Space-time precoding for asymmetric MIMO channels

We study space-time precoding techniques for asymmetric multiple-input multiple-output (MIMO) channels with more transmit than receive antennas which is a typical downlink setup. Under the condition that channel state information (CSI) is perfectly known at the receiver, but not known at the transmitter, we consider groupwise space-time block coding (GSTBC), quasi-orthogonal space-time block coding (QSTBC), linear dispersion coding (LDC), and spatial spreading (SS). We show that for narrow-band i.i.d. circularly symmetric complex Gaussian (CSCG) MIMO channels, SS has the lowest ergodic capacity, while GSTBC, QSTBC and LDC have almost the same ergodic capacity. We then compare their input-output mutual information when fixed-order modulation is deployed and show that the same performance order maintains. Finally we look into their bit error rate (BER) performance for uncoded systems, and show that LDC slightly outperforms GSTBC and QSTBC in high SNR regions and below BER of 10^-2, and SS has the worst performance. Due to the good flexibility for various asymmetric channels and its simple implementation, we therefore recommend GSTBC as the asymmetric MIMO precoding scheme