UAV-to-UAV MIMO Systems Under Multimodal Nonisotropic Scattering: Geometrical Channel Modeling and Outage Performance Analysis

An arbitrary-elevation two-sphere reference model is utilized to mimic the unmanned aerial vehicle (UAV) air-to-air fading channels. The model considers the Line-of-Sight (LoS), the single-bounced transmit (SBT), the single-bounced receive (SBR), and the double-bounced (DB) rays. Based on this model, the closed-form expression of the space-time correlation function (ST-CF) is obtained for the first time under the widely used assumption of von Mises-Fisher (vMF) distributed scatterers. To further improve the model's adaptability to realistic scattering environments, the distribution of the scatterers is generalized from a single unimodal vMF density into a mixture that can possess multimodality. Using the single-vMF ST-CF, the ST-CF under the mixture is also written in closed form. Corresponding to the reference channel model, both the deterministic and the stochastic simulation models are provided, which yield consistent results with the respective derived expressions. This validates the correctness of the suggested closed-form ST-CFs. Moreover, a detailed analysis of the outage probability and the outage capacity is reported, which offers revealing insights into the behaviors of the system performance with respect to change of some key model parameters under multimodal distributions of the scatterers.