Energy-Efficient Resource Allocation Scheme in Hybrid Heterogeneous Networks with mmWave Base Stations

In this paper, we study the problem of joint resource allocation and user association in a hybrid heterogeneous network (HetNet) with the coexistence of sub-6-GHz base stations (BSs) and millimeter wave (mmWave) BSs with different channel conditions. mmWave communications are characterized by many propagation problems and low power efficiency. In addition, the coexistence with sub-6GHz BSs can become challenging due to different channel characteristics. To tackle this problem, we formulate a joint optimization problem to maximize the energy efficiency of all users in a cell. The optimization problem formulated is a non-linear binary fractional programming problem and is proved to be NP-hard. Accordingly, we divide the optimization problem into two sub-problems; power allocation and user-base station association. We transform the sum-of-ratio form of the power allocation sub-problem into a subtractive form optimization problem and then closed-form optimal powers are calculated through a two-layer optimization. The user association sub-problem is solved using a graph based method. Simulation results show an increment in the energy efficiency of the proposed algorithm with an improved reduction in computational complexity.