Adaptive equalizer

Details Adaptive equalizer Adaptive equalizer

1995-04-17

1998-01-20

Chin, Stephen

Pulse or digital communications

Equalizers

375232, H03H 730

Patent

active

057107920

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to an adaptive equalizer which suppresses transmission distortion by adaptively estimating the transfer function of the transmission path.
For example, in a conventional TDMA digital mobile communication system, multipath propagation causes intersymbol interference, which may sometimes result in a failure of accurate carrier recovery by a demodulator circuit (a coherent detector circuit, for instance) at the receiving end. To avoid this, the receiving end is conventionally provided with a local oscillator which generates a reference carrier signal instead of the recovered carrier signal and an adaptive equalizer, which performs quasi-coherent detection based on the reference carrier signal and estimates the transfer function of the transmission path for equalization, is used to suppress the degradation of the transmission performance owing to the above-mentioned intersymbol interference. As typical adaptive equalizers there are known (i) a decision feedback equalizer that feeds back a decision signal at each time instant through a transmission path characteristic simulating filter to automatically remove multipath delayed signal components and makes a signal decision at the next time instant, and (ii) a maximum likelihood sequence estimation type adaptive equalizer which generates all symbol sequence candidates in a sequential order at each time instant, generates a replica by passing each candidate through a transmission path characteristic simulating filter and estimates a symbol sequence candidate of the maximum likelihood based on an estimation error between the replica and the received signal corresponding thereto. The latter needs a larger amount of operation than does the former but is appreciably superior in terms of the error rate performance.
FIG. 1 is a block diagram showing the configuration of a receiver loaded with the conventional maximum likelihood sequence estimation type adaptive equalizer. In FIG. 1, a multiplier 6 is supplied at one input terminal with a burst-like received signal r(t) by the TDMA scheme and is supplied at the other input terminal with a reference carrier signal of about the same frequence .omega..sub.c '/2.pi. as the carrier frequency .omega..sub.c /2.pi. of the received signal output by a local oscillator 7, and multiplies them. The output from the multiplier 6 is fed to a low-pass filter (LPF) 8 to extract a baseband signal (a complex signal containing a transmission distortion) Y.sub.b (t) which is a difference frequency component. The baseband signal Y.sub.b (t) thus obtained by such quasi-coherent detection is converted by an A/D converter 9 into a digital signal y(k), which is fed to an input terminal IN of an adaptive equalizer 10.
Letting the received signal r(t) with a carrier angular frequency .omega..sub.c be expressed as the sum of its in-phase and quadrature components by the following equation: output by the local oscillator 7 by .omega..sub.c ' and .theta..sub.c ', respectively, the in-phase component of the output signal from the multiplier 6 is expressed by the following equation: ##EQU1## Letting the baseband signal Y.sub.b (t) output by the low-pass filter 8 be the difference frequency component of Eq. (2), and hence can be expressed by the following equation: ')-Q(t)sin(.DELTA..omega..sub.c t-.theta..sub.c ')} (3) quadrature component of the output signal output by the multiplier 6 is expressed by the following equation: ##EQU2## and the difference frequency component (the quadrature component of the 8, is expressed by the following equation: ')+Q(t)cos(.DELTA..omega..sub.c t-.theta..sub.c ')} (5) and converts the received baseband signal y.sub.b (t) into digital signals y(k) at the timing t=kT (k=1, 2, 3, . . . ). The output y(k) from the A/D converter 9 is expressed as the sum of its in-phase and quadrature components by the following equation: memory 11. The received digital baseband signal y(k) will hereinafter be referred to as a received baseband signal or simply as a baseband signal. Furthermore,

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Fines, P., and Aghvami, A.H., "A Comparison Study of Low Bit Rate DE-QPSK and TCM 8-PSK Fully Digital Demodulators over a Land Mobile Satellite Link", IEEE Global Telecommunications Conference & Exhibition, Dec. 2-5, 1990, pp. 401.6.1-40.6.6.

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