Design of near-orthogonal LD space-time codes

This work considers a wireless multiinput-multioutput (MIMO) communication system in a frequency-nonselective scenario with spatially uncorrelated Rayleigh fading channel coefficients and investigates the design of linear dispersion (LD) space-time block codes. By optimizing the constituent weight matrices so that an upper bound on the union bound of the codeword error probability is minimized, good LD codes are obtained regardless of transmit antenna array size and code rate. We make the interesting observation that the proposed design procedure automatically generates LD codes that either correspond to, or are close to, the well-known class of orthogonal space-time block (OSTB) codes. A theoretical analysis confirms the correctness of the observation by proving that OSTB codes are indeed optimal, when the setup under study permit their existence. Simulation results demonstrate the excellent performance of the designed codes. In particular, the importance of the codes´ near-orthogonal property is illustrated by showing that low-complexity linear equalizer techniques can be used for decoding purposes while incurring a relatively small performance loss compared with optimal maximum likelihood (ML) decoding.