  This paper analyzes the downlink performance of ultra-dense networks with
elevated base stations (BSs). We consider a general dual-slope pathloss model
with distance-dependent probability of line-of-sight (LOS) transmission between
BSs and receivers. Specifically, we consider the scenario where each link may
be obstructed by randomly placed buildings. Using tools from stochastic
geometry, we show that both coverage probability and area spectral efficiency
decay to zero as the BS density grows large. Interestingly, we show that the BS
height alone has a detrimental effect on the system performance even when the
standard single-slope pathloss model is adopted.
