Multirate Schemes and Multiuser Decoding in DS/CDMA Systems

Licentiate thesis, 1995

Direct-Sequence Code Division Multiple-Access (DS/CDMA) has for some years been considered as a candidate for future personal and mobile communications. The reasons are mainly the properties of universal frequency reuse and the multi-path mitigation. Generally, the focus has been on low rate services such as speech, and very little has been done on evaluating the performance for a system that can support many types of services, such as speech, fax, image and data transmission. In this thesis, we present several schemes for multirate support and evaluate their relative performance. It is shown that the multi-channel scheme, using several channels (codes) in parallel, is the most efficient scheme. The main advantage is a high processing-gain even in “narrowband” systems (a few MHz). The drawback, though, is the need for linear amplifiers also in the mobile. Furthermore, we present a new multiuser receiver for the synchronous CDMA channel. The receiver is optimum in the maximum likelihood sense, and has a lower complexity than the previously known optimum receiver. Also, the thesis includes a survey of multiuser detection for the synchronous DS/DMA channel. The optimum and several suboptimal receivers are described and their advantages and disadvantages discussed. The third part of this thesis considers joint optimization of several components in a multiuser system. For a synchronous DS/DMA channel, we optimize the source and channel decoders, and the multiuser detector jointly, resulting in a soft multiuser decoder for transmission of vector quantized data. The optimum and several suboptimal receivers are presented. We also show that the performance gains for these decoders, with respect to individual optimization of the source and channel decoders and the detectors, are significant.