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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 …