Polarization-Space Based Interference Alignment for Cognitive Heterogeneous Cellular Network

In underlay cognitive heterogeneous cellular network (CHCN), small cells can transmit their signals as long as the interference to macro cell is below a threshold. Consider a two-layer CHCN with polarized MIMO small cells, a novel polarization-space based interference alignment scheme is proposed. The cross-tier interference between macro cell and small cells is addressed by two given algorithms with different purposes. Orthogonal projection based polarization-space interference alignment (OP-PSIA) for ensuring the minimum effect to macro cell and interference constrained polarization-space interference alignment (IC-PSIA) for maximizing the performances of small cells if permitted. The co-tier interference between small cells are reduced by a minimum total mean squared error (MMSE) algorithm. Then we give specific solutions for two algorithms both including orthogonal projection processing and analytically iterative calculations. Simulation results show the improvement of two algorithms in BER performance of small cells while ensuring the protection of macro cell and keeping maximum overall sum rate.