time of the frame and the positional relationship of surrounding satellite receiving stations.

A second aspect of the present disclosure is a communication method that operates as a satellite receiving station that orbits the Earth in a predetermined orbit and receives frames from a terrestrial terminal, the method comprising:
a receiving step of receiving and processing frames from the terminal;
a determination step of determining a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information at the time of receiving a frame from the terminal, a frame transmission interval of the terminal, and the positional relationship of surrounding satellite receiving stations;
It is a communication method having the following.

A third aspect of the present disclosure provides a system including a terminal installed on the ground and a plurality of satellite receiving stations each orbiting the Earth in a predetermined orbit;
Each of the plurality of satellite receiving stations comprises a receiving unit that receives and processes frames from the terminal, and a determining unit that determines a satellite receiving station that is suitable for receiving the next frame transmitted by the terminal based on frame reception information when frames are received from the terminal, a frame transmission interval of the terminal, and a positional relationship between neighboring satellite receiving stations.
It is a communication system.

However, the term "system" here refers to a logical collection of multiple devices (or functional modules that realize specific functions), and it does not matter whether each device or functional module is contained within a single housing.

The present disclosure provides a communication device and communication method that operates as a satellite receiving station, efficiently acquires information on radio resources of a terrestrial terminal, and receives frames from the terminal while suppressing unnecessary reception and demodulation processing, as well as a communication system consisting of a terrestrial terminal and a satellite receiving station that receives frames from the terminal while suppressing unnecessary reception and demodulation processing.

Please note that the effects described in this specification are merely examples, and the effects brought about by this disclosure are not limited to these. Furthermore, this disclosure may also bring about additional effects in addition to the effects described above.

Further objects, features and advantages of the present disclosure will become apparent from the following detailed description of the embodiments and accompanying drawings.

FIG. 1 is a diagram showing an example of the configuration of an LPWA wireless communication system. FIG. 2 is a diagram showing an example of a communication sequence between a terminal and a receiving station. FIG. 3 is a diagram showing an example of the functional configuration of the terminal 100. FIG. 4 is a diagram showing an example of the functional configuration of the receiving station 200. As shown in FIG. FIG. 5 is a diagram showing an example of a frame configuration. FIG. 6 is a diagram showing a method for generating radio resources, preambles and synchronization information, and scrambling patterns used in frame transmission. FIG. 7 is a diagram showing an example of the configuration of an LPWA wireless communication system using a satellite receiving station. FIG. 8 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station. FIG. 9 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station (first embodiment). FIG. 10 is a diagram showing an example of the functional configuration of the satellite receiving station 1000. FIG. 11 is a diagram showing an example of the frame structure of the data portion of the frame reception information frame (first embodiment). FIG. 12 is a flowchart showing the processing operations performed by the terminal. FIG. 13 is a flowchart showing a processing procedure (first embodiment) for a satellite receiving station to receive a notification frame. FIG. 14 is a flowchart showing a processing procedure (first embodiment) for a satellite receiving station to receive a data frame. FIG. 15 is a flowchart showing a processing procedure (first embodiment) for a satellite receiving station to receive a frame reception information frame. FIG. 16 is a diagram showing an example of the change over time in frequency error and propagation delay when the satellite is at an altitude of 600 km and the frequency is 2 GHz. FIG. 17 is a diagram showing an example of the frame structure of the data portion of the frame reception information frame (second embodiment). FIG. 18 is a flowchart showing a processing procedure (second embodiment) for a satellite receiving station to receive a frame reception information frame. FIG. 19 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station (third embodiment). FIG. 20 is a diagram showing an example of the frame configuration of the data portion of the target terminal list registration notification frame. FIG. 21 is a flow chart showing a processing procedure (third embodiment) for a satellite receiving station to receive a frame reception information frame. FIG. 22 is a flowchart showing a processing procedure for a satellite receiving station to receive a target terminal list registration notification frame. FIG. 23 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station (fourth embodiment). FIG. 24 is a diagram showing an example of the frame structure of the data portion of the frame reception information frame (fourth embodiment). FIG. 25A is a flowchart showing a processing procedure (fourth embodiment) for a satellite receiving station to receive a frame reception information frame. FIG. 25B is a flowchart showing a processing procedure (fourth embodiment) for a satellite receiving station to receive a frame reception information frame. FIG. 26 is a diagram showing an example of an adjacent satellite receiving station information list held by a satellite receiving station. FIG. 27 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station (fifth embodiment). FIG. 28 is a diagram showing an example of the frame structure of the data portion of the target terminal registration frame. FIG. 29 is a flowchart showing a processing procedure (fifth embodiment) for a satellite receiving station to receive a notification frame. FIG. 30 is a flowchart showing a processing procedure (fifth embodiment) for a satellite receiving station to receive a data frame. FIG. 31 is a flowchart showing the processing procedure for a satellite receiving station to receive a target terminal list registration frame. FIG. 32 is a diagram showing an example of a communication sequence between a terminal and a receiving station in a system in which the terminal can arbitrarily transmit a data frame. FIG. 33 is a diagram showing a configuration example (sixth embodiment) of an LPWA wireless communication system using a satellite receiving station. FIG. 34 is a diagram showing an example of a communication sequence between a terminal and a satellite receiving station (sixth embodiment). FIG. 35 is a diagram showing an example of the functional configuration of a satellite receiving station 3500. FIG. 36 is a diagram showing an example of the structure of a data frame. FIG. 37 is a diagram illustrating an example of the frame structure of the data portion of the reception target radio resource information notification frame. FIG. 38 is a flowchart showing the processing operations performed by the terminal. FIG. 39 is a flowchart showing the processing operation for the satellite receiving station to receive the initial data frame. FIG. 40 is a flowchart showing the processing procedure for the satellite receiving station to receive a normal data frame. FIG. 41 is a flowchart showing a processing procedure for a satellite receiving station to receive a reception target radio resource information notification frame.

The present disclosure will now be described in the following order with reference to the drawings.

A. Assumed system B. System configuration using a satellite receiving station C. First embodiment C-1. Example of communication sequence C-2. Device configuration C-3. Frame configuration C-4. Processing operation of terminal C-5. Processing operation of satellite receiving station D. Second embodiment D-1. Example of frame configuration D-2. Reception operation of frame reception information frame E. Third embodiment E-1. Example of communication sequence E-2. Example of frame configuration E-3. Processing operation of satellite receiving station F. Fourth embodiment F-1. Example of communication sequence F-2. Example of frame configuration F-3. Adjacent satellite receiving station information list G. Fifth embodiment G-1. Example of communication sequence G-2. Example of frame configuration G-3. Processing operation of satellite receiving station H. Effects I. Sixth embodiment I-1. System configuration I-2. Example of communication sequence I-3. Device configuration I-4. Frame configuration example I-5. Terminal processing operation I-6. Satellite receiving station processing operation I-7. Effects

A. In the assumed system wireless communication system, in order for the receiving station to demodulate data transmitted by a terminal, it is necessary to know, for each terminal, information required for demodulation, such as the modulation method, modulation rate, code, and encryption key, as well as the wireless resources (time and frequency) used for data transmission.

In wireless communication systems capable of two-way communication, such as LTE (Long Term Evolution), signaling is performed to exchange necessary information between the terminal and the receiving station before data transmission, allowing the receiving station to specify the wireless resources (time and frequency) for transmitting data to the terminal, and then demodulation can be performed using the modulation method and modulation rate stored in the data being transmitted using the specified wireless resources (time and frequency).

In contrast, in LPWA, a wireless communication method for IoT, signaling is undesirable from the perspective of terminal power consumption. Furthermore, if only one-way communication from the terminal to the receiving station is supported, signaling is not possible. Therefore, we consider a method for the receiving station to determine the wireless resources used by the terminal for data transmission in an LPWA wireless communication system.

Figure 1 shows a schematic example of the configuration of an LPWA wireless communication system. In the example shown in the figure, the system consists of a receiving station and multiple terminals. The terminal is a transmitter that periodically transmits information sensed by an on-board sensor, etc. The receiving station receives the data transmitted by the terminal and performs demodulation processing. The receiving station also transmits the demodulated results (user data) to an application server (not shown) on the cloud as necessary. The wireless communication system shown in Figure 1 assumes that time is synchronized within the system (between the receiving station and each terminal). One possible method of time synchronization is to use GPS information.

Figure 2 shows an example communication sequence between a terminal and a receiving station within the system shown in Figure 1.

First, the terminal transmits a notification frame to notify its own ID (SEQ201). To transmit the notification frame, it uses radio resources (time and frequency) randomly selected from those allocated in advance within the system for transmitting notification frames.

The receiving station receives and demodulates the notification frame using the radio resources used to transmit the notification frame (SEQ202). If the demodulation is successful, the receiving station registers the ID obtained from the notification frame in the list of target terminals (SEQ203).

Based on rules (described below) previously determined within the system, the terminal calculates the radio resources (time and frequency) for transmitting a data frame using its own ID and the code required to generate the data frame, and then generates and transmits the data frame (SEQ204).

Here, we assume that the time between the transmission of the notification frame and the transmission of the data frame is sufficiently short that the next transmitted data frame can be received by the same satellite receiving station.

The receiving station uses the ID registered in its list of target terminals to receive data, and calculates the radio resources (time and frequency) for the terminal to transmit the data frame and the code required to demodulate the data frame based on the same rules as the terminal, and receives and demodulates the data frame (SEQ205). The receiving station also transmits the demodulation results (user data) to an application server (not shown) on the cloud as needed.

Figure 3 shows an example of the functional configuration of a communication device 100 (hereinafter referred to as terminal 100) that operates as a terminal in the system shown in Figure 1. The illustrated terminal 100 comprises a wireless communication unit 101, a wireless control unit 102, a frame generation unit 103, a sensor 104, a wireless resource determination unit 105, and a memory unit 106. Note that the functional configuration of the terminal 100 may be the same regardless of whether the receiving station is a terrestrial station or a satellite receiving station.

The wireless communication unit 101 transmits wireless signals. Under control of the wireless control unit 102, the wireless communication unit 101 converts frames generated by the frame generation unit 103 into wireless signals and transmits them.

The wireless control unit 102 controls the wireless communication unit 101 to transmit frames at the transmission time and transmission frequency obtained from the wireless resource determination unit 105.

The frame generation unit 103 generates frames to be transmitted by the terminal 100. The frame generation unit 103 obtains the preamble, synchronization information, and scrambling pattern required for frame generation from the radio resource determination unit 105. The frame generation unit 103 also writes sensor information obtained by the sensor unit 104, for example, in the payload of the frame.

Sensor 104 is a sensor that acquires external or internal information of terminal 100, such as a temperature sensor or an acceleration sensor.

The radio resource determination unit 105 uses the terminal 100's own ID and initial values obtained from the memory unit 106 to generate the radio resources (time and frequency) for transmitting the frame, as well as the preamble, synchronization information, and scrambling pattern required for frame generation.

The memory unit 106 stores the ID and initial values of the terminal 100 itself, which are information necessary to determine wireless resources. Of course, the memory unit 106 may also store other information.

Figure 4 shows an example of the functional configuration of a communication device 200 (hereinafter referred to as receiving station 200) that operates as a receiving station in the system shown in Figure 1. The illustrated receiving station 200 comprises a wireless communication unit 201, a wireless control unit 202, a frame detection and demodulation unit 203, a wireless resource determination unit 204, and a memory unit 205. The illustrated receiving station 200 is assumed to be a terrestrial station installed on the ground.

The wireless communication unit 201 receives wireless signals. Under control of the wireless control unit 202, the wireless communication unit 201 receives radio waves, converts them into wireless signals, and passes them to the frame detection and demodulation unit 203.

The wireless control unit 202 controls the wireless communication unit 201 to receive frames at the reception time and reception frequency obtained from the wireless resource determination unit 204.

The frame detection and demodulation unit 203 detects and demodulates frames from the received signal. Specifically, the frame detection and demodulation unit 203 generates a known pattern from the preamble, synchronization information, and scrambling pattern obtained from the radio resource determination unit 204, calculates the correlation value between the received signal and the known pattern, and determines that a frame has been detected if the correlation value is equal to or greater than a certain value. If frame detection is successful, the frame detection and demodulation unit 203 extracts the signal corresponding to the frame from the received signal, descrambles it, extracts the payload, and performs error correction code decoding and error detection using a CRC (Cyclic Redundancy Code). If frame demodulation is successful, the frame detection and demodulation unit 203 notifies the upper layer (not shown) of the demodulated data.

The radio resource determination unit 204 uses the ID and initial values of the terminal 100 obtained from the memory unit 205 to generate the radio resources (time and frequency) with which the terminal 100 will transmit frames, as well as the preamble, synchronization information, and scrambling pattern required for frame demodulation.

The memory unit 205 stores the ID and initial values of the terminal 100, which are information necessary to determine the radio resources. The memory unit 205 also stores a list of target terminals that stores the IDs of the terminals 100 that the receiving station 200 is intended to receive from.

Figure 5 shows an example of the frame structure used in the system shown in Figure 1. The frame includes ID, DATA, and CRC fields. Both the notification frame and data frame sent from the terminal are assumed to have the frame structure shown in Figure 5.

The ID field stores an ID, which is a unique identifier of the transmitting terminal 100. The DATA field stores the transmitted data. In the case of a data frame, data obtained from the sensor 104 is stored. In the case of a notification frame, the transmission interval from the transmission of the notification frame to the transmission of the data frame, and the transmission interval of periodically transmitted data frames are stored. The CRC field stores a CRC value calculated from the information stored in the ID field and the DATA field. On the receiving side, the CRC value is used to determine whether demodulation was successful.

The sequence consisting of the ID, DATA, and CRC is then subjected to Forward Error Correction (FEC) coding and interleaving to generate the payload. A preamble, a known pattern used for frame detection and synchronization, and synchronization information are then added to the beginning of the payload, and the frame is generated by performing an exclusive OR (XOR) with the scrambling pattern on a bit-by-bit basis.

The radio resources (time and frequency) used for frame transmission, the preamble and synchronization information required for frame generation, and the method for generating the scrambling pattern are explained with reference to Figure 6.

The system has predefined rules 1 through 4 for determining the radio resources (time and frequency) used for frame transmission, the preamble and synchronization information required for frame generation, and the method for generating the scramble pattern. The time (Time) used for frame transmission is determined by entering two initial values for time calculation, Seed(T)-1 and Seed(T)-2, in rule 1. The frequency (Freq) used for frame transmission is determined by entering two initial values for frequency calculation, Seed(F)-1 and Seed(F)-2, in rule 2. The preamble and synchronization information (Preamble/Sync) is determined by entering two initial values for preamble and synchronization information calculation, Seed(P)-1 and Seed(P)-2, in rule 3. The scramble pattern is determined by entering two initial values for the scramble pattern, Seed(S)-1 and Seed(S)-2, in rule 4.

Each initial value is determined in advance within the system and is stored in the memory unit 106 in the terminal 100 and the memory unit 205 in the receiving station 200. When transmitting a data frame, the ID, which is the unique identifier of the terminal 100 that is transmitting the data frame, is input into each second initial value (Seed(T)-2, Seed(F)-2, Seed(P)-2, Seed(S)-2). On the other hand, when transmitting a notification frame, a value randomly selected from several initial values previously assigned for notification frames is input into each second initial value (Seed(T)-2, Seed(F)-2, Seed(P)-2, Seed(S)-2).

By using this method, the terminal 100 can simply notify the receiving station 200 of its terminal ID to obtain the radio resources (time and frequency) used for frame transmission and the information necessary for frame demodulation.

B. System Configuration Using Satellite Receiving Stations Figure 7 shows a schematic diagram of an example of the configuration of an LPWA wireless communication system using satellite receiving stations. The example shown in the figure is composed of multiple receiving stations installed on the ground (hereinafter referred to as "ground stations"), multiple satellite receiving stations orbiting the Earth (hereinafter simply referred to as "satellite receiving stations"), and numerous terminals scattered around the Earth. It is assumed that all terminals and receiving stations within the system are time-synchronized. One possible method for time synchronization is to use GPS information. Each satellite receiving station orbits the Earth in a sun-synchronous orbit, for example, near a polar orbit that passes over the poles, so that the angle of sunlight incident on the satellite's orbital plane is the same, and the satellite always passes the equator at the same local time.

The terminal is a transmitter that periodically transmits information sensed by onboard sensors, etc. The satellite receiving station receives the data transmitted by the terminal and performs demodulation processing. The satellite receiving station also transmits the demodulation results (user data) to an application server (not shown) on the cloud as necessary. The satellite receiving station also enables inter-satellite communication. There are no restrictions on the communication method between satellite receiving stations, but constant connection and high-speed communication are desirable. The ground station is a transmitter/receiver that communicates with the satellite receiving station. There are no restrictions on the communication method between the satellite receiving station and the ground station, but high-speed communication is desirable.

In the case of terrestrial receiving stations, if the terminal is stationary or has a limited range of movement, the receiving stations that can receive the data (radio waves) transmitted by the terminal will always be the same. Therefore, it is sufficient for the terminal to transmit a notification frame when it is turned on and then infrequently, such as once every few days.

On the other hand, because low-orbit satellites continue to move in their orbits at speeds of several kilometers per second, the receiving station that can receive data transmitted by a terminal is not fixed. For example, if satellite receiving station 701 is capable of receiving a notification frame when that terminal transmits it, satellite receiving station 701 can receive and demodulate the data frame transmitted by that terminal as long as the terminal is located within the receivable range 711 of satellite receiving station 701. On the other hand, if, over time, the terminal moves out of the receivable range 711 of satellite receiving station 701 and into the receivable range 712 of the subsequent satellite receiving station 702 on the same satellite orbit, satellite receiving station 701 will not be able to receive and demodulate the data frame transmitted by that terminal.

However, in the example shown in Figure 7, the network is designed so that the reception ranges 711 and 712 of adjacent satellite receiving stations 701 and 702 on the satellite orbit do not overlap. Satellite receiving stations 701 and 702 orbit in the same orbit, for example, but may also orbit in adjacent orbits.

Figure 8 shows an example of a communication sequence between satellite receiving stations 701 and 702, which are adjacent on a satellite orbit, and terminal 801. Satellite receiving stations 701 and 702 are each constantly moving on their satellite orbits at a speed of several kilometers per second, so there are time periods when the location of terminal 801 is within the reception range 711 of satellite receiving station 701, and time periods when it is outside the reception range 711 of satellite receiving station 701 and within the reception range 712 of satellite receiving station 702.

First, terminal 801 transmits (broadcasts) a notification frame to notify its own ID (SEQ 801). To transmit the notification frame, radio resources (time and frequency) that have been allocated in advance within the system for transmitting notification frames are used. At this point, terminal 801 is within the reception range 711 of satellite receiving station 701. Therefore, satellite receiving station 701 receives and demodulates the notification frame using the radio resources used for transmitting the notification frame (SEQ 811), and registers the ID obtained from the notification frame in its list of target terminals (SEQ 812). On the other hand, since the notification frame from terminal 801 does not reach satellite receiving station 702, satellite receiving station 702 does not register terminal 801's ID in its list of target terminals.

Next, based on rules 1 to 4 previously determined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the data frame (SEQ 802).

The satellite receiving station 701 uses the ID registered in the list of target terminals to receive data, and calculates the radio resources (time and frequency) for terminal 801 to transmit the data frame and the code required to demodulate the data frame based on the terminal's ID, based on the same rules 1 to 4 as terminal 801, and receives and demodulates the data frame (SEQ 813).

Furthermore, based on rules 1 to 4 predetermined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the next data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the next data frame (SEQ 803).

Using the ID registered in the target terminal list, satellite receiving station 701 calculates the radio resources (time and frequency) from which terminal 801 will transmit the data frame and the code required to demodulate the data frame based on the same rules 1 to 4 as terminal 801, and attempts to receive and demodulate the data frame (SEQ 814). However, because terminal 801 is located outside of reception range 711 and within satellite receiving station 702's reception range 712, demodulation of the data frame from terminal 801 fails. Furthermore, although the data frame from terminal 801 reaches satellite receiving station 702, satellite receiving station 702 does not register terminal 801's ID in its target terminal list, and therefore does not receive or demodulate the data frame from terminal 801.

To avoid a situation as shown in Figure 8 where the terminal's data frames cannot be received by any satellite receiving station due to the movement of each satellite receiving station in its satellite orbit, the terminal could transmit notification frames frequently, but this is undesirable from the perspective of terminal power consumption and efficient use of radio resources. Furthermore, it is undesirable from the perspective of efficient use of resources at the satellite receiving station for a satellite receiving station to perform reception and demodulation processing for a terminal that has moved out of its reception area.

Therefore, this disclosure proposes a method in which, by taking into account that a satellite receiving station moves in a satellite orbit, a satellite receiving station whose reception range covers the location where a terminal is located when the terminal transmits a data frame can target the terminal as a reception target, even if the terminal does not transmit notification frames frequently.

C. First Example As a first example, a method will be described in which, in a system configuration such as that shown in Fig. 7, frame reception information is shared among a plurality of satellite receiving stations, and a satellite receiving station whose reception range covers the location of a terminal targets that terminal as a reception target.

9 shows an example of a communication sequence between satellite receiving station 701, satellite receiving station 702, and terminal 801 when the method according to the present disclosure is applied. Satellite receiving station 701 and satellite receiving station 702 continue to move on their respective satellite orbits at a speed of several kilometers per second, and therefore there are time periods when the location of terminal 801 is within the receivable range 711 of satellite receiving station 701 and time periods when the location is outside the receivable range 711 of satellite receiving station 701 and within the receivable range 712 of satellite receiving station 702.

First, terminal 801 transmits (broadcasts) a notification frame to notify its own ID (SEQ 901). To transmit the notification frame, terminal 801 uses radio resources (time and frequency) that have been allocated in advance within the system for transmitting notification frames.

At this point, terminal 801 is within the reception range 711 of satellite receiving station 701. Therefore, satellite receiving station 701 receives and demodulates the notification frame using the radio resources for transmitting the notification frame (SEQ911), and registers the ID obtained from the notification frame in the list of target terminals (SE912).

Then, the satellite receiving station 701 transmits frame reception information regarding the notification frame received from the terminal 801 to the satellite receiving station 702 located behind in the same satellite orbit (SEQ 913). The frame reception information includes the terminal ID of the sender of the notification frame, the data transmission interval, the frequency error estimation result measured when the notification frame was received, and the propagation delay estimation result. The processing procedure for the satellite receiving station to receive the notification frame will be described later (see Figure 13).

On the other hand, since the notification frame from terminal 801 does not reach satellite receiving station 702, satellite receiving station 702 does not register the ID of terminal 801 in its list of target recipient terminals. Furthermore, satellite receiving station 702 determines whether or not to register terminal 801 in its list of target recipient terminals based on the frame reception information received from satellite receiving station 701, which is located ahead on the same satellite orbit (SEQ 921). The processing procedure and specific determination conditions for the satellite receiving station to receive the frame reception information frame will be described later (see Figure 15). In the example communication sequence shown in Figure 9, the determination conditions are not met because terminal 801 is still located outside of reception range 712 at the time when terminal 801 next transmits a data frame, and therefore satellite receiving station 702 does not register terminal 801 in its list of target recipient terminals.

Next, based on rules 1 to 4 previously determined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the data frame (SEQ 902).

The satellite receiving station 701 uses the ID registered in the list of target receiving terminals to calculate the radio resources (time and frequency) for terminal 801 to transmit the data frame and the code required to demodulate the data frame based on the same rules 1 to 4 as terminal 801, and receives and demodulates the data frame (SEQ 914). Then, the satellite receiving station 701 transmits frame reception information regarding the data frame received from terminal 801 to the satellite receiving station 702, which is located behind in the same satellite orbit (SEQ 915). The processing procedure for receiving data frames by a satellite receiving station will be described later (see Figure 14).

Based on the frame reception information received from satellite receiving station 701, which is located ahead on the same satellite orbit, satellite receiving station 702 determines whether or not to register terminal 801 in its list of target receiving terminals (SEQ 922). Here, since the location of terminal 801 satisfies the determination conditions, such as being within receivable range 712 at the time the next data frame is transmitted, satellite receiving station 702 registers terminal 801 in its list of target receiving terminals.

Furthermore, based on rules 1 to 4 predetermined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the next data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the next data frame (SEQ 903).

Using the ID registered in the list of target terminals to receive data, satellite receiving station 701 calculates the radio resources (time and frequency) for terminal 801 to transmit the data frame and the code required to demodulate the data frame from the terminal ID based on the same rules 1 to 4 as terminal 801, and attempts to receive and demodulate the data frame (SEQ 916 ). However, because terminal 801 is located outside receivable range 711 and within receivable range 712 of satellite receiving station 702, demodulation of the data frame from terminal 801 fails.

The data frame from terminal 801 also arrives at satellite receiving station 702. Satellite receiving station 702 has registered terminal 801's ID in its list of target receiving terminals based on the frame reception information received from terminal 801, and so, based on the same rules 1 to 4 as terminal 801, calculates the radio resources (time and frequency) from which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and attempts to receive and demodulate the data frame. Then, because terminal 801 is located within reception range 712, satellite receiving station 702 successfully demodulates the data frame from terminal 801 (SEQ 924).

C-2. Device Configuration Even in a system that shares frame reception information, the terminal configuration can be the same as that shown in Fig. 3, so in this section C, the explanation of the terminal configuration will be omitted.

Figure 10 shows an example of the functional configuration of a satellite receiving station 1000 in a system that shares frame reception information. The satellite receiving station 1000 is used on board a low-earth orbit satellite that orbits the Earth in one of the satellite orbits shown in Figure 7. The satellite receiving station 1000 has an LPWA unit 1200 and an inter-satellite communication unit 1300.

The LPWA unit 1200 receives frames transmitted by a terrestrial terminal. The same components of the LPWA unit 1200 as those of the receiving station 200 shown in Figure 4 are given the same reference numbers as in Figure 4. The LPWA unit 1200 further includes a target terminal determination unit 206. Based on frame reception information received from a satellite receiving station ahead on the same satellite orbit, the target terminal determination unit 206 determines whether or not to register the terminal in its own list of target terminals for reception. The frame detection and demodulation unit 203 also outputs the terminal ID of the sender of the received notification frame or data frame, the frame transmission interval, the frequency error estimation result measured when the notification frame was received, and the propagation delay estimation result to the frame generation unit 1303 on the inter-satellite communication unit 1300 side.

The inter-satellite communication unit 1300 includes a wireless communication unit 1301, a wireless control unit 1302, a frame generation unit 1303, and a frame detection/demodulation unit 1304.

The wireless communication unit 1301 transmits and receives wireless signals. Under the control of the wireless control unit 1302, the wireless communication unit 1301 receives radio waves, converts them into wireless signals, and passes them to the frame detection and demodulation unit 1304. Also, under the control of the wireless control unit 1302, the wireless communication unit 1301 converts frames generated by the frame generation unit 1303 into wireless signals and transmits them.

The wireless control unit 1302 controls the wireless communication unit 1301 so that frames can be sent and received between satellite receiving stations.

The frame generation unit 1303 stores the frame reception information obtained from the frame detection/demodulation unit 203 on the LPWA unit 1200 side in data and generates a frame according to a specified format.

The frame detection and demodulation unit 1304 detects and demodulates frames from the received signal of the wireless communication unit 1301. If the frame detection and demodulation unit 1304 successfully demodulates the frame, it passes the received data, i.e., frame reception information, to the target terminal determination unit 206.

C-3. Frame Structure This section describes the frame structure used in the LPWA wireless communication system according to the present disclosure. However, since the notification frame and data frame transmitted from the terminal to the satellite receiving station have the same frame structure as shown in FIG. 5, detailed description thereof will be omitted here.

Figure 11 shows an example of the frame structure of the data portion of a frame reception information frame. The format of the portion other than the data portion will depend on the communication method used. The frame reception information frame is a frame that notifies subsequent satellite receiving stations on the same satellite orbit of frame reception information regarding frames received by a satellite receiving station from a terminal.

The nSTA field stores the number of frame reception information (Frame Rx Info) stored in the data portion. The nSTA field is followed by the same number of Frame Rx Info fields as listed in the nSTA field. Each Frame Rx Info field stores frame reception information regarding frames received from each terminal. In the frame configuration example shown in Figure 11, the nSTA field stores information indicating the value n, and n Frame Rx Info fields are stored.

Each Frame Rx Info field includes an ID field, an Ftype field, a Freq. Error field, a Propagation Delay field, a Data Tx Interval field, and a Next Data Tx Interval field.

The ID field stores the ID (terminal ID) of the terminal that sent the received frame. The Ftype field stores a value indicating the type of frame received. Specifically, a value of 0 stored in the Ftype field indicates a notification frame, and a value of 1 indicates a data frame. The Freq. Error field stores the frequency error estimation result for the received frame. The Propagation Delay field stores the propagation delay estimation result for the received frame. The Data Tx Interval field stores the data frame transmission interval. The Next Data Tx Interval field stores the time until the next data frame is transmitted. The Next Data Tx Interval field is added only when Ftype is 0, i.e., when a notification frame is received.

C-4. Processing Operation of Terminal Fig. 12 shows the processing operation performed by the terminal in the form of a flowchart.

The terminal first determines whether α time has elapsed since the terminal was powered on or since the terminal last transmitted a notification frame (step S1201). The transmission interval α for notification frames is assumed to be preset.

Immediately after power-on or after α hours have passed since the last notification frame was sent (Yes in step S1201), the terminal executes the notification frame sending process.

When transmitting a notification frame, the terminal first calculates information such as the radio resources (time, frequency) to be used to transmit the notification frame, the preamble and synchronization information required for frame generation, and the scrambling pattern based on rules 1 to 4 described above (step S1202).

The terminal then generates a notification frame using the preamble, synchronization information, and scrambling pattern calculated in step S1202 (step S1203).

Next, the terminal determines whether the transmission time calculated in step S1202 has arrived (step S1204). When the transmission time for the notification frame arrives (Yes in step S1204), the terminal transmits the notification frame generated in step S1203 using the transmission frequency calculated in step S1202 (step S1205).

On the other hand, immediately after power-on or if α hours have not passed since the last notification frame was sent (No in step S1201), and after sending the notification frame, the terminal determines whether β hours have passed since the last data frame was sent (step S1206).

If β time has elapsed since the last data frame was transmitted (Yes in step S1206), the terminal calculates information such as the radio resources (time and frequency) to be used to transmit the data frame, the preamble and synchronization information required for frame generation, and the scrambling pattern based on rules 1 to 4 (step S1207).

The terminal then generates a data frame using the preamble, synchronization information, and scrambling pattern calculated in step S1207 (step S1208).

Next, the terminal determines whether the transmission time calculated in step S1207 has arrived (step S1209). When the transmission time arrives (Yes in step S1209), the terminal transmits the data frame generated in step S1208 using the transmission frequency calculated in step S1207 (step S1210).

If β time has not elapsed since the last data frame was transmitted (No in step S1206), or after the data frame has been transmitted, the terminal returns to step S1201 and repeatedly transmits the notification frame.

C-5. Processing Operation of Satellite Receiving Station In this disclosure, the satellite receiving station is capable of performing notification frame reception processing, data frame reception processing, and frame reception information frame transmission/reception processing in parallel.

Figure 13 shows the processing steps for a satellite receiving station to receive a notification frame in the form of a flowchart.

First, the satellite receiving station refers to the list of target terminals and calculates the radio resources (time and frequency) for receiving the notification frame from the registered terminal and the information necessary for demodulating the notification frame based on rules 1 to 4 described above (step S1301).

Next, the satellite receiving station determines whether the notification frame reception time calculated in step S1301 has arrived (step S1302). When the notification frame reception time arrives (Yes in step S1302), the satellite receiving station detects the notification frame from the terminal in question using the preamble and synchronization information calculated in step S1301 (step S1303), and demodulates the received notification frame using the scrambling pattern calculated in step S1301 (step S1304). The satellite receiving station then determines whether the demodulation of the notification frame was successful (step S1305).

Here, if the notification frame is successfully demodulated (Yes in step S1305), the satellite receiving station registers information such as the terminal ID and data transmission interval stored in the received notification frame in the list of target terminals (step S1306).

Next, the satellite receiving station generates a frame reception information frame (step S1307) using the terminal ID and data transmission interval stored in the notification frame, the frequency error estimation result and propagation delay estimation result calculated when detecting and demodulating the notification frame in steps S1303 to S1304, and the transmission interval from the notification frame transmission to the data frame transmission.

The satellite receiving station then transmits a frame reception information frame to a subsequent satellite receiving station in the same satellite orbit (step S1308).

On the other hand, if demodulation of the notification frame fails (No in step S1305), the satellite receiving station does not register the terminal to the list of target receiving terminals or send a frame reception information frame.

In the processing procedure shown in Figure 13, the satellite receiving station transmits a frame reception information frame each time it receives a notification frame, but the satellite receiving station may also receive multiple notification frames and then transmit the frame reception information frame all at once.

Figure 14 shows in the form of a flowchart the processing procedure for a satellite receiving station to receive a data frame. However, it is assumed that the satellite receiving station can execute the data frame reception process in parallel for each terminal registered in the list of target terminals to receive data.

First, the satellite receiving station obtains information about the target terminal, such as the terminal ID, from the list of target terminals (step S1401), and calculates the radio resources (time and frequency) for receiving the data frame from the target terminal, as well as the preamble, synchronization information, and scrambling pattern required for demodulating the data frame, based on rules 1 to 4 described above (step S1402).

The satellite receiving station then determines whether the data frame reception time calculated in step S1402 has arrived (step S1403). When the data frame reception time arrives (Yes in step S1403), the satellite receiving station detects the data frame from the terminal in question using the preamble and synchronization information calculated in step S1402 (step S1404), and demodulates the received data frame using the scrambling pattern calculated in step S1402 (step S1405). The satellite receiving station then determines whether the data frame demodulation was successful (step S1406).

Here, if the data frame is successfully demodulated (Yes in step S1406), the satellite receiving station generates a frame reception information frame (step S1407) using the data transmission interval and the frequency error estimation results and propagation delay estimation results calculated when detecting and demodulating the data frame in steps S1404 to S1405.

The satellite receiving station then transmits a frame reception information frame to a subsequent satellite receiving station in the same satellite orbit (step S1408).

On the other hand, if demodulation of the data frame fails (No in step S1406), the satellite receiving station does not transmit the frame reception information frame.

In the processing procedure shown in Figure 14, the satellite receiving station transmits a frame reception information frame each time it receives a data frame, but the satellite receiving station may also receive multiple data frames and then transmit the frame reception information frame all at once.

Figure 15 shows in flowchart form the processing procedure for a satellite receiving station to receive a frame reception information frame.

First, the satellite receiving station determines whether or not it has received a frame reception information frame (step S1501). Then, when the satellite receiving station receives a frame reception information frame (Yes in step S1501), it demodulates the received frame (step S1502) and determines whether or not the demodulation process was successful (step S1503).

If the satellite receiving station successfully demodulates the frame reception information frame (Yes in step S1503), it acquires nSTA, which indicates the number of frame reception information stored in the frame reception information frame (step S1504), and repeatedly processes each subsequent frame reception information (Frame Rx Info) for the number of frame reception information pieces.

The satellite receiving station first estimates the remaining time during which the notification frame or data frame can be received at the same satellite receiving station based on the frequency error estimation result (Freq. Error) and the propagation delay estimation result (Propagation Delay) (step S1505).

Next, based on the remaining reception time and data transmission interval estimated in step S1505, it is determined whether the next data frame transmitted from the terminal in question can be received by the same satellite receiving station (step S1506). Here, for the data transmission interval, if Ftype is 0, i.e., the received frame is a notification frame, the value stored in the Next Data Tx Interval field is used; if Ftype is 1, i.e., the received frame is a data frame, the value stored in the Data Tx Interval field is used.

If the next data frame transmitted from the terminal can be received by the same satellite receiving station (Yes in step S1506), the satellite receiving station returns to step S1505 and repeats the processing of the next frame reception information (Frame Rx Info).

On the other hand, if the next data frame transmitted from the terminal in question cannot be received by the same satellite receiving station (No in step S1506), the satellite receiving station registers the terminal ID and data transmission interval stored in the frame reception information (Frame Rx Info) in its own list of target terminals (step S1507), then returns to step S1505 and repeats the processing of the next frame reception information (Frame Rx Info).

A supplementary note is provided regarding the frequency error estimation and propagation delay estimation in step S1505 in the flowchart shown in Figure 15. Figure 16 shows an example of the change in frequency error and propagation delay over time when the satellite is at an altitude of 600 km and has a frequency of 2 GHz. Figure 16 shows an example of a case where a satellite passes overhead, and the satellite receiving station is capable of receiving signals from one terminal for six minutes, from 0 to 6 minutes. The frequency error was calculated using the model described in 3GPP TR 38.811 V15.1.0 (Non-Patent Document 1). In the example shown in Figure 16, the satellite receiving station is directly overhead at the third minute, and the propagation delay is minimized and the sign of the frequency error is reversed.

By comparing the frequency error estimation results and propagation delay estimation results of the received frame from the terminal with the chart in Figure 16, it is possible to determine which time within the six-minute period during which the frame from the terminal can be received. Furthermore, when the satellite receiving station moves in one direction at a predetermined speed above the terminal, it is possible to estimate the relative positional relationship between the terminal and the satellite receiving station that corresponds to the calculated time.

For example, if the data transmission interval is 3 minutes, the frequency error estimate reported in the frame reception information frame is 30 kHz, and the propagation delay estimate is 2.5 milliseconds, this corresponds to the second minute of the six minutes available for reception. Therefore, the next data frame corresponds to the fifth minute of the six minutes available for reception, and can be received by the same satellite receiving station.

On the other hand, if the data transmission interval is 3 minutes, the estimated frequency error reported in the frame reception information frame is -30 kHz, and the estimated propagation delay is 2.5 milliseconds, this corresponds to the fifth minute of the six minutes available for reception. Therefore, the next data frame will be received after the six minutes available for reception have elapsed, and therefore cannot be received by the same receiving station. In this way, considering that the satellite receiving station moves in one direction along the satellite orbit, it is possible to estimate the remaining time available for reception at the satellite receiving station that received the frame from the terminal by combining the frequency error and propagation delay.

In short, in the method of the first embodiment, each satellite receiving station shares frame reception information including the frequency error estimation result and propagation delay estimation result of the frame received from the terminal, and determines whether the next data frame transmitted from the terminal can be received by the same satellite receiving station based on the frequency error estimation result, propagation delay estimation result, frame transmission interval, and the positional relationship of adjacent satellite receiving stations.

According to the first embodiment, by a satellite receiving station transmitting frame reception information to a subsequent satellite receiving station in the same orbit, even if the terminal does not transmit notification frames frequently, it is possible for a satellite receiving station that has a reception range covering the location where the terminal is located to receive and demodulate the data frame from the terminal the next time the terminal transmits a data frame.

D. Second Embodiment In the first embodiment, a method was described in which satellite receiving stations notify each other of the frequency error estimation result and propagation delay estimation result of a frame received from a terminal to determine whether a data frame transmitted from the terminal can be received by the same satellite receiving station. In contrast, in the second embodiment, a method is described in which, when the terminal is equipped with a GPS receiver, satellite receiving stations notify each other of the terminal's position coordinates (latitude and longitude) to determine whether a data frame transmitted from the terminal can be received by the same satellite receiving station.

D-1. Frame Configuration Example Figure 17 shows an example of the frame configuration of the data portion of the frame reception information frame when notifying the terminal's position coordinates. The format of the portion other than the data portion depends on the communication method used. As described above, the frame reception information frame is a frame that notifies a subsequent satellite receiving station on the same satellite orbit of frame reception information regarding a frame received by a satellite receiving station from a terminal.

The nSTA field stores the number of frame reception information (Frame Rx Info) stored in the data portion. The nSTA field is followed by the same number of Frame Rx Info fields as listed in the nSTA field. Each Frame Rx Info field stores frame reception information regarding frames received from each terminal. In the frame configuration example shown in Figure 17, the nSTA field stores information indicating the value n, and n Frame Rx Info fields are stored.

Each Frame Rx Info field includes an ID field, an Ftype field, a Longitude field, a Latency field, a Frame Rx Time field, a Data Tx Interval field, and a Next Data Tx Interval field.

The ID field stores the ID (terminal ID) of the terminal that sent the received frame. The Ftype field stores a value indicating the type of frame received. Specifically, a value of 0 stored in the Ftype field indicates a notification frame, and a value of 1 indicates a data frame. The Longitude field stores the latitude at which the terminal is located. The Latitude field stores the longitude at which the terminal is located. The Frame Rx Time field stores the time at which the frame was received from the terminal. The Data Tx Interval field stores the data frame transmission interval. The Next Data Tx Interval field stores the time until the next data frame is transmitted. The Next Data Tx Interval field is only added when Ftype is 0, i.e., when a notification frame has been received.

D-2 Receiving Operation of Frame Reception Information Frame Fig. 18 shows in the form of a flowchart the processing procedure for the satellite receiving station to receive the frame reception information frame.

First, the satellite receiving station determines whether or not it has received a frame reception information frame (step S1801). Then, when the satellite receiving station receives a frame reception information frame (Yes in step S1801), it demodulates the received frame (step S1802) and determines whether or not the demodulation process was successful (step S1803).

If the satellite receiving station successfully demodulates the frame reception information frame (Yes in step S1803), it acquires nSTA, which indicates the number of frame reception information stored in the frame reception information frame (step S1804), and repeatedly processes each subsequent frame reception information (Frame Rx Info) for the number of frame reception information pieces.

First, the remaining time during which the notification frame or data frame can be received by the same satellite receiving station is estimated from the terminal's location coordinates and the time the frame was received from that terminal (step S1805).

Next, based on the remaining reception time and data transmission interval estimated in step S1805, it is determined whether the next data frame transmitted from the terminal in question can be received by the same satellite receiving station (step S1806). Here, for the data transmission interval, if Ftype is 0, i.e., the received frame is a notification frame, the value stored in the Next Data Tx Interval field is used; if Ftype is 1, i.e., the received frame is a data frame, the value stored in the Data Tx Interval field is used.

If the next data frame transmitted from the terminal can be received by the same satellite receiving station (Yes in step S1806), the satellite receiving station returns to step S1805 and repeats the processing of the next frame reception information (Frame Rx Info).

On the other hand, if the next data frame transmitted from the terminal in question cannot be received by the same satellite receiving station (No in step S1806), the satellite receiving station registers the terminal ID and data transmission interval stored in the frame reception information (Frame Rx Info) in its own list of target terminals (step S1807), then returns to step S1805 and repeats the processing of the next frame reception information (Frame Rx Info).

To transmit the data received by the LPWA to the ground, the satellite receiving station must communicate with the ground station via satellite communications. For this reason, the satellite receiving station generally knows its own location coordinates using GPS or other means.

In addition, when constructing a satellite constellation, information on satellite receiving stations orbiting in the same satellite orbit is stored. For example, suppose the information is such that a satellite in front of it passes the same point on Earth every six minutes.

Specifically, if the time the frame reception information frame is received is 12:00, the time the location information notified by the terminal in the data frame becomes within reception range is 12:05, and the frame is received at 11:58, the remaining reception time at the satellite receiving station that received the notification frame or data frame is 7 minutes. If the data transmission interval is 3 minutes, the next data frame can be received by the same receiving station.

On the other hand, if the time the frame reception information frame was received was 12:00, the time the location information notified by the terminal in the data frame became within reception range was 12:01, and the frame was received at 11:59, the remaining reception time at the satellite receiving station that received the notification frame or data frame would be 2 minutes. If the data transmission interval was 3 minutes, the next data frame would not be receivable at the same receiving station.

As such, considering that low-orbit satellites move in one direction in their satellite orbits and that the relative positions of each satellite receiving station can be determined using the constellation, it is possible to estimate the remaining reception time at the receiving station that received the frame from the location information notified by the terminal and the time the frame was received.

E. Third Embodiment In the first embodiment (the example of the communication sequence shown in FIG. 9 ), satellite receiving station 701 keeps terminal 801 registered in its list of target terminals even after terminal 801 has passed its own reception range 711, and performs unnecessary reception and demodulation processing on data frames from terminal 801. In contrast, in the third embodiment, a method will be described in which the satellite receiving station does not perform unnecessary reception and demodulation processing on terminals that have passed through its own reception range.

19 shows an example of a communication sequence between satellite receiving station 701, satellite receiving station 702, and terminal 801 when the method according to the third embodiment is applied. Satellite receiving station 701 and satellite receiving station 702 continue to move on their respective satellite orbits at a speed of several kilometers per second, so there are time periods when the location of terminal 801 is within the receivable range 711 of satellite receiving station 701, and time periods when the location is outside the receivable range 711 of satellite receiving station 701 and within the receivable range 712 of satellite receiving station 702.

First, terminal 801 transmits (broadcasts) a notification frame to notify its own ID (SEQ1901). To transmit the notification frame, terminal 801 uses radio resources (time and frequency) that have been allocated in advance within the system for transmitting notification frames.

At this point, terminal 801 is within the reception range 711 of satellite receiving station 701. Therefore, satellite receiving station 701 receives and demodulates the notification frame using the radio resources for transmitting the notification frame (SEQ1911), and registers the ID obtained from the notification frame in the list of target terminals (SE1912). Then, satellite receiving station 701 transmits frame reception information regarding the notification frame received from terminal 801 to satellite receiving station 702, which is located subsequent to it on the same satellite orbit (SEQ1913). The frame reception information includes the terminal ID of the sender of the notification frame, the data transmission interval, the position coordinates of the terminal in question, and the reception time of the notification frame. Satellite receiving station 701 receives and demodulates the notification frame and transmits the notification frame according to the processing procedure shown in FIG. 13.

On the other hand, since the notification frame from terminal 801 does not reach satellite receiving station 702, satellite receiving station 702 does not register the ID of terminal 801 in its list of target receiving terminals. Satellite receiving station 702 also receives and processes the frame reception information frame from satellite receiving station 701, which is located ahead on the same satellite orbit, and determines whether or not to register terminal 801 in its list of target receiving terminals (SEQ 1921). The processing procedure for a satellite receiving station to receive a frame reception information frame will be described later (see Figure 21). Here, satellite receiving station 702 determines that it cannot receive the next data frame transmitted from terminal 801 based on the remaining receivable time and data transmission interval estimated based on the frequency error estimation result and propagation delay estimation result, and therefore does not register terminal 801 in its list of target receiving terminals.

Next, based on rules 1 to 4 previously determined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the data frame (SEQ 1902).

The satellite receiving station 701 uses the ID registered in the list of target receiving terminals to calculate the radio resources (time and frequency) for terminal 801 to transmit the data frame and the code required to demodulate the data frame from the terminal's ID, based on the same rules 1 to 4 as terminal 801, and receives and demodulates the data frame (SEQ 1914). Then, the satellite receiving station 701 transmits frame reception information regarding the data frame received from terminal 801 to the satellite receiving station 702, which is located subsequent to it on the same satellite orbit (SEQ 1915). The satellite receiving station 701 receives and demodulates the data frame and transmits the notification frame according to the processing procedure shown in FIG. 14.

The satellite receiving station 702 receives and processes the frame reception information frame from the satellite receiving station 701, which is located ahead on the same satellite orbit, and determines whether or not to register the terminal 801 in its list of target receiving terminals (SEQ 1922). The processing procedure for the satellite receiving station to receive the frame reception information frame will be described later (see Figure 21). Here, the satellite receiving station 702 determines that the next data frame transmitted from the terminal 801 can be received by the satellite receiving station 702 based on the remaining receivable time and data transmission interval estimated based on the frequency error estimation result and the propagation delay estimation result, and registers the terminal 801 in its list of target receiving terminals (SEQ 1923).

Then, satellite receiving station 702 transmits a target terminal list registration notification frame to satellite receiving station 701, which is located ahead on the same satellite orbit, to notify it that terminal 801 has been registered in the target terminal list (SEQ 1924). Upon receiving the target terminal list registration notification frame from satellite receiving station 702, satellite receiving station 701 deletes the entry for terminal 801 from its own target terminal list (SEQ 1916).

Furthermore, based on rules 1 to 4 predetermined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the next data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the next data frame (SEQ 1903).

At this point, terminal 801 is located outside the reception range 711 of satellite receiving station 701. Since satellite receiving station 701 has already removed terminal 801 from its list of target terminals for reception, it does not perform unnecessary reception and demodulation processing on the data frame from terminal 801.

In addition, satellite receiving station 702 registers terminal 801's ID in its list of target receiving terminals based on the frame reception information received from satellite receiving station 701, and therefore, based on the same rules 1 to 4 as terminal 801, calculates the radio resources (time and frequency) from which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and attempts to receive and demodulate the data frame, successfully completing the demodulation (SEQ 1925).

According to the example communication sequence shown in Figure 19, the satellite receiving station 701 can prevent unnecessary reception and demodulation processing from being performed after the terminal 801 has passed out of its reception range 711 by receiving notification from a subsequent satellite receiving station 702 on the same satellite orbit.

E-2. Frame Configuration Example Fig. 20 shows an example of the frame configuration of the data portion of the target terminal list registration notification frame. The format other than the data portion depends on the communication method used. As shown in Fig. 19, the target terminal list registration notification frame is a frame that notifies a satellite receiving station that it has registered a terminal in its own target terminal list based on frame reception information received from a satellite receiving station ahead on the same satellite orbit.

The nSTA field stores the number of terminal IDs (STA Rx IDs) stored in the data section. The nSTA field is followed by the same number of STA ID fields as listed in the nSTA field. Each STA ID field stores information about the terminal ID registered in the target terminal list.

E-3 Processing Operation of Satellite Receiving Station Figure 21 shows in the form of a flowchart the processing procedure for the satellite receiving station to receive the frame reception information frame.

First, the satellite receiving station determines whether or not it has received a frame reception information frame (step S2101). Then, when it receives a frame reception information frame (Yes in step S2101), it demodulates the received frame (step S2102) and determines whether or not the demodulation process was successful (step S2103).

If the satellite receiving station successfully demodulates the frame reception information frame (Yes in step S2103), it acquires nSTA, which indicates the number of frame reception information stored in the frame reception information frame (step S2104), and repeatedly processes each subsequent frame reception information (Frame Rx Info) for the number of frame reception information pieces.

First, the remaining time during which the notification frame or data frame can be received at the same satellite receiving station is estimated from the frequency error estimation result (Freq. Error) and the propagation delay estimation result (Propagation Delay) (step S2105).

Next, based on the remaining reception time and data transmission interval estimated in step S2105, it is determined whether the next data frame transmitted from the terminal in question can be received by the same satellite receiving station (step S2106). Here, for the data transmission interval, if Ftype is 0, i.e., the received frame is a notification frame, the value stored in the Next Data Tx Interval field is used; if Ftype is 1, i.e., the received frame is a data frame, the value stored in the Data Tx Interval field is used.

If the next data frame transmitted from the terminal can be received by the same satellite receiving station (Yes in step S2106), the satellite receiving station returns to step S2105 and repeats the processing of the next frame reception information (Frame Rx Info).

On the other hand, if the next data frame transmitted from the terminal in question cannot be received by the same satellite receiving station (No in step S2106), the satellite receiving station registers the terminal ID and data transmission interval stored in the frame reception information (Frame Rx Info) in its own list of target receiving terminals (step S2107), then adds the terminal ID of the terminal in question to the target receiving terminal list registration notification frame (see Figure 20) (step S2108), returns to step S2105, and repeats the processing of the next frame reception information (Frame Rx Info).

When processing has been completed for all nSTA frame reception information stored in the frame reception information frame received in step S2101, the satellite receiving station determines whether it is necessary to transmit a target terminal list registration notification frame (step S2109). If it is necessary to transmit a target terminal list registration notification frame (Yes in step S2109), the satellite receiving station stores the number of stored target terminal list registration IDs in nSTA and transmits the target terminal list registration notification frame (step S2110).

Figure 22 shows in flowchart form the processing procedure for a satellite receiving station to receive a target terminal list registration notification frame.

First, the satellite receiving station determines whether it has received a target terminal list registration notification frame (step S2201). When the satellite receiving station receives a target terminal list registration notification frame (Yes in step S2201), it demodulates the received frame (step S2202) and determines whether the demodulation was successful (step S2203).

If the satellite receiving station successfully demodulates the target terminal list registration notification frame (Yes in step S2203), it acquires nSTA, which indicates the number of STA ID fields stored in the target terminal list registration notification frame (step S2204), and repeatedly processes each subsequent STA ID field the number of times equal to nSTA. During this repeated process, it deletes the entry for the terminal ID stored in the target terminal list registration notification frame from its own target terminal list (step S2205).

By processing the target terminal list registration notification frame according to the processing procedure shown in Figure 22, a satellite receiving station can delete from its own target terminal list a terminal that has become a new target for reception at a subsequent satellite receiving station in the same satellite orbit.

Therefore, according to the communication sequence of the third embodiment, the satellite receiving station can avoid performing unnecessary reception and demodulation processing for terminals that have passed through its reception range.

F. Fourth Embodiment In the first embodiment described above, it is assumed that when a terminal currently receiving data from a certain satellite receiving station transmits a data frame next, the location of the terminal will be within the receivable range of the terminal itself or the next satellite receiving station in the same satellite orbit (see, for example, the example communication sequence shown in FIG. 9).

However, if the data transmission interval of a terminal is long, such as several hours, it is possible that when the terminal next transmits a data frame, it will have moved into the reception range of a satellite receiving station several (two or more) behind the satellite receiving station that is the target of reception on the same satellite orbit.

Therefore, in the fourth embodiment, we describe a method that allows the appropriate satellite receiving station to receive the data frame even if the terminal moves within the reception range of several (two or more) satellite receiving stations on the same satellite orbit during the terminal's data transmission interval.

F-1. Example of a communication sequence Figure 23 shows an example of a communication sequence between satellite receiving stations 701 to 703 and terminal 801 when the method according to the fourth embodiment is applied. Since satellite receiving stations 701 to 703 each continue to move on their satellite orbits at a speed of several kilometers per second, there are time periods when the location of terminal 801 is within the receivable range 711 of satellite receiving station 701, time periods when the location is outside the receivable range 711 of satellite receiving station 701 and within the receivable range 712 of satellite receiving station 702, and time periods when the location is outside the receivable range 712 of satellite receiving station 702 and within the receivable range 713 of satellite receiving station 703.

Based on rules 1 to 4 previously determined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the data frame from its own ID and the code required to generate the data frame, and then generates and transmits (broadcasts) the data frame (SEQ2301).

Let us assume that terminal 801 is within the reception range 711 of satellite receiving station 701, and that satellite receiving station 701 has registered terminal 801 in its list of target receiving terminals. Therefore, using the ID registered in its list of target receiving terminals, satellite receiving station 701 calculates, based on the same rules 1 to 4 as terminal 801, the radio resources (time and frequency) with which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and receives and demodulates the data frame (SEQ 2311). Here, we assume that satellite receiving station 701 successfully receives and demodulates the data frame.

Then, the satellite receiving station 701 transmits frame reception information regarding the data frame received from the terminal 801 to the satellite receiving station 702 located subsequent to it on the same satellite orbit (SEQ2312). The frame reception information includes information such as the terminal ID that received the data frame, the frequency error estimation result, the propagation delay estimation result, and the ID of the receiving satellite receiving station.

Satellite receiving station 702 receives and processes the frame reception information frame from satellite receiving station 701, which is located ahead on the same satellite orbit, and determines whether or not to register terminal 801 in its list of target receiving terminals (SEQ 2321). Here, satellite receiving station 702 determines that it cannot receive the next data frame transmitted from terminal 801 based on the remaining receivable time and data transmission interval estimated based on the frequency error estimation result and propagation delay estimation result, and does not register terminal 801 in its list of target receiving terminals.

Next, based on the received frame reception information, satellite receiving station 702 determines whether or not to transmit this frame reception information to satellite receiving station 703, which is located subsequent to it on the same satellite orbit (SEQ 2322). Then, based on the determination result, satellite receiving station 702 transmits the frame reception information to satellite receiving station 703 (SEQ 2323).

Satellite receiving station 703 receives and processes the frame reception information frame from satellite receiving station 702, which is located ahead on the same satellite orbit, and determines whether or not to register terminal 801 in its list of target receiving terminals (SEQ2331). Here, satellite receiving station 703 determines, based on the remaining receivable time and data transmission interval estimated based on the frequency error estimation result and propagation delay estimation result, that the next data frame transmitted from terminal 801 can be received by satellite receiving station 703, and registers terminal 801 in its list of target receiving terminals (SEQ2332).

Then, the satellite receiving station 703 transmits a target terminal list registration notification frame to the satellite receiving station 701 that received the data frame from the terminal 801, notifying it that the terminal 801 has been registered in the target terminal list (SEQ2333). When the satellite receiving station 701 receives the target terminal list registration notification frame from the satellite receiving station 702, it deletes the entry for the terminal 801 from its own target terminal list (SEQ2313).

Furthermore, based on rules 1 to 4 predetermined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the next data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the next data frame (SEQ2302).

At this point, terminal 801 is located outside the reception range 711 of satellite receiving station 701 and the reception range 712 of satellite receiving station 702. Satellite receiving station 701 has already removed terminal 801 from its list of target receiving terminals, so it does not perform unnecessary reception and demodulation processing on data frames from terminal 801. Furthermore, satellite receiving station 702 has not registered terminal 801 in its list of target receiving terminals, so it does not perform reception and demodulation processing on data frames from terminal 801.

In addition, satellite receiving station 703 has registered terminal 801's ID in its list of target receiving terminals based on the frame reception information received from satellite receiving station 702, and therefore, based on the same rules 1 to 4 as terminal 801, calculates the radio resources (time and frequency) from which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and attempts to receive and demodulate the data frame, successfully completing the demodulation (SEQ 2334).

According to the example communication sequence shown in Figure 23, when terminal 801 next transmits a data frame, it has moved into the reception range 713 of satellite receiving station 703, which is two stations behind the satellite receiving station 701 that is the reception target for terminal 801, but satellite receiving station 703 can receive the next data frame from terminal 801. Furthermore, by receiving a notification from the subsequent satellite receiving station 703 on the same satellite orbit, satellite receiving station 701 can prevent unnecessary reception and demodulation processing from being performed after terminal 801 has passed out of its reception range 711.

F-2. Frame Structure Figure 24 shows an example of the frame structure of the data portion of a frame reception information frame. The format of the portion other than the data portion depends on the communication method used. The frame reception information frame is a frame that notifies subsequent satellite receiving stations on the same satellite orbit of frame reception information regarding a frame received by a satellite receiving station from a terminal.

The nSTA field stores the number of frame reception information (Frame Rx Info) stored in the data portion. The nSTA field is followed by the same number of Frame Rx Info fields as listed in the nSTA field. Each Frame Rx Info field stores frame reception information regarding frames received from each terminal. In the frame configuration example shown in Figure 24, the nSTA field stores information indicating the value n, and n Frame Rx Info fields are stored.

Each Frame Rx Info field includes an ID field, an Ftype field, a Freq. Error field, a Propagation Delay field, a Data Tx Interval field, a Next Data Tx Interval field, and a Data Rx SAT ID.

The ID field stores the ID (terminal ID) of the terminal that sent the received frame. The Ftype field stores a value indicating the type of the received frame. Specifically, a value of 0 in the Ftype field indicates a notification frame, and a value of 1 indicates a data frame. The Freq. Error field stores the frequency error estimation result for the received frame. The Propagation Delay field stores the propagation delay estimation result for the received frame. The Data Tx Interval field stores the data frame transmission interval. The Next Data Tx Interval field stores the time until the next data frame is transmitted. The Next Data Tx Interval field is only added when Ftype is 0, that is, when a notification frame is received. The Data Rx SAT ID field stores the ID (SAT ID) of the satellite receiving station that received the frame in question.

F-3 Receiving Operation of Frame Reception Information Frame Figures 25A and 25B show in the form of a flowchart the processing procedure for the satellite receiving station to receive the frame reception information frame.

First, the satellite receiving station determines whether or not it has received a frame reception information frame (step S2501). Then, when it receives a frame reception information frame (Yes in step S2501), it demodulates the received frame (step S2502) and determines whether or not the demodulation process was successful (step S2503).

If the satellite receiving station successfully demodulates the frame reception information frame (Yes in step S2503), it acquires nSTA, which indicates the number of frame reception information stored in the frame reception information frame (step S2504), and repeatedly processes each subsequent frame reception information (Frame Rx Info) for the number of frame reception information pieces.

When processing the frame reception information, the satellite receiving station first estimates the remaining time during which the notification frame or data frame can be received at the same satellite receiving station from the frequency error estimation result (Freq. Error) and the propagation delay estimation result (Propagation Delay) (step S2505).

Next, based on the remaining reception time and data transmission interval estimated in step S2505, it is determined whether the next data frame transmitted from the terminal in question can be received by the same satellite receiving station (step S2506). Here, for the data transmission interval, if Ftype is 0, i.e., the received frame is a notification frame, the value stored in the Next Data Tx Interval field is used; if Ftype is 1, i.e., the received frame is a data frame, the value stored in the Data Tx Interval field is used.

If the next data frame transmitted from the terminal can be received by the same satellite receiving station (Yes in step S2506), the satellite receiving station returns to step S2505 and repeats the processing of the next frame reception information (Frame Rx Info).

On the other hand, if the next data frame transmitted from the terminal cannot be received by the same satellite receiving station (No in step S2506), the satellite receiving station further determines whether it can receive the next data frame transmitted from the terminal based on the remaining receivable time and data transmission interval estimated in step S2505 (step S2507).

If the satellite receiving station is capable of receiving the next data frame transmitted from the terminal in question (Yes in step S2507), the satellite receiving station registers the terminal ID and data transmission interval stored in the frame reception information (Frame Rx Info) in its own list of target terminals (step S2508), then adds the terminal ID of the target terminal to the target terminal list registration notification frame (step S2509), and then returns to step S2505 to repeatedly process the next frame reception information (Frame Rx Info).

On the other hand, if the satellite receiving station is unable to receive the next data frame transmitted from the terminal (No in step S2507), the satellite receiving station adds the frame reception information to the frame reception information frame for transmission to the subsequent satellite receiving station on the same satellite orbit (step S2510), then returns to step S2505 and repeats the processing of the next frame reception information (Frame Rx Info).

When processing has been completed for all nSTA frame reception information stored in the frame reception information frame received in step S2501, the satellite receiving station determines whether it is necessary to transmit a target terminal list registration notification frame (step S2511). If it is necessary to transmit a target terminal list registration notification frame (Yes in step S2511), the satellite receiving station stores the number of stored target terminal list registration IDs in nSTA and transmits the target terminal list registration notification frame (step S2512).

Next, the satellite receiving station determines whether it is necessary to transmit a frame reception information frame to a subsequent satellite receiving station on the same satellite orbit (step S2513). If it is necessary to transmit a target terminal list registration notification frame (Yes in step S2513), the satellite receiving station stores the number of target terminal list registration IDs stored in nSTA and transmits the frame reception information frame (step S2514).

F-3. Adjacent Satellite Receiving Station Information List Figure 26 shows an example of an adjacent satellite receiving station information list held by a satellite receiving station. The adjacent satellite receiving station information list shown in the figure registers information on each adjacent satellite receiving station that orbits on the same satellite orbit as the satellite receiving station. The entry for each adjacent satellite receiving station stores the relative position from the own station, a satellite ID that identifies the satellite receiving station, and the distance from the own station. Here, the relative position is information indicating the positional position of the satellite on the same satellite orbit relative to the own station. A negative number indicates a position ahead on the satellite orbit, and a positive number indicates a position behind on the satellite orbit. A relative position of 0 indicates the own station. Furthermore, the distance is information indicating the positional distance from the own station. A negative number indicates a position ahead on the satellite orbit, and a positive number indicates a position behind on the satellite orbit. A distance of 0 indicates the own station.

For example, in the examples shown in Figures 16 and 26, if the data transmission interval is 10 minutes, the frequency error estimation result notified in the frame reception information frame is -30 kHz, the propagation delay estimation result is 2.5 milliseconds, and the satellite ID of the satellite receiving station that received the frame is 502, the next data frame can be received by the local station.

On the other hand, for example, if the data transmission interval is 10 minutes, the frequency error estimation result notified in the frame reception information frame is -30 kHz, the propagation delay estimation result is 2.5 milliseconds, and the satellite ID of the satellite receiving station that received the frame is 503, the next data frame cannot be received by the same satellite receiving station or by the station itself, and frame reception information must be sent to the subsequent satellite receiving station.

According to the fourth embodiment, the terminal does not need to transmit notification frames frequently, and even if the terminal moves into the reception range of a satellite receiving station several stations away during a long data transmission interval, the satellite receiving station within the reception range of the terminal in question can receive and demodulate data frames from the terminal in question.

G. Fifth Embodiment In the first, third, and fourth embodiments described above, each satellite receiving station must process the received frame reception information, which may increase the processing load of the satellite receiving station. Therefore, in the fifth embodiment, a method will be described in which a satellite receiving station that has received a notification frame or a data frame determines which satellite receiving station will receive the next data frame.

G-1. Example of a communication sequence Figure 27 shows an example of a communication sequence between satellite receiving stations 701 to 703 and terminal 801 when the method according to the fifth embodiment is applied. Since satellite receiving stations 701 to 703 each continue to move on their satellite orbits at a speed of several kilometers per second, there are time periods when the location of terminal 801 is within the receivable range 711 of satellite receiving station 701, time periods when the location is outside the receivable range 711 of satellite receiving station 701 and within the receivable range 712 of satellite receiving station 702, and time periods when the location is outside the receivable range 712 of satellite receiving station 702 and within the receivable range 713 of satellite receiving station 703.

Based on rules 1 to 4 previously determined within the system, terminal 801 calculates the radio resources (time and frequency) for transmitting the data frame from its own ID and the code required to generate the data frame, and generates and transmits (broadcasts) the data frame (SEQ2701).

Let us assume that terminal 801 is within the reception range 711 of satellite receiving station 701, and that satellite receiving station 701 has registered terminal 801 in its list of target receiving terminals. Therefore, using the ID registered in its list of target receiving terminals, satellite receiving station 701 calculates, based on the same rules 1 to 4 as terminal 801, the radio resources (time and frequency) with which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and receives and demodulates the data frame (SEQ 2711). Here, we assume that satellite receiving station 701 successfully receives and demodulates the data frame.

Then, based on the terminal ID at the time of receiving the data frame, the frequency error estimation result, the propagation delay estimation result, and information on adjacent satellite receiving stations, the satellite receiving station 701 determines that the satellite receiving station 703 can receive the next data frame transmitted from terminal 801 (SEQ2712), and transmits a target terminal registration frame to the satellite receiving station 703 (SEQ2713).

The satellite receiving station 703 registers the terminal ID and data transmission interval stored in the target terminal registration frame received from the satellite receiving station 701 in its own target terminal list (SEQ 2731).

Furthermore, since the satellite receiving station 701 determines that it is unable to receive the next data frame transmitted from the terminal 801 (SEQ 2712), it deletes the entry for the terminal 801 from its list of target terminals for reception (SEQ 2714).

After that, time passes and terminal 801 generates and transmits (broadcasts) the next data frame (SEQ2702).

At this point, terminal 801 is located outside the reception range 711 of satellite receiving station 701 and the reception range 712 of satellite receiving station 702. Satellite receiving station 701 has already removed terminal 801 from its list of target receiving terminals, so it does not perform unnecessary reception and demodulation processing on data frames from terminal 801. Furthermore, satellite receiving station 702 has not registered terminal 801 in its list of target receiving terminals, so it does not perform reception and demodulation processing on data frames from terminal 801.

In addition, satellite receiving station 703 has registered the ID of terminal 801 in its own list of target terminals based on the target terminal registration frame received from satellite receiving station 701, and therefore, based on the same rules 1 to 4 as terminal 801, calculates the radio resources (time and frequency) from which terminal 801 will transmit the data frame and the code required to demodulate the data frame from the terminal's ID, and attempts to receive and demodulate the data frame, successfully completing the demodulation (SEQ2732).

According to the example communication sequence shown in Figure 27, when terminal 801 next transmits a data frame, it has moved into the reception range 713 of satellite receiving station 703, which is two stations behind satellite receiving station 701 that is the target receiving station for terminal 801. However, based on the process by satellite receiving station 701 to determine which satellite receiving stations are available for reception, satellite receiving station 703 can receive the next data frame from terminal 801. Furthermore, by receiving a notification from the subsequent satellite receiving station 703 on the same satellite orbit, satellite receiving station 701 can prevent unnecessary reception and demodulation processing from being performed after terminal 801 has passed out of its reception range 711.

G-2. Example of frame configuration Figure 28 shows an example of the frame configuration of the data portion of a target terminal registration frame. The format of the portion other than the data portion shall be in accordance with the communication method used. The target terminal registration frame is a frame in which a satellite receiving station instructs a subsequent satellite receiving station on the same satellite orbit to register the relevant terminal in the target terminal list.

The nSTA field stores the number of terminal information (STA Info) stored in the data portion. The nSTA field is followed by the same number of STA Info fields as listed in the nSTA field. Each STA Info field stores terminal information about the terminal that instructs the destination satellite receiving station to register in the target terminal list. In the frame configuration example shown in Figure 28, the nSTA field stores information indicating the value n, and n STA Info fields are stored.

Each STA Info field includes an ID field and a Data Tx Interval field. The ID field stores the ID of the terminal to receive the data. The Data Tx Interval field stores the data frame transmission interval of the terminal.

G-3 Processing Operation of Satellite Receiving Station Figure 29 shows in the form of a flowchart the processing procedure for the satellite receiving station to receive a notification frame.

First, the satellite receiving station refers to the list of target terminals and calculates the radio resources (time and frequency) for receiving the notification frame from the registered terminal and the information necessary for demodulating the notification frame based on rules 1 to 4 described above (step S2901).

The satellite receiving station then determines whether the notification frame reception time calculated in step S2901 has arrived (step S2902). When the notification frame reception time arrives (Yes in step S2902), the satellite receiving station detects the notification frame from the terminal in question using the preamble and synchronization information calculated in step S2901 (step S2903), and demodulates the received notification frame using the scrambling pattern calculated in step S2901 (step S2904). The satellite receiving station then determines whether demodulation of the notification frame was successful (step S2905). If demodulation of the notification frame fails (No in step S2905), the process returns to step S2901, and the satellite receiving station attempts to receive the next notification frame.

On the other hand, if the notification frame is successfully demodulated (Yes in step S2905), the satellite receiving station determines which satellite receiving station can receive the next transmitted data frame based on the terminal ID and data transmission interval stored in the received notification frame (step S2906).The satellite receiving station then determines whether it can receive the next data frame transmitted from the terminal in question (step S2907).

Here, if the satellite receiving station itself is able to receive the next data frame transmitted from the terminal in question (Yes in step S2907), the satellite receiving station registers information such as the terminal ID and data transmission interval stored in the received notification frame in the list of target terminals (step S2908).

If the satellite receiving station itself is not capable of receiving the next data frame transmitted from the terminal in question (No in step S2907), the satellite receiving station generates a target terminal registration frame (step S2909) and transmits the target terminal registration frame to another satellite receiving station that is capable of receiving the next data frame transmitted from the terminal in question (step S2910).

Next, the satellite receiving station checks whether the terminal in question is present in its list of target terminals for reception (step S2911), and if so (Yes in step S2911), deletes the entry for the terminal in question from its list of target terminals for reception (step S2912).

Figure 30 shows in the form of a flowchart the processing procedure for a satellite receiving station to receive a data frame. However, it is assumed that the satellite receiving station can execute the data frame reception process in parallel for each terminal registered in the list of target terminals to receive data.

First, the satellite receiving station obtains information about the target terminal, such as the terminal ID, from the list of target terminals (step S3001), and calculates the radio resources (time and frequency) for receiving data frames from the target terminal and the information required for demodulating the data frames based on rules 1 to 4 described above (step S3002).

The satellite receiving station then determines whether the data frame reception time calculated in step S3002 has arrived (step S3003). When the data frame reception time arrives (Yes in step S3003), the satellite receiving station detects the data frame from the terminal in question using the preamble and synchronization information calculated in step S3002 (step S3004), and demodulates the received data frame using the scrambling pattern calculated in step S3002 (step S3005). The satellite receiving station then determines whether demodulation of the data frame was successful (step S3006). If demodulation of the data frame fails (No in step S3006), the process returns to step S3001, and the satellite receiving station attempts to receive the next notification frame.

On the other hand, if the data frame is successfully demodulated (Yes in step S3006), the satellite receiving station determines which satellite receiving station can receive the next transmitted data frame based on the terminal ID and data transmission interval stored in the data frame (step S3007).The satellite receiving station then determines whether it can receive the next data frame transmitted from the terminal in question (step S3008).

Here, if the satellite receiving station itself is able to receive the next data frame transmitted from the terminal in question (Yes in step S3008), the satellite receiving station registers information such as the terminal ID and data transmission interval stored in the received data frame in the list of target terminals (step S3009).

If the satellite receiving station itself is not capable of receiving the next data frame transmitted from the terminal in question (No in step S3008), the satellite receiving station generates a target terminal registration frame (step S3010) and transmits the target terminal registration frame to another satellite receiving station that is capable of receiving the next data frame transmitted from the terminal in question (step S3011).

The satellite receiving station then checks whether the terminal in question is present in its list of target terminals for reception (step S3012), and if so (Yes in step S3012), deletes the entry for the terminal in question from its list of target terminals for reception (step S3013).

Figure 31 shows in flowchart form the processing procedure for a satellite receiving station to receive a target terminal list registration frame.

First, the satellite receiving station determines whether or not it has received a target terminal list registration frame (step S3101). Then, when the satellite receiving station receives a target terminal list registration frame (Yes in step S3101), it demodulates the received frame (step S3102) and determines whether or not the demodulation process was successful (step S3103).

If the satellite receiving station successfully demodulates the target terminal list registration frame (Yes in step S3103), it acquires nSTA, which indicates the number of STA Info fields stored in the target terminal list registration frame (step S2204), and repeatedly processes each subsequent STA Info field the number of times equal to nSTA. During this repeated process, it registers terminal information such as the terminal ID and data frame transmission interval stored in the STA Info field in its own target terminal list (step S3105).

The satellite receiving station can register a newly selected terminal as a target terminal in its list of target terminals by processing the target terminal list registration frame according to the processing procedure shown in Figure 31.

Therefore, according to the fifth embodiment, the terminal does not need to transmit notification frames frequently, and even if the terminal moves into the reception range of a satellite receiving station several stations away during a long data transmission interval, the satellite receiving station within the reception range of the terminal in question can receive and demodulate data frames from the terminal in question.

H. Effects This section summarizes the effects brought about by the first to fifth embodiments described above. According to the first to fifth embodiments, the following effects (1) and (2) can be obtained.

(1) The terminal does not need to transmit notification frames frequently. A satellite receiving station that can receive data frames periodically transmitted from the terminal can receive and demodulate the data frames.
(2) It is possible to prevent a satellite receiving station from performing unnecessary reception and demodulation for a terminal that has passed through its coverage area.

The above (1) and (2) further lead to the following effects:

(3) Lower power consumption and lower costs of terminals (4) Increase in the number of devices that can be accommodated per receiving station (5) Reduction in the number of receiving stations required for the system (6) Lower costs of communication services

I. Sixth Embodiment In the first to fifth embodiments described above, the terminal transmits a notification frame and then a data frame in the communication sequence, and the satellite receiving station basically targets the terminal that has received the notification frame as the reception target. In contrast, in the sixth embodiment, the terminal transmits a data frame without transmitting a notification frame.

Figure 32 shows an example of a communication sequence between a terminal and a receiving station in a system in which the terminal can transmit data frames at will. The receiving station does not know the radio resources from which the terminal will transmit the data frame. The radio resources here include the transmission time, transmission frequency, and code used for transmission. Therefore, the receiving station must perform reception and demodulation processing for all radio resources (time, frequency, code).

However, if reception and demodulation processing is performed for all radio resources, the resources required for reception processing at the receiving station (memory usage, amount of calculation, etc.) will become enormous, resulting in problems such as high costs, large size, and increased power consumption of the receiving station. In particular, when installing a receiving station on a low-earth orbit satellite, it is necessary to make the receiving station smaller and less power-consuming.

In contrast, in the sixth embodiment, the terminal adds information regarding the transmission of the next data frame to the data frame. The satellite receiving station then manages the radio resources for receiving and demodulating the next data frame based on the information added to the data frame. Therefore, according to the sixth embodiment, the satellite receiving station can limit the radio resources for receiving and demodulating, making it easier to achieve low-cost, small-sized, and low-power-consumption receiving stations.

I-1. System Configuration Fig. 33 shows a schematic configuration example of an LPWA wireless communication system according to the sixth embodiment. The example shown in the figure is composed of multiple receiving stations installed on the ground (hereinafter referred to as "ground stations"), multiple low-orbit satellite receiving stations (hereinafter simply referred to as "satellite receiving stations") orbiting the Earth in low orbit, and numerous terminals scattered around the Earth.

The terminal is a transmitter that periodically transmits information sensed by onboard sensors, etc. The satellite receiving station receives the data transmitted by the terminal and performs demodulation processing. The satellite receiving station also transmits the demodulation results (user data) to an application server (not shown) on the cloud as necessary. The satellite receiving station also enables inter-satellite communication. There are no restrictions on the communication method between satellite receiving stations, but constant connection and high-speed communication are desirable. The ground station is a transmitter/receiver that communicates with the satellite receiving station. There are no restrictions on the communication method between the satellite receiving station and the ground station, but high-speed communication is desirable.

In the example shown in Figure 33, satellite receiving station 3301 and satellite receiving station 3302 are adjacent satellite receiving stations orbiting on the same satellite orbit. As satellite receiving station 3301 and satellite receiving station 3302 move on their satellite orbits, receivable range 3311 of satellite receiving station 3301 and receivable range 3312 of satellite receiving station 3302 also move on the ground.

Figure 34 shows an example communication sequence between terminal 3401, which is located in either reception range 3311 or reception range 3312, and satellite receiving station 3301 and satellite receiving station 3302.

First, terminal 3401 transmits (broadcasts) the initial data frame (SEQ 3401).

The initial data frame refers to the frame that terminal 3401 transmits for the first time, such as immediately after turning on the power. Terminal 3401 transmits using the radio resources (time, frequency, and code) that satellite receiving stations 3301 and 3302 always receive. In addition, terminal 3401 stores information about the radio resources to be used for transmitting the next normal data frame in the initial data frame.

In the example shown in Figure 34, when terminal 3401 transmits the first data frame, the location of terminal 3401 is within the reception range 3311 of satellite receiving station 3301, so satellite receiving station 3301 successfully receives and demodulates the first data frame (SEQ 3411). Next, satellite receiving station 3301 determines which satellite receiving station can receive the next normal data frame based on the radio resource for transmitting the next normal data frame stored in the first data frame. Then, since satellite receiving station 3301 itself can receive the next normal data frame, satellite receiving station 3301 registers information about the radio resource for the next normal data frame to be transmitted by terminal 3401 in its own reception list (SEQ 3412).

On the other hand, when terminal 3401 transmits the initial data frame, the location of terminal 3401 is outside the reception range 3312 of satellite receiving station 3302, so satellite receiving station 3302 cannot receive the initial data frame.

Next, terminal 3401 transmits a normal data frame using the radio resources stored in the initial data frame (SEQ 3402). Terminal 3401 also stores information about the radio resources to be used for transmitting the next normal data frame in the normal data frame.

Satellite receiving station 3301 can pinpoint and receive and demodulate the normal data frame transmitted by terminal 3401 by performing reception and demodulation processing based on the radio resources registered in its reception list (SEQ 3413). However, at the time terminal 3401 transmits this normal data frame, the location of terminal 3401 is outside the reception range 3312 of satellite receiving station 3302, so satellite receiving station 3302 cannot receive this normal data frame.

Furthermore, satellite receiving station 3301 determines the satellite receiving station that can receive the next normal data frame based on the radio resources for transmitting the next normal data frame stored in the received normal data frame. Since satellite receiving station 3301 itself can receive the next normal data frame, satellite receiving station 3301 again registers terminal 3401 in its reception list (SEQ 3414).

Next, terminal 3401 transmits the next normal data frame using the radio resources stored in the previous normal data frame (SEQ 3403). Terminal 3401 also stores information about the radio resources to be used to transmit the next normal data frame within the normal data frame.

Satellite receiving station 3301 can pinpoint and receive and demodulate the normal data frame transmitted by terminal 3401 by performing reception and demodulation processing based on the radio resources registered in its reception list (SEQ 3415). However, at the time terminal 3401 transmits this normal data frame, the location of terminal 3401 is outside the reception range 3312 of satellite receiving station 3302, so satellite receiving station 3302 cannot receive this normal data frame.

Furthermore, satellite receiving station 3301 determines which satellite receiving station can receive the next normal data frame based on the radio resources for transmitting the next normal data frame stored in the received normal data frame. Since satellite receiving station 3301 cannot receive the next normal data frame, it transmits a reception target radio resource information notification frame to satellite receiving station 3302 that can receive it (SEQ 3416).

Satellite receiving station 3302 registers the information on the radio resources stored in the reception target radio resource information notification frame received from satellite receiving station 3301 in its own reception list (SEQ 3421).

Next, terminal 3401 transmits the next normal data frame using the radio resources stored in the previous normal data frame (SEQ 3404).

When terminal 3401 transmits the next normal data frame, its location falls within satellite receiving station 3302's reception range 3312. Satellite receiving station 3302 can pinpoint and receive and demodulate the normal data frame transmitted by terminal 3401 by performing reception and demodulation processing based on the radio resources registered in its reception list (SEQ 3422). However, when terminal 3401 transmits this normal data frame, its location has moved outside satellite receiving station 3301's reception range 3312, so satellite receiving station 3301 cannot receive this normal data frame.

The terminal may transmit the initial data frame periodically, rather than immediately after turning on the power. Even if the satellite receiving station fails to receive the normal data frame, the terminal will be registered in the satellite receiving station's reception list again from the point at which it successfully receives the initial data frame, so it is desirable for the terminal to transmit the initial data frame periodically.

I-3. Device Configuration In the sixth embodiment, the terminal configuration can be the same as that shown in FIG. 3, so a description of the terminal configuration will be omitted in this section.

Figure 35 shows an example of the functional configuration of a satellite receiving station 3500 in a system relating to the sixth embodiment. The satellite receiving station 3500 is used by being mounted on a low-earth orbit satellite that orbits the Earth in one of the satellite orbits shown in Figure 33. The satellite receiving station 3500 comprises an LPWA unit 3520 and an inter-satellite communication unit 3530.

The LPWA unit 3520 receives frames transmitted by a terrestrial terminal. Components of the LPWA unit 3520 that are the same as those of the receiving station 200 shown in Figure 4 are given the same reference numbers as in Figure 4. The LPWA unit 3520 further includes a receivable receiving station determination unit 3526. The receivable receiving station determination unit 3526 determines the receiving station that can receive the next data frame based on the next data frame transmission information stored in the data frame received from the terminal.

The inter-satellite communication unit 3530 includes a wireless communication unit 3531, a wireless control unit 3532, a frame generation unit 3533, and a frame detection/demodulation unit 3534.

The wireless communication unit 3531 transmits and receives wireless signals. Under control of the wireless control unit 3532, the wireless communication unit 353 receives radio waves, converts them into wireless signals, and passes them to the frame detection and demodulation unit 3534. Also, under control of the wireless control unit 3532, the wireless communication unit 3531 converts frames generated by the frame generation unit 3533 into wireless signals and transmits them.

The wireless control unit 3532 controls the wireless communication unit 3531 so that frames can be sent and received between satellite receiving stations.

The frame generation unit 3533 stores the next data frame transmission information obtained from the receivable receiving station determination unit 3526 on the LPWA unit 3520 side and generates a frame according to a specified format.

The frame detection and demodulation unit 3534 detects and demodulates frames from the signal received by the wireless communication unit 3531. If the frame detection and demodulation unit 3534 successfully demodulates the frame, it passes the received data to the memory unit 205 in the LPWA unit 3520. Specifically, it stores information on the wireless resources of the target terminal received from a satellite receiving station ahead on the same orbit in a reception list in the memory unit 205.

I-4. Frame Structure Figure 36 shows an example of the structure of a data frame transmitted and received between a terminal and a satellite receiving station. However, the frame structure of the initial data frame and the normal data frame is assumed to be the same. The data frame includes the fields ID, DATA, Time Interval, Ch, Code, and CRC.

The ID field stores an ID, which is a unique identifier of the transmitting terminal 100. The DATA field stores the transmitted data. In the case of a data frame, data obtained from the sensor 104, etc. is stored. The Time Interval field stores information regarding the transmission interval from the transmission of the current data frame to the transmission of the next data frame. The Ch field stores information regarding the frequency to be used when transmitting the next data frame. The Code field stores information regarding the code to be used when transmitting the next data frame, specifically, information indicating the preamble, synchronization information (Sync), and four initial values required to generate the scrambling pattern. The CRC field stores a CRC value calculated from the information stored in the ID field, DATA field, Time Interval field, Ch field, and Code field. On the receiving side, the CRC value is used to determine whether demodulation was successful.

The sequence consisting of the ID, DATA, Time Interval, Ch, Code, and CRC is FEC encoded and reordered (interleaved) to generate the payload. A preamble, a known pattern used for frame detection and synchronization acquisition, and synchronization information are then concatenated to the beginning of the payload, and the frame is generated by performing an exclusive OR (XOR) with the scrambling pattern on a bit-by-bit basis.

The system has predefined rules for determining the preamble and synchronization information required for frame generation, and the method for generating the scramble pattern. The preamble and synchronization information required for frame generation and the method for generating the scramble pattern follow rules 3 and 4, respectively, shown in Figure 6. The preamble and synchronization information (Preamble/Sync) are determined by inputting two initial values, Seed(P)-1 and Seed(P)-2, used to calculate the preamble and synchronization information into rule 3. The scramble pattern is determined by inputting two initial values, Seed(S)-1 and Seed(S)-2, used to calculate the scramble pattern into rule 4.

Each initial value is determined in advance within the system and stored in the memory unit 106 in the terminal 100 and the memory unit 3525 in the satellite receiving station 3500. In addition, the initial values that can be used for the initial data frame and normal data frames are different.

Figure 37 shows an example of the frame structure of the data portion of a target radio resource information notification frame. The format of the portion other than the data portion depends on the communication method used. The target radio resource information notification frame is a frame that notifies other satellite receiving stations that can receive the next normal data frame of the radio resources for transmitting the next normal data frame when a satellite receiving station determines that it cannot receive the next normal data frame itself.

The data portion of the reception target radio resource information notification frame includes an Rx Time field, a Ch field, and a Code field. The Rx Time field stores information about the reception time of the next data frame. The Ch field stores information about the reception frequency of the next data frame. The Code field stores information about the reception code of the next data frame, specifically, information indicating the preamble, synchronization information (Sync), and four initial values required to generate the scramble pattern.

I-5. Processing Operation of Terminal FIG. 38 shows the processing operation performed by the terminal in the form of a flowchart.

First, the terminal determines the radio resources (time, frequency, code) to be used for transmitting the initial data frame (step S3801).

The terminal then determines the radio resources (time, frequency, code) to be used for the next normal data frame transmission (step S3802).

The terminal then generates an initial data frame (step S3803). The Time Interval, Ch, and Code fields of the initial data frame store information about the radio resources to be used for transmitting the next normal data frame, as determined in step S3802.

Then, when the time to transmit the first data frame arrives (Yes in step S3804), the terminal transmits the first data frame generated in step S3803 using the transmission frequency calculated in step S3801 (step S3805).

Next, the terminal determines whether the frame to be transmitted next is the initial data frame (step S3806).

If the frame to be transmitted two times is not the initial data frame (No in step S3808), the terminal determines the radio resources (time, frequency, code) to be used for transmitting the two times normal data frame (step S3807).

The terminal then generates a normal data frame (step S3808). The Time Interval, Ch, and Code fields of the normal data frame store information about the radio resources to be used for transmitting the next normal data frame, as determined in step S3807. However, if the next data frame to be transmitted is the initial data frame (Yes in step S3806), the Time Interval, Ch, and Code fields of the normal data frame are padded with zeros.

Then, when the time to transmit the normal data frame arrives (Yes in step S3809), the terminal transmits the normal data frame generated in step S3803 using the transmission frequency calculated in step S3802 (step S3810).

The terminal then determines whether the next frame to be transmitted is an initial data frame (step S3811). If the next frame to be transmitted is an initial data frame (Yes in step S3811), the terminal returns to step S3801 and repeats the process for transmitting the initial data frame. If the next frame to be transmitted is not an initial data frame (No in step S3811), the terminal returns to step S3806 and repeats the process for transmitting a normal data frame.

I-6 Processing Operation of Satellite Receiving Station Figure 39 shows in the form of a flowchart the processing operation for the satellite receiving station to receive the initial data frame.

First, the satellite receiving station performs reception and demodulation processing for all frequencies and codes assigned to the transmission of the initial data frame (step S3901).

If the reception and demodulation process of the first data frame is successful (Yes in step S3902), the satellite receiving station calculates the estimated reception time of the next normal data frame based on the transmission interval stored in the Time Interval field in the first data frame that was successfully demodulated (step S3903).

Next, the satellite receiving station determines whether it is the satellite receiving station that can receive the next normal data frame (step S3904).

If the satellite receiving station that can receive the next normal data frame is itself (Yes in step S3904), the satellite receiving station registers the estimated reception time calculated in step S3903 and the frequency and code stored in the initial data frame in its reception list (step S3905).

On the other hand, if the satellite receiving station that can receive the next normal data frame is not itself (No in step S3904), the satellite receiving station transmits a reception target radio resource information notification frame to the satellite receiving station that can receive the next normal data frame (step S3906).

The processing of steps S3903 and S3904 in the flowchart shown in Figure 39 will be explained with reference to Figure 16.

For example, if the reception time of the first data frame that was successfully demodulated in step S3902 was 12:00:01.020 and the Time Interval was 00:01:00.000, the predicted reception time of the next normal data frame would be 12:01:01.020. However, because the distance between the ground and the satellite receiving station is long, at several hundred kilometers, propagation delays occur. In addition, because the satellite receiving station moves, it is necessary to take into account that the amount of propagation delay varies for each data frame.

Figure 16 shows an example of the change in frequency error and propagation delay over time when the satellite is at an altitude of 600 km and has a frequency of 2 GHz. This is an example of a satellite passing overhead, and assumes that the satellite receiving station can receive signals from one terminal for six minutes. The frequency error was calculated using the model described in 3GPP TR 38.811 V15.1.0 (Non-Patent Document 1).

For example, suppose the estimated frequency error when receiving a data frame is 40 kHz. In this case, if the propagation delay is 3.5 milliseconds and the Time Interval is 00:01:00.000, the propagation delay when the next normal data frame is received will be 2.5 milliseconds. Therefore, the estimated time of reception of the next normal data frame will be 12:01:01.019.

Next, determine whether you are the satellite receiving station that will be able to receive the next normal data frame.

For example, in the example shown in Figure 16, if the frequency error estimation result when receiving a data frame is 40 kHz and the Time Interval is 00:01:00.000, the satellite receiving station that can receive the next normal data frame is itself.

On the other hand, if the estimated frequency error when receiving the data frame is -40 kHz and the Time Interval is 00:01:30.000, the satellite receiving station that can receive the next normal data frame will be the satellite receiving station one position behind orbiting the same satellite.

Figure 40 shows in flowchart form the processing procedure for a satellite receiving station to receive normal data frames. However, the satellite receiving station shall perform this processing for each radio resource to be received that is registered in the reception list.

First, the satellite receiving station obtains the radio resources (time, frequency, code) to be received from the reception list (step S4001). Then, the satellite receiving station determines whether the target reception time has arrived (step S4002).

When the target reception time arrives (Yes in step S4002), the satellite receiving station performs reception and demodulation processing on the target reception frequency using the target reception code (step S4003).The satellite receiving station then determines whether the demodulation of the normal data frame was successful (step S4004).

If demodulation of the normal data frame fails (No in step S4004), the satellite receiving station skips all subsequent processing steps and terminates the reception processing of the normal data frame in the target radio resource.

On the other hand, if the demodulation of the normal data frame is successful (Yes in step S4004), the satellite receiving station determines whether the demodulated normal data frame contains information about the radio resources to be used for transmitting the next normal data frame (step S4005).

If the demodulated normal data frame does not contain information about the radio resource to be used for transmitting the next normal data frame (No in step S4005), the satellite receiving station skips all subsequent processing steps and terminates the reception processing of the normal data frame on the radio resource to be received.

If the demodulated normal data frame contains information about the radio resources to be used for transmitting the next normal data frame (Yes in step S4005), the satellite receiving station calculates the estimated reception time of the next normal data frame based on the reception time of the normal data frame and the transmission interval stored in the normal data frame (step S4006).

Next, the satellite receiving station determines whether it is the satellite receiving station that can receive the next normal data frame (step S4007).

If the satellite receiving station that can receive the next normal data frame is itself (Yes in step S4007), the satellite receiving station registers the estimated reception time calculated in step S4006 and the frequency and code stored in the normal data frame demodulated in step S4003 in its reception list (step S4008).

Also, if the satellite receiving station that can receive the next normal data frame is not itself (No in step S4007), the satellite receiving station transmits a reception target radio resource information notification frame to the satellite receiving station that can receive the next normal data frame (step S4009).

Figure 41 shows in flowchart form the processing procedure for a satellite receiving station to receive a target radio resource information notification frame.

First, the satellite receiving station determines whether it has received a target radio resource information notification frame from another satellite receiving station (step S4101). If it has received a target radio resource information notification frame (Yes in step S4101), the satellite receiving station demodulates the target radio resource information notification frame (step S4102) and determines whether the demodulation of the frame was successful (step S4103).

If the demodulation of the radio resource information notification frame to be received is successful (Yes in step S4103), the satellite receiving station determines whether the frame is addressed to the station itself (step S4104).

If the demodulated reception target radio resource information notification frame is addressed to the satellite receiving station (Yes in step S4104), the satellite receiving station registers the information stored in the frame in its own reception list (step S4105).

Also, if the demodulated reception target radio resource information notification frame is not addressed to the satellite receiving station (No in step S4104), the satellite receiving station transfers the reception target radio resource information notification frame to the corresponding satellite receiving station (step S4106).

I-7. Effects The effects brought about by the sixth embodiment explained in this section I will now be summarized. According to the sixth embodiment, the following effects (1) and (2) can be obtained.

(1) There is no need for signaling between the terminal and the satellite receiving station for allocating radio resources, and the satellite receiving station can limit the radio resources for performing reception and demodulation processing.
(2) There is no need for signaling between the terminal and the satellite receiving station for wireless resource allocation, and it is possible to follow even if the satellite receiving station moves and the satellite receiving station that can receive the data frame transmitted by the terminal changes.

The above (1) and (2) further lead to the following effects:

(3) Lower power consumption and lower costs of terminals (4) Increase in the number of devices accommodated per receiving station (5) Smaller size and lower power consumption of receiving stations (6) Reduction in the number of receiving stations required for the system (7) Lower costs of communication services

The present disclosure has been described in detail above with reference to specific embodiments. However, it is clear that those skilled in the art may modify or substitute the embodiments without departing from the spirit and scope of the present disclosure.

While this specification has focused on embodiments in which this disclosure is applied to the LPWA communication system, the gist of this disclosure is not limited thereto. This disclosure can be similarly applied to various types of communication systems in which communication is carried out between terrestrial terminals and satellite receiving stations orbiting the Earth. Furthermore, while this specification has described embodiments specific to a specific satellite constellation for convenience, this disclosure can also be applied to communication systems that use other satellite constellations and can accommodate any number of satellite receiving stations.

In short, the present disclosure has been described in the form of examples, and the contents of this specification should not be interpreted in a limiting manner. The claims should be taken into consideration to determine the gist of the present disclosure.

In addition, this disclosure can also be configured as follows.

(1) Operates as one of the satellite receiving stations that orbits the Earth in a predetermined orbit and receives frames from a terminal on the ground;
a receiving unit that receives and processes frames from the terminal;
a determination unit that determines a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information when a frame is received from the terminal, a frame transmission interval of the terminal, and the positional relationship of surrounding satellite receiving stations;
A communication device comprising:

(2) The frame reception information includes a frequency error estimation result and a propagation delay estimation result at the time of frame reception.
The communication device according to (1) above.

(3) The frame reception information includes location information of the terminal.
The communication device according to (1) above.

(4) Exchanging frame reception information with surrounding satellite receiving stations;
A communication device according to any one of (1) to (3) above.

(5) The determination unit further determines a satellite receiving station suitable for receiving a frame to be transmitted next by the terminal based on frame reception information from surrounding satellite receiving stations.
The communication device according to (4) above.

(6) storing information about the terminal determined by the determination unit as being capable of receiving the frame to be transmitted next in a list of target terminals;
receiving and processing a frame to be transmitted next by the terminal based on the information stored in the target terminal list;
The communication device according to (5) above.

(7) Notifying the surrounding satellite receiving stations of information about terminals added to the target receiving terminal list based on frame reception information from the surrounding satellite receiving stations.
The communication device according to (6) above.

(8) Based on the notification from the surrounding satellite receiving station, delete the information of the corresponding terminal from the receiving terminal list of the own station.
The communication device according to (7) above.

(9) exchanging frame reception information including the station's own identification information with surrounding satellite receiving stations;
notifying the satellite receiving station whose identification information is indicated in the frame reception information of the terminal that has been added to the target receiving terminal list based on the frame reception information from the surrounding satellite receiving station;
The communication device according to (7) above.

(10) Notifying a nearby satellite receiving station that the determination unit has determined to be suitable for receiving the next frame transmitted from the terminal of the registration of the target terminal.
The communication device according to (1) above.

(11) The terminal that notified the surrounding satellite receiving station of its registration as a target terminal for reception is removed from the target terminal for reception of the own station.
The communication device according to (10) above.

(12) A terminal that has been notified of registration as a target terminal for reception by a nearby satellite receiving station is set as a target for reception by the own station.
A communication device according to any one of (10) and (11) above.

(13) The frame reception information includes a frame transmission interval notified from the terminal;
the determination unit estimates an expected reception time of the next frame transmitted from the terminal based on the frame transmission interval and the frequency error estimation result at the time of frame reception;
The communication device according to (1) above.

(14) The determination unit determines a satellite receiving station suitable for receiving the next frame transmitted from the terminal based on a predicted reception time of the next frame transmitted from the terminal and a positional relationship of surrounding satellite receiving stations.
The communication device according to (13) above.

(15) The terminal that the determination unit determines to be able to receive the next transmitted frame is set as the reception target of the next transmitted frame.
The communication device according to (14) above.

(16) Notifying a nearby satellite receiving station that is determined to be suitable for receiving the next frame transmitted by the terminal of wireless resource information for receiving the frame from the terminal;
The communication device according to (14) above.

(17) In response to notification of wireless resource information for receiving frames from a nearby satellite receiving station, the corresponding terminal is set as a reception target for the own station.
The communication device according to (16) above.

(18) The surrounding satellite receiving stations include at least one of a satellite receiving station moving in the same orbit and a satellite receiving station moving in an adjacent orbit.
A communication device according to any one of (1) to (17) above.

(19) A communication method that operates as a satellite receiving station that orbits the Earth in a predetermined orbit and receives frames from a terminal on the ground, comprising:
a receiving step of receiving and processing frames from the terminal;
a determination step of determining a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information at the time of receiving a frame from the terminal, a frame transmission interval of the terminal, and the positional relationship of surrounding satellite receiving stations;
A communication method comprising:

(20) A system comprising a terminal installed on the ground and a plurality of satellite receiving stations each orbiting the Earth in a predetermined orbit;
Each of the plurality of satellite receiving stations comprises a receiving unit that receives and processes frames from the terminal, and a determining unit that determines a satellite receiving station that is suitable for receiving the next frame transmitted by the terminal based on frame reception information when frames are received from the terminal, a frame transmission interval of the terminal, and a positional relationship between neighboring satellite receiving stations.
Communication system.

DESCRIPTION OF SYMBOLS 100...Terminal, 101...Wireless communication unit, 102...Wireless control unit 103...Frame generation unit, 104...Sensor 105...Wireless resource determination unit, 106...Memory unit 200...Receiving station, 201...Wireless communication unit, 202...Wireless control unit 203...Frame detection and demodulation unit, 204...Wireless resource determination unit 205...Memory unit 1000...Satellite receiving station, 1200...LPWA unit, 1300...Inter-satellite communication unit 1301...Wireless communication unit, 1302...Wireless control unit 1303...Frame generation unit, 1304...Frame detection and demodulation unit 3500...Satellite receiving station, 3520...LPWA unit 3526...Receivable receiving station determination unit 3531...Wireless communication unit, 3532...Wireless control unit 3533...Frame generation unit, 3534...Frame detection and demodulation unit

Claims (20)

The satellite operates as one of the satellite receiving stations that orbits the Earth in a predetermined orbit and receives frames from a terminal on the ground;
a receiving unit that receives and processes frames from the terminal;
a determination unit that determines a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information relating to the frame received from the terminal, the frame transmission interval of the terminal included in the frame reception information, and the positional relationship of surrounding satellite receiving stations on the same orbit ;
A communication device comprising: The frame reception information includes a frequency error estimation result and a propagation delay estimation result when the frame is received from the terminal .
The communication device according to claim 1 . The frame reception information includes location information of the terminal.
The communication device according to claim 1 . exchange frame reception information with surrounding satellite receiving stations;
The communication device according to claim 1 . the determination unit further determines, based on frame reception information from surrounding satellite receiving stations, a satellite receiving station that is suitable for receiving the frame that the terminal will next transmit.
The communication device according to claim 4. storing information about a terminal that the determination unit has determined to be capable of receiving the next frame to be transmitted by the own station in a list of target terminals;
receiving and processing a frame to be transmitted next by the terminal based on the information stored in the target terminal list;
The communication device according to claim 5 . notifying the surrounding satellite receiving stations of information about the terminals added to the target receiving terminal list based on frame reception information from the surrounding satellite receiving stations;
The communication device according to claim 6. Based on the notification from the surrounding satellite receiving station, delete the information of the corresponding terminal from the receiving terminal list of the own station.
The communication device according to claim 7. The frame reception information including the station's own identification information is exchanged with surrounding satellite receiving stations,
notifying the satellite receiving station whose identification information is indicated in the frame reception information of the terminal that has been added to the target receiving terminal list based on the frame reception information from the surrounding satellite receiving station;
The communication device according to claim 7. notifying a nearby satellite receiving station that the determining unit has determined to be suitable for receiving the next frame transmitted from the terminal of the registration of the target terminal;
The communication device according to claim 1 . removing the terminal that has notified the surrounding satellite receiving station of its registration as a target terminal for reception from the target terminal for reception of the own station;
The communication device according to claim 10. The terminal that has been notified of the registration of the target terminal from the surrounding satellite receiving station is set as the target of reception for the own station.
The communication device according to claim 10. the determination unit estimates an expected reception time of the next frame transmitted from the terminal based on a frame transmission interval of the terminal and a frequency error estimation result at the time of receiving a frame from the terminal;
The communication device according to claim 1 . the determination unit determines a satellite receiving station suitable for receiving the next frame transmitted from the terminal based on a positional relationship of surrounding satellite receiving stations on the same orbit as the predicted time of reception of the next frame transmitted from the terminal;
14. The communication device of claim 13. The terminal that the determination unit determines to be able to receive the next transmitted frame is set as the reception target of the terminal.
15. The communication device of claim 14. notifying a nearby satellite receiving station on the same orbit that is determined to be suitable for receiving the next frame transmitted by the terminal of radio resource information for receiving the frame from the terminal;
15. The communication device of claim 14. In response to notification of wireless resource information for receiving frames from the terminal from a nearby satellite receiving station, the terminal is set as a reception target of the terminal.
17. The communication device of claim 16. The surrounding satellite receiving stations further include satellite receiving stations traveling in adjacent orbits.
The communication device according to claim 1 . A communication method that operates as a satellite receiving station that orbits the Earth in a predetermined orbit and receives frames from a terminal on the ground, comprising:
a receiving step of receiving and processing frames from the terminal;
a determination step of determining a satellite receiving station suitable for receiving the next frame transmitted by the terminal based on frame reception information regarding the frame received from the terminal, the frame transmission interval of the terminal included in the frame reception information, and the positional relationship of surrounding satellite receiving stations on the same orbit ;
A communication method comprising: It consists of a terminal installed on the ground and multiple satellite receiving stations that orbit the Earth in their own designated orbits.
Each of the plurality of satellite receiving stations comprises a receiving unit that receives and processes frames from the terminal, and a determining unit that determines a satellite receiving station that is suitable for receiving the next frame transmitted by the terminal based on frame reception information regarding the frames received from the terminal, the frame transmission interval of the terminal included in the frame reception information, and the positional relationship of surrounding satellite receiving stations on the same orbit.
Communication system.