  We develop a framework for downlink heterogeneous cellular networks with
line-of-sight (LoS) and non-line-of-sight (NLoS) transmissions. Using
stochastic geometry, we derive tight approximation of achievable downlink rate
that enables us to compare the performance between densifying small cells and
expanding BS antenna arrays. Interestingly, we find that adding small cells
into the network improves the achievable rate much faster than expanding
antenna arrays at the macro BS. However, when the small cell density exceeds a
critical threshold, the spacial densification will lose its benefits and
further impair the network capacity. To this end, we present the optimal small
cell density that maximizes the rate as practical deployment guidance. In
contrast, expanding macro BS antenna array can always benefit the capacity
until an upper bound caused by pilot contamination, and this bound also
surpasses the peak rate obtained from deployment of small cells. Furthermore,
we find that allocating part of antennas to distributed small cell BSs works
better than centralizing all antennas at the macro BS, and the optimal
allocation proportion is also given for practical configuration reference. In
summary, this work provides a further understanding on how to leverage small
cells and massive MIMO in future heterogeneous cellular networks deployment.
