Maximum likelihood differential detecting method and differentia

Details Maximum likelihood differential detecting method and differentia Maximum likelihood differential detecting method and differentia

1995-01-30

1997-11-04

Chin, Stephen

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

375331, 375341, 371 43, 329306, H04L 512, H04L 2302

Patent

active

056848323

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a differential detecting method for differentially detecting a digital signal transmitted as a phase difference sequence in a symbol interval and for obtaining a decoded sequence, and, more particularly it relates to a differential detecting method and a differential detector thereof corresponding to a maximum likelihood sequence estimation technique.


RELATED ART

Phase modulated waves are conventionally demodulated by coherent detection and differential detection. In coherent detection, the receiver side reproduces a carrier wave as a reference signal, measures the phase of a received wave corresponding to the reference signal, and estimates a transmitted code. In this case, since the absolute phase is unknown, the sender side generally uses differential phase shift-keying modulation (DPSK) that modulates information corresponding to the variation of the phase of the carrier wave. Since the reproduced reference signal is not affected by noise and the like, a low error rate can be accomplished.
On the other hand, as differential detection, differential phase detection and quadrature differential detection have been widely used. In differential detection, the reference wave is formed of a received wave with a delay of one symbol interval. Thus, since no carrier wave reproducing circuit is required, the detecting circuit can be simply constructed and the detecting operation can be performed at high speed. Consequently, differential detection is suitable for receiving a burst signal in time division multiple access (TDMA) communication. However, since the signal with a delay of one symbol interval is used as the reference signal, the reference signal tends to be adversely affected by thermal noise and the like. Thus, the error rate of differential detection degrades in comparison with that of coherent detection. As a result, depending on whether the detecting circuit is complicated, the burst signal is received, and so forth, either coherent detection or differential detection is selected.
For example, for four-phase DPSK, at a bit error rate 0.1%, the difference in bit energy-to-noise rate (Eb/No) between differential detection and coherent detection is 1.8 dB. To reduce the difference, a maximum likelihood quadrature differential detection for estimating a transmitted data sequence has been proposed as in Reference 1. The quadrature differential detector is composed of delay devices and multipliers. (Reference 1: D. Divsalar and M. K. Simon, "Multiple-symbol differential detection of MPSK," IEEE Trans. Commun., vol. 38, pp. 300-308, March 1990.) In addition, a technique for recursive estimation using the Viterbi algorithm has been proposed as in Reference 2. (Reference 2: D. Makrakis and K. Feher, "Optimal noncoherent detection of PSK signals," Electronics Letters, vol. 26, pp. 398-400, March 1990.)
Assuming that an N-symbol phase difference sequence .DELTA..phi..sub.n (where n=1, 2, . . . , N) is being transmitted, the received signal is quadrature-differentially detected and a maximum likelihood sequence estimation is applied. M-phase DPSK signals that are received in an interval (n-1)T.ltoreq.t<nT can be given in the complex representation as follows: is energy per symbol; T is one symbol interval; .theta. is a phase difference between a received wave and a locally oscillated wave of the receiver; w(t) is noise of the receiver; and .DELTA..phi..sub.n =.phi..sub.n -.phi..sub.n-1 is the n-th phase difference. After z(t) is filtered, it is sampled in the symbol interval. The obtained signal sample sequence is denoted by {Z.sub.n ; n=0, 1, . . . , N}. In the technique of Reference 1, a sequence that maximizes a metric given by the following equation is selected: exp j(.DELTA..phi..sub.N +.DELTA..phi..sub.N-1)+ . . . +z.sub.0 exp j (.DELTA..phi..sub.N +.DELTA..phi..sub.N-1 + . . . +.DELTA..phi..sub.1).vertline..sup.2 ( 02) ##EQU1## complex. In the technique of Reference 2, phase difference sequences are successively estimated based on the M.sup.L-1 state

REFERENCES:
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patent: 5272726 (1993-12-01), Furuya et al.
patent: 5325402 (1994-06-01), Ushirokawa
patent: 5331666 (1994-07-01), Dent
Makrakis et al., "Optimal Noncoherent Detection of PSK Signals", Electronics Letters, vol. 26, No. 6, pp. 398-400, Mar. 1990.
Divsalar et al., "Multiple-Symbol Differential Detection of MPSK", IEEE, vol. 38, No. 3, pp. 300-308, Mar. 1990.

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