Performance Analysis of SDMA with Inter-tier Interference Nulling in HetNets

The downlink performance of two-tier (macro/pico) multi-antenna cellular heterogeneous networks employing space division multiple access (SDMA) technique with zero-forcing precoding is analyzed in this paper. The number of users simultaneously served with SDMA by a base-station (BS) depends on the number of active users in its cell, with the maximum served users limited to $L_{\text {max}}$ . To protect the pico users from severe macro-interference, part of the antennas at each macro BS is proposed to be utilized toward interference nulling to pico users. The partitioning of macro antenna resources to serve macro-users and to null interference to pico users for optimal performance is investigated in this paper. Biased-nearest-distance-based user association scheme is proposed, where the bias value accounts for the natural bias due to the differences in multi-antenna transmission schemes across tiers, as well as the artificial bias for load balancing. The signal-to-interference-ratio coverage probability, rate distribution, and average rate of a typical user are then derived. Our results demonstrate that the proposed interference nulling scheme has strong potential for improving performance if the macro antennas partitioning is carefully done. The optimal $L_{\text {max}}^{*}$ for both macro and pico-tier, which maximize the average data rate, is also investigated and it is found to outperform both single-user beamforming and full-SDMA. Finally, the impact of imperfect channel state information due to limited feedback is analyzed.