JPS634982B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an automatic frequency control method using a reference pilot signal, for example, SCPC (Single
This is suitable for use in the FDMA (Frequency Division Multiple Access) system, which allocates one voice line to one carrier wave, which is called the Channel Per Carrier system. The SCPC method is widely used in maritime satellite communication systems and domestic satellite communication systems;
In this method, the occupied frequency bandwidth of each communication carrier wave is narrow, and the frequency interval between carrier waves is narrow, so the transmission frequency and reception frequency must match well.
Therefore, in order to compensate for the frequency deviation of the communication carrier wave caused by the frequency fluctuation of the local oscillator of the satellite repeater (transponder), the frequency fluctuation of various local oscillators in the earth station equipment, or the Doppler shift due to the movement of the satellite, the standard Automatic frequency control (AFC) using a pilot is performed. That is, each earth station transmits a reference pilot to the satellite, either commonly or individually, and performs AFC using its own pilot, which is relayed by the satellite and received. This makes it possible to correct most of the frequency deviations caused by the various frequency fluctuation factors mentioned above. Note that in a satellite communication system, frequency fluctuations of transponders play a large role, so even when a common reference pilot is used, a considerable part of the frequency deviation can be corrected. AFC using individual reference pilots is called the individual pilot method, and can achieve highly accurate AFC, but in the past, this method generally used an unmodulated carrier wave for the reference pilot. However, the non-modulated pilot has the following drawbacks. That is, (1) It is susceptible to noise such as cross-modulation noise where energy is concentrated in a narrow band, and there is a risk of synchronization being lost due to this. Satellite transponders have strong nonlinearity, so when many narrowband carrier waves in the SCPC system are commonly amplified by the transponder, many narrowband cross-modulation products occur, which can be mistaken for non-modulated pilots. In order to eliminate the influence of cross-modulation noise, it is sufficient to provide a sufficient S/N margin after pilot demodulation, but this is not appropriate because it means an increase in the satellite power consumed for pilot amplification. (2) The protection frequency band required to identify without error the source of the reference pilot transmitted by each earth station is large, and the frequency usage efficiency of the satellite communication system is significantly reduced. Each earth station must not mistakenly capture the reference pilot of another station, so each reference pilot must have a dedicated band that is at least twice the expected maximum frequency deviation, with sufficient frequency spacing. A typical satellite communication system requires an interval of 100KHz to 200KHz. Incidentally, like the Inmarsat (International Maritime Satellite Organization) system, the maximum frequency deviation is ±
Assuming a system expected to be 55KHz and accessed by 20 earth stations, the frequency band dedicated to the 20 reference pilots is 55 x 2 x 20 =
It becomes 2200KHz (=2.2MHz). In the Inmarsat system, where the transponder's frequency band is 7.5MHz, this figure reaches 30% of the total, resulting in extremely poor frequency utilization efficiency. Note that the drawback (1) above also applies to AFC using a common reference pilot. In view of the above-mentioned prior art, it is an object of the present invention to provide an automatic frequency control system that is resistant to noise where energy is concentrated in a narrow band and that has a narrow frequency band occupied for AFC. The above purpose is to use a carrier wave as the reference pilot.
Using the PN code sequence modulated, the carrier wave component of the received reference pilot is reproduced by determining the correlation with the PN code sequence of the same pattern used for modulation, and the frequency of the reproduced carrier wave component and a predetermined nominal frequency are determined. This can be achieved by controlling the oscillation frequency of a local oscillator for frequency conversion in at least one of the receiving system and the transmitting system so as to correct the frequency deviation of the communication carrier wave from the difference between the two. The present invention will be described below with reference to the drawings. Figures 1a to 1c show typical examples of communication systems to which the present invention can be applied, and Figure 1a shows a communication satellite.
Figure b shows a satellite communication system between a land station E and a mobile station M such as an aircraft, ship, or automobile via a communication satellite ST. shows a normal communication system between fixed stations L that does not use communication satellites. In addition, it can be applied to various communication systems. In the present invention, the carrier wave is modulated with a pulse pattern having an extremely long repetition period, which is also called a PN code sequence, or a pseudo-random pattern, and used as a reference pilot.
By correlating with the PN code sequence, it can be clearly distinguished from other reference pilots that have the same carrier frequency but different PN code sequences. Of course, if the carrier frequencies are different, the distinction will become clearer. On the other hand, when the carrier wave is modulated with a PN code sequence, the spectrum is spread, and then by calculating the correlation, the spread spectrum is concentrated.
For noises whose energy is concentrated within a narrow band, such as cross-modulation noise, the energy is diffused when correlation is determined. Therefore, even if a large number of strong intermodulation noises are received, the energy is spread by determining the correlation, so that only the carrier wave of the reference pilot can be captured, and no synchronization occurs. The present invention applies frequency control in the transmitting system so that the frequency of the communication carrier wave for the other station remains constant during reception, and two cases in which frequency control is performed on the receiving side so that the frequency of the communication carrier wave for the other station changes. There are two ways to follow this, but in the satellite communication system between mobile station M and land station E in Figure 1a, it is not space-friendly or economical to provide the mobile station with frequency control equipment. Since there are many inconveniences, it is convenient to equip the land station E with AFC equipment that controls the frequency of communication carrier waves transmitted by the land station E and AFC equipment that follows and receives uncontrolled communication carrier waves from the mobile station M. In addition, in systems such as satellite communication systems where one station can receive the reference pilot modulated with a PN code sequence, each station does not have to transmit it individually, and only one pilot can be used within the receivable area. If a station transmits a reference pilot on its behalf (common pilot method), other stations can also use this to transmit or receive signals.
AFC can be applied. Note that when applying AFC to the transmission system using the common pilot method, it is important to control only the frequency of the communication carrier wave and not the frequency of the reference pilot. Of course, when AFC is applied to the communication system using the individual pilot method, the frequency may be controlled by linking the communication carrier wave and the reference pilot. FIG. 2 shows an example of a circuit for creating a reference pilot. In the figure, 1 is a reference pilot carrier oscillator (REFOSC), 2 is a balanced modulator (MOD), 3 is a PN code generator, 4 is a bandpass filter, and 5 is an output terminal. The operation of the reference pilot creation circuit 6 shown in the figure is as follows. A carrier wave from a carrier wave oscillator 1 is balanced modulated by a balanced modulator 2 such as a ring modulator using a PN code sequence from a PN code generator 3, and the modulated output is output after removing unnecessary frequency components by a bandpass filter 4. Output to terminal 5. In other words, the signal at output terminal 5 is a two-phase signal whose transmission rate is the bit rate of the PN code sequence used for modulation, that is, the clock.
It is a PSK wave, and its spectrum is spread within a frequency band equal to the clock of the PN code sequence. The reference pilot thus created is converted to a desired radio frequency by an up converter within the communication equipment, and then sent to the communication satellite or directly to the other station. FIG. 3 shows an example of a circuit that regenerates a reference pilot carrier wave and generates a frequency control signal. In the figure, 7 is an input terminal for the received signal from the down converter, 8 is a bandpass filter, 9 is a correlator, 10 is a variable phase PN code generator, and 11 is a
12 is a carrier extraction circuit, 13 is a phase-locked loop (PLL), 1
4 is an input terminal for a nominal frequency signal, 15 and 15a are output terminals for control signals having opposite polarities, and 16 is an inverting amplifier. Note that a balanced modulator is used as the correlator 9. The carrier extraction circuit 12 passes input signals above a certain level, and limits the amplitude of excessive signals. The signal entering input terminal 14 is the nominal intermediate frequency signal of the carrier of the reference pilot to be acquired. The operation of the carrier wave regeneration and control signal generation circuit 17 shown in FIG. 3 will be explained below. The received signal, which has been converted to an intermediate frequency by a down converter in the communication equipment, is separated from the communication signal by a bandpass filter 8, and only the received reference pilot is input to a correlator 9. The correlator 9 multiplies the PN code sequence from the PN code generator 10 by the reception reference pilot of the two-phase PSK wave, that is, balance-modulates the reception reference pilot with the PN code sequence and correlates the two. As described above, if the patterns of the PN code sequences do not match, the correlation output will be low, but even if they match, if the phases are shifted, the correlation output will be low. Therefore, the output signal of the correlator 9 is input to the PN code phase synchronization circuit 11, and this phase synchronization circuit 11 adjusts the phase of the PN code sequence generated by the phase variable PN code generator 10 so that the correlation output is maximized. Control. In other words, the correlator 9, the phase synchronization circuit 11 and the phase variable
A feedback loop constituted by the PN code generator 10 reproduces the carrier wave component of the original reference pilot from the reception reference pilot undergoing spectrum spreading, and outputs it from the correlator 9. The output signal of the correlator 9 is input to the carrier extraction circuit 12,
If the input level exceeds a predetermined threshold, it is determined that the desired reference pilot is being received, and the input carrier wave component is directly transmitted to the PLL 13.
is output to. If it is below the threshold, it is another reference pilot or noise, and these are not adopted. The PLL 13 detects the frequency difference between the input signal from the carrier extraction circuit 12 and the nominal frequency signal from the reference input terminal 14, and outputs a signal indicating the frequency difference as a control signal to output terminals 15, 15a. This control signal is applied to the local oscillator of the up converter in the transmission system from one output terminal, e.g. 15, when applying AFC to the communication carrier wave, and is applied from the other output terminal, e.g. is applied to the local oscillator of the down converter, and the local oscillation frequency is controlled so that the frequency difference becomes zero in both cases. Note that as the signal input to the reference input terminal 14, if the reference pilot is transmitted and received at the same station, the signal from the carrier wave oscillator 1 in the reference pilot creation circuit 6 can be used, but if the transmission and reception are different. In the case of a station, a separate oscillator with the same oscillation frequency is provided. FIG. 4 shows an example in which the present invention is applied to a maritime satellite communication system. In this example, the land station has the function of automatic frequency control for both transmission and reception, thereby eliminating the burden on the ship station's equipment. FIG. 4a shows an automatic frequency control system for the transmission system of a land station, and FIG. 4b shows an automatic frequency control system for the reception system of the land station. First, to explain the circuit shown in FIG. 4a, 101 is a telephone line, 102 is an audio signal modulator, 103 is a reference pilot and modulation signal synthesizer, and the output of the synthesizer is sent to the first and second up converters 104. ,10
5, the frequency is converted to the 6GHz band, and antenna 1
From 06, it is sent to a communication satellite. Communication satellite transponders convert 6GHz to 1.5GHz and transmit it to ship stations, so land stations also use this 1.5GHz.
and the first and second down converters 107,1
At step 08, the signal is converted to an intermediate frequency and input to the carrier wave regeneration and control signal generation circuit 17. The control output of this circuit 17 is given from a terminal 15 to a variable frequency local oscillator 104a of the first up converter 104 of the transmission system, and the local oscillation frequency is controlled so that the carrier wave component of the reproduced reference pilot matches the nominal frequency. be done. In other words, the local oscillation frequency changes by the deviation amount in the opposite direction to the frequency deviation. This corrects frequency deviations caused by transponders, etc., and keeps the 1.5GHz band communication carrier wave received by the ship station constant. Therefore, the ship station does not need to apply AFC to reception. To explain the circuit shown in FIG. 4b, the communication carrier wave transmitted by the ship station is in the 1.6 GHz band, which is converted to the 4 GHz band by the satellite transponder and transmitted to the land station. Therefore, the land station uses the first and second up converters 109 and 110 to convert the reference pilot.
Convert to 1.6GHz and transmit. The land station in question receives the reference pilot sent out by the station and is relayed by satellite.
The frequency of 4 GHz is received, converted to an intermediate frequency by the first and second down converters 111 and 112, and then input to the carrier wave regeneration and control signal generation circuit 17. The control output of this circuit 17 is at terminal 15a.
is applied to the variable frequency local oscillator 112a of the second down converter 112, and the local oscillation frequency is controlled so that the carrier wave component of the regenerated reference pilot matches the nominal frequency. In other words, the local oscillation frequency changes by the deviation amount in the same direction as the frequency deviation. This corrects frequency deviations caused by transponders, etc., and allows land stations to follow and receive communication carrier waves with frequency deviations even if the ship station does not apply AFC to transmission. Therefore, the received signal obtained from the second down converter 112 to the output terminal 113 becomes a frequency-corrected signal, and the received signal is transmitted to the demodulator 114.
The audio signal of the desired ship station can be obtained from. In Figure 4a, AFC is applied to the reference pilot that is sent out along with the communication carrier wave, and the frequency of the reference pilot fluctuates, so other stations can use this reference pilot to control the transmitting or receiving system of the station in question.
There are some inconveniences when applying AFC. In other words, Fig. 4a can be said to be an AFC suitable for the individual pilot method. On the other hand, in FIG. 4b, since AFC is not applied to the transmitted reference pilot itself, other stations can use this reference pilot to apply AFC to their receiving systems. In this case, the other station does not need the reference pilot transmission equipment, and the antenna 106 and the first down converter 111 in FIG.
Thereafter, it is sufficient to include up to the carrier wave regeneration and control signal generation circuit 17 and an oscillator for a nominal frequency signal. In other words, FIG. 4b can be applied to both the individual pilot system and the common pilot system. A specific example of applying AFC to a communication carrier wave using the common pilot method is shown in FIG. Figure 5 is the fourth
The major difference from Figure A is that no AFC is applied to the up-converter system of the reference pilot, and AFC is applied only to the up-converter system of the communication carrier. That is, the output of the reference pilot generation circuit 6 and the output of the modulator 102 are passed through separate first up converters 104' and 104, respectively, and then combined in the combiner 103, and the control signal of the carrier wave regeneration and control signal generation circuit 17 is sent to the terminal 15. It is applied only to the variable frequency local oscillator 104a of the up converter 104 for communication carrier waves. As a result, AFC is not applied to the transmitted reference pilot, so other stations receive this reference pilot and compare the frequency of its carrier wave component with the nominal frequency from a separately prepared oscillator, thereby applying AFC to the communication carrier wave. You can send it out over the phone. In the example shown in FIG.
08a and AFC to the reference pilot's receiving system.
is being applied. This is done in consideration of the fact that since AFC is not applied to the transmitted reference pilot, there is a possibility that it may fall outside the receivable frequency band due to frequency deviations received by the satellite's transponder etc. during reception. In other words, even if a large frequency deviation occurs, the reference pilot can always be received within the receivable frequency band. Other embodiments of the present invention are shown in FIGS. 6 and 7. This embodiment is intended to reduce the synchronization acquisition time required for initial acquisition of the reference pilot as much as possible, and each of the embodiments shown in FIGS. 2 and 3 always uses a spread spectrum reference pilot. On the other hand, a non-modulated pilot is used only during initial acquisition. In other words,
If only a spread spectrum reference pilot is used, it takes time for the frequencies of the PN code generators 10, which have the same frequency and variable phase, to synchronize in phase when correlation is taken for carrier wave recovery. Therefore, if a non-modulated pilot is used only during initial acquisition, there is no need for phase synchronization of the PN code, which saves time.
In this case, there are concerns about the same drawbacks as in the past, such as the widening of the occupied frequency band, but the fact that the probability of multiple pilots becoming unmodulated pilots at the same time is extremely low, and for spread spectrum pilots, Even if the frequencies of unmodulated pilots overlap, they do not cause interference; conversely, even to unmodulated pilots, a spread spectrum reference pilot looks like noise; and the frequency change in a short period of time, such as initial acquisition, is extremely small, so acquisition is difficult. If the spread spectrum is applied later, the disadvantages of the conventional method virtually do not occur because frequency synchronization is less likely to be lost during the establishment of phase synchronization of the PN code. Figure 6 shows the reference pilot creation circuit.
Mode switching is performed by adding changeover switches 18 and 19 to the circuit shown in the figure. During the time period when the reference pilot has already been acquired and the AFC loop is in synchronization, the switches 18 and 19 are turned on to the diffusion mode side contacts 18a and 19a, respectively, and the circuit is switched to the second
It works the same as the example in the figure. During the period when the AFC loop is out of synchronization and an attempt is made to capture the reference pilot, the switch is activated by the initial acquisition mode side contacts 18b and 19.
The carrier wave of the reference pilot is not subjected to spectrum spreading and is output to the output terminal 5 without being modulated. In addition, when the AFC loop is out of synchronization,
For example, if the output of the carrier extraction circuit 12 is zero, that is, AFC
This can be detected when a state in which the period is considered to be out of alignment continues for a certain period of time or more predetermined in consideration of the operating characteristics of the phase synchronization circuit 11 and the like. Figure 7 shows a carrier wave regeneration and control signal generation circuit, with changeover switches 20 and 21 added to the circuit in Figure 3.

【table】

[Table] As explained above, in the present invention, the spectrum of the reference pilot is spread by direct spreading using a PN code sequence, so even if the occupied frequency bands of two or more reference pilots overlap, each station can can separate the reference pilot of a desired station, for example, the own station, from others. Therefore, the frequency spacing between reference pilots can be made considerably smaller than twice the expected maximum frequency deviation, and furthermore, if the number of reference pilots is not very large, all reference pilots can be sent out at the same frequency. You can also do it. That is, the frequency bandwidth occupied by the reference pilot becomes much narrower than in the conventional system, and the frequency band utilization efficiency of the entire system is increased. Furthermore, since the present invention uses spread spectrum, even if noise such as cross-modulation noise whose energy is concentrated in a narrow frequency band is received, the energy of these noises will be spread out during demodulation, resulting in noise. The impact of this will be greatly reduced. Note that, if the carrier frequencies of the reference pilots are sufficiently shifted, a plurality of reference pilots with the same PN code sequence for spread spectrum can be used in the same communication system. However, if the PN code sequences are different, it is possible to identify that the pilot captured by PN code synchronization is the desired pilot for transmission to the local station.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of a communication system to which the present invention can be applied, FIGS. 2 and 3 are block diagrams showing an embodiment of the present invention, and FIG. FIG. 5 is a block diagram showing a specific example applied to a satellite communication system, FIG. 5 is a block diagram showing another specific example, FIGS. 6 and 7 are block diagrams showing other embodiments of the present invention, and FIG. FIG. 3 is an explanatory diagram showing an example of pilot arrangement. In the drawing, 1 is a reference pilot carrier wave oscillator,
2 is a balanced modulator, 3 is a PN code generator, 4 is a bandpass filter, 5 is an output terminal, 6 is a spread spectrum reference pilot creation circuit, 7 is an input terminal for received signals, 8 is a bandpass filter, 9 is a correlator, 10 is a variable phase PN code generator, 11 is a phase locked circuit, 12 is a carrier extraction circuit, 13 is a phase locked loop, 14 is an input terminal for a nominal frequency signal, 15 and 15a are signals for frequency control 16 is an inverting amplifier, 17 is a carrier wave regeneration and control signal generation circuit, 18 to 21 are mode changeover switches, 22 is a narrowband bandpass filter, 23
is an envelope detector, 101 is a telephone line, 102 is a modulator, 103 is a combiner, 104, 104', 10
5, 109 and 110 are up converters, 10
6 is an antenna, 107, 108, 111 and 11
2 is a down converter; 104a, 108a and 112a are variable frequency local oscillators; 113 is a received signal output terminal; and 114 is a demodulator.

Claims (1)

[Claims] 1 On the transmitting side, the carrier wave serving as the reference pilot signal is modulated using a PN code sequence and transmitted.
On the receiving side, the received reference pilot carrier wave component is regenerated by determining the correlation between the received signal and a signal of a code sequence equal to the PN code sequence, and the frequency of the regenerated carrier wave component is matched with a predetermined nominal carrier frequency. An automatic frequency control method characterized in that the oscillation frequency of a local oscillator for frequency conversion in at least one of a receiving system and a transmitting system is controlled so as to correct a frequency deviation of a communication carrier wave from the difference in frequency.