On the construction of high-rate quasi-orthogonal STBC for MIMO QR demodulation

Abstract

Orthogonal space-time block codes (STBCs) [1], [2] appear to be a very fascinating means of enhancing reception quality in quasistatic MIMO channels due to their full diversity, and especially their simple maximum-likelihood (ML) decoders. However, full-rate full-diversity orthogonal STBCs do not exist for more than two transmit antennas. Vertical layered space-time architecture (so-called the V-BLAST) [3] with a nulling- and cancelling-based detection algorithm, in contrast, has an ability of achieving high transmission rates at the cost of having very low diversity gain, an undesirable consequence caused by the interference nulling and cancelling processes. The uncoded V-BLAST system is able to reach its ML performance with the aid of the sphere decoder algorithm [4] at the expense of higher detection complexity. Undoubtedly, the tradeoff between transmission rates, diversity, and complexity is inherent in designing space-time codes. This paper investigates a method to increase the "nulling diversity gains" for a general high-rate space-time code and introduces a new design strategy for high-rate space-time codes detected based on interference nulling and cancelling processes, thanks to which high-rate quasi-orthogonal space-time codes for MIMO applications are proposed. We show that when n(T) transmit and n(R) = n(T) receive antennas are deployed, the first code offers a transmission rate of (n(T) - 1) with a minimum nulling diversity order of 3, whereas the second one offers a transmission rate of (n(T) - 2) with a minimum nulling diversity order of 5. Therefore, the proposed codes significantly outperform the V-BLAST as n(R) = n(T). Simulation results and discussions on the performance of the proposed codes are provided.