Adaptive equalisers for wideband time division multiple access mobile radio

Cheung, Joseph Chung Shing (1992) Adaptive equalisers for wideband time division multiple access mobile radio. University of Southampton, Doctoral Thesis.

Abstract

The detection of partial response continuous phase modulated (CPM) signals transmission over wideband time division multiple access (TDMA) mobile radio channels is addressed. The design of adaptive equalisers to accommodate channels having long excess delays while maintaining an acceptable performance is the central core of the investigation. Maximum likelihood sequence estimation (MLSE) detector in the form of the Viterbi equaliser is described and its performance in narrowband and wideband radio channels is studied. Binary digital phase modulation (DPM) employing a three-bit raised-cosine (3-RC) phase shaping filter and a modulation index of 1.08 was used to convey data over a flat Rayleigh fading channel. A signal-to-noise ratio of 27dB was required to achieve a bit error rate (BER) of 10^-3. For transmission to DPM data over Rician fading channels, the BER improved with the K-factor values. The DPM transmissions over wideband channels yielded performance gains provided the entire length of the channel impulse response was utilised by the Viterbi equaliser. A 9dB gain in SNR was obtained with two-path Rayleigh fading channels compared to flat Rayleigh fading channel. In channels exhibiting exceedingly large excess path delays, as found in mountainous regions, the complexity required by the conventional Viterbi equaliser is prohibitively high. To accommodate these massive path delays we propose a modified Viterbi equaliser which explicitly exploits the inherent diversity effect innate in wideband radio channels. A separate metric calculator is used for each resolvable component in the channel impulse response and these metric calculator outputs are combined to give the overall metric for a simple Viterbi processor. The equaliser performance was found to be nearly identical to that of the original Viterbi equaliser for SNR< 15dB. The complexity was only about 25% of that required by the original Viterbi equaliser capable of handling channels having the same excess delay. Decision feedback equalisation (DFE) for CPM signals is also considered. The error propagation caused by incorrect decision in the feedback filter is addressed. A hybrid combination of the DFE and the Viterbi processor is proposed which allows soft-decisions to be made and incorrect decisions to be changed to curtail error propagation in the DFE. This arrangement we designated as the Soft-Decision Feedback Equaliser (SDFE). The performance of the SDFE approaches that of the original Viterbi equaliser with discrepancies in channel SNR of 1.5dB and 2dB at BERs of 10^-2 and 10^-3, respectively when transmission as over a two-path Rayleigh fading channel. For the GSM hilly terrain (HT) and the equaliser test (EQ) channel models, up to 75% of the sequence feedback changes made to the DFE were successful for moderate SNR, say 15dB. The SDFE has the attractive characteristic of being able to trade complexity and performance by operating between a full Viterbi equaliser and a simple DFE. For typical implementation of the SDFE, the complexity ranged from 25% to 50% of the original Viterbi equaliser.

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