  In this paper, we introduce a sophisticated path loss model into the
stochastic geometry analysis incorporating both line-of-sight (LoS) and
non-line-of-sight (NLoS) transmissions to study their performance impact in
small cell networks (SCNs). Analytical results are obtained on the coverage
probability and the area spectral efficiency (ASE) assuming both a general path
loss model and a special case of path loss model recommended by the 3rd
Generation Partnership Project (3GPP) standards. The performance impact of LoS
and NLoS transmissions in SCNs in terms of the coverage probability and the ASE
is shown to be significant both quantitatively and qualitatively, compared with
previous work that does not differentiate LoS and NLoS transmissions.
Particularly, our analysis demonstrates that when the density of small cells is
larger than a threshold, the network coverage probability will decrease as
small cells become denser, which in turn makes the ASE suffer from a slow
growth or even a notable decrease. For practical regime of small cell density,
the performance results derived from our analysis are distinctively different
from previous results, and shed new insights on the design and deployment of
future dense/ultra-dense SCNs.
