JP2005191743A - Demodulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a demodulator preventing occurrence of out of carrier synchronism when the quality of a received signal is low and suppressing deterioration in the BER when the quality of the received signal is not low during reception. <P>SOLUTION: A phase difference detection circuit 80 detects the phase error of an output signal from an endless phase shifter 60 to output phase error information. A quality monitor circuit 100 generates quality information on the basis of the output signal from the endless phase shifter 60 and outputs he quality information to a loop filter 90. The loop filter 90 uses the value of a parameter corresponding to the received quality information, carries out arithmetic processing on the basis of the phase error information and outputs an APC value. A numerical controlled oscillator 70 outputs the correction value of the phase error on the basis of the APC value. The endless phase shifter 60 carries out rotation symmetric conversion on the basis of the correction value of the phase error to correct the phase of a demodulation signal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a demodulation device used in a reception device of a wireless communication system.

  In a wireless communication system, a local oscillator (LO) is used in a transmission circuit and a reception circuit to modulate a baseband signal into an intermediate frequency (IF) signal and to demodulate an intermediate frequency signal into a baseband signal. Used. Further, a local oscillator is used in the transmission circuit and the reception circuit in order to frequency convert an intermediate frequency signal to a radio frequency (RF) signal and to convert a radio frequency signal to an intermediate frequency signal.

  In a transmission circuit or a reception circuit, a synthesizer type local oscillator may be used for the purpose of making the radio frequency variable at low cost. A synthesizer type local oscillator includes a lot of phase noise in its output. Therefore, when a synthesizer type local oscillator is used, a modulation signal (modulated wave) containing a lot of phase noise is transmitted from the transmission device. Therefore, a means for suppressing the influence of the phase noise is necessary in the demodulation circuit of the receiving apparatus in order to reduce the deterioration of the communication quality due to the phase noise.

  In the receiving apparatus, synchronous detection is used in which a carrier wave is reproduced from the received signal and the received signal is demodulated using the reproduced carrier wave as a reference signal. As a method for realizing synchronous detection, there is a case where quasi-synchronous detection using an approximate reference signal is used in which the frequency and phase do not completely match the carrier wave.

  When using synchronous detection, the receiver uses, for example, a voltage controlled oscillator (VCO) whose frequency of the oscillation signal is variable to generate a reference signal that is phase-synchronized with the carrier wave from the received signal. Is demodulated. In addition, when quasi-synchronous detection is used, the receiver uses a local oscillator (LO) that outputs an oscillation signal having a predetermined fixed frequency, and the carrier wave in the received signal does not completely match the frequency and phase. Is generated, and the received signal is demodulated using the reference signal.

  In synchronous detection, a carrier recovery circuit (carrier recovery loop) is used to recover a carrier from a received signal. Also in quasi-synchronous detection, a carrier recovery circuit that recovers a carrier is used to compensate the phase of the demodulated signal.

  In a receiver using synchronous detection or quasi-synchronous detection, in order to suppress the influence of phase noise included in the received signal, the bandwidth (hereinafter referred to as loop bandwidth) of the carrier recovery circuit is widened. It is necessary to speed up the response of the carrier reproduction loop of the reproduction circuit. However, when the loop bandwidth is widened, the jitter of the regenerated carrier wave (carrier) increases due to noise generated inside the carrier reproduction loop. Therefore, when the loop bandwidth is widened, the carrier synchronization holding limit, which is the limit level at which the carrier wave can be recovered from the received signal, is deteriorated as compared with the case where the loop bandwidth is narrow.

FIG. 5 shows a loop bandwidth and a degree of deterioration of a CN ratio (Carrier to Noise ratio (hereinafter referred to as C / N)) with respect to a BER (Bit Error Rate) characteristic (hereinafter referred to as C / N). And C / N deterioration amount) are qualitative illustrations. FIG. 5 shows values of C / N for obtaining a predetermined BER (for example, 10 ⁻³ ) and ideal values where there is no influence of phase noise and carrier jitter, and various loops. The difference from the C / N value in the bandwidth is shown by a solid line. For example, in order to obtain a BER of 10 ⁻³ , the theoretical C / N may be 20 dB. However, in an actual apparatus, a BER of 10 ⁻³ was obtained when the C / N was 20.3 dB. Then, the C / N deterioration amount with respect to the BER characteristic is 0.3 dB.

Further, in FIG. 5, the influence of phase noise and the influence of carrier jitter on the C / N deterioration amount are indicated by broken lines. As shown in FIG. 5, the effect of phase noise becomes smaller as the loop bandwidth is increased. The influence of carrier jitter increases as the loop bandwidth is increased. A loop bandwidth (A in FIG. 5) having the smallest C / N degradation amount with respect to the BER characteristic, in other words, a loop bandwidth having the smallest BER with respect to a certain C / N exists. That is, for a certain C / N (for example, 20 dB), there is a loop bandwidth that minimizes the amount of deterioration from a predetermined BER (for example, 10 ⁻³ ).

  FIG. 6 is an explanatory diagram qualitatively showing the relationship between the loop bandwidth and the carrier synchronization retention limit. For example, suppose that 15 dB is required as C / N to recover a carrier wave from a received signal in a certain loop bandwidth. If the C / N for reproducing the carrier wave from the received signal is 15.5 dB when the loop bandwidth is widened, the C / N deterioration amount with respect to the carrier synchronization holding limit is 0.5 dB.

  In FIG. 6, the influence of phase noise and the influence of carrier jitter on the carrier synchronization retention limit are indicated by broken lines. As shown in FIG. 6, the deterioration amount due to carrier jitter is more dominant than the deterioration amount due to phase noise with respect to the C / N of the carrier synchronization holding limit. For this reason, the carrier synchronization retention limit C / N is smaller as the loop bandwidth is narrower. Therefore, in order to maintain carrier synchronization up to a place where the C / N of the received signal is lower, the loop bandwidth may be narrowed.

  The loop bandwidth of the carrier recovery loop is mainly determined by the bandwidth of the loop filter included in the carrier recovery circuit. The loop filter functions as a high-pass filter against phase fluctuations caused by external disturbances such as phase noise and microphonics. Therefore, as shown in FIG. 5, in order to reduce the influence of phase noise, it is better to widen the bandwidth of the loop filter so as to widen the loop bandwidth. On the other hand, the loop filter functions as a low-pass filter for carrier jitter caused by noise generated inside the carrier reproduction loop. Therefore, as shown in FIG. 5, in order to reduce the influence of carrier jitter, it is better to narrow the bandwidth of the loop filter so that the loop bandwidth is narrowed.

  As shown in FIG. 5, the loop bandwidth, that is, the BER degradation, that minimizes the sum of the influence of the phase noise on the C / N deterioration and the influence of the carrier jitter on the C / N deterioration with respect to the BER. There is a loop bandwidth that minimizes. In general, in the demodulation circuit, the loop bandwidth is determined so as to minimize the BER degradation.

  However, when a synthesizer type local oscillator is used, the influence of phase noise is large, and therefore, in the loop bandwidth that minimizes the BER degradation, as shown in FIG. Becomes larger. That is, with the loop bandwidth that minimizes BER degradation, carrier synchronization is easily lost when the quality of the received signal is degraded. In particular, when error correction is performed on demodulated data using forward error correction (FEC), carrier synchronization is performed even though the BER is apparently not deteriorated due to the effect of FEC. The phenomenon of coming off occurs.

  Patent Document 1 describes a quasi-synchronous detection demodulator that is resistant to disturbances and can obtain constant synchronization characteristics regardless of the frequency difference between the carrier frequency of the input signal and the internal local oscillation frequency. . The carrier wave recovery circuit of the demodulator described in Patent Document 1 includes a replacement circuit that executes a process of bringing the value of the low-pass filter closer to the center value after the synchronization of the carrier wave is established. In the demodulator described in Patent Document 1, the tolerance to phase noise is enhanced by controlling the low-pass filter to operate near the center of the operable range.

  Patent Document 2 describes a communication system that can prevent the occurrence of clock synchronization loss when switching to a protection line when the line quality of a working line deteriorates. In the communication system described in Patent Document 2, the protection line demodulator includes a variable bandwidth loop filter. Further, the value of the tap coefficient is input from the demodulator of each working line to the demodulator of the protection line. Then, based on the tap coefficient, the bandwidth of the variable bandwidth loop filter is set in accordance with the working line having the most deteriorated line quality.

JP 2000-41074 A (page 4-6, FIG. 1-9) JP-A-11-68622 (page 8-10, Fig. 1-3)

  According to the demodulator described in Patent Document 1, the resistance to phase noise can be increased by performing processing for bringing the value of the low-pass filter closer to the center value after synchronization of the carrier wave is established. However, after the synchronization is established, the low-pass filter is merely controlled so as to operate near the center of the operable range, and it is not possible to prevent the carrier synchronization from being lost depending on the reception state.

  Further, according to the communication system described in Patent Document 2, since the bandwidth is set in accordance with the working line having the most deteriorated line quality, when switching from the working line to the protection line is performed, It is possible to prevent loss of clock synchronization in the standby line demodulator. However, Patent Document 2 does not disclose a configuration for controlling so as to suppress BER degradation during reception.

  Therefore, the present invention can prevent the carrier synchronization from being lost when the received signal quality is low during reception, and can suppress the BER degradation when the received signal quality is not low. An object is to provide an apparatus.

  A demodulator according to the present invention includes a loop filter having a variable bandwidth, and includes a carrier recovery loop that recovers a carrier signal that is phase-synchronized with a carrier wave in the input signal. Is higher, the loop filter is caused to change the loop bandwidth so that the BER degradation is minimized, and the quality of the input signal may be the second predetermined quality (may be the same as the first predetermined quality). In the following cases, the loop filter is provided with a quality determination circuit that makes the loop bandwidth narrower than the predetermined bandwidth.

  The demodulator includes a demodulator that reproduces a demodulated signal from a received modulated wave as an input signal using a local oscillation frequency signal, and a phase compensator that performs phase compensation of the demodulated signal, and the loop filter includes a phase compensator. The structure provided in may be sufficient. That is, the present invention can be applied to a demodulator that performs quasi-synchronous detection.

  The phase compensator performs complex multiplication of a numerically controlled oscillator that outputs a numerical value obtained by converting the data output from the loop filter into a phase, and a digital demodulated signal that is a digitized demodulated signal and the output of the numerically controlled oscillator. A phase shifter that outputs a multiplication result; and a phase difference detection circuit that detects a phase shift of the output of the phase shifter and generates phase difference data. The loop filter generates phase difference data generated by the phase difference detection circuit. And may be configured to output data in which noise components are suppressed. That is, in a demodulator that performs quasi-synchronous detection, the present invention can be applied to a carrier recovery loop in a phase compensation unit.

  In the demodulator, for example, the loop filter includes a multiplier that multiplies the phase difference data by a coefficient, and the quality determination circuit loops data indicating the coefficient used by the multiplier that is data corresponding to the quality of the input signal. It is configured to output to a filter.

  The demodulator includes a demodulator that reproduces a demodulated signal from the input signal using the recovered carrier signal, and the output of the loop filter is input to a frequency variable oscillator that outputs the carrier signal to the demodulator. May be. That is, the present invention can be applied to a demodulator that performs synchronous detection.

  A phase difference detection circuit that generates a phase difference signal by detecting the phase shift of the demodulated signal (with respect to the regular signal point arrangement) is input. The loop filter receives the phase difference signal generated by the phase difference detection circuit, A voltage according to the phase difference may be output to a voltage controlled oscillator as a frequency variable oscillator. That is, the present invention can be applied to a carrier recovery loop that recovers a carrier signal for detection in a demodulator that performs synchronous detection.

  In the demodulator, for example, the loop filter is configured by a lag lead filter having a variable capacitance unit and a variable resistance unit, and the quality determination circuit determines the capacitance of the variable capacitance unit and the resistance of the variable resistance unit according to the quality of the input signal. It is configured to output a specified signal.

  For example, the quality monitoring circuit estimates the carrier signal power to noise power ratio of the input signal from the demodulated signal, and determines the quality of the input signal based on the estimated carrier signal power to noise power ratio.

  According to the present invention, the bandwidth of the carrier recovery loop can be adaptively varied according to the reception state. Therefore, it is possible to achieve both prevention of loss of carrier synchronization and suppression of BER degradation during reception.

Embodiment 1 FIG.
A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of a demodulator according to the present invention. In this embodiment, a received signal is demodulated using a local oscillator (LO) that generates a signal having a fixed frequency approximating the frequency and phase of the carrier wave of the received signal, and a slight remaining phase difference (or frequency difference) is obtained. A demodulator using quasi-synchronous detection that is corrected using a carrier recovery circuit is taken as an example.

  As shown in FIG. 1, the demodulator includes a quadrature detector (hereinafter referred to as a demodulator (DEM)) 10 that demodulates a received signal, and the carrier wave of the received signal does not completely match the frequency and phase. A local oscillator (LO) 20 that outputs an oscillation signal having a fixed frequency approximated to the above, an A / D converter 30 that converts an analog signal of the I (in-phase) channel output from the demodulator 10 into a digital signal, and an output from the demodulator 10 An analog-to-digital converter 40 that converts the Q (orthogonal) channel analog signal to a digital signal, a circuit corresponding to a phase compensation unit that performs phase compensation of the demodulated signal, and a carrier recovery circuit 50 that performs carrier recovery processing; And a quality monitoring circuit (quality judgment circuit) 100 for monitoring the quality of the demodulated signal. The demodulator 10 demodulates the received signal as an input signal using the oscillation signal (reference signal) output from the local oscillator 20, and reproduces the analog signals of I channel and Q channel.

  As shown in FIG. 1, a carrier recovery circuit 50 includes an infinite phase shifter (EPS) 60, a numerically controlled oscillator (NCO) 70, and a phase difference detection circuit (PD). 80 and loop filter 90.

  The infinite phase shifter 60 is a complex that performs rotationally symmetric conversion by multiplying the I-channel and Q-channel signals from the AD converters 30 and 40 by the phase error correction value output from the numerically controlled oscillator 70. It is a multiplier. The infinite phase shifter 60 outputs I-channel and Q-channel signals subjected to rotational symmetry conversion. The infinite phase shifter 60 performs control so that the phase error of each of the I channel signal and the Q channel signal approaches 0 by performing rotational symmetry conversion.

  The numerically controlled oscillator 70 converts an input value from the loop filter 90 (hereinafter also referred to as an APC (Automatic Phase Control) value) into a phase θ. The numerically controlled oscillator 70 obtains values of sin θ and cos θ with respect to the converted phase θ. The numerically controlled oscillator 70 outputs the calculated values of sin θ and cos θ to the infinite phase shifter 60 as phase error correction values.

  The phase difference detection circuit 80 detects how much the signal point arrangement determined by the I channel signal and the Q channel signal from the infinite phase shifter 60 deviates from the normal signal point arrangement. That is, the phase difference detection circuit 80 detects a phase error between the output signal of the infinite phase shifter 60 and the normal signal. In addition, the phase difference detection circuit 80 outputs phase difference data indicating the detected phase error to the loop filter 90.

  The loop filter 90 performs arithmetic processing based on the phase difference data from the phase difference detection circuit 80. Further, the loop filter 90 outputs the value obtained by the arithmetic processing to the numerical control oscillator 70. Further, the loop filter 90 changes the parameters used in the calculation based on the quality information from the quality monitoring circuit 100.

  FIG. 2 is a block diagram illustrating an example of the configuration of the loop filter 90. As shown in FIG. 2, the loop filter 90 is a perfect integration circuit type filter generally used in quasi-synchronous detection. The loop filter 90 includes a multiplier 90a that multiplies the phase difference data from the phase difference detection circuit 80 by a coefficient α, a multiplier 90b that multiplies the phase difference data from the phase difference detection circuit 80 by a coefficient β, and according to the quality information. ROM 90c for outputting coefficients α and β, adder 90d for adding the output of multiplier 90a and the output of adder 90f, delay circuit 90e for delaying the output of adder 90d, the output of delay circuit 90e and the output of multiplier 90b An adder 90f for adding the output is included.

  The ROM 90c stores a coefficient α and a coefficient β corresponding to each of a plurality of types of quality information. By changing the coefficient α and the coefficient β in the loop filter 90, the bandwidth (loop bandwidth) of the carrier recovery circuit 50 is changed.

  In the present embodiment, after demodulator 10 demodulates the received signal using the fixed frequency reference signal, carrier recovery circuit 50 corrects the phase error of the demodulated signal. In the present embodiment, the carrier recovery circuit 50 does not directly recover the carrier wave for demodulating the received signal, but when correcting the phase error of the demodulated signal, a process corresponding to the carrier recovery process is performed. ing.

  The quality monitoring circuit 100 determines the quality of the received signal based on the I channel and Q channel signals from the infinite phase shifter 60, and generates quality information indicating the quality of the received signal. Further, the quality monitoring circuit 100 outputs the generated quality information to the loop filter 90.

  The quality monitoring circuit 100 determines whether there is a possibility that the carrier synchronization is lost. Specifically, the quality monitoring circuit 100 estimates the C / N of the received signal by obtaining the variance of signal points based on the I channel and Q channel signals from the infinite phase shifter 60. The quality monitoring circuit 100 determines whether the carrier synchronization may be lost by determining whether the estimated C / N is equal to or less than a predetermined C / N value (corresponding to the second predetermined quality). Judging. When the quality monitoring circuit 100 determines that the carrier synchronization may be lost, the quality monitoring circuit 100 causes the loop filter 90 to change the coefficients α and β to values that make the loop bandwidth narrower than the predetermined bandwidth. . Note that the predetermined C / N value corresponding to the second predetermined quality is a value determined according to the device configuration and the modulation method used, and is, for example, the C / N value of the synchronization holding limit. This is a value with a margin (see FIG. 6).

  Further, when the quality monitoring circuit 100 determines that there is no risk of loss of carrier synchronization, the quality monitoring circuit 100 selects the coefficient α and coefficient β corresponding to the loop bandwidth (see FIG. 5) that minimizes the BER degradation. To the loop filter 90. Note that the C / N threshold value (corresponding to the first predetermined quality) for determining that there is no possibility of loss of carrier synchronization is larger than the predetermined C / N value corresponding to the second predetermined quality. It may be a value.

  Next, the operation will be described. When the input of the reception signal to the demodulator starts, the demodulator 10 demodulates the reception signal using the oscillation signal from the local oscillator 20 and performs A / D conversion on the I-channel analog signal and the Q-channel analog signal. Output to the units 30 and 40. The AD converter 30 converts the I-channel signal into a digital signal and outputs the digital signal to the infinite phase shifter 60. The A-D converter 40 converts the Q channel signal into a digital signal and outputs the digital signal to the infinite phase shifter 60.

  When the I-channel and Q-channel digital signals are input, the infinite phase shifter 60 determines the equations (1) and (1) for the I-channel and Q-channel signals based on the phase error correction value from the numerically controlled oscillator 70. The rotational symmetry transformation shown in Equation (2) is performed and output.

I _out = I _in × cos θ−Q _in × sin θ Formula (1)
Q _out = I _in × sin θ + Q _in × cos θ Equation (2)

In Expressions (1) and (2), I _out indicates an I channel output signal of the infinite phase shifter 60, and Q _out indicates an Q channel output signal of the infinite phase shifter 60. I _in indicates an I channel input signal from the A-D converter 30, and Q _in indicates a Q channel input signal from the A-D converter 40. Further, sin θ and cos θ indicate phase error correction values from the numerically controlled oscillator 70. The signal output from the infinite phase shifter 60 is a signal from which the phase rotation corresponding to the difference between the carrier frequency and the oscillation frequency is removed.

  The quality monitoring circuit 100 estimates the C / N of the received signal based on the output signal of the infinite phase shifter 60. When the quality monitoring circuit 100 determines that the estimated C / N is equal to or less than the value of C / N corresponding to the second predetermined quality, the quality monitoring circuit 100 sets the loop bandwidth of the carrier regeneration circuit 50 to be greater than the predetermined bandwidth. Decide to narrow. Then, data specifying the coefficient α and the coefficient β that realize such a loop bandwidth is output to the loop filter 90. Further, when the quality monitoring circuit 100 determines that the estimated C / N is larger than the C / N corresponding to the first predetermined quality, the loop bandwidth of the carrier reproduction circuit 50 is minimized. It is determined that the bandwidth becomes. Then, the quality monitoring circuit 100 outputs to the loop filter 90 data specifying the coefficient α and the coefficient β that realize the loop bandwidth that minimizes the BER degradation.

  In the loop filter 90, the ROM 90c outputs the value of the coefficient α designated by the data (quality information) input from the quality monitoring circuit 100 to the multiplier 90a. The ROM 90c outputs the value of the coefficient β designated by the input quality information to the multiplier 90b. The loop filter 90 performs arithmetic processing using the phase difference data, the coefficient α and the coefficient β output from the ROM 90 c, and outputs an APC value to the numerical control oscillator 70. The loop filter 90 performs arithmetic processing to suppress noise components included in the phase difference data from the phase difference detection circuit 80.

  When the APC value is input, the numerically controlled oscillator 70 converts the APC value into the phase θ. Also, the numerically controlled oscillator 70 obtains sin θ and cos θ using the converted phase θ. The numerically controlled oscillator 70 outputs the calculated values of sin θ and cos θ to the infinite phase shifter 60 as phase error correction values.

  Based on the phase error correction value from the numerically controlled oscillator 70, the infinite phase shifter 60 uses the equations (1) and (2) for the I channel and Q channel signals from the AD converters 30 and 40. It repeats the rotational symmetry transformation shown in. While the received signal is input to the demodulation circuit, the infinite phase shifter 60 performs control so that the phase difference between the I-channel output signal and the Q-channel output signal approaches 0 by repeatedly performing line symmetry conversion. To do.

  As described above, according to the present embodiment, in the demodulator, the quality monitoring circuit 100 determines the quality of the demodulated signal, determines whether there is a possibility that the carrier synchronization may be lost based on the determination result, and the carrier If it is determined that there is no risk of loss of synchronization, the loop bandwidth is optimized so that BER degradation is minimized. Further, when the demodulator determines that there is a possibility that the carrier synchronization is lost, the demodulator makes the loop bandwidth narrower than the predetermined bandwidth.

  In other words, by adaptively varying the loop bandwidth according to the reception state, it is possible to prevent the carrier synchronization from being lost when the received signal quality is low during reception, and the received signal quality is not low. In this case, BER deterioration can be suppressed. That is, it is possible to achieve both the characteristic of minimizing BER degradation and the conflicting characteristic of making it difficult for carrier synchronization to be lost. In addition, since minimization of BER degradation and prevention of loss of carrier synchronization can be achieved at the same time, it becomes easy to employ a synthesizer type local oscillator that is easily affected by phase noise. Therefore, the cost of the demodulator can be reduced, and the receiver can easily have a variable function of radio frequency (RF).

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a block diagram illustrating another configuration example of the demodulation device. In the present embodiment, a demodulator using synchronous detection instead of quasi-synchronous detection is taken as an example of a demodulation method. In this embodiment, the demodulator reproduces a carrier wave from a received signal in a carrier wave reproduction loop including a voltage controlled oscillator (VCO), and demodulates the received signal using the reproduced carrier wave as a reference signal.

  As shown in FIG. 3, the demodulator includes a demodulator (DEM) 110 that detects a modulated wave as an input signal, a voltage-controlled oscillator (VCO) 120, AD converters 130 and 140, a phase difference detection circuit ( PD) 150, loop filter 160, and quality monitoring circuit 170.

  The voltage controlled oscillator 120 outputs an oscillation signal having an oscillation frequency corresponding to the control voltage from the loop filter 160. That is, the loop filter 160 can change the oscillation frequency of the oscillation signal from the voltage controlled oscillator 120 by changing the voltage value of the control voltage. In the phase difference detection circuit 150, the signal point arrangement determined by the I-channel signal and the Q-channel signal from the AD converters 130 and 140 for digitizing the output of the demodulator 110 is changed from the normal signal point arrangement. The phase error is detected by detecting whether or not the phase error has occurred, and a phase difference signal indicating the detected phase error is output to the loop filter 160. The loop filter 160 outputs a control voltage having a voltage value corresponding to the value indicated by the phase difference signal from the phase difference detection circuit 150.

  In this embodiment, the loop including demodulator 110, A / D converters 130 and 140, phase difference detection circuit 150, loop filter 160, and voltage controlled oscillator 120 shown in FIG. 3 reproduces the carrier from the received signal. Form a playback loop.

  FIG. 4 is a block diagram illustrating an example of the configuration of the loop filter 160. As shown in FIG. 4, the loop filter 160 is a lag reed filter. Loop filter 160 includes variable resistors 160a and 160b and a variable capacitor 160c. Further, the quality information from the quality monitoring circuit 170 is input to the loop filter 160 so as to change the resistance values of the variable resistors 160a and 106b and the capacitance of the variable capacitor 160c. In the present embodiment, the loop bandwidth of the carrier recovery circuit is changed by changing the resistance values of the variable resistors 160a and 160b and the capacitance of the variable capacitor 160c in the loop filter 160.

  The quality monitoring circuit 170 outputs a control signal designating the resistance values of the variable resistors 160a and 160b of the loop filter 160 and the capacitance of the variable capacitor 160c. The quality monitoring circuit 170 determines the quality of the received signal based on the I channel and Q channel signals from the infinite phase shifter 60, and determines whether there is a possibility that the carrier synchronization may be lost based on the determination result. . The quality monitoring circuit 170 outputs a control signal for changing the resistance values of the variable resistors 160a and 160b of the loop filter 160 and the capacitance of the variable capacitor 160c based on the determination result of whether or not carrier synchronization is lost.

  Next, the operation will be described. When the input of the reception signal to the demodulator starts, the demodulator 110 demodulates the reception signal using the oscillation signal from the voltage controlled oscillator 120, and converts the analog signal of the I channel and the analog signal of the Q channel to AD. Output to the converters 130 and 140. The AD converter 130 converts the I channel signal into a digital signal and outputs the digital signal. The A-D converter 140 converts the Q channel signal into a digital signal and outputs the digital signal.

  The phase difference detection circuit 150 detects an error between the signal point arrangement of the output signals of the A / D converters 130 and 140 and the normal signal point arrangement and outputs a phase difference signal. When the quality monitoring circuit 170 estimates the C / N of the received signal and determines that the estimated C / N is equal to or lower than the value of C / N corresponding to the second predetermined quality, the quality monitoring circuit 170 It is decided to make the loop bandwidth narrower than the predetermined bandwidth. Then, control signals indicating the resistance values of the variable resistors 160a and 160b and the capacitance of the variable capacitor 160c that realize such a loop bandwidth are output to the loop filter 160 as quality information.

  When the quality monitoring circuit 100 determines that the estimated C / N is larger than the C / N corresponding to the first predetermined quality, the quality monitoring circuit 100 reduces the loop bandwidth of the carrier reproduction circuit loop to the minimum BER degradation. It is determined that the bandwidth becomes. The quality monitoring circuit 100 outputs, as quality information, a control signal indicating the resistance values of the variable resistors 160a and 160b and the capacitance of the variable capacitor 160c that realize a loop bandwidth that minimizes the BER degradation. The meanings of the first predetermined quality and the second predetermined quality are the same as in the case of the first embodiment.

  In the loop filter 160, the resistance values of the variable resistors 160a and 160b and the capacitance of the variable capacitor 160c change according to the control signal from the quality monitoring circuit 170.

  As described above, even when synchronous detection is used as the demodulation method, the demodulator optimizes the loop bandwidth so as to minimize the BER degradation based on the quality information when there is no possibility of loss of carrier synchronization. To do. Further, when there is a possibility that the carrier synchronization is lost, the demodulator makes the loop bandwidth narrower than a predetermined bandwidth. Therefore, even when synchronous detection is used, it is possible to prevent carrier synchronization from being lost when the quality of the received signal is low during reception, and to suppress BER degradation when the quality of the received signal is not low. Can do.

  Note that the demodulating devices shown in the first and second embodiments are not limited to differences in modulation schemes, analog signals, or digital signals, as long as they perform quasi-synchronous detection or synchronous detection. Is possible.

  In the first embodiment and the second embodiment, the case where the parameter of the loop filter and the value of the circuit element are changed has been described. However, the method for changing the loop bandwidth is the first embodiment. The method is not limited to the method described in the second embodiment. For example, the loop bandwidth may be changed by changing the modulation sensitivity of the voltage controlled oscillator (VCO) in the case of synchronous detection, or by changing the gain of the amplifier when using an amplifier (amplifier) in the demodulator. .

  In the first embodiment and the second embodiment, the quality monitoring circuit calculates the estimated value of the C / N of the received signal by obtaining the variance of the signal points based on the demodulated signal. As described above, the method for obtaining the quality information is not limited to the methods shown in the first embodiment and the second embodiment. For example, the quality of the received signal may be determined on the basis of the error occurrence frequency by monitoring the error occurrence frequency when forward error correction (FEC) is performed.

  The demodulator according to the present invention can be applied to a receiver in a wireless communication system. By configuring the receiving apparatus using the demodulating apparatus according to the present invention, it is possible to prevent the carrier synchronization from being lost when the quality of the received signal is low during reception, and when the quality of the received signal is not low. Can suppress BER degradation.

It is a block diagram which shows an example of a structure of the demodulation apparatus by this invention. 3 is a block diagram illustrating an example of a configuration of a loop filter 90. FIG. It is a block diagram which shows the other structural example of a demodulation apparatus. 3 is a block diagram illustrating an example of a configuration of a loop filter 160. FIG. It is explanatory drawing which shows the relationship between a loop bandwidth and the grade of BER degradation qualitatively. It is explanatory drawing which shows the relationship between a loop bandwidth and a carrier-synchronization maintenance limit qualitatively.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Demodulator 20 Local oscillator 30,40 AD converter 50 Carrier reproduction | regeneration circuit 60 Infinite phase shifter 70 Numerical control oscillator 80 Phase difference detection circuit 90 Loop filter 100 Quality monitoring circuit 90a, 90b Multiplier 90c ROM
90d, 90f Adder 90e Delay circuit

Claims (8)

In a demodulator comprising a carrier recovery loop that includes a variable bandwidth loop filter and recovers a carrier signal that is phase-synchronized with the carrier in the input signal,
When the quality of the input signal is higher than the first predetermined quality, the loop filter is caused to change the loop bandwidth so that the bit error rate deterioration is minimized, and the quality of the input signal is equal to or lower than the second predetermined quality. In this case, the demodulating device is characterized in that the loop filter is provided with a quality determination circuit for narrowing a loop bandwidth to be smaller than a predetermined bandwidth. A demodulator that reproduces a demodulated signal using a local oscillation frequency signal from a received modulated wave as an input signal;
A phase compensation unit that performs phase compensation of the demodulated signal,
The demodulation device according to claim 1, wherein a loop filter is provided in the phase compensation unit. The phase compensation unit performs complex multiplication of a numerically controlled oscillator that outputs a numerical value obtained by converting data output from the loop filter into a phase, a digital demodulated signal that is a digitized demodulated signal, and an output of the numerically controlled oscillator. A phase shifter that outputs a multiplication result, and a phase difference detection circuit that detects a phase shift of the output of the phase shifter and generates phase difference data,
The demodulator according to claim 2, wherein the loop filter inputs the phase difference data generated by the phase difference detection circuit and outputs data in which a noise component is suppressed. The loop filter includes a multiplier that multiplies the phase difference data by a coefficient,
The demodulator according to claim 3, wherein the quality determination circuit outputs data corresponding to a quality of an input signal and indicating data used by the multiplier to the loop filter. A demodulator that reproduces a demodulated signal from the input signal using the regenerated carrier wave signal;
The demodulator according to claim 1, wherein an output of the loop filter is input to a frequency variable oscillator that outputs the carrier wave signal to the demodulator. A phase difference detection circuit that detects a phase shift of the demodulated signal and generates a phase difference signal,
The demodulator according to claim 4, wherein the loop filter receives the phase difference signal generated by the phase difference detection circuit and outputs a voltage corresponding to the phase difference to a voltage controlled oscillator as a frequency variable oscillator. The loop filter is composed of a lag lead filter having a variable capacitance portion and a variable resistance portion,
The demodulator according to claim 6, wherein the quality determination circuit outputs a signal designating the capacitance of the variable capacitance unit and the resistance of the variable resistance unit according to the quality of the input signal. The quality determination circuit estimates the carrier signal power to noise power ratio of the input signal from the demodulated signal, and determines the quality of the input signal based on the estimated carrier signal power to noise power ratio. The demodulator according to any one of the above.

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