Adaptive MIMO-OFDM with low signalling overhead for unbalanced antenna systems

The knowledge of channel state information (CSI) at the transmitter (TX), which in case of time division duplex (TDD) can be easily obtained due to radio channel reciprocity, can dramatically increase the spectral efficiency of a multiple-input multiple-output (MIMO) system. This paper presents a robust link adaptation method for TDD systems employing MIMO-OFDM eigenmode based signalling. We propose a rather simple logarithm-free bit and power loading algorithm which requires low signalling overhead. For unbalanced MIMO systems with larger number of transmit than receive antennas, the achieved spectral efficiency at low and medium SNR is higher than the outage MlMO capacity with unknown CSI at the TX. The simulation results show that for a constant frame error rate, the throughput degradation comparing to the universally accepted Hughes-Hartogs algorithm is negligible.