Hybrid beamforming for millimeter-wave massive MIMO heterogeneous networks

Abstract

In this paper, we investigate the hybrid beamforming designs in millimeter wave (mmWave) massive MIMO relaying heterogeneous networks (HetNet), where small cells are overlaid onto the service area of a macrocell. The deployment of HetNet and utilization of the adequate bandwidth in the mmWave band are capable of addressing the explosive growth of data rates in next-generation wireless networks. However, the severe path loss and attenuation in the mmWave band and complex interference in HetNet should be addressed. For the macrocell, we propose a minimum mean squared error (MMSE)-based alternative iterative hybrid analog/digital beamforming algorithm for the downlink multi-user mmWave relaying system. With the constant modulus constraints and the higher-order polynomial function, we reformulate the optimization problem as three convex quadratic subproblems, and the coupling approximate optimal precoders and combiners are jointly obtained by the proposed alternative iterative algorithm. For the picocells, we propose a hybrid beamforming algorithm for downlink mmWave massive MIMO system. An alternative iterative optimization algorithm based on manifold optimization is developed to approximate the optimal beamforming designs. The baseband digital beamforming scheme is investigated by leveraging the mean square error (MSE) criteria for macrocell users and picocell users to manage cross-tier and inter-user interference. Simulation results demonstrate the obvious advantages of the proposed hybrid beamforming algorithms in terms of achievable sum-rate for various setting scenarios.