Two-stage interference alignment for partially connected heterogeneous networks

In this paper, we propose a two-stage interference alignment (IA) scheme for the downlink of partially connected heterogeneity networks (HetNets), where the partial connectivity is induced by the path loss and the low transmission power of small cells. Particularly, in the first stage, the macro cell carries out conventional IA with a set of fully connected small cells. We derive the maximum number of small cells that can be accommodate into this stage. In the second stage, the beamforming matrices are designed for a set of small cells that are partially connected with each other and also partially connected with the set of fully connected small cells. In the above two-stage IA, one user is served by each base station (BS). Since the macro BS usually has more antennas than small cell BSs, we extend the two-stage IA to a scenario where multiple users are served by the macro BS. Moreover, we investigate the influence of the number of served macro users to the total number of data streams achieved by the system. The simulation results show that two-stage IA can significantly improve system sum rate.