JPH0413893B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a communication system for transmitting narrowband signals between small earth stations via a communication satellite using small antennas and small-scale transmitting/receiving equipment.

[Conventional technology]

In recent years, improvements in the performance of communication satellites have made it possible to create small earth station communication systems that include small antennas and small-scale transmitting and receiving equipment. The capabilities of small earth stations are limited in both transmission power and signal bandwidth, forcing narrowband communications. However, in narrowband communication, phase noise caused by local frequency signal sources is added each time frequency conversion is performed several times in a transmission path. This causes a significant deterioration in communication quality, and in some cases, communication itself becomes impossible. These phase noises are low frequency noises that have an extremely large spectral density in the very low frequency region.

Conventionally, in satellite communications, the synchronous detection method has been mainly used from the viewpoint of effective use of transmission power, and the bandwidth of the carrier regeneration circuit is set to about 1/100 of the transmission speed to improve S/N. is normal. However, in the case of narrowband communication, the follow-up characteristics of the carrier regeneration circuit deteriorate, causing large phase jitter and frequent phase slips, which impede normal communication.

FIG. 8 shows the local signal system of the satellite communication system. In the figure, 50 is a modulator, 51 is a carrier oscillator, 54 is an up converter, 55 is a local oscillator with an oscillation frequency _U , 56 is a frequency conversion circuit, 57 is a local oscillator with an oscillation frequency _L , 58
59 is a local oscillator with an oscillation frequency _D , and 64 is a receiving IF system.

In the transmitter, the carrier of frequency _C is modulated by the transmission data, and further frequency-converted to the uplink RF frequency _U by a local signal of frequency _U , and then sent out. On the satellite, frequency conversion to downlink is performed using local signal _L , and further, in the receiving section, frequency conversion is performed using frequency _D to a specified IF frequency band and demodulation.

[Problem that the invention seeks to solve]

The received signal includes the phase tones of all the local signal sources mentioned above. In particular, when building a narrowband communication system between small earth stations using existing communication satellites, an overwhelming portion of the phase noise is generated in the local oscillator on the satellite. Since each earth station's transmission signal must accurately transmit a signal on the assigned frequency, an extremely stable frequency source is used. However, in the receiving section, the local oscillation frequency of the down converter cannot be expected to be so highly stable due to the scale of the device. Therefore, most of the phase noise is generated in the transmission path and the receiving device.

For this reason, FSK modulation is conventionally used when communicating through a transmission path with large phase noise.
In FSK modulation, data 0 is transmitted at a frequency ⁽⁰⁾ , and data 1 is transmitted at a frequency ⁽¹⁾ .

Figure 9 shows the FSK signal receiving circuit. 70 is a down converter, 71 is a channel frequency synthesizer, 72 is a band waver tuned to the frequency corresponding to data 0, 73 is a band wave generator tuned to the frequency corresponding to data 1, 74 and 75 are square law detectors, 76 is an amplitude comparator, 77 is a clock recovery circuit, and 78 is a data discriminator.

The method using FSK modulation is effective in systems with large phase noise, but since it is a square law detection method, theoretically the BER characteristics are significantly worse than in the case of synchronous detection, and the FSK modulation Due to its characteristics, it has no choice but to have a wide band.

An object of the present invention is to overcome the above-mentioned drawbacks and to realize a communication system that can perform high-performance narrowband communication even in a transmission system with large phase noise.

[Means for solving problems]

In the present invention, a predetermined pilot station transmits a pilot signal at a specified frequency, and each earth station transmits the pilot signal by passing the received signal through a narrowband bandpass waver tuned to the pilot frequency. A frequency synthesizer that reproduces a signal and generates a channel frequency signal to be received, and a tracking type bandpass waver that uses the frequency synthesizer as a local signal and has wave characteristics tuned to the local signal, The signal of the channel to be received is selected by passing the received signal through a type band-pass transducer, and the signal of the received channel is frequency-converted to a low frequency band using the regenerated pilot signal as a local signal. Part of the frequency synthesizer output as a local signal
This is a narrowband communication method that converts the frequency to the IF frequency band to obtain the channel signal to be received in a fixed frequency band regardless of the channel, and demodulates and reproduces it.

〔Example〕

FIG. 2 shows a communication system to which the present invention is applied. 91 is a communication satellite, 92 is a pilot station, 93
is a large number of small earth stations.

FIG. 3 shows the transmission signal spectrum according to the method of the present invention.

FIG. 1 shows a narrowband signal receiving circuit according to the present invention. This is a part corresponding to the reception IF system 64 in FIG. 1 and 2 are frequency tracking type band wave generators, 3 is a mixer, 4 is a local oscillator for pilot signals, 5 is a channel frequency synthesizer, 6 is a low frequency wave generator, 7 is a mixer,
8 is a band wave generator, and 10 is a demodulator.

The frequency tracking type band wave generator used in the present invention is
As shown in the figure. 11 and 12 are mixers, 13 is a π/2 phase shifter, 14 and 15 are low frequency filters, 16 and 17 are mixers, and 18 is an IF signal synthesizer.

The basic circuit of demodulator 10 is shown in FIG. 101
1 is a carrier recovery circuit, 102 is a clock recovery circuit, 103 is a demodulator, and 104 is a data recovery circuit.

The communication principle of the communication system according to the present invention will be explained below. First, the received signal is the pilot signal; υ _p (t) = cos (ω _p t + θ _j + θ _po ) (1) Each channel signal; υ _k (t) = cos (ω _k t + θ _j + _k + θ _ko ) (2) It can be expressed as However, ω _p : Pilot frequency θ _po : Transmission line thermal noise added to the pilot signal ω _k : Frequency of channel k (k=1, 2,...N) θ _ko : Thermal noise _k (t) added to the k-th channel signal ; Modulation phase (0 or π) θ _j ; Phase noise generated in the transmission path.

The important fact here is that the phase noise θ _j is common to all channels.

Referring to FIG. 1, the local oscillator 4 is tuned to the pilot frequency, and the pilot signal of equation (1) is selectively extracted at the output of the bandpass generator 1. Similarly,
At the output of the bandpass filter 2, a signal to be received (referred to as the k-th channel) is selected from among the signals expressed by equation (2). As a result of the multiplication in the mixer 3, υ ₃ (t)=cos {(ω _k −ω _p )t+ _k +θ _ko −θ _po } (3) is obtained, and the phase noise θ _j is canceled out. Instead, phase thermal noise _θpo is added, but this term can be reduced by making the power of the pilot signal larger than that of other channels, or by making the bandwidth of bandpass converter 1 narrower than that of bandpass converter 2. , θ _po can be suppressed to a value much smaller than θ _ko , and the S/N deterioration in the mixer 3 can be suppressed to a negligible value.

Next, in mixer 7, υ ₇ (t)=cos {(ω _k −ω _k ′−ω _p )t + _k +θ _ko −θ _po }+ω _k +ω _k ′−ω _p (4) is obtained. It will be done. Here, ω _k ' is the output frequency of the channel frequency synthesizer 5, and has a value extremely close to ω _k . Bandwidth wave generator 8 is tuned to ω _p and formula (4)
By passing only the first term of , the channel signal to be received in the frequency band ω _p can be obtained regardless of the channel frequency. The output of the bandpass converter 8 is set to the IF frequency ω _p
The data and clock are input to a demodulator 10 which operates at

In some cases, as shown in Fig. 1, the output of the bandpass generator 8 may be frequency-converted to the baseband band (OHz band) using a part of the output of the local oscillator 4 for synchronization or delay detection. Therefore, signal reproduction can be performed.

As described above, according to the present invention, it is possible to cancel out the phase noise generated in the transmission path and the receiving device.
Note that the phase noise generated in the transmitter of each earth station may be handled as follows. That is, it is sufficient if each earth station independently transmits a pilot signal. The transmission spectrum of each channel in this case is shown in Figure 6.
The receiving device is shown in FIG. 52 is an IF signal synthesizer;
53 is a pilot oscillator, 55', 59' are channel frequency synthesizers, 60 is a bandpass generator, 61
is a narrowband wave generator, 62 is a mixer, 63 is a low frequency wave generator, and 10 is a demodulator.

The operation of this system is basically the same as the system described above. Each station transmits a pilot signal in addition to the modulated signal at a frequency position that differs by ( _p - _c ) from the carrier frequency of the modulated signal within the assigned frequency band. In the receiving section, first, a channel frequency synthesizer 59' and a down converter 58,
A bandpass filter 60 selectively outputs the signal of the receiving channel in a specified frequency band. Next, the pilot signal of the channel is extracted by the narrowband wave generator 61, and the frequency is converted by the mixer ₆₂ , so that the transmitting _section and the transmission By obtaining a highly stable modulated signal from which phase noise generated in the path section and the receiving device section is removed, signal reproduction can be performed using a normal synchronous demodulation method. Although this method has some loss in terms of power efficiency and bandwidth efficiency, it is extremely effective when performing ultra-low-speed communication with a simple device through a transmission path with large phase noise. That is,
Since the phase noise generated in the entire RF signal path including the transmitter can be removed, it becomes possible to perform ultra-low speed communication using a micro-compact station, and mobile communication using communication satellites becomes possible.

〔Effect of the invention〕

The following effects can be achieved by the present invention.

(1) First, the system and receiver of the present invention can remove phase noise generated in the transmission line and receiver, and the synchronous demodulation method enables narrowband communication with stable and good characteristics.

(2) Therefore, it is possible to construct a system that performs narrowband communication between many small stations using existing communication satellites.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a receiving device according to the present invention, FIG. 2 is a diagram for explaining a narrowband communication system to which the present invention is applied, and FIG. 3 is a diagram showing a transmission signal spectrum according to the present invention. FIG. 4 is a block diagram of a frequency tracking type band waver used in the present invention, FIG. 5 is a block diagram showing the basic configuration of a synchronous demodulator used in the present invention, and FIG. 6 is another example of the present invention, that is, for each channel. FIG. 7 is a block diagram showing the communication system of the channel-by-channel pilot signal superimposition method; FIG. 8 shows the local signal system in the satellite communication system; FIG. 9 shows a conventional FSK modulated signal receiving circuit. In the figure, 1 and 2 are frequency tracking type band wave generators, 3,
7 is a mixer, 4 is a local oscillator, 5 is a channel frequency synthesizer, 6 is a low frequency generator, 8 is a band frequency generator, and 10 is a demodulator.

Claims (1)

[Claims] 1. In a system that performs narrowband communication between multiple small earth stations via communication satellites, a predetermined pilot station sends out a pilot signal at a specified frequency, and each earth station transmits a pilot signal at a specified frequency. A frequency synthesizer that regenerates the pilot signal by passing the received signal through a narrowband bandpass waver tuned to the pilot frequency and also generates a channel frequency signal to be received; and a tracking type bandpass transducer having tuned wave characteristics, and by passing the received signal through the tracking type bandpass transducer, the signal of the channel to be received is selected, and the signal of the receiving channel is selected. By converting the frequency of the reproduced pilot signal as a local signal to a low frequency band, and further frequency converting a part of the output of the frequency synthesizer to the IF frequency band as a local signal, the signal can be received in a constant frequency band regardless of the channel. A narrowband signal communication method that obtains, demodulates, and reproduces the desired channel signal.