Drones, unmanned aerial vehicles (UAVs), or unmanned aerial systems (UAS) are expected to be an important component of 5G/beyond 5G (B5G) communications. This includes their use within cellular architectures (5G UAVs), in which they can facilitate both wireless broadcast and point-topoint transmissions, usually using small UAS (sUAS). Allowing UAS to operate within airspace along with commercial, cargo, and other piloted aircraft will likely require dedicated and protected aviation spectrum-at least in the near term, while regulatory authorities adapt to their use. The command and control (C2), or control and non-payload communications (CNPC) link provides safety critical information for the control of the UAV both in terrestrial-based line of sight (LOS) conditions and in satellite communication links for so-called beyond LOS (BLOS) conditions. In this paper, we provide an overview of these CNPC links as they may be used in 5G and satellite systems by describing basic concepts and challenges. We review new entrant technologies that might be used for UAV C2 as well as for payload communication, such as millimeter wave (mmWave) systems, and also review navigation and surveillance challenges. A brief discussion of UAV-to-UAV communication and hardware issues are also provided.

Multi user orthogonal chirp spread spectrum (OCSS) improves the spectral inefficiency of CSS but is only feasible with perfect synchronism and without any channel distortion. Either asynchronism or channel distortion causes multiple access interference (MAI), which degrades performance. Conditions with small timing offsets we term quasi-synchronous (QS) transmission. In this paper, we investigate CSS signaling in QS conditions. We do this for the classical linear chirp, for which cross correlations can be derived analytically, and also propose two sets of nonlinear chirps to improve CSS system performance. We numerically evaluate cross-correlation distributions, and show that with an FSK-based chirp modulation, our two new nonlinear chirp designs outperform the classical linear chirp and multiple existing nonlinear chirps from the literature, over the QS additive white Gaussian noise channel in both partially and fully-loaded systems. We also demonstrate our nonlinear CSS designs outperform the existing chirps in a realistic dispersive channel, via simulations using an empirical air-ground channel.

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