Massive-MIMO Meets HetNet: Interference Coordination Through Spatial Blanking

In this paper, we study the downlink performance of a heterogeneous cellular network (HetNet) where both macro and small cells share the same spectrum and hence interfere with each other. We assume that the users are concentrated at certain areas in the cell, i.e., they form hotspots. While some of the hotspots are assumed to have a small cell in their vicinity, the others are directly served by the macrocell. Due to a relatively small area of each hotspot, the users lying in a particular hotspot appear to be almost co-located to the macrocells, which are typically deployed at some elevation. We assume a large number of antennas at the macrocell relative to the number of users simultaneously served. In this massive MIMO regime, the channel vectors become highly directional. We exploit this directionality in the channel vectors to obtain spatial blanking, i.e., concentrating transmission energy only in certain directions while creating transmission opportunities for the small cells lying in the other directions. In addition to this inherent interference suppression, we also develop three low-complexity interference coordination strategies: 1) turn off small cells based on the amount of cross-tier interference they receive or cause to the scheduled macrocell hotspots; 2) schedule hotspots such that treating interference as noise is approximately optimal for the resulting Gaussian interference channel; and 3) offload some of the macrocell hotspots to nearby small cells to improve throughput fairness across all hotspots. For all these schemes, we study the relative merits and demerits of uniform deployment of small cells vs. deploying more small cells towards the cell center or the cell edge.