I. Introduction
Heterogeneous Networks (HetNets) consist of low power base stations (BSs) such as pico cells and femto cells deployed to operate in same region as the traditional macro cellular infrastructure [1]. These small cells increase the capacity due to better spatial re-use of spectrum. Future 5G networks are expected to be even more heterogeneous with wireless access to user equipments (UEs) simultaneously available via multiple technologies, including new technologies such as mmWave and aerial BSs. As demand increases and cells get smaller, there will be an increased number of tiers in future HetNet architectures.