Compensation of channel state estimation errors in adaptive MIMO-OFDM systems

The knowledge of channel state information (CSI) at the transmitter (TX) can dramatically increase the spectral efficiency of a multiple-input multiple-output (MIMO) system. In the case of time division duplex (TDD) systems, CSI can be easily obtained due to radio channel reciprocity. However, perfect CSI is too strong an assumption for a wireless system. The paper presents a robust link adaptation method for TDD systems employing MIMO-OFDM channel eigenmode based signalling. The scheme takes into account channel estimation errors at the TX side, and, by analyzing the noise enhancement at the equalizer output, we propose a simple method to maintain a constant frame error rate (FER) by controlling the instantaneous transmitted power in such a way that the average SNR at the equalizer output is kept constant. Simulation results show that the target FER can he maintained even in the presence of CSI errors; promising results were obtained by using a low complexity LS channel estimator at the TX side and a linear equalizer at the receiver. Thus, the proposed scheme constitutes a practical solution for high data rate wireless applications.