Fast PSK carrier recovery in a mobile satellite communication network.

Description: TECHNICAL FIELD [Detailed description of the invention]

[Industrial application field] The present invention relates to a mobile satellite communication system, and more particularly to a carrier wave reproduction method for recovering out-of-synchronization of a demodulator due to abrupt attenuation of a signal generated in a propagation path.

[Conventional technology] Conventionally, the demodulator has been composed of a modulation removal circuit by operating frequency multiplication or the like from the received modulated wave, a narrow band tuning circuit for the purpose of improving the S / N of the extracted carrier wave, or a PLL circuit or the like.

[Problems to be solved by the invention] The above-mentioned conventional carrier wave reproduction method has no particular problem when a signal is constantly received at a certain S / N or higher. However, in mobile satellite communication, due to the nature of various obstacles such as trees and buildings in the propagation path, it is not possible to obtain S / N above a certain level. Under such conditions, if the attenuation of the propagation path becomes extremely large, the signal will not reach the receiving side at all, and the phase synchronization of the carrier wave reproduction loop will be lost. In this out-of-range section, the loop becomes self-propelled, and naturally, a phase and frequency shift occurs in the reproduction carrier. Therefore, even if the signal is received again due to the position movement, there is a drawback that it takes a long time to resynchronize the reproduced carrier wave due to this deviation. In addition, this has been a cause of unnecessary click noise and the like occurring for a long time in the case of voice communication. Therefore, in view of the above drawbacks, the technical subject of the present invention is to provide a carrier wave reproduction method for accelerating the recovery of the carrier wave out-of-synchronization caused by shadowing or the like.

[Means to solve problems] According to the present invention, a synchronous demodulator that performs synchronous demodulation by inputting a modulated signal in which a fixed word is periodically inserted and an output signal of the synchronous demodulator are input and a mutual correlation value with the fixed word is output. The absolute value of the output signal of the mutual correlation circuit, the phase calculation circuit that obtains the phase error of the reproduced carrier from the real part and the imaginary part of the output signal of the mutual correlation circuit, and the absolute value of the output signal of the mutual correlation circuit exceeds a predetermined threshold value. If so, the aperture signal is output to the synchronous demodulator using a frame synchronization circuit that generates an aperture signal indicating the timing in the cycle in which the fixed word is inserted and an aperture signal obtained from the frame synchronization circuit. A carrier regenerator can be obtained, which comprises means for using the phase of the output of the phase calculation circuit at the time of the operation as the phase of the regenerated carrier used for demodulation. That is, in the present invention, the signal string synchronously demodulated by the output signal of the carrier wave reproduction circuit in the synchronous demodulator that has become self-propelled due to signal interruption is inserted synchronously on the transmitting side. This is a carrier wave regeneration method characterized in that fixed words are detected for mutual correlation, the phase error of the reproduced carrier wave is detected from the detected signal, and this is set in the carrier wave reproduction circuit to accelerate the restart. Further, according to the present invention, a synchronous demodulator that performs synchronous demodulation by inputting a modulated signal in which a fixed word is periodically inserted and an output signal of the synchronous demodulator as an input, and a mutual correlation value with the fixed word. A mutual correlation circuit that outputs the above, a phase calculation circuit that obtains the phase error of the reproduced carrier from the real part and the imaginary part of the output signal of the mutual correlation circuit, and a predetermined threshold value of the absolute value of the output signal of the mutual correlation circuit. If it exceeds, then at the time when the frame synchronization circuit that generates the aperture signal indicating the timing in the period in which the fixed word is inserted and the aperture signal that is periodically output from the frame synchronization circuit are output. A frequency calculation circuit that obtains a carrier frequency error from the difference between two consecutive output values of the phase calculation circuit output, and a reproduction that uses the output signal of the frequency calculation circuit in the synchronous demodulator when the aperture signal is output. A carrier regenerator can be obtained that comprises means to be used as the frequency of the carrier. That is, the present invention is a fixed word that is synchronously inserted on the transmitting side with respect to a signal sequence synchronously demodulated by the output signal of the carrier wave reproduction circuit in the synchronous demodulator that has become self-propelled due to signal interruption. The phase error of the reproduced carrier wave is detected from the detected signal, and by detecting this error for each frame period, the frequency offset is also detected and the resynchronization time of the demodulator is shortened. It is a carrier wave reproduction method characterized by.

[Example] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a first embodiment of the present invention. 1 is a normal synchronous demodulator with a function to set the phase value of 114 at the time when the 113 aperture signal is output as the initial phase of the regenerative carrier, and its main component is 101 synchronous detection. Circuits, 102 clock regeneration circuits, 103 carrier regeneration circuits. 2 is a cross-correlation circuit for fixed words that are periodically inserted in the data, 3 is a phase calculation circuit that calculates the phase difference between the received signal and the regenerated carrier self-propelled phase from the cross-correlation circuit output signal, and 4 is the phase. It is a frame synchronization circuit that establishes frame synchronization for a fixed word period using the correlation output. The input modulated wave signal applied to the terminal 110 is subjected to normal synchronous demodulation processing by the synchronous demodulation unit 1, and the demodulated data series is output to the terminal 111. The cross-correlation circuit 2 detects the fixed word y (t) included in the demodulated data series by the operation of the following equation.<img file="JP2513331B2_D0001.tif" /> Here, r (t) is the received signal, and if only the fixed word pattern part is considered r (t) = y (t) + n (t) (N (t) is noise). On the other hand, 2N indicates a fixed word length and T is a time slot. In addition, r (t) and y (t) each represent two series of signal sequences as complex signals, and * represents complex conjugate. When (1) is transformed<img file="JP2513331B2_D0002.tif" />The first term is the autocorrelation function of y (t), and the second term is the same as the signal when the noise signal passes through the filter y (-t). The simulation result of φ (τ) is shown in Fig. 3. Figure 3 shows the case where a fixed word with a length of 64 bits is transmitted by QPSK modulation with a frame period of 256 bits and the received Eb / No is 0 dB. In this simulation, the carrier regenerative circuit 103 is still tracking correctly, and fixed words are normally received. Fig. 4 shows this correlation signal in terms of phase. In the figure, the magnitude from the center indicates the signal level and the phase rotation amount is<img file="JP2513331B2_D0003.tif" />Is shown. Here, Real indicates the real part and Imag indicates the imaginary part. The phase calculation circuit 3 performs the calculation of Eq. (3) (output signal is 114). From these figures, the problem is the influence of false positive signals generated by noise, patterns, and so on. For that purpose, it is possible to generate an aperture and prevent erroneous detection by utilizing the fact that the frame period is constant. When the absolute value of the cross-correlation circuit output exceeds the threshold value continuously determined for the number of times in this frame cycle, this aperture signal is output at the timing when the threshold value is exceeded in this frame cycle. Figure 5 shows only the relevant time portion using this aperture. From this, it can be seen that the influence of the false positive signal has almost disappeared. On the other hand, Fig. 6 shows the cross-correlator output signal when fading is applied so that the carrier wave reproduction loop cannot be synchronized (frame synchronization circuit 4 is operating). In this case as well, the phase relationship between the received signal and the reproduced carrier wave should be read from the cross-correlation circuit output. Therefore, for the demodulated sequence, the phase difference is obtained by Eq. (3) using the correlator output signal using fixed words, and the phase difference of the carrier regenerative circuit 103 is calculated using the signal 114 and the aperture signal 113 as the timing. Even if the carrier regenerative circuit 103 is in an asynchronous state, its synchronization can be quickly established. Next, according to the second embodiment of the present invention shown in FIG. 2, the frequency deviation is obtained by the following method by providing the frequency calculation circuit 5 between the phase calculation circuit 3 and the carrier wave reproduction circuit 103. .. First, the phase of Eq. (3) obtained at each frame bit is set to θ.₁, θ₂, ... θ_nAnd. If the function of the frequency offset to be obtained is ω (t), the phase relationship Θ (t) is Θ (t) = ω (t) dt ... (4) Will be. Initial phase θ_xAnd let the frame bit length be l<img file="JP2513331B2_D0004.tif" />Is obtained. This and the above series θ₁... θ_nMake a comparison with. That is, for θ2-θ1, it is a comparison of Θ (2lT) -Θ (lT). The optimum ω (t) is the evaluation function<img file="JP2513331B2_D0005.tif" />Is chosen to minimize. The 0th order term is extracted from this obtained ω (t), that is, ω (t) = ω₀+ ω₁t + ω₂t²+ ... Ω₀Take out. If you need fast sync<img file="JP2513331B2_D0006.tif" />Even a straight line approximation of is almost acceptable. Ω obtained in this way₀And, θ_iHigh-speed synchronization of the regenerative circuit can be achieved by setting (when loading at the frame position of i) to the carrier regenerative circuit. Here, FIG. 7 describes a case where a digital VCO (voltage controlled oscillator) is used for the carrier regenerative circuit 103. First, input ω_nIs frequency, output θ_nCorresponds to the phase. Therefore, ω in the figure_e, θ_eWhen is zero<img file="JP2513331B2_D0007.tif" />It is a simple integrator. If you want to correct the phase error for this, θ in the figure_eIf you substitute the phase error in place,<img file="JP2513331B2_D0008.tif" />And the output θ with the corrected phase error_n Is obtained. If you want to correct the frequency error, ω in the figure_eIf you substitute the frequency error in place<img file="JP2513331B2_D0009.tif" />ω_n = Ω_n+ ω_eAnd the output θ with the corrected frequency error_nIs obtained.

[Effect of the invention] As described above, the present invention detects the cross-correlation of fixed words periodically included in the demodulated data string, measures the phase angle from the real part and the imaginary part of the detected signal, and measures the output signal of the carrier wave reproduction circuit. By correcting only the phase angle, it is possible to accelerate the recovery of the carrier out-of-synchronization caused by shadowing or the like. Further, according to the present invention, the same effect can be obtained by measuring the phase difference and the frequency difference from the detection signal obtained by cross-correlating the fixed words and setting them in the carrier wave reproduction circuit.

[Simple explanation of drawings]

FIG. 1 is a block circuit diagram showing a first embodiment of the present invention, FIG. 2 is a block circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a simulation result of φ (τ). Figures and 4 are diagrams showing the waveform of the correlation signal output when the phase of the fixed word is 0 degrees, and in this figure, the level of the correlation signal is the highest in the vicinity of 0 degrees. However, the correlation signal is output in a relatively large circular region smaller than 0 degrees from the center of the phase plane. This indicates that a correlated signal is output in an all-phase state due to an uncorrelated signal such as noise. Fig. 5 is a diagram showing only the corresponding time part using the aperture. In this figure, the noise generated in the circular region from the center of the phase plane in Fig. 4 by generating the aperture, etc. It shows that the correlation signal output due to is completely eliminated. Fig. 6 shows the correlation signal output when the phases of fixed words exist at 0 degrees, 90 degrees, 180 degrees, and 270 degrees, respectively. In this figure, each phase state varies due to fading. Is large, but it shows that the correlation signal levels are distributed around the phase states of 0 degrees, 90 degrees, 180 degrees, and 270 degrees. Figure 7 is a circuit diagram of a digital VCO. 1 ...... Synchronous demodulator, 2 ...... Cross-correlation circuit, 3 ...... Phase calculation circuit, 4 ...... Frame synchronization circuit, 5 ..... .Frequency calculation circuit.