Reduced Complexity Iterative Multiuser Detector for beyond 3 G CDMA Systems

A reduced complexity multiuser detector for DSCDMA is proposed for beyond 3G (B3G) systems with large bandwidth. We first argue that the resulting increase in intersymbol interference and data rate in B3G systems makes the computational load of multiuser receivers very high. Two solutions are proposed to reduce complexity. First, a frequency domain implementation of the RAKE receiver is described. Then, iterative methods for solving linear systems are used to reduce the complexity of the MMSE multiuser detector. Simulation results show that the proposed schemes achieve similar performance than a receiver with exact MMSE matrix inversion. I. I NTRODUCTION Emerging wireless services will demand higher data rates at lower cost, which in technological terms translates into more bandwidth, higher spectral efficiency, and reduced complexity terminals. This presents a major challenge to system designers, since each technology (e.g. TDMA, CDMA, or OFDM) has different tradeoffs in these terms. This paper addresses the design of an uplink multiple access scheme based on direct sequence code division multiple access (DS-CDMA) [1]. CDMA has been shown to have the potential to achieve the capacity of multiple access channels, albeit at the cost of higher complexity than TDMA and FDMA. The main challenge for DS-CDMA is dealing with a channel response spanning many symbols, resulting from the combination of large transmission bandwidth and channel time dispersion. The high bandwidth and corresponding high resolution of the receiver envisioned for beyond 3G (B3G) systems raise questions about the feasibility of the Rake receiver, since with bandwidths of 20-100 MHz [2], the signal energy is likely to be dispersed among many resolvable paths, and many Rake fingers may be needed. An alternative solution being proposed for this problem is a frequency domain (FD) Rake [3] which performs maximal ratio combining (MRC) in the frequency domain. Besides the matched filter stage, implemented with a time or frequency domain Rake, multiuser detection will be required in order to achieve high spectral efficiency. The fundamental challenge of multiuser detection for B3G systems is that delay spread creates interference among several symbol intervals, resulting in very large correlation matrices. In order to make the MMSE receiver feasible, iterative methods for the solution of linear systems are proposed [4]. The implementation of successive over-relaxation (SOR) and the Chebishev method show that, in both cases, results close to exact matrix inversion are achieved in a few iterations. The outline of this paper is as follows. First in Section II we describe continuous and block transmission schemes suitable for time domain (TD) and FD implementations of the Rake receiver, respectively. The reduced complexity receiver is described in Section III. The performance of the proposed reduced complexity receivers is evaluated in Section IV, and, finally, Section V contains the conclusions. II. SYSTEM MODEL Two transmission schemes are considered: continuous transmission (CT) and block transmission (BT). These two schemes are introduced in order to consider processing in time domain (TD) and frequency domain (FD), respectively. For continuous transmission,