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


Runhua Chen, Robert W. Heath Jr., and Jeffrey G. Andrews


Accepted subject to minor revisions to IEEE Transactions on Signal Processing, in January 2005.


Multiuser spatial multiplexing is a downlink transmission technique that uses linear transmit precoding to multiplex multiple users and pre-cancel inter-user 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 inter-user 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 ser MIMO methods and static allocation methods. For example, the proposed techniques achieve an SNR improvement of 6-10 dB at an uncoded SER of $10^{-3}$, with only one extra transmit antenna.

This preprint is available in .IEEE Xplore format.