Linear MMSE Interference Suppression in Asynchronous Random-CDMA

{ This paper considers interference suppression , based on linear minimum mean square error (MMSE) ltering, in random code-division multiple-access (Random-CDMA) systems such as IS-95 and Wideband-CDMA using long pseudo-noise (PN) sequences. It examines the design of the MMSE receiver and its performance in terms of probability of bit error (Pe). Expressions for Pe are derived for systems with multiple bit rates (and spreading factors), variable signal powers and chip pulse shaping. Numerical examples show substantial Pe gains in systems with one to two high-powered users and square-root raised cosine (Sqrt-RC) pulse shaping with 100% excess bandwidth. This paper investigates the design and performance of a linear MMSE receiver for one-shot bit symbol detection in asynchronous Random-CDMA. Random-CDMA refers to systems, such as IS-95 1] and Wideband-CDMA 2], in which the sequence of chips used to spread a single bit eeectively changes with each transmitted bit. Previous investigations of MMSE receivers demonstrated two desirable attributes: excellent performance gains over the conventional matched lter (CMF) and the possibility of adaptive implementations with low computational complexity relative to those of most multiuser detectors (see 3, 4] and other references listed therein). However, these investigations were largely restricted to an incompatible CDMA system where the sequence of chips remain constant with each bit. As a result, the majority of these receivers are impractical in Random-CDMA where the lter response would change and require updating at the bit-rate 1]. The idea of designing a linear MMSE receiver speciically for Random-CDMA was recently proposed by Wong et al. in 5]. Their analysis showed promising results in terms of near-far resistance for a two-user system and signal-to-noise ratio (SNR) for more general conditions. However, probability of bit error (P e) performance was not investigated. Moreover , such P e analyses and results for the MMSE receiver currently do not exist. Accordingly, the primary goal of this paper is to investigate P e performance of the MMSE receiver in Random-CDMA. Its performance is examined in systems where multiple bit-rate services and non-uniform signal power allocations are possible. This is of particular signiicance to next-generation systems where certain users with high data rates and demanding quality-of-service (QoS) requirements may form the dominant source of multiple-access interference (MAI) 2]. In addition, the analysis can be used to gauge P e performance of adaptive implementations of the MMSE receiver 5, 6]. To help facilitate the analysis, a secondary goal is …