Equalizer with a sequence estimation method with state reduction

Details Equalizer with a sequence estimation method with state reduction Equalizer with a sequence estimation method with state reduction

1997-11-26

2000-03-28

Pham, Chi H.

Pulse or digital communications

Equalizers

Automatic

375232, 3647242, H03H 730, H03H 740, H03K 5159

Patent

active

060441119

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a digital transmission system comprising a receiver, to receiver including an equalizer for estimating transmitted binary symbols from a sequence of sample values of a received signal distorted by a transmission channel, by implementing a reduced-state sequence estimation method or reduced-state single symbol estimation method.
Furthermore, the invention relates to a receiver which includes an equalizer for estimating transmitted binary symbols from a sequence of sample values of a received signal distorted by a transmission channel, by implementing a reduced-state sequence estimation method or reduced-state single symbol estimation method.
2. Description of the Related Art
Such receivers are used, for example, in digital mobile radio according to the GSM standard. According to the GSM standard, digital signals are transmitted in a TDMA method by a GMSK modulation. The data transmission is then influenced by a time-variant transmission channel. More particularly, multipath propagation and reflections determine differences of delay and phase shifts for the transmitted digital data symbols in the received signal and lead to a superpositioning of adjacent data symbols. The fact that a received signal for a data symbol, is influenced by d previous data symbols is known as intersymbol interference (ISI). Then d is an integer defining the memory depth of the transmission channel.
For the equalization of the received signal which is linearly distorted as a result of multipath propagation and transmitting-end and receiving-end band limitation filters (intrinsic impulse noise with linear demodulation of the GMSK signal), the receiver is to be adapted, for data reconstruction, to the respective time-variant transmission properties of the transmission channel. Therefore, an estimation is made of the respective impulse response of the currently distorting transmission system, this system comprising not only the transmission channel, but also the influences of the GMSK modulation and a receiver input stage which produces sample values of the received digital signal. For this purpose, a substitute system describing the transmission system is formed, with the aid of which, estimated impulse response data are estimated according to the Maximum Likelihood Sequence Estimation (MLSE) method via the execution of a Viterbi algorithm, more particularly, a soft-output Viterbi algorithm, or a single symbol estimation method.
With this method, the most probable transmit sequence is determined from all possible data sequences, while taking into account the received sequence and the estimated impulse response of the transmission system. More particularly, the Viterbi algorithm is suitable for estimating the data symbols according to the MLSE method. The Viterbi algorithm is known from "The Viterbi algorithm", G. D. Forney, Jr., IEEE Proceedings, vol. 61, pp. 268-278, 1973. Additional information to the hard-decision estimates of data symbols is produced by the Soft-Output Viterbi Algorithm, which is known, for example, from "A Viterbi algorithm with soft-decision outputs and its applications", J. Hagenauer and P. Hoher, Proceedings of the GLOBECOM 1989, pp. 47.1.1-47.1.7, Dallas, 1989. With single symbol estimation, optimum maximum a-posteriori symbol-by-symbol decoder algorithms are used according to "Optimal decoding of linear codes for minimizing symbol error rate", L. R. Bahl, J. Cocke, F. Jelinek, and J. Raviv, IEEE Transactions on Information Theory, IT-20: pp. 284-287, 1974, or modification of this algorithm, respectively, found in "Optimum and Sub-Optimum Detection of Coded Data Disturbed by Time-Varying Intersymbol Interference", W. Koch and A. Baier, Proceedings of the GLOBECOM 1990, pp. 807.5.1-807.5.6, San Diego, December 1990. With an equal value of the estimation of the received signals, the manufacturing costs of the equalizer in a first approximation proportionally rise by 2.sup.d i.e., they rise exponentially with the depth d of the memo

REFERENCES:
patent: 5307374 (1994-04-01), Baier
patent: 5673294 (1997-09-01), Namekata
patent: 5867538 (1999-02-01), Liu
patent: 5887035 (1999-03-01), Molnar
Duel-Hallen et al, "Delayed Decision-Feedback Sequence Estimation" IEEE Transactions on Communications, vol.37, No.5, May, 1989.
Muller et al., "Reduced-State Soft-Output Trellis Equalization incorporating Soft Feedback", May, 1996.

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