Joint optimization for cell association and vertical downtilts adjustment in 3D MIMO enabled HetNets

This paper addresses the cell association issue in the downlink of 3-dimension (3D) multiple input multiple output (MIMO) enabled heterogeneous networks (HetNets). In order to serve specific users more efficiently, the downtilts of cell-center users and cell-edge users are configured through dynamic vertical beamforming. Therefore, it is necessary to consider the factor of downtilt in cell association for 3D MIMO enabled HetNets, so that the users are more likely to access to an appropriate cell. In this paper, a novel joint processing algorithm based on Lagrange dual decomposition (JP-LDD) is proposed, where cell association and downtilts adjustment are jointly optimized. The objective of JP-LDD algorithm is to maximize the spectral efficiency of HetNets and NP-hard cell association problem is transformed into a linear format to facilitate the analysis, with access control principle considered as well. Finally, simulation results show the superiority of JP-LDD algorithm in both performance of spectral efficiency and offloading effects in HetNets. Furthermore, the feasibility of its application in massive MIMO systems is also verified.