Digital mobile radio

Wassell, Ian James (1990) Digital mobile radio. University of Southampton, Doctoral Thesis.

Abstract

The transmission of digital signals over both narrowband and wideband mobile radio channels is investigated. When the work for this thesis commenced, the S900-D system was a contender for the pan-European digital cellular mobile radio standard. Although the principle of its operation was known, its performance had not been evaluated. Accordingly, this thesis provides an analysis of the BER in the presence of noise, co-channel interference and flat Rayleigh fading. Similarly, the performance of the Viterbi equalisers proposed for NB-TDMA systems using either DPM or GMSK modulation was also unknown. Complex baseband simulations of CPM data transmissions over AWGN, narrowband fading and wideband fading channels were performed. These simulations showed that for transmission over a 2-Ray channel at a BER of 10^-2, DPM (3RC, h_p = 1.0) provided a 1/2 bit duration advantage in delay spread capacity over GMSK (B_n = 0.3, h_f = 0.5). DPM also held a 1 dB SNR advantage when GMSK transmissions were performed over a Gaussian channel. This thesis also addresses the BER improvements brought about by the addition of interleaving and channel coding. A systematic (12, 8)₄ RS code was found to reduce the BER by a factor of up to 55 when applied to error patterns gathered during trials in Central London and Southampton. A punctured 2/3 rate convolutional code of constraint length 5 and hard Viterbi decoding only achieved a maximum 7 fold reduction in BER compared to when no channel coding was used. The proposed DECT radio interface was investigated and a novel bit timing recovery circuit was designed and constructed. The technique gave only a 0.5 dB performance penalty at a BER of 10^-2 in the presence of Gaussian noise when operating at 1.024 Mbits/s. Synchronisation was achieved within a maximum of eight bits. The circuit is currently being used by a company in their DECT testbed over fading channels. For light weight hand-held personal communicators, we advocate the use of Embedded Delta Modulation (EDM). This speech codec provided 5 dB and 8 dB improvements in peak SNR performance when compared with linear delta modulation (LDM) and PCM respectively, at 32 kbits/s. Being an embedded codec, speech quality may easily be exchanged for capacity at times of higher user demand. Mobile radio communication is entering a very exciting phase in its development. VLSI technology will enable the widespread application of some of the sophisticated speech codecs, channel codecs and modems we have described in our discourse.

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