QAM techniques for digital mobile radio

Webb, William Timothy (1992) QAM techniques for digital mobile radio. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Quadrature amplitude modulation (QAM) schemes to convey digital signals over mobile radio channels are proposed and examined. By deploying QAM the spectral efficiency is considerably enhanced compared to binary modulation methods. However, QAM does require higher signal-to-noise ratios (SNRs) for the same bit error rates (BERs), and is more vulnerable to co-channel interference. Our intention is to use QAM in microcells where these drawbacks are outweighed by the increase in spectral efficiency. Different QAM constellations were examined, leading to the choice of a twin amplitude 8-level PSK constellation which we called Star QAM. When coupled with differential amplitude and phase coding, our simulations and quasi-theoretical arguments demonstrated that BERs of < 10^-3 were achieved for channel SNRs of > 35dB in a Rayleigh fading envionment. In an attempt to improve this performance, Trellis Code Modulation (TCM) and a block coding derivation of TCM were simulated, the latter providing a substantial improvement in performance over the uncoded system, giving BERs of < 10^-6 for channel SNRs in excess of 35dB. Wideband systems were considered with bit rates up to 32 Mb/s for use in office environments and street microcells. Equalisers were found to be necessary and a number of conventional equalisers were considered before a novel system comprising a linear, decision feedback, and RAKE-type equaliser was simulated and shown to give a BER of < 10^-3 for channel SNRs in excess of 30dB. Our investigation concluded with the implementation of our fixed rate Star QAM transmitter and receiver modem with its associated clock recovery system. Its performance aligns well with our simulation results and demonstrates that by using Star QAM with differential coding, multi-level transmissions within micro-cellular mobile radio environments are feasible.

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