Performance of different multi-rate schemes in DS/CDMA systems
Tony Ottosson Gadd, Arne Svensson · 1994
Abstract In this paper the performance of different multi-rateschemes in Direct Sequence Code Division MultipleAccess (DS/CDMA) systems are evaluated. Theassumptions are random code sequences and AWGNchannel. We have looked at four different solutions:different spreading factors, mixed modulation, paral-lel channels and Parallel Combinatory Spread Spec-trum (PC/SS). Our conclusion is that parallel channelsis the best scheme in supporting many data-rates, if wealso take into account code-design and receiver com-plexity. Finally we suggest a combined system withparallel channels and mixed modulation, which is cur-rently evaluated.Keywords: DS/CDMA, multi-rate, different spreadingfactors, mixed modulation, parallel channels, ParallelCombinatory Spread Spectrum (PC/SS). 1 Introduction Most DS/CDMA systems considered in the literature iseither single bit rate or in a couple of cases few-bit rate(less than 5) systems. Future personal communicationssystems (PCS) must support various services and be veryflexible. This means that bit rates from about 10 kbps (e.g.speech) to 2 Mbps (e.g. video) and even higher rates areneeded.The aim of this paper is to analyse different systems in theview of multi-rate performance and give some clues onhow to construct such a system. In Section 2, we presentthe different system models and assumptions used. A dis-cussion on performance calculation is then done (for fur-ther details, see [5]). Some numerical examples for thedifferent schemes is shown in Section 3. The results areinterpreted and some conclusions are then drawn in Sec-tion 4. Finally we present a proposal that we currentlyinvestigate.