Partial Interference Alignment for Heterogeneous Cellular Networks

The deployment of small cell base stations (BSs) can efficiently boost the network capacity. Cross-tier interference management is critical for the performance enhancement in such heterogeneous networks. Interference alignment provides an efficient way to achieve optimal degree-of-freedom in interference channels. However, it is not necessarily optimal in terms of system sum capacity in the intermediate SNR region. In this paper, we propose a partial interference alignment scheme for heterogeneous network downlink transmission at intermediate SNR by considering the power imbalance among the macro cell and small cell BSs. The hard partial interference alignment adaptively selects the subspace for transmission and captures the trade-off of the transmission dimensions of small cell users and macro cell user. The soft partial interference alignment is used to further improve the hard interference alignment by iteratively balancing the interference and useful signals in each user side. Also, large dimensional analysis is performed to show that the number of transmit dimensions, which can be exploited by the small cell BS is determined by the SNR of the system and singular value distribution of the channel matrix. Simulation results suggest that the proposed partial interference alignment can achieve better achievable sum capacity in the intermediate SNR regime and the large dimension result is accurately verified.