Precoding of Full-Rate Full-Diversity STBCs with Covariance Feedback

Full-rate full-diversity (FRFD) space-time block codes (STBCs), which are originally designed for independent and identically distributed (i.i.d.) multiple-input multiple-output (MIMO) channels, may suffer from severe performance degradation in the presence of channel correlation. A feasible solution is to adopt a linear precoder to match the eigen beams of these codes with that of the channels. The precoder design addressed in the literature is applicable only to the cases of orthogonal/quasiorthogonal STBCs in semi-correlated MIMO channels, where the minimum codeword difference matrices (CDM) are either equal, or reducible, to the identity matrix. A general FRFD STBC, however, can have multiple CDMs which, when interacting with a general doubly correlated MIMO channel of covariance matrix feedback, call for a new design criterion for precoding which is the focus of this paper. Furthermore, a modified sphere decoder is also proposed to resolve the problem of degenerated equivalent channel matrix caused by the precoding at low SNRs. Numerical results are presented to demonstrate the superior performance of the resulting optimal precoder when used alongside various real- and complex-rotation based FRFD STBCs.