Measurement and Characterization of Low-Altitude Air-to-Ground MIMO Channels

To support the development of high-capacity air-to-ground links for range extension, measurements of the low-altitude air-to-ground channel were made at 915 MHz. Two transmit antennas were mounted on an unmanned aerial vehicle (UAV), which was flown in loops at an altitude of approximately 200 m above ground level. The received signals were recorded at each of eight antenna elements mounted on a van at locations outside and inside the flight loop. The analysis of the measurements shows that there are regions where the spatial diversity is significant, despite the sparse multipath environment, indicating spatial decorrelation at both the ground and air terminals. The variations in spatial correlation across the receiver array indicate the presence of nonplanar wavefronts produced by the signals' interaction with objects in the array near field, in particular the measurement vehicle. A similar effect is probable at the UAV, and it is expected that more significant near-field effects would arise on a more conventional air platform. These support significant reductions in outage probability at both receiver locations: With appropriate signaling strategies, an airborne platform could provide a viable relay or broadcast node for high-capacity communications using a multiple-input-multiple-output (MIMO) system.