Weighted rank constrained rank minimization interference alignment for 3-tier downlink heterogeneous networks

In this paper, interference alignment (IA) in 3-tier heterogeneous networks (HetNets) is addressed. The rank constrained rank minimization (RCRM) approach is modified to deal with the nature of tiers with different quality-of-service (QoS) requirements. In the proposed approach, which we name weighted RCRM (WRCRM), different tiers are treated with different weighting factors to reflect their tolerance to interference. We first study the achievable degrees of freedom (DoFs) for the 3-user mutually interfering broadcast channels (MIBCs) and derive an inner and a loose outer bound on the achievable DoFs. Then, using the derived expressions and the condition of IA feasibility, we show that, by choosing the appropriate weighting factors, the proposed WRCRM-IA solution can outperform the RCRM-IA approach in terms of the average sum rate and the average number of DoFs. Moreover, the WRCRM-IA solution comes closer to the perfect IA DoFs outer bound.