Linear receivers for spread spectrum communication over multipath dispersive channels

This paper studies single-signature and orthogonal multiple-signature (multicode) spread spectrum communication over frequency-selective Rayleigh fading channels. We analyze the influence of the spreading factor, which is defined here as the ratio of transmission bandwidth to user symbol rate. More specifically, we focus on the impact of small spreading factors and highly-dispersive channels which, together, introduce potentially detrimental intersymbol or inter-signature interference in the detection process. Such links are important for high-speed data transmission in future wideband code-division multiple access (WCDMA) systems. We present theoretical results for the output signal-to-distortion ratio of a range of important linear receiver types: matched filter (MF), minimum mean-square error (MMSE), and pre-equalizing (PE). We show that, at any specified spreading factor, single-signature transmission, orthogonal multiple-signature transmission, and simple frequency diversity all yield about the same link performance for these receivers. Moreover, we demonstrate the significant gain of MMSE processing over MF processing (equivalent to RAKE processing) in links with small spreading factors