Millimeter-wave channels in urban environments

Since millimeter-wave (mm-wave) communications systems deployed in urban environments will be an important component of next-generation cellular communications, it is important to understand the propagation channels those systems will be working in. This paper provides an overview of the state of the art in urban mm-wave channel modeling, and outlines directions for future research. Propagation at mm-wave differs in several critical aspects from those at microwave frequencies: much higher free-space pathloss, a smaller number of multipath components, the inefficiency of diffraction as a propagation process, the importance of body shadowing, and possibly more diffuse scattering all impacts the channel characteristics. The paper will also review the new challenges for channel sounding in this frequency band, in particular for the pathloss, and finally review stochastic and semi-deterministic modeling approaches suitable for these channels. It is clear that while some important work has been done, a lot of topics remain open for future work.