PSEUDO-ANALOGUE TRANSMISSION USING WEIGHTED QAM

Abstract

This thesis describes the development and simulation of various 256-level weighted QAM structures for the transmission of PCM speech over both AWGN and Rayleigh fading channels. A source matched system approach is employed. Structured mapping schemes are used in place of traditional Gray codes and bit-mapping. The optimisation of weights is based on the evaluation of the digital error power and the overall received SNR is used as an objective measure of performance. Weighted constellations evaluated include square QAM, rectangular QAM, star QAM and sub-constellation QAM structures. These are compared with Skewed Phase Shift Keying (SPSK) and Skewed Differential Phase Shift Keying (SDPSK) benchmarks. Weighted QAM structures are found to offer significant gains in performance over unweighted constellations. Performance of such systems met or exceeded that of the benchmark systems.