Optimal Load Balancing in Millimeter Wave Cellular Heterogeneous Networks

In this paper, we propose a novel and effective approach to optimizing the load balancing in a millimeter wave (mmWave) cellular heterogeneous network (HetNet) with a macro-tier and a micro-tier. The unique characteristics of mmWave transmission are incorporated into the network by adopting the Poisson point process (PPP) for base station (BS) location, the line-of-sight (LoS) ball model for mmWave links, the sectored antenna model for key antenna array characteristics, and Nakagami-m fading for wireless channels. To reduce the load of macro-tier BSs, we consider a bias factor As in the network for offloading user equipments (UEs) to micro-tier BSs. For this network, we first analyze the loads of macroand micro-tier BSs. Then we derive a new expression for the rate coverage probability of the network, based on which the optimal As maximizing the rate coverage probability is found. Through numerical results, we demonstrate the correctness of our analysis and the validity of the optimal As. Importantly, the optimal As can bring a profound improvement in the rate coverage probability relative to a fixed As. Furthermore, we evaluate the impact of various network parameters, e.g., the densities and the beamwidths of BSs, on the rate coverage probability and the optimal As, offering valuable guidelines into practical mmWave HetNet design.