  In this article, a network model incorporating both line-of-sight (LOS) and
non-line-of-sight (NLOS) transmissions is proposed to investigate impacts of
blockages in urban areas on heterogeneous network coverage performance. Results
show that co-existence of NLOS and LOS transmissions has a significant impact
on network performance. We find in urban areas, that deploying more BSs in
different tiers is better than merely deploying all BSs in the same tier in
terms of coverage probability.
