  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. Assuming large
number of antennas at the macrocell, 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: (i) turn off small cells based on the amount of
cross-tier interference they receive or cause to the scheduled macrocell
hotspots, (ii) schedule hotspots such that treating interference as noise is
approximately optimal for the resulting Gaussian interference channel, and
(iii) offload some of the macrocell hotspots to nearby small cells in order 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.
