Multiuser detection for wideband CDMA communications

We are on the edge of the evolution from the second generation (2G) to the third generation (3G) wireless personal communication systems. In the coming 3G mobile communications systems, Code-Division Multiple Access (CDMA) has been adopted as the main air interface due to its high spectrum efficiency, soft handover and so on. However, because the users in CDMA systems transmit at the same time on the same frequency, the multiple access interference (MAI) and near-far problem will degrade the system performance significantly. Multiuser Detection (MUD) is a key technique for combating these problems. The optimal MUD proposed by Verdu is a Maximum-Likelihood detector whose complexity increases exponentially with the number of users. Therefore, many sub-optimal MUD receivers with lower complexity have been proposed.;In this thesis, we will concentrate on MUD for CDMA systems. Firstly, we will propose a new MUD scheme which is referred to as Objective Function based Multiuser Detection (OFMUD). It is based on the local optimization of two selected objective functions, the ML function and the MSE function. We will show that our proposed receiver can achieve the same or better performance than some popular MUD schemes with even less computational complexity. However, the proposed MUD scheme requires the information of all the active users and as such can only be used in the base station. For the mobile terminal, we will propose the use of the linear MMSE receiver which only requires the information of the desired user. However, the MMSE receiver has a major drawback; namely, the number of filter taps in the receiver is typically very long. Therefore, we will propose a novel reduced-rank MMSE receiver, which can reduce the receiver's complexity significantly while good performance is maintained. We will demonstrate that the proposed scheme far outperforms some previously proposed reduced-rank MMSE receivers with even fewer filter taps. Furthermore, we will consider the adaptive MMSE in CDMA systems with an antenna array. An efficient and simplified receiver structure is presented and correspondingly, an alternate adaptation method is proposed to jointly optimize the MMSE filter and the beamforming weights. Some direction of arrival (DOA) estimation methods for the antenna array will also be presented. Given that multirate transmission is one of the key drivers for the next-generation wireless systems, we will also evaluate the performance of two multirate schemes; namely, Variable Spreading Length (VSL-CDMA) and Multicode (MC-CDMA). With both an in-depth analysis and numerical results, we will show that MC-CDMA essentially has the same performance as VSL-CDMA with low-rate detection, but much better performance than VSL-CDMA with high-rate detection. Some insightful differences between single-rate and multirate systems will also be shown. In particular, we will prove that the linear receiver in multirate CDMA systems is no longer optimal in the sense of near-far resistance.