Soft-Decision Decoding for DPSK-Modulated Wireless Voice Communications

Abstract

This thesis addresses some techniques that enhance a receiver's performance in a wireless voice communication system where differential phase shift keying (DPSK) is the adopted modulation scheme and soft-decision decoding is used to improve the effectiveness of the channel coding scheme.<P>First, several fundamental issues regarding the statistical properties of fading channels are provided. We demonstrate the constraints, that must be satisfied so that the channel can be regarded as impaired by ﲦlat (i.e., non-frequency-selective) fading with a constant fading factor over each symbol duration. Throughout this thesis these constraints are assumed to be satisfied.<P>We next investigate the channel capacity and cutoff rates for fading channels with DPSK-modulated input signals and perfect symbol interleaving. The impact of the channel state information (CSI), on these information-theoretic limits is also discussed. We introduce several symbol metrics for soft-decision decoding, and their performance is investigated by analytical derivation as well as by simulation. Furthermore, we define a bit metric for DQPSK modulation, and compare this bit metric to dibit (symbol) metrics.<P>We then consider the problem of error concealment for mobile radio, communications with a maximum-likelihood soft- decision decoder. A normalized codeword reliability is defined as the decision reliability information when CSI is not available. We employ a given, interpolation algorithm on a particular land mobile radio system and design a rule for selecting unreliable codewords. Simulation results show that error concealment can decrease the minimum operational signal-noise ratio (SNR) by 3 dB or more.<P>Finally, we address the problem of exploiting the residual redundancy in the source to enhance the channel decoder's performance -i.e., maximum a posteriori (MAP) decoding. We use two simple source models to demonstrate that MAP decoding can achieve significant gain over maximum-likelihood decoding. We employ a practical CELP-based land mobile radio system to show that significant residual redundancy does exist in the output of some source encoders. Simulation results show that a 2 - 3 dB gain can be achieved by MAP decoding (over ML) at low SNR.<P>