Practical Interference alignment in the frequency domain for OFDM-based wireless access networks

Interference alignment (IA) is often considered in the spatial domain in combination with MIMO systems. In contrast, aligning interference in the frequency domain among multiple subcarriers can also benefit single-antenna OFDM-based access networks. It allows for flexible operation on a per-subcarrier basis. We investigate the gains achievable by frequency IA in practice for a scenario with multiple access points and clients. Previous work is predominantly theoretical and focuses on idealized cases where all nodes have the same average signal-to-noise ratio (SNR). On the contrary, in practical networks, nodes typically have heterogeneous SNRs depending on channel conditions, which might have a significant impact on IA performance. We tackle this problem by designing mechanisms that adaptively choose which nodes shall perform IA on which subcarriers depending on current channel conditions. We implement and validate our approach on software-defined radios. To the best of our knowledge, this is the first practical implementation of IA in the frequency domain. Our measurements show that (1) frequency IA is feasible in practice, and (2) choosing appropriate nodes and subcarriers overcomes the main limitations due to heterogeneous SNRs. Our mechanisms enable IA in scenarios where it would be infeasible otherwise, achieving throughput gains close to the 33% theoretical maximum.