A Distributed Approach to Interference Alignment in OFDM-Based Two-Tiered Networks

In this paper, we consider a two-tiered network and focus on the coexistence between two tiers at the physical layer. We target our efforts on a Long-Term Evolution Advanced (LTE-A) orthogonal frequency-division multiple-access (OFDMA) macrocell sharing the spectrum with a randomly deployed second tier of small cells. In such networks, high levels of cochannel interference between the macrocell base station and the small-cell base station (SBS) may largely limit the potential spectral efficiency gains provided by frequency reuse 1. To address this issue, we propose a novel cognitive interference alignment-based scheme to protect the macrocell from cross-tier interference while mitigating the co-tier interference in the second tier. Remarkably, only local channel state information (CSI) and autonomous operations are required in the second tier, resulting in a completely self-organizing approach for the SBSs. The optimal precoder that maximizes the spectral efficiency of the link between each SBS and its served user equipment is found by means of a distributed one-shot strategy. Numerical findings reveal nonnegligible spectral efficiency enhancements with respect to traditional time division multiple access (TDMA) approaches at any signal-to-noise ratio (SNR) regime. Additionally, the proposed technique exhibits significant robustness to channel estimation errors, achieving remarkable results for the imperfect CSI case and yielding consistent performance enhancements to the network.