Multiuser detection for CDMA antenna array receivers using spatial equivalence classes

Abstract

The reverse link (mobile to base) of a cellular direct-sequence code-division multiple-access system is considered with the aim of increasing system capacity while avoiding system complexity. The capacity of a conventional (matched filter) receiver can be increased by employing sectorised antenna arrays or multiuser detection techniques. Results confirm that the potentially complex approach of beamforming followed by multiuser detection further increases capacity when the uplink transmissions are asynchronous for both additive white gaussian noise channels and Rayleigh fading multipath channels. A method by which the computational cost of multiuser detection applied after beamforming can be significantly reduced without greatly affecting performance is proposed. Bit detection techniques based on partitioning the users into spatial equivalence classes by beamforming and then employing standard multiuser detection techniques within each class are investigated. Classes of users can be determined so that multiple-access interference terms corresponding to users in different classes are of little significance. These terms can be discarded before applying multiuser detection, thus reducing computation.