Energy Efficient Hybrid Precoding in Heterogeneous Networks with Limited Wireless Backhaul Capacity

This paper investigates a two-tier heterogeneous networks (HetNets) with wireless backhaul, where millimeter wave (mmWave) frequency is adopted at the macro base station (MBS), and the cellular frequency is considered at small cell BS (SBS) with orthogonal frequency division multiple access (OFDMA). Subarray structure based hybrid analog/digital precoding scheme is investigated to reduce the hardware cost and energy consumption. Our goal is to maximize the energy efficiency (EE) of the HetNets with limited wireless backhaul capacity and all users’ quality of service (QoS) constraints. The formulated problem is non-convex mixed integer nonlinear fraction programming (MINLFP), which is non-trivial to solve directly. In order to circumvent this issue, we propose a two-loop iterative resource allocation algorithm. Specifically, we transform the outer-loop problem into a difference of convex programming (DCP) by employing integer relaxation and Dinkelback method. In addition, the first-order approximation is considered to linearize this inner-loop DCP problem into a convex optimization framework. Lagrange dual method is adapted to achieve the optimal closed-form power allocation. Furthermore, we analyze the convergence of the proposed iterative algorithm. Numerical results are presented to demonstrate our proposed schemes