JPH0554747B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a small earth station in a satellite communication system consisting of a plurality of small earth stations, and particularly relates to FDM (FDM) as a communication method between small earth stations. This invention relates to a small earth station that can use both Frequency Division Multiplexing (Frequency Division Multiplexing) and CDM (Code Division Multiplexing) communications.

(Prior Art) Satellite communication systems, which establish communication channels between multiple small-capacity earth stations scattered over a vast area, through satellites, are attracting a great deal of attention and are expected to develop rapidly in the future. It is well known that this is considered a certainty.

Incidentally, small earth stations include fixed stations as well as automobiles, aircraft, people, etc., and the antennas equipped thereon are small in antenna diameter, so broadening of the antenna beam is unavoidable. In other words, in a satellite communication system consisting of a small earth station with an antenna that does not have very high directivity, it is desirable to adopt a communication method that can reduce interference with satellite lines and terrestrial lines of adjacent satellites. Therefore, conventional satellite communication systems consisting of small earth stations are known to employ the CDM communication method using spread spectrum modulation technology. This is MICRO EARTH
It is called the STATION system (small earth station system) and is developed by the American company EQUATORIAL.

(Problems to be Solved by the Invention) However, the CDM communication system inherently has a problem of poor frequency utilization efficiency. One of the important issues in wireless communication systems is to utilize frequencies effectively, and the same is true for satellite communication systems. Therefore,
Considering from the perspective of effective use of frequencies, TDM
The (time division multiplex) communication method is unsuitable for small earth stations due to the complicated equipment configuration.
It can be said that SCPC (Single Channel Per Carrier)-FDM communication method is optimal. In other words, the CDM communication method and FDM communication method are used together.

By the way, the problem at this time is the transmission band structure. For example, as shown in Figure 9, the total used bandwidth is divided into two, the FDM band and the CDM band, and the FDM
It is conceivable that a band is made up of a number of frequency slots. However, with such a transmission band structure, the bandwidth that can be used in the CDM communication system is limited, which reduces the processing gain by half, making it difficult to suppress interference with satellite lines and terrestrial lines of neighboring satellites. There is a problem that.

The present invention was made in view of these problems, and its purpose is to provide a communication system between small earth stations in a satellite communication system consisting of a plurality of small earth stations having various antenna directivity characteristics. The objective is to provide a small earth station that uses both the FDM communication system and the CDM communication system to suppress interference with satellite lines and terrestrial lines of adjacent satellites, and that can utilize frequencies effectively.

(Means for Solving the Problems) In order to achieve the above object, the small earth station of the present invention has the following configuration.

That is, the small earth station of the present invention has a transmission band structure including a first channel consisting of l (where l is a natural number) adjacent frequency slots, and m to do
(m is a natural number) frequency slots.
A plurality of small earth stations having various antenna directivity characteristics that communicate with each other using satellite lines set in an alternating structure with channels;
Among the small earth stations, the small earth station having a highly directional antenna has a transmitter that transmits an FDM (frequency division multiplexing) signal using the first channel.
FDM transmitting means; and CDM transmitting means for transmitting a CDM (code division multiplexed) signal subjected to spread modulation over the entire used bandwidth; FDM receiving means having a channel selection filter that selects a predetermined frequency slot of a first channel; and an FDM demodulator that receives the output of the channel selection filter and performs demodulation processing; a comb filter bank having the first channel as a stop band and the second channel as a pass band; a matched filter that receives an output of the comb filter bank and performs despreading processing; A CDM receiving means having a CDM demodulator that receives and demodulates the output of The apparatus is characterized in that it includes the CDM transmitting means, and the receiving section includes the CDM receiving means.

(Function) Next, the function of the small earth station of the present invention configured as described above will be explained.

The transmission band structure of the satellite line is that among the many frequency slots that divide the total used bandwidth, adjacent l
(l is a natural number) frequency slots.
channel and a second channel consisting of m (m is a natural number) adjacent frequency slots. Multiple small earth stations communicate with each other using this satellite link.
It is a mixture of small earth stations having various antenna directivity characteristics, and for example, small earth station A having a highly directional antenna transmits an FDM (frequency division multiplexing) signal using the first channel.
The small earth station B, which is equipped with an FDM transmission means and has a low directivity antenna, is equipped with a CDM transmission means for transmitting a CDM (code division multiplexed) signal that has been spread-modulated over the entire usable bandwidth, and has a high directivity. A small earth station C having an antenna has an FDM receiving means having a channel selection filter that selects a predetermined frequency slot of the first channel from the received signal, and an FDM demodulator that receives the output of the channel selection filter and performs demodulation processing. and a comb-shaped filter bank that performs filtering processing on the received signal, the first channel being a stopband and the second channel being a passband, and a comb-shaped filter bank that receives the output of the comb-shaped filter bank. The CDM receiving means can be provided with a matching filter that performs despreading processing and a CDM demodulator that receives the output of the matching filter and performs demodulation processing.

Thus, according to the small earth station of the present invention,
By using a combination of FDM and CDM communication systems, it is possible to compensate for the low frequency efficiency, which is a drawback of CDM communication systems. In addition, since the CDM communication system performs spectrum spreading using the entire available band, it is possible to obtain a desired processing gain and suppress interference with satellite lines and terrestrial lines of adjacent satellites. Furthermore, depending on the antenna directional characteristics, small earth stations may be stations using only the FDM communication method or
It is possible to have a station using only the CDM communication method or a station using both methods, and it is possible to optimally design a satellite communication system consisting of multiple small earth stations with various antenna directivity characteristics. effective.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 shows an example of the configuration of a satellite communication system comprising a small earth station according to the present invention. In Figure 1,
A small earth station that communicates with each other via satellite 1 is
FD communication station 2-1 to 2-M and CDM communication station 3-
It consists of stations 1 to 3-K, and stations 4-1 to 4-N that use both FDM communication and CDM communication.

Since the configurations of FDM communication stations 2-1 to 2-N and CDM communication stations 3-1 to 3-K are well known, their explanations will be omitted, but the combined stations 4-1 to 4-K will be omitted.
For example, as shown in FIG. 2, the N receiving system includes a receiving device 21 including a low-noise amplifier and a down converter, a channel selection filter 22, and an FDM.
An FDM receiving section including a demodulator 23, a comb filter bank 24, a matching filter 25 and a CDM
and a CDM receiving section including a demodulator 26.

In the present invention, the transmission band structure of the satellite line consists of a first channel consisting of l (where l is a natural number) adjacent frequency slots out of a large number of frequency slots obtained by dividing the total used bandwidth, and a first channel consisting of l (l is a natural number) adjacent frequency slots; The channel is set to have an alternating structure with a second channel consisting of frequency slots (m is a natural number). Therefore, the channel selection filter 22
A predetermined frequency slot of the first channel is selected based on the output of the FDM signal, and an FDM signal is extracted from the predetermined frequency slot of the first channel. In addition, the comb-shaped filter bank 24
has the first channel as a rejection zone and the second channel as a rejection zone.
Since the second channel is the passband, the signal of only the second channel, that is, the CDM signal, is separated and output upon receiving the output of the receiving device 21. In addition, CDM
For more information about signal shaping, or spread spectrum modulation techniques, read Spread Spectrum by RC Dixon.
Systems (John-Wiley & Suns 1976), so the explanation will be omitted.

Next, the reception operation of the combined station when the FDM communication station and the CDM communication station transmit to the combined station will be explained with reference to FIG.

Now, if l = 1 and m = 1, the transmission band structure of the satellite line is that the first channel and the second channel are 1
It has a structure that repeats each frequency slot.

Therefore, if the bandwidth of one frequency slot is Δf, the output of the receiving device 21, that is, the spectrum of the received signal, is as shown in FIG.
A CDM signal exists for each frequency slot, and the CDM signal exists over the entire available bandwidth. If the channel selection filter 22 is a bandpass filter whose passband is the nth frequency slot in the first channel (Fig. 3b), then the FDM signal transmitted at the nth frequency slot is is extracted (FIG. 3c) and input to the FDM demodulator 23 for data reproduction.

On the other hand, the filtering characteristics of the comb-shaped filter bank 23 are such that the stopband and passband repeat for each frequency slot, as shown in FIG. A CDM signal in the passband of the comb filter bank 23 having a width N·Δf is extracted (FIG. 3e). This does not include FDM signals. This CDM signal is subjected to despreading processing in matching filter 25, and then input to CDM demodulator 26 where it is subjected to data reproduction processing.

Here, since the CDM signal shown in FIG. 3e has distortion in its frequency characteristics, it is necessary to evaluate its influence. If the original CDM signal is s(t), its frequency conversion is S(ω)=∫s(t)e ^-j 〓 ^t dt...(1). On the other hand, the frequency characteristic of the comb filter is F(ω)=〓 ^na _o cos(nω/Δf) (2). However, {a _o } is a Fourier coefficient determined by the frequency characteristics of the comb filter.

The output of this comb filter is S^(t)=1/2π∫S(ω)・F(ω)e ^-j 〓 ^t dω=
s(t)+1/2 〓 ⁿ a _o {s(t-n/Δf)+s(t+n/Δf)}...(3). In equation (3), the first term is the original CDM signal (main response output), and the second term is the so-called echo response output. The echo response output has the same signal waveform as the original CDM signal {s(t)}, but appears before and after the main response output at timings that are integral multiples of 1/Δf. The situation is shown in Figure 4. FIG. 4 shows the basic response of the matched filter 25 that performs pulse compression, and the pulse width of each pulse is proportional to 1/N·Δf, that is, the reciprocal of the entire band used.

As is clear from FIG. 4, the present invention
In the CDM receiver, temporal dispersion occurs over several sections every 1/∆f (sec), so if the data transmission rate R (b/s) and ∆f are equal, the preceding and succeeding data will overlap. This results in intersymbol interference. However, this is shown in Figure 5 (when R<Δf)
The problem of distortion can be solved by simply shifting R slightly from Δf, as shown in FIG. 6 (in the case of R>Δf). That is, since the pulse width of each pulse is 1/N・Δf, in order to prevent the previous and subsequent pulses from overlapping, |1/R−1/Δf|>1/N・Δf ...(4) It is sufficient. In other words, it is sufficient if Δf/R<1-1/N...(5) or Δf/R>1+1/N...(6), but normally N is a value between 100 and 1000,
Since it is sufficiently large, it can be defined as shown in FIGS. 5 and 6, and inter-symbol interference can be avoided for any R.

Finally, the CDM demodulator 26 of the present invention will be explained with reference to FIG. In FIG. 7, reference numerals 71 to
74 is a delay device with a delay time of 1/Δf, code 75
is an adder, 76 is a sampler, 77 is a demodulation circuit, 78 is a square-law detector, 79 and 80 are
is a sampler, 81 is a differential amplifier, 82 is a low-pass filter, 83 is a voltage controlled oscillator,
Reference numeral 84 is a counter, and reference numeral 85 is a digital delay device.

The CDM signal to be received and input is, for example, 1 in Fig. 5.
5 types as shown in ~5, or 1 to 6 in Figure 6
Assuming that there are six types as shown in the figure, matching filter 2
The output of 5 is as shown in FIG. 5b and FIG. 6. The output of this matching filter 25 is
The echo components of each CDM signal are also added to the adder 74 and the adder 75, and as a result, the maximum value of the signal is sent from the adder 75 to the sampler 76 and the square law detector 78.

The circuit from the square-law detector 78 to the digital delay device 85 is a clock synchronization circuit, and as shown in FIG. 8, when the clock phase error is zero, the sample value A (output of the sampler 79) and the sample value B ( The values of the sampler 80 (output) are equal, and if a clock phase error occurs, a difference will occur between the two sample values.

When the clock phase error becomes zero and clock synchronization is established, the output (maximum signal value) of the adder 75 is sampled by the sampler 76 and sent to the demodulation circuit 77.
, and the demodulated data is reproduced there.

As described above, since the adder 75 adds the echo response component to the main response component, the carrier-to-noise ratio (C/N) can be maximized and the reliability of reproduced data can be ensured.

Note that the small earth station in the present invention includes not only fixed stations but also mobile objects such as automobiles, airplanes, and people. In addition, the antenna used is not limited to a small diameter parabolic antenna, which is a highly directional antenna.
For example, a helical antenna, which is a low-directivity antenna, can also be used, and may be selected and used appropriately depending on the characteristics of the small earth station and the communication method to be adopted (FDM method, CDM method, or a combination of these methods). In the case of FDM communication, a parabolic antenna is used to avoid interference with external systems as much as possible. Furthermore, in the combination method of the present invention, audio transmission at, for example, 64 kbps is also possible, and in this case, a parabolic antenna is used.
It becomes FDM communication. Since the CDM system utilizes the entire available band of the satellite line, it is easy to obtain a desired processing gain, and communication at a desired transmission rate can be performed without interference with external systems.

(Effects of the Invention) As detailed above, according to the small earth station of the present invention, since the FDM communication method and the CDM communication method are used in combination, the low frequency utilization efficiency, which is a drawback of the CDM communication method, can be solved. It can be supplemented. Also,
Since the CDM communication method uses the entire available bandwidth to perform spectrum spreading, it is possible to obtain the desired processing gain and suppress interference with satellite lines and terrestrial lines of adjacent satellites. Furthermore, depending on the antenna directional characteristics, a small earth station can be a station using only the FDM communication method, a station using only the CDM communication method, or a station using both methods. It has various excellent effects, such as making it possible to optimally design a satellite communication system composed of earth stations.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a satellite communication system consisting of the small earth station of the present invention, and Figure 2 is an FDM
A receiving system block diagram of a small earth station that uses communication and CDM communication in combination; Fig. 3 is an explanatory diagram of the reception operation of the small earth station that uses both communication and CDM communication; Fig. 4 is a spectrum diagram of the basic response of the matching filter output; Figs. Figure 6 is
An explanatory diagram of the operation of the CDM receiving section, Fig. 7 is a configuration block diagram of the CDM demodulator, Fig. 8 is an explanatory diagram of the clock phase error detection operation performed in the clock synchronization circuit in the CDM demodulator, and Fig. 9 is a diagram of the satellite line. Divide the total used band into two, one being FDM and the other being CDM.
FIG. 2 is a diagram showing an example of a transmission band structure as a band. 1...Satellite, 2-1~2-M, 3-1~3-
K, 4-1 to 4-N...small earth station, 21...receiving device, 22...channel selection filter, 23...
...FDM demodulator, 24...comb filter bank,
25... Matching filter, 26... CDM demodulator,
71-74...delay device, 75...adder, 76,
79, 80... Sampler, 77... Demodulation circuit, 7
8...square law detector, 81...differential amplifier, 82...
...Low pass filter, 83...Voltage controlled oscillator,
84...Counter, 85...Digital delay device.

Claims (1)

[Claims] 1 The transmission band structure consists of a first channel consisting of l (where l is a natural number) adjacent frequency slots, and a first channel consisting of m (where m is a natural number) adjacent frequency slots, out of a large number of frequency slots into which the total used bandwidth is divided. A plurality of small earth stations having various antenna directivity characteristics that perform mutual communication using a satellite line set in an alternating structure with a second channel consisting of frequency slots; The small earth station having a highly directional antenna includes: a transmitting unit that transmits an FDM (frequency division multiplexed) signal using the first channel; a CDM transmitting means for transmitting a CDM (code division multiplexed) signal that has been transmitted; FDM receiving means having an FDM demodulator for receiving the output of the channel selection filter and performing demodulation processing; A comb-shaped filter bank whose passband is the channel of
CDM receiving means; A small earth station characterized by: