Beamforming Utilizing Channel Norm Feedback in Multiuser MIMO Systems

Abstract: Cellular wireless communication like GSM and WLAN has become an important part of the infrastructure. The next generation of wireless systems is believed to be based on multiple-input multiple-output (MIMO), where all units are equipped with multiple antennas. In contrast to the single antenna case, MIMO systems may exploit beamforming to concentrate the transmission in the direction of the receiver. The receiver may in turn use beamforming to maximize the received signal power and to suppress the interference from other transmissions. The capacity of a MIMO system has the potential of increasing linearly with the number of antennas, but the performance gain is limited in practice by the lack of channel information at the transmitter side. This thesis considers downlink strategies where the transmitter utilizes channel norm feedback to perform beamforming that maximizes the signal-to-noise ratio (SNR) for a single beam. Two optimal strategies with feedback of, either the channel squared norm to each receive antenna, or the maximum of them are introduced and analyzed in terms of conditional covariance, eigenbeamforming, minimum mean-square error (MMSE) estimation of the SNR and the corresponding estimation variance. These strategies are compared under fair conditions to the upper bound and strategies without feedback or with pure SNR feedback. Simulations show that both strategies perform well, even if spatial division multiple access (SDMA) is required to exploit the full potential. The beamforming strategies are generalized to the multiuser case where a scheduler schedule users in time slots in which their channel realization seems to be strong and thereby support high data rates. The gain of exploiting multiuser diversity is shown in simulations. The thesis is concluded by a generalization to a multi-cell environment with intercell interference. Optimal and suboptimal receive beamforming is analyzed and used to propose approximate beamforming strategies based on channel norm feedback.