Rooftop Relay Nodes to Enhance URLLC in UAV-Assisted Cellular Networks

Recently, communication in cellular networks assisted by Unmanned Aerial Vehicles (UAVs) has attracted considerable attention, as it provides wireless connectivity to devices in areas with poor coverage. With a single UAV deployed, it is difficult to achieve Line-of-Sight (LoS) probability and network availability targets for critical Ultra-Reliable Low-Latency Communication (URLLC) applications while meeting cost and system complexity requirements. To harness the advantages of UAVs in these situations, an alternative solution is to deploy a multi-UAV system, exploiting inter-connectivity to maintain uninterrupted communication with a ground transmitter. The idea is to deploy a fixed UAV on the side of a building rooftop, which acts as a relay between the ground transmitter and the flying UAV base station, thus increasing the LoS probability. Notably, a two-hop amplify-and-forward relay can provide significant improvements in the channel capacity, channel gain, and thus overall quality of service. In our study, simulations were carried out in four general environments as specified by ITU-R, namely Suburban, Urban, Dense Urban, and High Rise Urban, based on data collected in Los Angeles, USA. Numerical results demonstrate that two-hop communication via a relay UAV increases LoS probability in all environments, thus improving system reliability and feasibility.