On the performance of DS/SS systems in band-limited multipath environments.

Abstract

Wireless communication systems or wired systems operating in a bandwidth limited multipath environment are of interest in personal communication systems. Direct sequence spread spectrum (DS/SS) is a well known technique for communicating in a multipath environment. However, conventional implementations are not very bandwidth efficient. In this dissertation, the performance of direct-sequence spread spectrum systems with various bandwidth limiting filters is analyzed in a multipath environment. First, the trade-off between spreading factor and intersymbol interference with fixed bandwidth and symbol rate is investigated for both filtered rectangular and raised-cosine chip waveforms. Though a larger spreading factor results in more intersymbol interference due to bandwidth limitations, it is shown that the increased interference suppression capability is able to overcome the degradation and provides a better signal-to-interference ratio in specular multipath fading channels. Second, a partial-response DS/SS system using a RAKE receiver is proposed and analyzed for specular multipath environments. Though partial-response DS/SS systems are marginally outperformed by DS/SS systems using filtered rectangular chips for most of the cases surveyed, they have easier filter implementations and can be specially designed to fit the channel response. Third, the error probability of a partial-response DS/SS system for WSSUS multipath fading channels is calculated. With random spreading sequence, partial-response signals perform worse than filtered rectangular chips. However, when m-sequences are used in a DS/SS system, partial-response signals achieve a much better performance than filtered rectangular chips. Lastly, the possibility of applying DS/SS systems to intra-building power-line channels is investigated. If correct operating frequency bands and modulation schemes are chosen, a DS/SS system has enough signal-to-noise margin and processing gain to establish a high speed data communication network even for an extremely hostile channel.