  We consider a heterogeneous cellular network (HetNet) where a macrocell tier
with a large antenna array base station (BS) is overlaid with a dense tier of
small cells (SCs). We investigate the potential benefits of incorporating a
massive MIMO BS in a TDD-based HetNet and we provide analytical expressions for
the coverage probability and the area spectral efficiency using stochastic
geometry. The duplexing mode in which SCs should operate during uplink
macrocell transmissions is optimized. Furthermore, we consider a reverse TDD
scheme, in which the massive MIMO BS can estimate the SC interference
covariance matrix. Our results suggest that significant throughput improvement
can be achieved by exploiting interference nulling and implicit coordination
across the tiers due to flexible and asymmetric TDD operation.
