Coverage and Robustness of mm-Wave Urban Cellular Networks: Multi-Frequency HetNets Are the 5G Future

Cellular deployments in the spectrum-rich mm-wave bands are anticipated to provide multi-Gbps connectivity in future 5G networks. Although measurements have demonstrated the feasibility of outdoor mm-wave links, the network-level opportunities and challenges of mm-wave cellular deployments are yet to be fully understood. In this paper we present a study of mm-wave urban network coverage and robustness - i.e. handover and beamsteering opportunities for supporting user mobility and resolving blockage of directional mm-wave links by moving obstacles. We use real 3D building data with ray tracing and investigate the impact of base station placement and density, building materials, and antenna directivity. Our results show that mm-wave networks can provide multi-Gbps connectivity locally, but that over 25% of the network area remains in outage even with dense pico-cellular deployments and that supporting mobility is very challenging. Moreover, primary link blockage can be resolved via handover or beamsteering to a secondary link at only up to 40% and 25% of high-throughput locations, respectively. Given these networking challenges, we argue that multi-frequency heterogeneous networks are the key to exploiting the great promise of mm-wave for future 5G deployments, i.e. mm-wave as a hotspot capacity booster, rather than a solution for comprehensive cellular coverage.