Multirate Schemes and Multiuser Decoding in DS/CDMA Systems

Sammanfattning: Direct-Sequence Code Division Multiple-Access (DS/CDMA) has for some yearsbeen considered as a candidate for future personal and mobile communications.The reasons are mainly the properties of universal frequency reuse andthe multi-path mitigation. Generally, the focus has been on low rate servicessuch as speech, and very little has been done on evaluating the performancefor 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 andevaluate their relative performance. It is shown that the multi-channelscheme, using several channels (codes) in parallel, is the most efficientscheme. The main advantage is a high processing-gain even in “narrowband”systems (a few MHz). The drawback, though, is the need for linear amplifiersalso in the mobile.Furthermore, we present a new multiuser receiver for the synchronousCDMA 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 synchronousDS/DMA channel. The optimum and several suboptimal receivers are describedand their advantages and disadvantages discussed.The third part of this thesis considers joint optimization of severalcomponents in a multiuser system. For a synchronous DS/DMA channel, weoptimize the source and channel decoders, and the multiuser detector jointly,resulting in a soft multiuser decoder for transmission of vector quantizeddata. The optimum and several suboptimal receivers are presented. We alsoshow that the performance gains for these decoders, with respect to individualoptimization of the source and channel decoders and the detectors, aresignificant.