Outage Constrained Robust Hybrid Coordinated Beamforming for Massive MIMO Enabled Heterogeneous Cellular Networks

Abstract

Heterogeneous network (HetNet), employing massive multiple-input multiple-output (MIMO), has been recognized as a promising technique to enhance network capacity, and to improve energy efficiency for the fifth generation of wireless communications. However, most existing schemes for coordinated beamforming (CoBF) for a massive MIMO HetNet unrealistically assume the availability of perfect channel state information (CSI) on one hand, and cascade of each antenna with a distinct radio-frequency chain in massive MIMO is neither power nor cost-efficient on the other hand. In this paper, we consider a massive MIMO-enabled HetNet framework, consisting of one macrocell base station (MBS) equipped with an analog beamformer, followed by a digital beamformer, and one femtocell base station (FBS) equipped with a digital beamformer. In the presence of Gaussian CSI errors, we propose a robust hybrid CoBF (HyCoBF) design, including an analog beamforming design for MBS, and a digital CoBF design for both MBS and FBS. To this end, an outage probability-constrained robust HyCoBF problem is formulated by minimizing the total transmit power. The analog beamforming mechanism at MBS is a newly devised low-complexity beam selection scheme by selecting analog beams from a discrete Fourier transform matrix codebook. Then, a conservative approximate CoBF solution is obtained via semidefinite relaxation and an extended Bernsteintype inequality. Furthermore, a distributed implementation for the obtained CoBF solution using alternating direction method of multipliers is proposed. Finally, numerical simulations are provided to demonstrate the efficacy of the proposed robust HyCoBF algorithm.

Keywords

• Heterogeneous network (HetNet)
• massive MIMO
• hybrid coordinated beamforming (HyCoBF)
• semidefinite relaxation (SDR)
• alternating direction method of multipliers (ADMM)