Cognitive interference alignment between small cells and a macrocell

Small cells have attracted large interest lately in the research community, mainly due to easy and quick deployment. In addition, a large number of small cells is in general more energy efficient than macrocells since less power is required to combat path loss and get across the wireless medium. However, in current network configurations these two types of cells have to coexist over the same spectrum because existing cellular systems are mainly based on macrocells and additional bandwidth for small cell deployments is scarce and too expensive to acquire. In this context, this contribution investigates an underlay cognitive communication technique which exploits interference alignment across multiple antennas in order to mitigate the interference of small cell User Terminals (UTs) towards the macrocell Base Station (BS). More specifically, three techniques for aligning interference are investigated, namely static, coordinated and un-coordinated. The system performance is evaluated and compared based on sum-rate capacity, primary rate protection ratio and primary to secondary rate ratio.