JP2696948B2 - Carrier recovery circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carrier recovery system used for synchronous detection and demodulation of digitally modulated signals, and a carrier recovery system for GMSK modulation and offset QPSK modulation applied to mobile communication and the like. It is related to the circuit.

2. Description of the Related Art Conventionally, as means for demodulating a digitally modulated signal, synchronous detection and delay detection have been used for digital phase modulation waves, and synchronous detection, delay detection and frequency detection have been used for digital frequency modulation waves. Are known. It is known that a synchronous detection system theoretically exhibits the best characteristics when the propagation characteristics of the space to be transmitted are ideal, that is, when the transmission channel can be defined only by white Gaussian noise. .

As a digital modulation method applied to mobile communication and the like, a C-class transmission power amplifier is used with the aim of improving the power efficiency of a transmission power amplifier of a mobile body. Modulation systems such as QPSK modulation are promising.

The synchronous detection system requires a carrier recovery system for extracting the carrier component of the received modulated wave. Conventionally, as a carrier recovery circuit of a four-phase phase modulated wave (hereinafter abbreviated as QPSK) system,
Four-signal signaling, inverse modulation, modified Costas loop, and the like are known.

Problems to be Solved by the Invention Meanwhile, in mobile communication and the like, a fading of a received electric field level and a sudden fluctuation of a carrier phase of a received modulated wave occur due to fading due to obstacles or multiple reflections on a propagation path. Abrupt fluctuations in the received electric field can be absorbed to some extent by automatic gain adjustment or limiters, but the carrier wave recovery system cannot follow the sudden fluctuations in the phase, Since normal synchronous detection is not performed, there is a problem that the characteristic of synchronous detection is deteriorated under fading as compared with other demodulation methods.

An object of the present invention is to solve the problems described above and to provide a carrier recovery circuit suitable for GMSK modulation or offset QPSK modulation that shows good characteristics even on a propagation path having abrupt fading such as mobile communication.

Means for Solving the Problems The present invention is used for synchronous detection demodulation of a digital modulation wave, a quadrature demodulator, an inverse modulation circuit, and a carrier recovery device having a phase locked loop including a voltage controlled oscillator.
An extraction unit for extracting the in-phase output and the quadrature-phase output of the quadrature demodulator at the symbol data identification points and the change points of the symbol data of each phase, and receiving modulation using the voltages at the extraction points extracted by the extraction unit. A phase error detection unit that detects a phase error between the phase of the carrier wave of the wave and the phase of the reproduced carrier wave, and when the phase error detected by the phase error detection unit is a certain value or more, the control voltage according to the phase error is A control voltage generator for outputting a step-wise output to the voltage-controlled oscillator and outputting a constant control voltage to the voltage-controlled oscillator when the phase error is equal to or smaller than a predetermined value is added.

Operation The present invention provides a GMSK modulation and an offset QP
An in-phase output and a quadrature output of a quadrature demodulator used for demodulation of an offset modulation wave such as SK modulation are alternately sampled at the maximum aperture of each eye pattern. When the in-phase component is the maximum aperture point, the sample value of the in-phase output is hard-decided, and at the same time, the sample value of the quadrature output at the data change point is sign-inverted / non-inverted based on the in-phase output hard decision result, and the result is Output. Conversely, when the quadrature component is at the maximum aperture point, the sample value of the quadrature output is hard-decided. And output the result. However, since this system is an offset system, since the in-phase maximum aperture point and the quadrature maximum aperture point alternately exist, the two output values representing the detected phase error are inverted in their polarities. , The polarity is forcibly and alternately changed to output in the order of sampling. By observing this output value, the phase difference between the phase of the carrier signal of the received modulated wave and the phase of the reproduced carrier signal is detected, and when the phase difference is smaller than a certain value, the normal carrier wave reproduction is performed. By operating the system, if the phase difference is a certain value or more, by changing the control voltage of the voltage controlled oscillator of the normal carrier recovery system from a fixed value in a stepwise manner, it is possible to follow a sudden phase change due to fading. Carrier wave regeneration is performed.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a block connection of a carrier recovery circuit for an offset QPSK system such as GMSK modulation or OQPSK modulation according to the present invention.

In this embodiment, an inverse modulation type carrier recovery system is used as a normal carrier recovery system, and is indicated by a dotted line 1 in the figure. The blocks in the solid line 2 are based on this embodiment.

In the figure, 4, 5, 9, 10, 15 are multipliers for multiplying two input signals, 8, 13 are phase shifters for shifting the phase by 90 °, 6, 7, 16 are low-pass filters, and 11, 12 are This is a comparator for hard decision.

In the figure, a quadrature demodulator is constituted by multipliers 4 and 5, low-pass filters 6 and 7, and a 90 ° phase shifter 8, while a delay circuit 3, multipliers 9 and 10, comparators 11, 12, 90逆 The phase modulator 13 and the sum circuit 14 constitute an inverse modulation circuit.

In the above configuration, the carrier signal from which the modulation component has been removed by the inverse modulation circuit is input to one input terminal of the multiplier 15 operating as a phase detector, and the output of the voltage controlled oscillator 17 is input to the other input terminal. Is done. A phase locked loop is formed by the loop filter 16, the multiplier 15, and the voltage controlled oscillator 17.

On the other hand, the A / D converters 18 and 19 convert the in-phase output and quadrature-phase output of the quadrature demodulator to the maximum aperture point of each eye pattern and the symbol data change using a clock synchronized with the data clock of the received modulated wave. At this point, it is sampled and A / D converted. The outputs of the A / D converters 18 and 19 detect the phase difference between the phase of the received modulated wave signal by the phase difference detection circuit 20 and the phase of the reproduced carrier signal generated by the voltage-controlled oscillator 17, and the phase difference is detected. If the phase difference is equal to or greater than a certain value, a constant control voltage corresponding to the phase error is input from the control voltage generator 21 to one control terminal 23 of the voltage controlled oscillator 17 in a stepwise manner.

When the phase difference is equal to or less than a certain value, the output voltage of the control voltage generator 21 is maintained at a certain value, and the carrier recovery operates as a normal inverse modulation circuit.

Now, when the phase of the carrier component of the received modulated wave fluctuates abruptly and the phase difference from the reproduced carrier phase exceeds a certain value, the multiplier 15, the low-pass filter 16, the voltage-controlled oscillator 17
Is in a state in which the carrier is not reproduced. However, the phase error detection circuit
Since a step voltage corresponding to the phase error detected by 20 is applied to the voltage controlled oscillator 17, the voltage controlled oscillator 22
The output signal frequency rapidly changes, and the phase rapidly changes due to fading or the like, that is, becomes closer to the frequency of the carrier component of the received modulated wave whose frequency component has changed. Normally, in the phase locked loop, the phase pull-in operation is faster than the frequency pull-in operation, and the frequency is rapidly changed, and the frequency is rapidly reproduced by the output signal of the voltage controlled oscillator 17 closer to the carrier frequency component of the received modulated wave. The carrier will be regenerated.

In the present embodiment, an inverse modulation type carrier wave reproducing system has been described, but the same applies to the case where a carrier wave reproducing system of another system is used.

As described above, according to the present invention, according to the present invention, a carrier recovery required for synchronous detection of a received digital modulation wave having a sudden phase change in a fading propagation path such as mobile communication or the like. The system can be provided in a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram of a carrier recovery circuit according to an embodiment of the present invention. 4, 5, 9, 10 ... multiplier, 18, 19 ... A / D converter, 20 ... phase difference detection circuit, 21 ... control voltage generator, 22 ... voltage controlled oscillator.

Claims (1)

(57) [Claims] A carrier recovery means used for synchronous detection demodulation of a digital modulation wave and having a quadrature demodulator, an inverse modulation circuit, and a phase locked loop including a voltage controlled oscillator.
An extraction unit for extracting the in-phase output and the quadrature-phase output of the quadrature demodulator at the symbol data identification points and the change points of the symbol data of each phase, and receiving modulation using the voltages at the extraction points extracted by the extraction unit. A phase error detection unit that detects a phase error between the phase of the carrier wave of the wave and the phase of the reproduced carrier wave, and when the phase error detected by the phase error detection unit is a certain value or more, the control voltage according to the phase error is And a control voltage generator for outputting a stepwise output to the voltage controlled oscillator and outputting a constant control voltage to the voltage controlled oscillator when the phase error is equal to or less than a predetermined value.