Coverage Analysis of Heterogeneous Cellular Networks in Urban Areas
  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. 
Bin Yang, Guoqiang Mao, Xiaohu Ge, Hsiao-Hwa Chen, Tao Han, Xuefei   Zhang
cs.NI cs.IT math.IT