Transmitter and receiver designs for the MIMO fading broadcast channel

We study the downlink of a MIMO multi-user system when no information about the channel is assumed at the transmitter. Instead of applying direct-sequence code-division multiple access (DS-CDMA) over the codewords of conventional space-time coding (STC) schemes, a modulation technique that can be seen as two-dimensional space-time spreading (2D-STSC) is described. It is based on well-known Walsh codes, provides full transmit diversity and high spectral efficiency, and produces groups of users that are orthogonal to each other. This last point translates into simplified detection strategies without loss of performance. The main detector structure of interest is a two-stage interference canceller which employs serial interference cancellation (SIC) in the first stage. We will demonstrate that in conjunction with an unequal power allocation scheme, this receiver is able to provide full diversity and suffers from only a small performance loss compared to the full-complexity maximum likelihood receiver. The proposed scheme compares favorably with related ones in terms of spectral efficiency, bit error probability, and complexity at the receiver.