User association in energy cooperation enabled HetNets with renewable energy powered base stations

In the conventional cellular network, each base station (BS) manages its own energy consumption. When a BS lacks sufficient energy, user equipments (UEs) have to be offloaded to the other BSs which leads to higher signaling cost and lower throughput. Energy cooperation schemes allow energy to be transferred among BSs through power grids which reducing the frequency of UEs offloading due to the lack of sufficient energy. In this paper, we study the user association (UA) problem with energy cooperation in heterogeneous networks (HetNets) where BSs are powered by renewable energy sources. We formulate the UA problem as an optimization problem that seeks to maximize the throughput while allowing energy to be exchanged between BSs. We implement a two-step approach to solve the UA problem. First we use a gradient descent method to achieve the optimal UA solution. Then we use a heuristic algorithm that determines the exchange of energy between BSs based on the acquired UA scheme. We compare the performance of our UA scheme with two other schemes namely, reference signal received power-based UA and gradient descent UA without energy cooperation. Our simulation results indicate that our UA scheme with energy cooperation could improve the throughput and increase the number of accepted UEs of the whole network.