Air-ground channels & models: Comprehensive review and considerations for unmanned aircraft systems

Use of unmanned aircraft systems (UASs) for multiple applications is expected to grow dramatically in the coming decades; this fact has motivated this paper's focus on fundamental physical layer characteristics relevant to UAS communications. In the past, for aeronautical communications with high transmitted power levels, narrow signal bandwidths, elevated ground site antennas in open areas, and low duty cycle transmissions, simple models for channel attenuation sufficed. In the future, when UAS ground stations may not all be in cleared areas with elevated antennas, higher data rates (wider bandwidths) are required, and small UASs with stringent power limitations still require high reliability, more comprehensive air-ground (AG) channel characteristics will be required in order to ensure robust signal designs for high-reliability AG links. We have found that no accurate, validated wideband models exist for the AG channel, particularly not in the Land C-bands that are being proposed for UASs. Airframe shadowing models also do not yet exist. We thus provide a comprehensive review of past work on the AG channel, and follow this with a brief description of plans for an AG channel measurement and modeling campaign. Resulting AG channel models will subsequently be used in the evaluation of candidate air interfaces for UAS control and non-payload communications (CNPC). The air interface must operate in the presence of both delay and Doppler spreads, and shadowing. It should also be spectrally efficient, low-latency, and reasonably robust to interference. We discuss these AG air interface considerations, and also show some initial modeling results based on both analysis and measurements.