Space-time multiplexing for MIMO multiuser downlink channels

In this paper, we study the downlink of a multiuser system, in which antenna arrays are employed at both the transmitter (base station) and the receivers (clients). A space-time modulation technique that can be seen as two-dimensional spreading is introduced. It provides full transmit diversity for every user, and accommodates N-t times the number of users as a single-antenna code-division multi-access (CDMA) scheme, where Nt is the number of transmit antennas. Thus multiple access is provided through spatial as well as code dimensions. In addition, the scheme forms groups of users that are orthogonal to each other. This feature translates into simplified detection strategies without loss of performance. The main detector structure of interest is a two-stage interference canceller because of its low complexity compared to other joint detectors. We will demonstrate that in conjunction with an unequal power allocation scheme, this receiver provides full diversity and suffers from only a small performance loss compared to the full-complexity maximum likelihood (NIL) receiver. In a single-user multiple antenna system, the same spreading scheme and unequal power allocation yields a new approach to designing full-rate, full-diversity space-time codes having good performance with successive interference cancellers.