Relative contribution of transmitted, diffracted and reflected signals to millimetre-wave propagation in urban environments

In recent years, the possibility of gaining access to significant amounts of new spectrum for mobile access by exploiting the millimetre-wave bands at 30, 38 and 60 GHz has attracted considerable interest from wireless equipment manufacturers and wireless standards bodies. In the past, conventional wisdom has held that such frequencies are only useful for very short-range non-line of sight links (tens of metres) in indoor environments or short-range line-of-sight links (hundreds of metres) due to the relatively poor transmission of such signals through rain, vegetation, building materials and human presence and a perceived requirement that such links operate under line-of-sight conditions. However, recent work by researchers in the United States and Korea has suggested that millimetre-wave links deployed in microcell configurations in urban environments can provide useful ranges in the hundreds of metres. Because diffraction contributes very little to propagation at these frequencies, these results suggest that reflection plays a far stronger role than originally believed. What is lacking, however, is a quantitative understanding of the relative contribution of transmitted, diffracted and reflected signals in different urban environments and different transmitter-receiver combinations across a broad range of frequencies between 2 and 60 GHz. Here, we present the results of a multiplicity of simulations using the Wireless InSite coverage prediction tool that we have validated, where possible, using published measurements. The results provide a clear indication of the manner in which wireless NLOS propagation transitions from the diffraction-dominated coverage observed in conventional cellular bands to the reflection-dominated coverage observed in millimeter-wave bands and the manner in which deployment environment and strategies affect the results.