This paper considers to mitigate the uplink cross-tier interference (CI) in heterogeneous networks (HetNets) by the idea of opportunistic transmission. Firstly, we introduce the conventional opportunistic interference alignment (OIA) into HetNets and it indeed reduces the CI from the macrocell users (MUs) effectively. Interestingly, based on the unique characteristic in HetNets, the performance of OIA can be further improved. Therefore, the novel opportunistic interference mitigation (OIM) schemes based on adaptive reference signal spaces (A-RSSs) are proposed. In the novel OIM, an A-RSS is defined for each FBS and the A-RSS of a FBS constantly updates as the channel state information (CSI) of its FUs changes. According to the A-RSSs of all FBSs, each MU calculates its scheduling metric and the macrocell base station (MBS) selects MUs by the metrics. However, broadcasting the A-RSS constantly makes the system more complicated, so a complexity-performance tradeoff is considered. To decrease the complexity of the system, the quantization codebook is introduced and the quantized A-RSS based OIM is proposed. At last, extensive simulations are conducted. It is shown that, in terms of femtocells, the performance of novel OIM is better than that of the codebook based OIM (CB-OIM) and the performance of CB-OIM is better than that of conventional OIA.
