Complete Characterization of the Equivalent MIMO Channel for Quasi-Orthogonal Space-Time Codes

摘要

Recently, a quasi-orthogonal space-time block code (QSTBC) capable ofachieving a significant fraction of the outage mutual information of amultiple-input-multiple output (MIMO) wireless communication system for thecase of four transmit and one receive antennas was proposed. We generalizethese results to $n_T=2^n$ transmit and an arbitrary number of receive antennas$n_R$. Furthermore, we completely characterize the structure of the equivalentchannel for the general case and show that for all $n_T=2^n$ and $n_R$ theeigenvectors of the equivalent channel are fixed and independent from thechannel realization. Furthermore, the eigenvalues of the equivalent channel areindependent identically distributed random variables each following anoncentral chi-square distribution with $4n_R$ degrees of freedom. Based on these important insights into the structure of the QSTBC, we derivean analytical lower bound for the fraction of outage probability achieved withQSTBC and show that this bound is tight for low signal-to-noise-ratios (SNR)values and also for increasing number of receive antennas. We also present anupper bound, which is tight for high SNR values and derive analyticalexpressions for the case of four transmit antennas. Finally, by utilizing thespecial structure of the QSTBC we propose a new transmit strategy, whichdecouples the signals transmitted from different antennas in order to detectthe symbols separately with a linear ML-detector rather than joint detection,an up to now only known advantage of orthogonal space-time block codes (OSTBC).