JPH047942A - Automatic frequency control system - Google Patents

Abstract

PURPOSE:To attain automatic frequency control without pattern jitter by using a converging signal at two points at a different time in a modulation period so as to detect a frequency error and using its output so as to compensate the frequency fluctuation. CONSTITUTION:A sampler 2 receives a signal outputted from a complex number multiplier 1 and samples a signal by a clock being four times the modulation clock and outputs sample signals S101, S102, S103 at times T/4, 3T/4, T/2 for each modulation period T. The sample signal S103 is outputted externally as a reception signal whose frequency fluctuation is compensated and filters 3, 4, 5, 6 all have a same characteristic to apply equalization to the output from a sampler 2 so as to eliminate inter-code interference. Delay means 7, 8 give a delay of a time T/2 only to a signal outputted from the filters 3, 4. A signal representing the frequency fluctuation is obtained via multipliers 9, 10 and a subtractor 11. The frequency of the output from the subtractor 11 is controlled via the loop filter 12 and a voltage controlled oscillator VCO and the result is outputted to the complex number multiplier 1 as a compensation signal. Thus, the fluctuation of the carrier frequency without pattern jitter is compensated.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is an automatic frequency system that compensates for uncertain carrier frequency fluctuations occurring on a transmission path on the receiver side in a digital communication system using phase shift keying. Regarding control method.

(Prior Art) In a digital communication system using a high carrier wave frequency of several GHz, large fluctuations occur in the carrier wave frequency due to frequency conversion on a transmission path, the Doppler effect due to the movement of a wireless station, and the like. Particularly in low modulation rate communication systems, the maximum frequency offset may be comparable to the modulation frequency. Generally, this carrier frequency variation is
Compensated by automatic frequency control at the receiver side.

As a conventional automatic frequency control system, there is an automatic frequency control system using a loss product type frequency discriminator as shown in FIG. In FIG. 2, connections indicate real signals, and thick lines indicate orthogonal signals. The complex multiplier 14 inputs an orthogonal signal obtained by quasi-synchronously demodulating a PSK (phase shift keying) signal, and
A compensation signal supplied from the voltage controlled oscillator (voltage controlled oscillator) 23 compensates for frequency fluctuations of the input orthogonal signal. Sampler 1
5 receives the signal output from the complex multiplier 14, samples the signal using a modulation clock supplied from the outside, and samples S(T/2) at time T/2 of 1/2 modulation period every modulation period T. 2), that is, output samples of signal points. This sample S(T/2> is outputted to the outside as a received signal with frequency fluctuations compensated for. Modulation removal means 1
6 is the sample S(T/2
＞, the input signal sample S(
T/2) modulation is removed. The delay means 17 receives the real part of the signal output from the modulation removal means 16, delays the real part by the time of the modulation period T, and outputs the delayed real part. Delay means 1
8 receives the imaginary part of the signal output from the modulation removal means 16, delays the imaginary part by the time of the modulation period T, and outputs the imaginary part. Multiplier 19 multiplies the output of delay means 17 and the imaginary part of the signal output from modulation removal means 16. Multiplier 2
0 multiplies the output of the delay means 18 and the real part of the signal output from the modulation removal means 16. Subtractor 21 subtracts the output of multiplier 20 from the output of multiplier 19. The output of this subtracter 21 is a signal indicating the frequency fluctuation. Loop filter 22 averages this frequency fluctuation signal. V
C023 has the frequency of the output signal controlled by the signal output from the loop filter 22, and outputs the output signal to the complex multiplier 14 as a compensation signal that compensates for the frequency fluctuation. Here, the frequency fluctuation of the received carrier wave is expressed as Δf, PS
Letting the number of modulation phases of the K signal be M (M is a positive integer), the signal r(t) from which modulation has been removed by the modulation removal means 16 is expressed as r (t) = exp (j 2πMΔft) . Therefore, the output d(nT) of the subtractor 21 is d(nT)=s i n (2rMΔfT>(n=o
, 1.2. ...) becomes. From the above formula, the range of frequency fluctuation that can be drawn in is 1.
Δf1 satisfies Δfl<fs/2M. Here, fs is a modulation frequency expressed as 1/T.

(Problem to be solved by the invention) As explained above, in the conventional automatic frequency control system,
In order to detect frequency errors, it is necessary to remove the modulation of the received signal by a multiplication operation. As a result, as the number of modulation phases increases, the frequency pull-in range becomes narrower. Furthermore, when the SN is low, nonlinear loss becomes a problem.

Therefore, the present invention detects frequency errors at different times in the modulation period using convergence signals at two points, each of which is equalized separately, and uses the output to control the VCO to compensate for carrier frequency fluctuations. The purpose of the present invention is to provide an automatic frequency control method that eliminates pattern jitter at the end of synchronization and at the same time realizes a wide frequency pull-in range.

(Means for Solving the Problems) The automatic frequency control method of the present invention provides a signal obtained by orthogonal frequency conversion of a phase shift keying signal whose carrier frequency fluctuates uncertainly, or a quadrature signal obtained by quasi-synchronously demodulating the phase shift keying signal. Frequency fluctuation compensating means receives a signal as an input signal and compensates for the frequency fluctuation of the input signal using a compensation signal supplied from the outside, and receives a signal output from the frequency fluctuation compensating means and receives the signal from the outside. Samples are performed using a clock synchronized with the supplied modulation clock, and the first and second times within the modulation cycle are sampled for each modulation cycle. a sampler that outputs samples at the time of the /2 modulation period as a first sample, a second sample, and a frequency fluctuation-compensated received signal, respectively; and receiving a real part of the first sample output from the sampler; a first filter that performs equalization on the real part so as to eliminate intersymbol interference; and a first filter that has the same characteristics as the first filter and receives the imaginary part of the first sample output from the sampler. , a second filter that performs equalization on the imaginary part so as to eliminate intersymbol interference; and a second filter that receives the real part of the second sample output from the sampler and equalizes the real part to eliminate intersymbol interference. a third filter that performs equalization to eliminate intersymbol interference; a fourth filter that performs equalization so as to eliminate the noise; and a first filter that receives the signal output from the first filter and applies a time delay to the signal from the first time to the second time. with delay means.

a second delay means that receives a signal output from the second filter and applies a time delay to the signal from the first time to the second time; a first multiplier for multiplying a signal output from the fourth filter by a signal output from the fourth filter; and a first multiplier for multiplying a signal output from the second delay means by a signal output from the third filter. a second multiplier; a subtracter that generates a difference between the output of the first multiplier and the output of the second multiplier and outputs the difference as a signal of the frequency fluctuation; and from the subtracter. a loop filter that receives the output frequency fluctuation signal and averages the frequency fluctuation signal; and a compensation circuit that controls the frequency of the output signal by the signal output from the loop filter and compensates for the frequency fluctuation of the output signal. and a voltage controlled oscillator that outputs a signal to the frequency fluctuation compensation means.

(Example) Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the automatic frequency control system of the present invention. In this embodiment, if a clock that is four times the modulation clock is used as the tarok that is supplied to the sampler 2, the circuit configuration becomes easier and it is most practical. Clock. In order to compensate for frequency fluctuations of a PSK (phase shift keying) signal whose carrier frequency fluctuates uncertainly, the complex multiplier 1 inputs an orthogonal signal obtained by quasi-synchronously demodulating the PSK signal and converts it into a VCO (voltage controlled oscillator).
The sampler 2 receives the signal output from the complex multiplier 1, samples the signal at a clock four times the modulation clock, and performs compensation for the frequency fluctuation using the compensation signal supplied from the complex multiplier 1. At time T/4 of 1/4 modulation period, at time 83 T/4 of 3/4 modulation period, 1. Sample S(T/4)10 at time T/2 of /2 modulation period
1, S (3T/4) 102, and S (T/2) 103 are output. Sample S (T/2>103 is a signal point sample, and is outputted to the outside as a received signal with frequency fluctuations compensated for. Filter 3 uses time T/2 of 1/4 modulation period output from sunglasser 2. 4 sample S(T/
4) Receive the real part Re[S (T/4) ] of 101,
Equalization is performed on the real part to eliminate intersymbol interference.

Filter 4 has the same characteristics as filter 3, and sampler 2
Imaginary part I m [S (T/4) of sample S (T/4) 101 at time T/4 of 1/4 modulation period output from
], and the imaginary part is equalized to eliminate intersymbol interference. The filter 5 receives the sample S<3T/ at time 3T/4 of the 3/4 modulation period output from the sampler 2.
4) Receive the real part Re of 102 [3(3T,”4)1,
Equalization is performed on the real part to eliminate intersymbol interference.

Filter 6 has the same characteristics as filter 5, and sampler 2
The imaginary part 1 m [S (3T
/4) ], and equalizes the imaginary part so as to eliminate intersymbol interference. The delay means 7 receives the signal output from the filter 3, and applies a time T/2 of the 1/2 modulation period to the signal.
Gives a delay of 2. The delay means 8 receives the signal output from the filter 4, and applies a time T/2 of 1/2 modulation period to the signal.
Gives a delay of 2. Multiplier 9 multiplies the output of delay means 7 and the output of filter 6. The multiplier 10 includes a 6-subtractor 1 that multiplies the output of the delay means 8 and the output of the filter 5.
1 subtracts the output of multiplier 10 from the output of multiplier 9. The output of this subtracter 11 is a signal indicating the frequency fluctuation. The loop filter 12 receives the frequency fluctuation signal output from the subtracter 11 and averages the frequency fluctuation signal. The frequency of the output signal of the VCO 13 is controlled by the signal output from the loop filter 12, and outputs the output signal to the complex multiplier 1 as the compensation signal for compensating for frequency fluctuations.

FIG. 3 is a diagram showing a demodulated signal obtained by quasi-synchronously demodulating a PSK signal with no carrier frequency fluctuation and passing it through a generally used optimal reception filter. In this figure, the modulation period of the same modulation code is represented by T. FIG. 4 is a diagram showing a demodulated signal after the quasi-synchronous demodulated signal is equalized by filter 3 or 4 in the embodiment of FIG. In this figure, the signal is equalized at time T/4 of the 1/4 modulation period. FIG. 5 is a diagram showing a demodulated signal after the quasi-synchronous demodulated signal is equalized by filter 5 or 6 in the embodiment of FIG. In this diagram, 3T
The signal is equalized at time 3T/4 of the /4 modulated signal6
Since no change in code occurs due to modulation at time T, the modulation codes at time T/4 and time 3T/4 are the same. Therefore, the phase change that occurs during time T/2 from time T/4 to time 3T/4 is not affected by modulation and is caused only by frequency fluctuations. Therefore, by inputting the convergence signals equalized at time T/4 and time 3T/4 to the cross-product type frequency discriminator, the difference in frequency numbers can be used horizontally. A feature of the present invention is that the VC013 is controlled based on the frequency error detected by such a method to compensate for carrier frequency fluctuations. Figures 3, 4 and 5 used in the above explanation
In the diagram, whether a 2-phase or 4-phase variable barrel signal is assumed, or M
It goes without saying that similar effects can be obtained with phase modulation signals.

Although FIG. 1 shows the configuration of an automatic frequency control system that performs processing entirely digitally, it is also possible to partially incorporate analog processing. For example, the complex multiplier 1 is replaced with an analog multiplier, an IP band analog VCO is used as the VC013, and an A/D converter is installed between the analog multiplier and the sampler 2 and between the loop filter 12 and the VC013, respectively. Of course, a configuration in which a D/A converter is provided is also conceivable. However, it is essentially the same as Figure 1.

(Effects of the Invention) As described above, in the present invention, by observing the phase changes of convergent signals at two points that are separately equalized at different times within the modulation period, the dependence on the sign change due to modulation is eliminated. frequency error can be detected.

Therefore, at the end of synchronization when the average frequency error becomes almost zero, pattern jitter due to modulation disappears. Furthermore, since no modulation removal means is used, a wide frequency pull-in range can be achieved, and nonlinear losses due to multiplication operations can be avoided.

3.4.5.6...F ...delay means, 9,1 1.21...Subtractor, 13 23・VCO

Claims (1)

[Claims] A signal obtained by orthogonal frequency conversion of a phase shift keying signal whose carrier frequency fluctuates uncertainly or a quadrature signal obtained by quasi-synchronous demodulation of the phase shift keying signal is input as an input signal, and frequency fluctuation compensating means for compensating for frequency fluctuations of the input signal using a compensation signal; receiving a signal output from the frequency fluctuation compensating means; , outputs samples at a first time, a second time, and a time of 1/2 modulation period within the modulation period as the first sample, the second sample, and a frequency fluctuation-compensated received signal, respectively, for each modulation period. a sampler; a first filter that receives a real part of a first sample output from the sampler and performs equalization on the real part so as to eliminate intersymbol interference; and a first filter having the same characteristics as the first filter. a second filter that receives the imaginary part of the first sample output from the sampler and performs equalization on the imaginary part so as to eliminate intersymbol interference; a third filter that receives the real part of the sample of No. 2 and equalizes the real part so as to eliminate intersymbol interference; and a third filter that has the same characteristics as the third filter and is output from the sampler. a fourth filter that receives the imaginary part of the second sample and performs equalization on the imaginary part so as to eliminate intersymbol interference; and a fourth filter that receives the signal output from the first filter and applies the a first delay means for providing a time delay from the first time to the second time; a second delay means that provides a time delay until the time; a first multiplier that multiplies the signal output from the first delay means and the signal output from the fourth filter; a second multiplier that multiplies the signal output from the second delay means by the signal output from the third filter; and the output of the first multiplier and the second multiplier. a subtracter that generates a difference and outputs the difference as the frequency fluctuation signal; a loop filter that receives the frequency fluctuation signal output from the subtracter and averages the frequency fluctuation signal; and an output from the loop filter. an automatic frequency control system, comprising: a voltage controlled oscillator whose frequency of an output signal is controlled by a signal generated by the frequency fluctuation, and outputs the output signal to the frequency fluctuation compensating means as the compensation signal for compensating for the frequency fluctuation.