Multirate Schemes and Multiuser Decoding in DS/CDMA Systems
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
Direct-Sequence Code Division Multiple-Access (DS/CDMA). multirate