JPS5910097B2 - Frequency control device at ground station - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is particularly applicable to a satellite communication system in which each earth station uses a repeater line on a communication satellite by frequency division. This invention relates to a frequency control device in a communication system with a relatively narrow bandwidth close to the amount of frequency fluctuation due to Doppler frequency transition due to frequency fluctuation of a local oscillator of a satellite repeater or drifting of a satellite.

First, a so-called frequency division multiple access method (hereinafter referred to as FDMA method) in which each earth station uses the relay transmission band of the satellite by frequency division when configuring a multiple communication system via communication satellites will be described.

FIG. 10 generally shows a conceptual diagram of satellite communications.
In FIG. 1, 1 is a communication satellite, and 2, 3, and 4 are satellite communication earth stations. In the FDMA system, each station transmits to a satellite using a frequency band assigned to it in advance or each time, and the satellite relays the signals transmitted from each station 15 and transmits them to each earth station. Each earth station extracts a signal in a frequency band specified by the other station from among the received signals and detects and reproduces it. As a modulation method for putting an information signal on a carrier wave, 20 different conventional modulation methods such as amplitude modulation and frequency modulation can be applied, but the present invention is independent of the modulation method, so it will not be described here. I will omit it here. FIG. 2 shows the concept of frequency allocation as an example. f
o is the frequency of the pilot signal, and f1 to f4, etc. indicate the center frequency 25 of each frequency band used. Δf is the occupied bandwidth of each channel. Generally, each channel has a different occupied bandwidth, but in this figure, for simplicity, each channel is represented as having the same occupied bandwidth. △fg
is a guard pad installed between each channel, taking into account the 30-wave number fluctuation of each station's transmission frequency and the amount of Doppler frequency shift caused by drifting of the satellite in the uplink from each station to the satellite. The settings are made so that the interference is below the allowable value. Pilot signal f. is usually transmitted by a specific reference earth station, and the other 35 earth stations detect the pilot signal from the received signal and perform automatic gain control (AGC) on the receiver using that level as a reference, while at the same time controlling the downlink. This pilot frequency is used to frequency convert the received signal in order to avoid the effects of frequency fluctuations caused by satellite transponders and receiver local oscillators, and Doppler frequency shifts in the downlink.

Now, consider a system as described above in which a very large number of stations each require a small capacity transmission band.

If each station requires, for example, one audio channel or one data signal channel, the required bandwidth for each channel may be 4KH density (depending on the modulation method) or, in extreme cases, about 100 Hz. obtain. In such a case, from the viewpoint of effective use of the frequency band, it is desirable to narrow the channel spacing as much as possible and set as many channels as possible within the satellite relay transmission bandwidth. However, as mentioned above, the guard band that must be provided between each channel prevents frequency fluctuations that occur in the uplink, that is,
It must be determined by taking into account transmission frequency fluctuations and Doppler frequency shifts of each station. For example, when using the 6 GHz band for uplink, this amount is determined by the fact that the transmission frequency stability is ±1X.
10-7, it is necessary to allow for ±60011 z and a Doppler frequency shift of about ±150 Hz, so a guard band of at least ±750 Hz in total is required.
Furthermore, considering the Doppler frequency shift that the pilot signal undergoes, a guard band of at least ±1 kHz or more is required, and an extra frequency band that is equal to or greater than the frequency band required by each of the channels described above is required. That is, the frequency band utilization efficiency will be significantly reduced. An object of the present invention is to provide an FDMA satellite communication system that eliminates such drawbacks and significantly improves the efficiency of use of the transmission frequency band.

FIG. 3 shows in chronological order the method for transmitting pilot signals according to the present invention. In this diagram, A, B, C
etc. indicate the distinction between each earth station, with the horizontal axis representing time and the vertical axis representing frequency. That is, in the present invention, each earth station transmits pilot signals of frequencies within predetermined error ranges in a time-division manner. The pilot signals received at each earth station are received in the order of A-B-C-A-B-..., but due to the frequency fluctuations mentioned above,
FA, fB, fc (indicating the receiving frequencies of pilot signals transmitted from stations A, B, and C, respectively), etc. do not match. Therefore, in the present invention, when station A is used as a reference station, station B extracts the signal from station A and the signal from its own station from the received pilot signal time series, and their frequencies, that is, FA and FB. Controls the transmission frequency of the own station pilot signal so that it matches. Station C also performs the same control. In this way, each station adjusts the reception frequency of the pilot signal transmitted from one reference station so that the reception frequency of the pilot signal transmitted from the own station and received via the satellite matches the reception frequency of the pilot signal transmitted from the own station and received via the satellite. Controls the station's pilot signal transmission frequency. As a result, the frequencies FA, fB, fC, etc. of each station in the reception pilot time series exactly match the reception frequency FA of the reference station pilot. FIG. 3 also shows how the reception pilot frequencies of each station gradually match from the initial state of such control. After completing the transmitting pilot frequency control as described above and confirming that the pilot frequency from the reference station matches that from the own station in the received pilot signal time series, each earth station assigns the frequency to the own station in advance. The information signal is transmitted using a designated frequency band (designated with the pilot frequency as a reference).

On the receiving side, the received signal is frequency-converted using the received pilot signal of the reference station or the own station, the frequency band used by the other station is extracted, and the information signal is detected and reproduced. Thus, according to the present invention, fluctuations in the frequency allocation of the received signal due to frequency fluctuations in the uplink are completely absorbed using the frequency of the reference station pilot signal as a reference, so it is not necessary to consider the frequency fluctuation in the uplink in the frequency allocation. This makes it possible to use the transmission frequency band efficiently. In the above description, no mention was made of frequency fluctuations in the downlink, but it will be easily understood that these fluctuations do not affect the frequency allocation based on the pilot frequency.
FIG. 4 shows an embodiment of the present invention, and shows the configuration of a transmitting/receiving device in a slave station.

In FIG. 4, 5 is a received RF signal input terminal, 6 is a signal distribution circuit, 7 is a pilot signal receiver, 8 is a station identification circuit, 9 is a frequency difference detection circuit, 10 is a voltage controlled oscillator, 11
and 13 is a sample hold circuit, 12 is a loop filter, 14 is a transmission voltage controlled oscillator, 15 is a gate circuit, 1
6 is a transmission frequency converter, 17 is a signal synthesis circuit, 18 is a reception frequency conversion circuit, 19 is a sample signal for reference station, 20
is the sample signal for own station, 21 is the transmission signal input terminal, 22
is a transmission output signal terminal, and 23 is a reception signal output terminal. The signal relayed by the communication satellite and received at the slave station is applied to the received RF signal input terminal 5.

As is well known, the received signal is generally converted to an intermediate frequency band and amplified to a specified level (at this time, the automatic gain adjustment function maintains a constant level regardless of fluctuations in the received level). However, these functions are omitted here for the sake of brevity as they are not directly related to the content of the present invention. That is,
It is assumed that a signal at a constant level is input to the input terminal 5. This input has a signal configuration as shown in FIGS. 2 and 3. The RF received signal is branched into two by the signal distribution circuit 6, one of which is input to the pilot receiver 7 and the other to the frequency converter 18.

The pilot receiver 7 uses a band divider to filter pilot signals transmitted in a time-division manner from the reference station and other slave stations, including the own station, within a predetermined frequency range from among the received signals. Output. The pilot signal thus extracted is applied to a station identification circuit 8 and a frequency difference detection circuit 9. The station identification circuit 8 detects the pilot signal, detects the pilot bursts of the reference station and the own station, and sends sample pulses to sample and hold circuits 11 and 13, which will be described later. Here, it is assumed that the reference station pilot burst is subjected to some kind of modulation to enable station identification on the receiving side. This modulation may be, for example, a method of applying shallow amplitude modulation, frequency modulation, or phase modulation using a pulse signal having a specific code sequence, or, for example,
It may be possible to have a burst duration that is clearly different from other slave bursts, but the method design requires that the detection speed is fast, the detection reliability is high, and the required frequency band is as narrow as possible. . The own station pilot burst may be detected using the same method as for the reference station, but as an example, the transmission time of the own station pilot burst is controlled based on the point in time when the reference station burst is detected, and the round trip propagation time to the satellite is known in advance. In consideration of this, a method of determining the reception time of the own station's pilot burst may also be considered. Which method is adopted is irrelevant to the essence of the present invention, and furthermore, these various methods can be fully realized using techniques known in conventional terrestrial or satellite communication systems.

In this way, the station identification circuit 8 detects the pilot bursts of the reference station and its own station from among the bursts of the received pilot signal, generates signals 19 and 20 specifying the reception time position, and also detects the reference station pilot burst reception time point. A gate signal for sending out a local pilot burst is sent to the gate circuit 15 based on the reference point.

On the other hand, the frequency difference detection circuit 9 inputs the output signals from the pilot receiver 7 and the voltage controlled oscillator 10, detects the frequency difference between the two, and sends a DC signal proportional to the difference to the sample and hold circuits 11 and 13.

Various methods can be considered for this frequency difference detection circuit, but communication technology experts will easily understand that it is fully possible using existing technology. In the sample and hold circuit 11, a signal generated at a time position corresponding to the reference station from among the input frequency difference signals is sampled using a sample pulse 19, and held until the next sampling point. The output signal is applied to the control terminal of the voltage controlled oscillator 10 through the rube filter 12, so that the output frequency of the voltage controlled oscillator 10 is controlled to match the reception frequency of the reference station pilot signal. On the other hand, the sample and hold circuit 13 receives the sample pulse 20 at the time corresponding to the reception position of the pilot burst signal of its own station.
The output signal of the frequency difference detection circuit is sampled and held. Its output is applied to the control terminal of the voltage controlled oscillator 14 to control the transmission frequency of the own station pilot signal. In this way, the output frequency of the voltage controlled oscillator 14 is controlled so that the reception reference station pilot frequency and the reception own station pilot frequency match. This signal is gate circuit 1
5, the signal is gated by a gate pulse generated by the station identification circuit 18 with reference to the reference station pilot reception time position, and is guided to the signal synthesis circuit 17. At the same time, this signal is applied to the transmission frequency converter 16 to up-convert the transmission signal 21, and then synthesized into a repilot burst signal by the signal synthesizer, which is output from the output terminal 22. The signals input to the input terminal 21 are, for example, F and -F in FIG. It is a signal modulated by an information signal, with the center frequency being F. The frequency is converted by the output signal of the voltage controlled oscillator 14 having a center frequency f
It becomes a modulated wave with 1. On the receiving side, these frequencies are different from the transmitting frequencies as described above.

F these. ′, F, ′, since the frequency of the reference station pilot signal and the local station pilot signal are controlled to match at the reception point as described above, f1′-FO
It is clear that '=f1-FO. That is, the signal branched by the received signal distribution circuit 6 is sent to the voltage controlled oscillator 10.
It is possible to obtain a signal having a center frequency f1-FO by performing a DOWN-CONVERT with the output of .
In reality, this station receives signals transmitted from other stations, but other slave stations also perform similar frequency control, so fluctuations in frequency allocation due to frequency fluctuations in the uplink mentioned above may occur. There is no frequency at all, and it is strictly fixed based on the pilot frequency. Needless to say, the reference station sends out a pilot signal for a predetermined period of time at regular intervals using its highly stable oscillator.

Furthermore, it is a well-known fact in the time division multiple access system (TDMA system) that each slave station synchronizes the phase of the received clock signal from the reference station with the received clock signal of its own station. The method itself for performing frequency synchronization or phase synchronization by receiving a burst signal transmitted to a target is only one means for realizing the present invention.

The present invention applies this technology to the FDMA system to control the pilot frequency of each slave station based on the reference pilot frequency, so that the frequency allocation on the satellite is not affected by uplink frequency fluctuations, and thus the transmission It is characterized by enabling effective use of frequency bands.
As explained above, according to the present invention, there is almost no need for a guard band to absorb the frequency fluctuation difference that occurs in the uplink from each station to the satellite, and the limited transmission bandwidth can be used effectively. It becomes possible to use it.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the satellite communication system, FIG. 2 is an explanatory diagram of frequency allocation in the FDMA communication system, FIG. 3 is a time chart showing the pilot signal sending method in the present invention, and FIG. An example is shown. 1... Communication satellite; 2, 3, 4... Satellite communication ground station; 5... Received RF signal input terminal; 6
...... Signal distribution circuit; 7... Pilot signal receiver; 8... Station identification circuit; 9...
・Frequency difference detection circuit; 10... Voltage controlled oscillator; 11, 13... Sample hold circuit; 12
...... Loop filter; 14... Transmission voltage controlled oscillator; 15... Gate circuit; 16...
...Transmission frequency converter; 17... Signal synthesis circuit; 18... Receiving frequency conversion circuit; 19...
... Sample signal for reference station; 20 ... Sample signal for own station; 21 ... Transmission signal input terminal;
22... Transmission output signal terminal; 23...
Received signal output terminal.

Claims (1)

[Claims] 1 In a frequency division multiple access satellite communication system in which multiple ground stations communicate via a communication satellite using frequency bands assigned to each station, time-division transmission is performed from one station to the other in the time slot assigned to that station. The own station receives one pilot signal transmitted from the reference ground station in a time division manner during the time slot assigned to that station, and uses the received signals to determine the received pilot signal of the own station and the reference station. Separate the received pilot signal from the station, control the frequency of the transmitting pilot signal of the own station so that the frequencies of these signals match, and set the transmitting or receiving carrier frequency of the own station based on this controlled frequency. A frequency control device in a ground station characterized by: