A cascaded precoding scheme for two-tier heterogeneous networks

In a two-tier heterogeneous network with shared spectrum, the cross-tier interference and co-tier interference significantly affect the network performance. In this paper, we investigate a cascaded precoding scheme in orthogonal frequency-division multiplexing systems to protect macro-cell user equipments (MUEs) from the interference caused by co-located small cells and at the same time to satisfy the quality-of-service (QoS) requirement of small-cell user equipments (SUEs). An outer precoder ensures that the signals intended for the SUEs are orthogonal to the MUEs thus avoids the cross-tier interference from the second tier. Moreover, optimal power allocation through an inner precoder at each small base station (SBS) yields better performance of the SUEs and guarantees their QoS requirement. Considering the dense deployment of SBSs, we further propose an SBS selection algorithm to reduce the computational complexity of our scheme. Simulation results demonstrate that the cascaded precoding structure is effective in mitigating the interference and enhancing the capacity of small cells.