On interference alignment over frequency selective channel

Interference alignment (IA) has proven to approach achieving optimal degrees of freedom (DoF) of interference channel. While IA over space dimensions is usually achieved through applying iterative algorithms, closed-form solutions exist for IA over frequency. However, these solutions put strict constraints on the number of subcarriers required and the number of DoF assigned to each user rendering it impractical to support more than 5 users [1]. In this paper, we propose two new approaches to relax these constraints and make it practical to use IA over frequency selective channel in SISO systems for arbitrary number of users, subcarriers and DoF. The two approaches apply the iterative methods developed for MIMO channels on SISO systems in both time and frequency domains. Moreover, we show the limitations of these iterative methods when applied on SISO systems and revisit the IA feasibility conditions deriving an upper bound on the DoF allowable for the users. Simulation results show the benefits of the proposed techniques over existing solutions and confirm the conjectures made in this paper.