Transmission of Vector Quantization over a Frequency-selective Rayleigh Fading Cdma Channel

Recently, the transmission of vector quantization (VQ) over a code-division multiple access (CDMA) channel has received a considerable attention in research community. The complexity of the optimal decoding for VQ in CDMA communications is prohibitive for implementation, especially for systems with a medium or large number of users. A suboptimal approach to VQ decoding over a CDMA channel, disturbed by additive white Gaussian noise (AWGN), was recently developed in [1], [2]. Such a suboptimal decoder is built from a soft-output multiuser detector (MUD), a soft bit estimator and the optimal soft VQ decoders of individual users. Due to its lower complexity and good performance, such a decoding scheme is an attractive alternative to the complicated optimal decoder. It is necessary to extend this decoding scheme for a frequency-selective Rayleigh fading CDMA channel, a channel model typically seen in mobile wireless communications. This is precisely the objective of this thesis. A frequency-selective Rayleigh fading channel is typically modeled as a tappeddelay line [3]. In this channel model, the received amplitude over each path of each iii user is a complex random variable. The delay between paths is an integer multiple of the chip duration. Therefore, the formulation of the suboptimal decoding proposed for an AWGN channel in [1], [2] needs to be carefully examined. In the suboptimal decoding under consideration, the received signal waveform is first correlated with delayed replicas of the users’ signature waveforms to form the sufficient statistic. The sufficient statistic is then processed by the MUD and the VQ decoder in order to make the final decision for the source data. The soft-output MUD can be the jointly optimal MUD (OPT-MUD), the minimum mean-square error MUD (MMSE-MUD) or the decorrelating MUD (DC-MUD). For each type of MUD, the soft-bit estimates are calculated from the sufficient statistic and then fed into the soft VQ decoders [1]. Furthermore, the suboptimal decoders are obtained not only for binary phase shift keying (BPSK), but also for M -ary pulse amplitude modulation (M -PAM). This extension offers a flexible trade-off between spectrum efficiency and performance of the systems. In addition, two algorithms based on distance measure and reliability processing are introduced as other alternatives to the suboptimal decoder. Simulation results indicate that the suboptimal decoders studied in this thesis also performs very well over a frequency-selective Rayleigh fading CDMA channel.