Femtocell exclusion regions in hierarchical 3-sector macrocells for co-channel deployments

In co-channel deployments of macro and femto cells, the much higher transmit power of a macrocell base station (MBS) than that of a femtocell access point (FAP) may create a femtocell exclusion region (FER) around an MBS, where reliable femto downlink (DL) services cannot be guaranteed. In this paper, we analyze the DL outage probability (OP) of orthogonal frequency division multiple access (OFDMA) based femtocells in terms of their locations with respect to nearby 3-sector MBSs, and derive the expression of FER radius as a function of the azimuth angle relative to the MBS, which can be directly used to divide the macrocell coverage area into inner and outer regions for hybrid frequency assignments. Simulation results show that the obtained closed-form expressions can be used to accurately predict the location-dependent femto DL OP and the non-circular FER in hierarchical 3-sector macrocells. Our results also show that the FER shrinks at edges between macro sectors, giving more space for co-channel deployment of femtocells to improve spatial reuse.