Offloading With Hybrid Cell Association in Non-Uniform Heterogeneous Cellular Networks: Modeling and Performance Analysis

Flexible/biased cell association in uniform Heterogeneous Cellular Network (HCN) can lead to poor offloading, particularly near to the Macrocell Base Station (MBS). Additionally, in the cell-edge region, a user might offload to an SBS even if unbiased cell association is employed. In this paper, firstly, we consider a Non-uniform HCN (NuHCN) model in which SBSs are not deployed in a circular region around each MBS within a predefined radius, which we call the inner region. Secondly, we define the outer region, which is disjoint to the inner region, as the cell-edge region. In these two regions, based on maximum received power; we propose hard cell association scheme, where users located in the inner region shall only be served by MBS, whereas users located in the outer region shall be served only by SBS. Finally, for the region between the inner and the outer region, called the annular region, we shall use the flexible cell association scheme. Based on the proposed system model, analytical closed forms for the downlink coverage and single user throughput are derived. It is observed that with non-uniform deployment of SBSs and precisely selecting the size of inner, annular, and outer region, both coverage and single user throughput are significantly improved compared to uniform HCN.