Transmit Selection Diversity for Unitary Precoded Multiuser Spatial Multiplexing Systems With Linear Receivers

Multiuser spatial multiplexing is a downlink transmission technique that uses linear transmit precoding to multiplex multiple users and precancel interuser interference. In such a system, the spatial degrees of freedom are used for interference mitigation and generally come at the expense of diversity gain. This paper proposes two precoding methods that use extra transmit antennas, beyond the minimum required, to provide additional degrees of diversity. The approach taken is to solve for a unitary transmit precoder, under a zero interuser interference constraint, that minimizes an upper bound on the symbol error rate (SER) for each user. Solutions where all transmit antennas are employed, as well as subsets of antennas (to reduce analog components), are described. Numerical results confirm a dramatic improvement in terms of SER and mutual information over single-user multiple-input multiple-output (MIMO) methods and static allocation methods. For example, the proposed techniques achieve a signal-to-noise ratio (SNR) improvement of 6-10 dB at an uncoded SER of 10^-3, with only one extra transmit antenna