MIMO transmission over nonlinear fading channels: A study of the effects of amplifier nonlinear distortions on space-time codes and MIMO beamforming systems

MIMO systems using multiple transmit and receive antennas on both ends of linear wireless communication links are by now well studied. Several axes of these schemes have been explored with the underlying MIMO channel assumed linear. However, when high-power amplifiers (HPA) operating near their peak efficiency operating points, or saturation points, are employed in the communication chain, non-linear distortions are introduced in the transmitted signals, and the resulting MIMO channel will be non-linear. In this thesis, we investigate the effect of Non-linearity in the MIMO communication channel. In order to gain in-depth understanding of the problem, two case studies have been considered for this investigation. In the first study, MIMO Beamforming (MIMO-MRC) system is used. New results on the symbol error rate performances of several M-QAM constellations in linear and non-linear MIMO channels are obtained, and system performance degradations owing to non-linearity in the MIMO channels are reported. In the second part of the study, Space-Time coding system is considered. Both the pairwise error probability (PWEP) and the bit error probability (BEP) performance of the system are evaluated over linear and non-linear MIMO channels. The effect of HPA non-linearity on the error rate performance of space-time coding system is then reported. The results show that for any MIMO configuration, performance degradation due to non-linearity in the channel reduces as the fading gets more severe, and for a particular fading channel, the degradation increases as the MIMO dimension is increased. This observation is consistent for both the MIMO Beamforming and the Space-time coding systems. Optimum operating points for non-linear amplifiers in MIMO channels are then derived.; These operating points present excellent trade-offs between amplifier efficiency and performance degradations due to HPA non-linearity. Then we studied the impact of HPA nonlinearity on the performance of multiuser MIMO system. It is similarly found that the diversity benefit inherent in such system is heavily degraded when the transmitted codewords are nonlinearly amplified. Next we investigate the use of MIMO subset antenna selection, and it is shown that the deployment of this scheme over Nonlinear MIMO channel helps cut back on the performance deterioration due to HPA nonlinear distortions. Finally we examine HPA nonlinearity compensation techniques suitable for space-time codes, and we illustrate the effects of such schemes on the coding and diversity gain performance of the system.