JP2002152802A - Base station for cdma system and initial synchronization acquisition method for the base station - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base station for the CDMA system that can reduce number of failures in acquiring initial synchronization so as to stably estimate a position. SOLUTION: The base station is provided with an analog/digital(A/D) conversion section 12 that detects an electric field strength of a received signal, with a search section 14 having a most advanced wave information collection circuit 21 that collects information of a most advanced wave and detection threshold value information and having a timer circuit 22 that measures a time from a reference start time of initial synchronization acquisition until a setting time to discriminate disabled initial synchronization acquisition, and with a control section 13 that stores threshold values of correlation power set by the search section 14 to detect the most advanced wave, outputs an instruction to decrease the threshold value by one step when receiving an end of time count of the timer circuit and estimates a distance up to a mobile station 60 on the basis of delay time information of the most advanced wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to position estimation of a mobile station in a base station of a CDMA system, and more particularly to initial synchronization of a position estimation signal transmitted from the mobile station when the base station estimates the position of the mobile station. It relates to a control method when capturing.

[0002]

2. Description of the Related Art Conventionally, in a CDMA system, the position of a mobile station is estimated.

A method of estimating the position of a mobile station in a conventional CDMA system will be described below with reference to the drawings.

FIG. 6 is a diagram showing the relationship among stations in position estimation in a CDMA system.

[0005] The control base station 50 and the first to third base stations 3 to 3 are connected by dedicated lines. Mobile station 6
0 indicates that the mobile station is moving in an area surrounded by each of the base stations 1 to 3, but is currently communicating via the second base station because the second base station 2 is closest. .

The method for estimating the position of the mobile station 60 in the case of FIG. 6 is carried out as follows.

First, the supervising base station 50 transmits the mobile station 60 from all base stations around the second base station 2 communicating with the mobile station 60.
By selecting two base stations (the first base station 1 and the third base station 3) to be involved in the position estimation, three base stations 1 to 3 can be combined with the second base station 2 in communication. The selected base stations 1 to 3 are synchronized using a global positioning system (GPS), which is one of the higher-level systems that generates the reference time. Since the mobile station 60 is in communication with the second base station 2 and is synchronized with the second base station 2, it is not necessary to newly synchronize the mobile station 60. After that, the controlling base station 50
Causes the mobile station 60 to transmit the position estimation signals 31 to 33 at the reference time via the second base station 2. In FIG. 6, the position estimation signals 31 to 33 are separately drawn in three directions. However, this is because when one position estimation signal transmitted from the mobile station 60 reaches each base station. Are received separately through different reception delay times D1 to D3, they are separately described for convenience, and there are not three types of position estimation signals transmitted from the mobile station 60.

FIG. 7 is a diagram showing the relationship between each of the base stations 1 to 3 and the position estimation signals 31 to 33.

When a position estimation signal is transmitted from mobile station 60 at reference time t0, first base station 1 delays signal D1.
The position estimating signal 31
Start receiving. Similarly, the second base station 2 starts receiving the position estimation signal 32 at the earliest preceding wave reception time t2 delayed by the delay time D2, and the third base station 3 delays the delay time D2.
At the reception time t3 of the earliest preceding wave delayed by 3
Start receiving 1.

As described above, the three base stations 1 to 3 around the mobile station 60 receive the position estimation signals transmitted from the mobile station 60 at the reference time t0, respectively. To obtain the delay times D1 to D3 from the reference time t0 to the reception of the position estimation signal.
By calculating the product of .about.D3 and the propagation speed of the radio wave, the distance between each of the base stations 1 to 3 and the mobile station 60 is obtained. At this point, the distance between each of the base stations 1 to 3 and the mobile station 60 is obtained, but it is unknown which direction the mobile station 60 is located in each of the base stations 1 to 3. Therefore, each of the base stations 1 to 3 transmits data on the distance to the mobile station 60 to the control base station 50.

[0011] In the control base station 50, based on the distance data to the mobile station 60 received from each of the base stations 1 to 3, FIG.
As shown in (3), three position estimation circles 41 to 43 are drawn around the base stations 1 to 3 on the map data, and the intersection of the three circles 41 to 43 is estimated as the position of the mobile station 60. Thus, the position of the mobile station 60 can be estimated on the map data.

In the above-described position estimation of the mobile station 60, the delay times D1 to D3 from the reference time t0 to the reception of the position estimation signals 31 to 33 are simply obtained.
In each of the base stations 1 to 3 of the actual CDMA system, since it is necessary to decode the signal received from the mobile station 60, the phase of the signal received by the base station from the mobile station 60 at the start of communication and the base stations 1 to 3 3, it is necessary to synchronize the phase of the signal generated inside, and by detecting a signal for synchronizing the signal, each delay time from the reference time t0 to the reception of the position estimation signals 31 to 33 is detected. D1 to D3
I want to ask.

For example, a narrow band signal (for example, position estimation signals 31 to 31) obtained by spreading (secondary modulation) the mobile station 60 from the mobile station 60 using a spreading code that can identify the mobile station 60 from other mobile stations.
When 33) is transmitted to base stations 1 to 3, base stations 1 to 3 that have received the signals despread using the same spreading code (spreading code replica) as the spreading code used in mobile station 60. To obtain the original signal.

In the despreading process in the base stations 1 to 3, the calculation for obtaining the correlation between the received signal and the spreading code replica is performed. It is necessary to synchronize the phases of the spreading code and the internally generated spreading code replica. The process of synchronizing the phase of the received spread code and the spread code replica at the start of communication in this manner is called initial synchronization acquisition.
, The respective delay times D1 to D3 in the position estimation are calculated.

In the initial synchronization acquisition, for example, a phase range is set in accordance with a time at which a signal for position estimation is transmitted from the mobile station 60, and the base stations 1 to 3 receive signals in the phase range. A correlation operation is performed on all the received signals to obtain a correlation power value for each received signal. Then, the correlation power value for each received signal is compared with a preset threshold value (threshold value for correlation power determination). Arrange in ascending order.

Next, a signal having a higher correlation power value is selected.
The phase positions of those signals are set as candidates for the phase position for synchronization. In other words, any one of the signals having higher correlation power values is regarded as the most advanced wave of the position estimation signal, and the phase of the signal is compensated by a distance away from the reference time, so that the source Is synchronized with the mobile station.

In the base stations 1 to 3 of the conventional CDMA system, the threshold value used when selecting a signal having a higher correlation power value is determined by the service area covered by the base stations 1 to 3. This is a fixed value set according to conditions such as the area. Therefore, the base stations 1 to 3 of the conventional CDMA system determine the correlation power value of the received signal using a fixed threshold value, and perform initial synchronization acquisition based on the correlation power value.

[0018]

However, among the base stations 1 to 3 of the conventional CDMA system, the mobile station 6
0, which is not in communication with the base station 1 selected by the controlling base station 50 to perform position estimation of the mobile station 60.
In the case of or 2, when the threshold value is a fixed value as described above, the initial synchronization acquisition may fail.

FIG. 8 shows that the adjacent base station 1 or 2
FIG. 9 is a diagram illustrating an example of a case where initial synchronization acquisition of a signal from 0 fails due to erroneous detection of noise. In FIG. 8, the horizontal axis indicates the phase, the vertical axis indicates the correlation power value, and when the signal of the correlation power value (position estimation signal PS) is equal to or greater than threshold value FS1, it is determined as a candidate for the most advanced wave W1. Is detected.

In FIG. 8, the correlation power value level of the noise floor NF rises above the threshold value FS1 set to a fixed value due to the size of the service area and the like, and the leading wave W1 of the position estimation signal PS is increased. Noise N generated earlier
1 shows an example in which 1 is erroneously detected as a candidate for a synchronization phase.

FIG. 9 is a diagram showing a case where the threshold value set to a fixed value is set high to avoid the situation of the example of FIG. 8, the horizontal axis indicates the phase, the vertical axis indicates the correlation power value, and when the signal of the correlation power value (position estimation signal PS) is equal to or greater than threshold value FS2, the most advanced wave W
1 candidate.

In the example shown in FIG. 9, since the threshold value FS2 is set high, the most advanced wave W1 of the position estimation signal PS to be detected as a candidate for the synchronization phase cannot be detected.

As shown in FIGS. 8 and 9, the conventional CDMA system base station using a fixed threshold value for determining the correlation power value has a problem that the initial synchronization acquisition may fail.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and it is an object of the present invention to reduce a failure in initial synchronization acquisition and to provide a base station and a base station of a CDMA system capable of stably estimating a position. It is intended to provide a capture method.

[0025]

In order to achieve the above-mentioned object, a base station of a CDMA system according to the present invention described in claim 1 is an analog base station which receives a signal delayed from a reference time by a distance from a mobile station. An analog / digital (A / D) converter for converting a received signal into a digital signal and detecting an electric field strength value of the received signal, and moving by a result of despreading the digital signal output from the A / D converter A despreading circuit for detecting a leading wave of the position estimation signal output from the station; delay time information indicating a time when the leading wave is delayed from a reference time; power value information of the leading wave; A search unit having a most advanced wave information collection circuit that collects threshold information of correlation power set to detect a wave, and at least the search unit configured to detect the most advanced wave A storage circuit for storing a threshold value of one correlation power and a distance to the mobile station calculated from the delay time information; and a mobile circuit for estimating a distance to the mobile station based on the delay time information of the most advanced wave. A base unit of a CDMA system comprising a control unit having a station distance estimating circuit, wherein the search unit is configured to set the despreading circuit from a reference start time of an initial synchronization acquisition set using a reference time of an upper system. A timer circuit for measuring up to a time set to determine that initial synchronization acquisition using the set threshold value cannot be performed, and the control unit detects the time detected by the A / D conversion unit. A conversion table used to set the first threshold based on the electric field strength value of the received signal, and lowering the threshold by one step when the timer circuit finishes timing. And a threshold changing circuit that outputs an instruction to cause the threshold changing circuit to store a plurality of threshold values corresponding to the change in the electric field intensity value of the received signal from the storage circuit. When the above-mentioned instruction is input, the threshold value selected by the instruction and the initial synchronization acquisition instruction are output to the despreading circuit.

According to a second aspect of the present invention, in the method for initial synchronization acquisition of the base station of the CDMA system, the most advanced wave of the position estimation signal output from the mobile station of the CDMA system is received with a delay from the reference time. A base station of a CDMA system for estimating a distance to the mobile station based on the time of the received signal, wherein a position at which a correlation power value is despread based on the electric field strength of a received signal. Setting an initial value of a threshold value to detect the leading wave of the estimation signal, and starting initial synchronization acquisition by the set threshold value, and determining that initial synchronization acquisition cannot be performed. Measuring the set time; and, if the initial synchronization acquisition is not completed before the timer ends, lowering the threshold value by one step and reopening the initial synchronization acquisition. And having a step of.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on illustrated embodiments.

FIG. 1 is a block diagram showing a configuration of a reception-related block of a base station according to an embodiment of the present invention.

The base station 1 of this embodiment shown in FIG.
A radio unit 11 that demodulates and converts a received radio signal into an analog electric signal with a delay from the reference time by a distance from 0
An analog / digital (A / D) converter 12 for converting an analog signal into a digital signal and detecting an electric field strength value of the received signal; a mobile station 60 and the base station 1 or the base station 1 and the control base station 50 And a control unit 13 for outputting various instructions for controlling communication with the base station 1 and for controlling each unit in the base station 1, and a search unit 14 for despreading a received signal and calculating a correlation power value according to an instruction from the control unit 13. And an output unit 15 that performs decoding processing and the like on the received signal having the despread correlation power value as necessary, and outputs the result to the control base station 50.

The search section 14 includes a despreading circuit 20 for despreading the received signal to calculate a correlation power value, delay time information indicating the reception timing of the earliest wave from the received signal having the correlation power value, and the reception power. The information is set using the most advanced wave information collection circuit 21 that collects information and threshold information at the time of detection of the most advanced wave, and a global positioning system (GPS) as one of the higher-level systems that generates the reference time. A timer circuit 22 measures the time from the reference start time of the initial synchronization acquisition to the set time in order to determine that the initial synchronization acquisition using the threshold value currently set in the despreading circuit cannot be performed. . In addition, as the higher-level system that generates the above-mentioned reference time, for example,
A master clock may be installed in the control station or any of the base stations, and the time of the master clock may be supplied as a reference time to another base station.

The despreading circuit 20 further includes a code replica generation circuit 23 for generating a set spreading code replica.
A correlation operation circuit 24 that receives a spreading code replica and a correlation operation start instruction and performs a correlation operation on a received signal;
A searcher circuit for determining whether or not the correlation power value of the received signal is equal to or larger than a threshold value based on the calculation result of the correlation calculation circuit.

FIG. 2 is a block diagram showing the configuration of the control unit 13 of the base station 1 in FIG.

The control unit 13 includes a conversion table 26 used for setting an initial threshold value based on the electric field strength value of the received signal detected by the A / D conversion unit 12, At least set to detect waves
A storage circuit 27 for storing the distance to the mobile station 60 calculated from one threshold value of correlation power and delay time information
A mobile station distance estimating circuit 29 for estimating the distance to the mobile station based on the delay time information of the leading wave, and a timer circuit 2
And a threshold changing circuit 28 for outputting an instruction to lower the threshold by one step in response to the completion of the time counting of 2. The storage circuit 27 stores a plurality of threshold values. The storage circuit 27 sets the threshold value from the threshold value change circuit 28 to 1
When an instruction to lower the level is input, the threshold value selected by the instruction is output to the despreading circuit 20.

Next, the operation of the base station 1 according to this embodiment will be described.

FIG. 3 shows the base station 1 shown in FIG. 1 and FIG.
6 is a flowchart showing the operation of the embodiment.

The received signal demodulated by the radio unit 11 is A /
The digital signal is converted into a digital signal by the D conversion unit 12, and the electric field strength is detected (step S1). Control unit 13
Then, an initial value of a threshold for detecting a correlation power value is determined by the conversion table 26 based on the electric field intensity value obtained from the A / D converter 12 (step S2), and the threshold and the initial synchronization are determined. The capture start instruction is notified to the searcher circuit 25 of the despreading circuit 20 (step S3).

Here, as shown in step S2, with respect to the initial value of the threshold value for detecting the correlation power value,
The effect of the case where the conversion table 26 determines the value based on the electric field intensity value obtained from the A / D converter 12 will be described.

FIG. 4 is a diagram showing a case where the threshold value is set based on the electric field strength value. In FIG. 4, the horizontal axis indicates the phase, the vertical axis indicates the correlation power value, and when the signal of the correlation power value (position estimation signal PS) is equal to or larger than the threshold value VS0, it is determined as a candidate for the most advanced wave W1. Is detected.

In FIG. 4, the threshold value VS0 is determined based on the electric field intensity value obtained from the A / D converter 12 by the conversion table 26.
When the noise floor NF rises, the threshold value VS0 also rises and is set higher than the correlation power value level of the noise floor NF, so that the most advanced wave W1 of the position estimation signal PS Noise N that occurs earlier than
This eliminates erroneous detection of 1 as a candidate for the synchronization phase. Therefore, the situation in which the initial synchronization acquisition fails can be reduced.

In the searcher circuit 25 that has received the initial synchronization acquisition start instruction from the control unit 13 shown in step S3, the searcher circuit 25 uses a global positioning system (GPS), which is one of the higher-level systems that generates the reference time. The timer 22 measures the time from the set reference start time of the initial synchronization acquisition to the time set to determine that the initial synchronization acquisition using the threshold value currently set in the despreading circuit 20 cannot be performed. Is set to start the initial synchronization acquisition (step S4). As a higher-level system that generates the reference time, as described above, for example, a master clock is installed in the control station or any one of the base stations, and the time of this master clock is used as a reference time for other base stations. The one supplied to the station may be used.

The control unit 13 determines whether the time set by the timer 22 has expired (step S5). If the time has not ended (step S5).
5: NO), it is determined whether or not the initial synchronization acquisition has been completed (step S6). If the clocking time has ended (step S5: YES), the current information from the most advanced wave information collection circuit 21 is output. The threshold information is obtained, an instruction is output from the threshold value changing circuit 28, and the threshold value output from the storage circuit 27 to the searcher circuit 25 is reduced by one step (step S8).

Next, the control section 13 determines whether or not the threshold value lowered by one step is equal to or more than the lower limit value of the correlation power (step S9).
9: YES), the threshold value of step S3 and the initial synchronization acquisition start instruction are again sent to the searcher circuit 25 of the despreading circuit 20.
Perform the process of notifying to. If it is less than the lower limit (step S9: NO), it means that the initial synchronization acquisition has failed, and the process ends. Note that the lower limit value of the correlation power is set in advance according to the range of the service area of each base station.

On the other hand, when the initial synchronization acquisition is completed (step S6: YES), the control unit 13 estimates the distance to the mobile station by the mobile station distance estimating circuit 29,
When initial synchronization acquisition is not completed (step S6: NO)
, The determination in step S5 is performed again.

Here, as shown in step S8, if the initial synchronization cannot be acquired until the timer times out, the current threshold value VS0 is set to 1 as to the threshold value for detecting the correlation power value. The effect of the case where the initial synchronization is acquired again after being lowered step by step will be described.

FIG. 5 is a diagram showing an example in which the threshold value VS0 is lowered one step at a time and the initial synchronization is acquired again. In FIG. 5, the horizontal axis indicates the phase, the vertical axis indicates the correlation power value, and when the signal of the correlation power value (position estimation signal PS) is greater than or equal to threshold value VS0, VS1, or VS2, The wave W1 is detected as a candidate.

In FIG. 5, the first threshold value VS0 is
This is the same as in FIG. In this state, the most advanced wave W1
Cannot be detected. However, the threshold value is reduced by one step by the processing shown in step S8 in FIG.
After the first threshold value is set to VS1, the initial synchronization acquisition is performed again, and the leading wave W1 still cannot be detected. Therefore, the threshold value is further reduced by one step, and the third threshold value is set to VS2. By performing the initial synchronization acquisition again after that, the earliest preceding wave W1 can be detected.

As described above, for example, since the transmission power from the mobile station 60 is not sufficiently larger than the noise floor NF, the conventional fixed threshold values FS1 and FS2 and the A / A as shown in step S2 are used. Although the threshold value VS0 determined by the conversion table 26 based on the electric field intensity value obtained from the D conversion unit 12 alone cannot detect the most advanced wave W1, the present embodiment has shown in step S8. As described above, since the thresholds VS1 and VS2 are gradually reduced, not only can the most advanced wave W1 be detected, but also the highest threshold value among the thresholds at which the most advanced wave W1 can be detected is set. be able to. Therefore, the detection accuracy of the earliest wave W1 in the initial synchronization acquisition can be improved.

Therefore, since the accuracy of estimating the distance between the base station 1 and the mobile station 60 can be improved, the position estimation circles 41 to 43 around the base stations 1 to 3 in the control base station 50 can be improved. The position estimation accuracy when the position of the mobile station 60 is estimated based on the intersection can also be improved.

As described above, in the present embodiment, when the noise floor NF rises, the initial threshold VS0 is also raised to be set higher than the correlation power value level of the noise floor NF. Since the noise N1 generated earlier than the leading wave W1 of the position estimation signal PS is not erroneously detected as a candidate for the synchronization phase, the initial synchronization acquisition can be performed correctly, and the initial synchronization acquisition fails. Can be reduced.

Further, in this embodiment, if the initial synchronization acquisition fails, the threshold is lowered by one step and the synchronization acquisition is performed again. Since the highest preceding wave W1 can be set to the highest threshold value among the detectable threshold values, the detection accuracy of the highest preceding wave W1 in the initial synchronization acquisition can be improved. Therefore, the base station 1 and the mobile station 60
Can improve the accuracy of distance estimation between
The position estimation accuracy of the mobile station 60 in the control base station 50 can also be improved.

In the first base station 1 of the present embodiment shown in FIG. 1, the earliest wave information collecting circuit 21 and the timer circuit 22 are described as being incorporated in the search unit 14, but these circuits are, for example, , The control unit 13 or another circuit. Further, the threshold value changing circuit 28 and the mobile station distance estimating circuit 29 in the control unit 13 may be incorporated in the search unit 14 or another circuit, for example.
Further, in the present embodiment, the thresholds of a plurality of stages are described in three stages, but the present invention is not limited to this. For example, by storing more stages in the storage circuit 27,
The threshold may be lowered at a finer stage.

[0052]

As described above, according to the present invention, when the noise floor rises, the initial threshold value is also raised to be set higher than the correlation power value level of the noise floor. Noise that occurs earlier than the earliest wave of the estimation signal is not erroneously detected as a candidate for the synchronization phase, so that the initial synchronization acquisition can be performed correctly, and the situation in which the initial synchronization acquisition fails is reduced. Can be.

Further, according to the present invention, if the initial synchronization acquisition fails, the threshold value is reduced one step at a time and the synchronization acquisition is performed again. Since the highest threshold can be set among the thresholds at which the preceding wave can be detected, it is possible to improve the detection accuracy of the leading wave in the initial synchronization acquisition. Therefore, since the accuracy of estimating the distance between the base station and the mobile station can be improved, the accuracy of estimating the position of the mobile station in the supervising base station can also be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a reception-related block of a base station according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a control unit 13 of the base station 1 in FIG.

FIG. 3 is a flowchart showing an operation of the base station 1 shown in FIGS. 1 and 2.

FIG. 4 is a diagram showing a case where a threshold value is set based on an electric field strength value.

FIG. 5 is a diagram illustrating an example of a case where the threshold value is reduced one step at a time and initial synchronization is acquired again.

FIG. 6 is a diagram illustrating a relationship among stations in position estimation of a CDMA system.

FIG. 7 is a diagram illustrating a relationship between each base station and a position estimation signal.

FIG. 8 is a diagram illustrating an example in which an adjacent base station fails to acquire an initial synchronization of a signal from a mobile station due to erroneous detection of noise.

FIG. 9 is a diagram showing a case where a threshold value set to a fixed value is set high in order to avoid the situation of the example of FIG. 8;

[Explanation of symbols]

1 first base station, 11 radio section, 12 analog /
Digital (A / D) conversion unit, 13 control unit, 14
Search section, 15 output section, 20 despreading circuit,
21 first leading wave information collecting circuit, 22 timer circuit,
23 code replica generation circuit, 24 correlation operation circuit,
25 Searcher circuit, 26 Conversion table, 27
Storage circuit, 28 threshold value changing circuit, 29 mobile station distance estimation circuit, 31 position estimation signal.

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Claims (2)

[Claims] 1. An analog / digital (A / D) converter for converting an analog reception signal received with a delay from a reference time by a distance from a mobile station to a digital signal and detecting an electric field strength value of the reception signal. A despreading circuit for detecting the most advanced wave of the position estimation signal output from the mobile station based on the result of despreading the digital signal output from the A / D conversion unit; A delay time information indicating a time delayed from the power supply, a power value information of the most-preceding wave, and a most-preceding-wave information collecting circuit for collecting threshold information of correlation power set for detecting the most-preceding wave. A search unit, and a storage circuit for storing a distance to a mobile station calculated from at least one threshold value of the correlation power and the delay time information set for detecting the most advanced wave in the search unit. And a control unit having a mobile station distance estimating circuit for estimating the distance to the mobile station based on the delay time information of the most advanced wave, the base station of a CDMA system, The time from the reference start time of the initial synchronization acquisition set using the reference time to the time set to determine that the initial synchronization acquisition using the threshold value currently set in the despreading circuit cannot be performed is measured. A control table configured to set the first threshold based on an electric field intensity value of the received signal detected by the A / D converter; And a threshold changing circuit for outputting an instruction to lower the threshold by one step when the timer circuit finishes measuring the time. A plurality of threshold values are stored in correspondence with the change in the degree value, and when the instruction from the threshold value change circuit is input, the threshold value selected by the instruction and the initial synchronization acquisition instruction again are stored. A base station for a CDMA system, which outputs the signal to the despreading circuit. 2. A CDMA system for estimating a distance to a mobile station based on a time at which a leading wave of a position estimation signal output from a mobile station of the CDMA system is received with a delay from a reference time. An initial synchronization acquisition method for a base station, wherein an initial value of a threshold value is set based on the electric field strength of a received signal in order to detect a leading wave of a position estimation signal which is a despread correlation power value. Starting the initial synchronization acquisition according to the set threshold value, and measuring a time set to determine that the initial synchronization acquisition cannot be performed. Lowering the threshold value by one step and restarting initial synchronization acquisition when synchronization acquisition is not completed. Synchronous acquisition method.