Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Patent
1996-02-22
1997-08-05
Grimm, Siegfried H.
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
375324, 375331, H04L 2722
Patent
active
056546670
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a differential detection method that is used, for example, in mobile radio communications to demodulate phase-modulated signals by estimating their most likely state from a previous or past detected symbol sequence in an M-level differential phase modulation system commonly referred to as M-phase DPSK (M being a positive integer).
Coherent detection and differential detection are widely used for the demodulation of phase-modulated signals. In the coherent detection, the receiving side regenerates a carrier, then measures the phase of the received signal, using the regenerated carrier as a reference signal, and decides a transmitted symbol. In this instance, since the absolute phase of the transmitted signal is unknown, the transmitting side usually employs a differential phase modulation (DPSK) scheme which puts information on a phase difference .DELTA..phi.. This coherent detection features an excellent error rate characteristic because the reference signal regenerated for coherent detection is insusceptible to agitation by thermal noise, but in a fading environment where the phase of the received signal varies, the phase of the regenerated reference signal cannot follow the phase variation and the performance deteriorates accordingly.
On the other hand, the conventional differential detection method uses, as the reference signal, a received signal z.sub.n-1 delayed by one symbol period to decide that .DELTA..phi..sub.n =.DELTA..phi..sub.n which maximizes Re{z.sub.n z*.sub.n-1 exp(-j.DELTA..phi..sub.n)} is a transmitted symbol. Here, z.sub.n is a complex representation of the received signal, Re a real part and * a complex conjugate. Thus, the differential detection does not need a carrier regeneration circuit, and hence simplifies the detection circuit used and provides a performance superior to that of the coherent detection in the fading environment.
In mobile radio communications, radio waves are received after being reflected by buildings or the like, so that when a mobile station performs transmission and reception while moving, multipath fading occurs in the received signal, causing a transmission error. In such a fading channel the differential detection provides an error rate performance more excellent than does the coherent detection, but as fading becomes faster, a random phase rotation is caused in the received signal and the phase difference between the signals z.sub.n and z.sub.n-1 becomes different from the transmission phase difference, readily causing an error. Contrary to this, in a no fading channel, the error rate performance is inferior to that obtainable with the coherent detection.
In the meantime, it is proposed, in D. Makrakis and K. Feher, "Optimal noncoherent detection of PSK signals," Electronics Letters, vol. 26, pp. 398-400, March 1990, to bring the error rate in quadrature differential detection close to that obtainable with coherent-detection with differential decoding by a differential detection scheme that makes a maximum likelihood sequence estimation through the use of the Viterbi algorithm.
With the proposed method, however, as fading becomes faster, the error rate performance rather deteriorates because of the random phase rotation of the received signal.
It is therefore an object of the present invention to provide a differential detection method for the M-phase DPSK modulated signal which possesses an excellent error rate performance even in a fast-varying fading environment.
Another object of the present invention is to provide a differential detection method for the M-phase DPSK modulated signal which has a fast fading tracking property and is capable of obtaining an error rate performance close to that by the coherent detection even in no fading environment.
DISCLOSURE OF THE INVENTION
In a first aspect, the present invention is directed to a differential detection method for an M-level differentially phase-modulated signal, M being an integer equal to or greater than 3, the method comprising the steps of: rece
REFERENCES:
patent: 5202643 (1993-04-01), Sato
Adachi, F. and Sawahasi, M., "Viterbi-Decoding Differential Detection of DPSK," Electronics Letters, vol. 28, No. 23, Nov. 5, 1992, pp. 2196-2198.
Adachi, F. and Sawahasi, M., "Decision Feedback Differential Detection of Differentially Encoded 16APSK Signals," Technical Report of the IEICE, MW93-19, RCS93-2, vol. 93, No. 77, May 27, 1993, pp. 7-14.
Grimm Siegfried H.
NTT Mobile Communications Network Inc.
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