Downlink Interference Alignment in Two-Tier Femtocell Networks

Femtocell networks have recently been widely investigated due to their effective commercial deployment by different operators. However, cross-tier interference between the two layers needs wise treatment. A recently developed area of research for interference management is called interference alignment (IA) where data and interference subspaces are separated at each receiver so that the network can achieve its deserved degrees of freedom (DOF). Although both femtocells and IA are individually attractive topics for many recent researches, the convergence point of these two lines of activities in which femtocells incorporate IA to reduce cross-tier interference impacts on the sum capacity of the network, has not received much attention yet. In this paper, we propose a structured framework for interference alignment in a femtocell overlaid cellular network with shared frequency channels where for the first time we also consider coverage area of each transmission. The algorithm captures proper participants for each individual alignment so that the strongest interfering nodes are removed at each receiver's side. Considering this constraint enables us to evaluate a more realistic ad-hoc interference alignment analytically where femtocells and macrocell users are scattered uniformly in the cell. We investigated three different femtocell channel selection scenarios including random baseline, max-min distance and min-average interference schemes. Using simulation, we verify our analytical results and obtain a comprehensive plot of the performance versus different simulation parameters. We also show that our proposed IA deployed network is superior to conventional two-tier networks in terms of cross-tier interference.