A New Heuristic Algorithm for Energy and Spectrum Efficient User Association in 5G Heterogeneous Networks

Macro base stations are densely overlaid by small cells to satisfy the demands of user equipments in heterogeneous networks. Due to their dense deployment, some small cells are not directly connected to macro base stations and thus backhaul connections are required to connect small cells to macro base stations. Millimeter wave backhauls which have high bandwidths are preferred for small cell backhaul communication, since they can increase the capacity of network considerably. In this context, association of user equipments to base stations becomes challenging due to the backhaul architecture. Considering environmental concerns, energy efficiency is a vital criterion in designing the user association algorithms. In this paper, we study the user association problem aiming at the maximization of energy efficiency given a specific spectral efficiency target. Firstly, we derive a quadratic upper bound on the total power consumption in heterogeneous network (access network and backhaul). Then we develop an energy and spectrum efficient user association method based on minimizing the derived quadratic upper bound, which indeed turns our target problem into a generalized quadratic assignment problem. Since this problem is NP-hard in general, we propose a heuristic algorithm for solving it. Extensive simulations show that our approach provides an enhanced level of energy efficiency compared to other well-known alternatives in the literature.