Rate adaptive modulation with MIMO mode switching between spatial multiplexing and diversity

Multiple-input multiple-output (MIMO) systems give high data rates through spatial multiplexing or diversity using space-time codes. We propose a rate-adaptive modulation scheme combined with mode switching between spatial multiplexing and orthogonal space-time block coding. We first find out which modulation level and mode maximize the spectral efficiency with a given target bit error ratio (BER). If the rates of the two MIMO modes are the same, we select one mode which gives lower BER than the other. We also propose a simple mode switching algorithm using effective SNR and constellation specific constant. Numerical results demonstrate improvement over each fixed mode with adaptive modulation in terms of average spectral efficiency.