Precoding for Asymmetric MIMO-OFDM Channels

We study space-time precoding techniques for asymmetric multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) channels with more transmit than receive antennas which is a typical downlink setup. Under the condition of perfect channel state information (CSI) at the receiver but no CSI at the transmitter, we consider linear dispersion coding (LDC), groupwise space-time block coding (GSTBC), quasi-orthogonal space-time block coding (QSTBC), and spatial spreading (SS) and study their capacity and diversity performances. Our results show that for both ergodic and outage capacities, SS is worse than LDC, GSTBC and QSTBC, while the latter three have very close/same performance. We also show that by adjusting the phase shift values in the SS matrices, its outage capacities can be improved. In terms of diversity, LDC, GSTBC and QSTBC have both transmit and receive spatial diversity, and LDC´s diversity order is slightly higher than the latter two. SS, on the other hand, has not only receive spatial diversity, but also introduces additional frequency diversity. This contributes to their very close bit error rate (BER) performance when turbo processing is deployed at the receiver. Considering the implementation flexibility and complexity, we therefore recommend SS and GSTBC as the promising precoding schemes for asymmetric MIMO-OFDM channels.