Characterization of UAV-based Wireless Channels With Diverse Antenna Configurations

In the next wave of swarm-based applications, unmanned aerial vehicles (UAVs) need to communicate with peer drones in any direction of a three-dimensional (3D) space. On a given drone and across drones, various antenna positions and orientations are possible. We know that, in free space, high levels of signal loss are expected if the transmitting and receiving antennas are cross polarized. However, increasing the reflective and scattering objects in the channel between a transmitter and receiver can cause the received polarization to become completely independent from the transmitted polarization, making the cross-polarization of antennas insignificant. Usually, these effects are studied in the context of cellular and terrestrial networks and have not been analyzed when those objects are the actual bodies of the communicating drones that can take different relative directions or move at various elevations. In this work, we show that the body of the drone can affect the received power across various antenna orientations and positions and act as a local scatterer that increases channel depolarization, reducing the cross-polarization discrimination (XPD). In addition to communicating with other UAVs in a swarm, UAVs can also serve users on the ground. For example, at ultra-low altitudes, an unmanned aerial vehicle (UAV) can act as a personal base station where it communicates only with one or two users on the ground. The communication device used by a user can be in their pocket, held by hand, or attached to their bodies. In these scenarios, the wireless channel can go through different fading levels, depending on the UAV’s location, user orientation, the location of the UE near the user’s body, and the frequency of the transmitted signal. The extent to which these factors can affect Air-to-Ground channels at ultra-low altitudes is studied in this work. We answer questions regarding how the human body and different use-cases of holding a communication device on the ground can affect the quality of the wireless channel and the optimal UAV hovering location. Furthermore, we demonstrate how the observed effects can be leveraged to our advantage and increase the physical layer security of UAV-assisted networks relying on the human-induced effects. Finally, in situations where a UAV swarm needs to communicate with a target that is far or surrounded by undesired receivers, beamforming can be an attractive solution. With beamforming, the transmitted signal becomes shaped towards a certain direction confining its spatial signature and increasing the received signal-to-noise-ratio (SNR) at the receiver. However, phase synchronization across the swarm is difficult to achieve and there will always exist some degree of phase incoherency across the transmitted signals from the distributed UAVs. Phase differences between the distributed nodes would result in signals arriving at different times and their phases might not align with each other resulting in reductions in beamforming gain. Hence, a method to increase phase coherency at the receiver with limited channel overhead is desired. To this end, we propose a UAV rotation-based method through which the UAV, relying on its heterogeneous body structure, can alter the phase of the incoming signals and increase the beamformed signal level