Abstract
Massive multiple-input multiple-output (MIMO) technology is considered as one of the enabling technologies to scale up the data rates for the future communication systems. Traditional MIMO systems employ digital beamforming where each antenna element is equipped with one radio frequency (RF) chain. When the number of the antennas are scaled up, the cost and power consumption of massive MIMO systems also increase significantly. Recently, hybrid analog-and-digital beamformers have attracted a lot of attention as a cost effective approach to benefit from the advantages of massive MIMO. In hybrid structure, a small number of RF chains are connected to a large number of antennas through a network of phase shifters. The optimal hybrid beamforming problem is a complex nonconvex optimization due to the nonconvex constraint imposed by phase shifters. The overall objective of this thesis is to provide simple and effective hybrid beamforming solutions for narrowband point-to-point and multiuser massive MIMO scenarios. Firstly, hybrid beamforming problem for a point-to-point communication system with perfect channel state information (CSI) is investigated, and an effective codebook based hybrid beamforming with low resolution phase shifters is proposed which is suitable for sparse scattering channels. Then, by leveraging the properties of massive MIMO, an asymptotically optimal hybrid beamforming solution as well as its closed-form formula will be presented. It will be shown that the proposed method is effective in both sparse and rich scattering propagation environments. In addition, the closed-form expression and lower-bounds for the achievable rates are derived when analog and digital phase shifters are employed. Secondly, hybrid beamforming problem to maximise the total sum-rate for the downlink of multiuser MIMO is investigated, and an effective solution as well as its closed-form expression for this system is proposed. The presented solutions for the single-antenna and multiantenna scenarios are shown to be effective as they can achieve a similar sum-rate as digital beamforming can reach. In addition, it is shown that the proposed technique with low-cost low resolution phase shifters at the RF beamformer demonstrates a comparable performance to that of a hybrid beamformer with an expensive analog beamformer. Finally, two novel hybrid beamforming techniques are proposed to reduce the power consumption at the RF beamformer. Defining a threshold level, it is shown that half of the phase shifters could be turned off without a performance loss when the wireless channel matrix is modeled by Rayleigh fading. Then, we reduce the number of the phase shifters by using a combination of phase shifters and switches at the RF beamformer. The proposed methods can significantly reduce the power consumption as switches, in general, have lower power consumption compared to phase shifters. It is noted that the presented algorithms and the closed-form expressions of their performance are derived by using the asymptotic properties of the elements of the singular vectors for the rich scattering channel matrix.