An Analysis of Unmanned Airborne Vehicle Relay Coverage in Urban Environments

Unmanned Airborne Vehicles, UAVs, can be used as radio relay platforms in environments characterized by poor RF connectivity. Those environments can be urban, forested or mountainous regions where no Line of Sight (LOS) exists between ground transmitters and receivers. The effectiveness of a UAV air relay was assessed in an urban area using the ray tracing method. UAVs were placed at various positions and heights over an approximately 500 square meter urban area, and excess path loss relative to unblocked LOS propagation was computed for outdoor receivers. UAV altitudes were 500, 1000 and 2000 meters and frequencies were 400 and 1800 MHZ. Excess path loss is the appropriate metric when UAV transmitter power is limited due to Size, Weight, and Power (SWAP) constraints and the need to minimize friendly interference. We found that a UAV at 2000 m provided coverage for over 90% of the ground receivers within 10 dB of LOS path loss. Most of the connectivity was obtained via unblocked LOS paths due to the small urban area considered. Diffraction around buildings played a larger role in providing connectivity than reflections at 400 MHz and the roles were approximately equal at 1800 MHz. The percentage coverage results were found to be stable to within +/-5% when poorly known building and ground electrical properties or ray tracing computational parameters were varied over reasonable values. Channel characteristics for time of arrival, time delay spread and angle of arrival were also computed.