Outage Performance Analysis and Downlink Beamforming Designs for Multi-User Massive MIMO Wireless Communication Systems

Abstract

In recent years, there has been great interest in the multi-user multiple input multiple output (MIMO) wireless communication systems with large antenna arrays at the base station (BS). The concept and technology of massive MIMO systems have emerged to be one of key concepts for the next generation wireless communication systems. Compared to the existing wireless communication technologies, the massive MIMO system equipped with a large number of BS antennas could potentially allow for orders of magnitude improvements in spectral efficiency and radiated energy efficiency while using relatively simple linear processing. Asymptotic analytical results show that uncorrelated noise and small-scale fading effects can be eliminated as the number of antennas of a massive MIMO system increases to infinity. And simple linear signal processing approaches can be used inmassive MIMO systems to achieve these advantages. Moreover, the technology of beamforming could be utilized to transmit signals only in the desired direction in order to minimize interference and concentrate the radiated energy as the number of antennas at the BS increases.In this dissertation, first we analyze the outage probability performance of down-link multi-user massive MIMO wireless systems over flat-fading channels in which a matched filter (MF) is applied at the base station (BS). We investigate the statistics of the SINR of each user, and further derive a closed-form expression for the outage probability of the massive MIMO system. Based on the analysis of the exact outage probability derivations, we also provide a high-SNR approximation for the outage probability performance and develop an error floor analysis for the equal-power transmission scenario. The theoretical results characterize the performance of the multi-user massive MIMO systems in terms of the number of antennas equipped and the number of users served in the system. Extensive numerical studies are provided to illustrate and validate the theoretical derivations.Secondly, we consider beamforming designs for the down-link multi-user massive MIMO system within one cell which has one BS equipped with M antennas and serves K single-antenna users. We consider a power consumption model with a practical power amplifier (PA) and power limits of the transmitter radio-frequency (RF) chains, and develop a problem formulation of finding the optimal beamforming matrix which maximizes the minimum SINR value for all users in the system. Based on the problem formulation, we further provide analysis and discussions on the optimal beamforming and power constraints, and evaluate the results on a set of sub-optimal linear beamforming designs. Unlike the conventional problem formulation considering only the average total power consumption, the proposed problem formulation considers both the PA power limit constraint for each transmit antenna and the average total power consumption constraint. Our analysis shows that the conventional constraint of total power consumption is only a necessary but not sufficient condition for the practical beamforming designs. We also show that the power constraint could be further simplified when the upper and lower bounds of the average total power consumption are considered. Moreover, we show that, unlike the conventional beamforming problem formulation in which the resulting SINR values are equal when the solution achieves its optimal value, the statement is not true for the proposed new problem formulation for beamforming designs.Finally, an optimal power and time assignment strategy is developed for hybrid automatic-repeat-request (H-ARQ) communication protocol over quasi-static Rayleigh fading channels. For any given average total time duration and energy budget, we try to find the sequences of power values and time durations for H-ARQ retransmission rounds that minimize the outage probability of the H-ARQ protocol. We solve the joint optimization of power and time duration assignment for the H-ARQ protocol and derive a set of equations that describe the optimal transmission power and time duration sequence which enable a recursive calculation. The complexity of the recursive algorithm is fixed no matter what the maximum number of retransmission rounds L is. For comparison purpose, we also derive the optimal power assignment for the H-ARQ protocol with equal-time retransmissions. Numerical results show that the performance of the proposed optimal-time and optimal-power assignment scheme is substantially better than that of the conventional equal-time and equal-power scheme as well as the equal-time and optimal-power scheme.