Analysis and evaluation of three symbol timing recovery techniques for digital wireless personal communication systems

This thesis investigates the performance in the presence of CCI and multipath fading of three symbol timing recovery schemes, maximum likelihood (ML) non-data aided (NDA) and data-aided (DA) tracking loops, and a ML block symbol synchronizer, for QPSK, π/4—shift DQPSK, and 16—QAM modulation schemes. In the first part of the thesis, analysis of the three symbol synchronizers in a combined AWGN and CCI environment is performed, based on a linear assumption for small jitter, and expressions for RMS symbol timing jitter are presented for each technique, assuming Nyquist pulse shaping. Computer simulations are also performed in order to validate the results and provide bit-error rate (BER) performance of each synchronizer. CCI was found to be the chief limit on symbol timing jitter performance at high signal-to-noise ratios (SNR) for all three synchronizers. Simulations matched theory in most cases, except where the linear assumption is no longer valid. Simulation showed that for carrier-to-interference (C/I) ratios of ≥ 10 dB, CCI effects on symbol timing recovery result in negligible BER degradation over ideal sampling. The second part of the thesis investigates the three symbol synchronizers in a fading environment. Irreducable BER and RMS timing jitter estimates were made by computer simulation using π/4—shift DQPSK modulation in frequency non-selective Rayleigh and Ricean fading, as well as in frequency selective fading. Rayleigh fading had little effect on symbol timing jitter at high SNR unless the fading was extremely fast, that is where the fading bandwidth (B[sub F]) times the information symbol duration (T) product is relatively high (i.e. Β[sub F]Τ>0.1), and had no noticeable effect on irreducable BER for the range simulated. In frequency selective fading with a two-ray model, the synchronizers were observed to track the optimum sampling instant well at high SNR, for delay spreads of the two-ray model used of τ[sub d] < 0.2T. For more severe fading (i.e. τ[sub d]> 0.2T), the ML block synchronizer performed best, tracking the optimum sampling instant with no increase in the irreducable BER until τ[sub d] = T/2. The ML NDA technique was found to be the worst performer for frequency selective multipath fading, with double the irreducable BER in the worst case simulated.

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