Spatial blanking and inter-tier coordination in massive-MIMO heterogeneous cellular networks

This paper studies inter-tier interference coordination in a heterogeneous cellular network (HetNet) consisting of "massive-MIMO" macrocells and multi-antenna small cells. We assume that the users are concentrated at certain areas of the cell forming hotspots. Assuming the hotspot size to be much smaller than the macrocell size, the users of a given hotspot are seen under a relatively narrow angular spread from the macrocell, thus appearing almost co-located to the macrocell. This gives rise to directional channel vectors, which can be exploited to simplify precoder design and obtain spatial blanking, a means of concentrating energy in the direction of scheduled hotspots, while mitigating interference caused to the active small cells located in the other directions. We further show that significant throughput gains can be achieved by complementing spatial blanking with active interference coordination strategies, such as turning OFF a small cell when it suffers from or causes excessive interference, or offloading macrocell hotspots to small cells.