Hierarchical multi-stage interference alignment for downlink heterogeneous networks

The occurrence of the intercell interference (ICI) is inevitable in downlink heterogeneous networks because user terminals (UTs) often receive the signals transmitted from different base stations (BSs) at the same time. To mitigate ICI, a hierarchical interference alignment (HIA) is proposed. HIA is based on the interference alignment (IA), which aligns the interference signals within a reduced dimensional subspace at each UT by multiplying the signals to be transmitted from each BS by the transmit beamforming matrix. However, HIA can be applied only to a network in which two picocells are placed within a macrocell. In this paper, we propose a hierarchical multi-stage interference alignment (HMIA) to cope with the restriction. In HMIA, by dividing the aligning process into multiple stages, every transmit beamforming matrix can be calculated in closed form. Furthermore, since the alignment is carried out in descending order of signal strength, strong interference signals are aligned preferentially. Simulation results can show that HMIA successfully deal with the network in which more than three picocells are placed within a macrocell although there is slight loss of the per-user capacity.