Low complexity group detectors for multirate transmission in TD-CDMA 3G systems

In this paper, we address the problem of the downlink detection in a mobile radio system with hybrid time-division and code-division multiple access, wherein the deployment of orthogonal variable spreading factor codes allows for multirate communication. Accounting for a multipath propagation channel, we focus on the detection of data carried by a group of codes (ranging from a single code, to all active intracell codes) destined to the same mobile station and on the simultaneous rejection of the interference due to the complementary set of active intracell codes. We tackle both the problems of intersymbol interference (ISI) suppression, and multiple access interference (MAI) mitigation, and show when it is convenient to treat these issues together. Specifically, the feasibility of linear detectors based on the zero forcing and the minimum mean square error (MMSE) criterion is investigated, with the aim to mitigate both ISI and MAI. A unified low-complexity formulation based on a sliding window algorithm is proposed for this class of linear detectors. The numerical results validate the proposed receiver structure, indicating that the attainable performance is very close to the single user lower bound up to moderate system loads.