The upcoming 5G network needs to achieve substantially larger link capacity and ultra-low latency to support emerging mobile applications. While conventional techniques have reached their limits, uplifting the carrier frequency to the millimeter wave (mm-wave) band stands out as an effective approach to further boost the network capacity, as it provides orders of magnitude greater spectrum than current cellular bands. Large-scale antenna arrays are needed to fully exploit the performance gains of mm-wave communications, which, however, brings formidable challenges to performance analysis, algorithm design, and hardware implementation. Conventional fully digital precoding techniques are inapplicable, as they require a separate radio frequency (RF) chain for each antenna element....[ Read more ]

The upcoming 5G network needs to achieve substantially larger link capacity and ultra-low latency to support emerging mobile applications. While conventional techniques have reached their limits, uplifting the carrier frequency to the millimeter wave (mm-wave) band stands out as an effective approach to further boost the network capacity, as it provides orders of magnitude greater spectrum than current cellular bands. Large-scale antenna arrays are needed to fully exploit the performance gains of mm-wave communications, which, however, brings formidable challenges to performance analysis, algorithm design, and hardware implementation. Conventional fully digital precoding techniques are inapplicable, as they require a separate radio frequency (RF) chain for each antenna element. Hybrid precoding was recently proposed as a cost-effective alternative, which requires a small number of RF chains and thus can significantly reduce the hardware cost and power consumption. In this thesis, we investigate different hybrid precoder structures and corresponding design methodologies.

First, by leveraging a single RF chain, analog beamforming serves as an initial solution, where the direction of the beam can be adjusted via a low-complexity phase shifter network. While the optimal analog beamforming strategy can be readily determined, it is difficult to characterize the performance of mm-wave networks with analog beamforming, due to complex intercell interference. Using tools from stochastic geometry, we propose an analytical framework for performance analysis of mm-wave networks with arbitrary antenna patterns, based on which a comprehensive investigation on the impact of directional antenna arrays in mm-wave networks is carried out. In particular, it is shown that the coverage probabilities of mm-wave networks increase as a non-decreasing concave function with the antenna array size.

To further improve the spectral efficiency, we then consider hybrid precoding which utilizes multiple RF chains and therefore supports spatial multiplexing. We first design the hybrid precoder with the state-of-the-art hardware implementation, called the single phase shifter (SPS) implementation, via non-convex optimization. Two alternating minimization algorithms based on manifold optimization and semidefinite relaxation (SDR) are proposed for the fully- and partially-connected structures, respectively. Simulation results validate that the proposed algorithms achieve much higher spectral efficiency than existing ones.

To reduce the computational complexity of hybrid precoding, a convex relaxation approach is then proposed. In this approach, the unit modulus constraint, which is the main difficulty in hybrid precoding, is relaxed as a convex one. In this way, the hybrid precoder design problem turns out to be a low-rank matrix approximation problem and an eigenvalue problem for the two typical structures, which yields very efficient hybrid precoding algorithms. Moreover, we propose a novel double phase shifter (DPS) implementation based on which the relaxed precoder can be implemented in practice.

Finally, an innovative hybrid precoder structure is introduced, namely, the fixed phase shifter (FPS) group-connected structure, which achieves a higher hardware efficiency than existing ones and also offers a flexible trade-off between spectral efficiency and hardware efficiency. It only requires a small number of phase shifters with quantized and fixed phases, and thus significantly reduces the cost and power consumption. Based on the proposed FPS group-connected structure, an effective alternating minimization algorithm is developed to design the hybrid precoder.