Space-time multiplexing for multi-antenna channels

Space-time multiplexing or linear dispersion coding spreads transmit signals across spacetime dimensions and linearly multiplexes spread signals at the transmitter. It subsumes various linear signaling techniques including Bell Laboratory Layered Space-Time (BLAST) architecture and spacetime block codes. In this paper, we present space-time multiplexing from a signal processing perspective that views design of space-time multiplexing as that of multiple access in space-time dimensions. The structure of symbol space-time spreading codes significantly affects symbol separability and diversity-rate trade-off intrinsic to multi-antenna systems. It is shown that BLAST and space-time block codes represent two extremes. The former separates symbols in spatial dimension to achieve high rate, while the latter uses temporal dimension to construct orthogonal spreading codes with full diversity but with large rate penalty. Efficient exploitation of spatial and temporal dimensions calls for a balance of diversity and rate. We propose such an approach for transmit diversity utilization - layered transmit diversity - that combines spatial and temporal dimensions for symbol separation and is able to operate very close to channel capacity.