  The application domains of civilian unmanned aerial systems (UASs) include
agriculture, exploration, transportation, and entertainment. The expected
growth of the UAS industry brings along new challenges: Unmanned aerial vehicle
(UAV) flight control signaling requires low throughput, but extremely high
reliability, whereas the data rate for payload data can be significant. This
paper develops UAV number projections and concludes that small and micro UAVs
will dominate the US airspace with accelerated growth between 2028 and 2032. We
analyze the orthogonal frequency division multiplexing (OFDM) waveform because
it can provide the much needed flexibility, spectral efficiency, and,
potentially, reliability and derive suitable OFDM waveform parameters as a
function of UAV flight characteristics. OFDM also lends itself to agile
spectrum access. Based on our UAV growth predictions, we conclude that dynamic
spectrum access is needed and discuss the applicability of spectrum sharing
techniques for future UAS communications.
