Outage probability minimization by optimal beamwidth selection for UAV-to-HAP FSO links under pointing inaccuracies

Abstract

The design of a free-space optical (FSO) link between an unmanned aerial vehicle (UAV) and a high-altitude platform (HAP) in the practical scenario of generalized pointing errors (PE) and beam wandering (BWAN) is addressed. We propose to select appropriately the system parameters such as optical beamwidth at the UAV and its flight altitude so as to minimize the outage probability. First, we characterize the channel statistics by taking into account the joint influence of attenuation loss, weak atmospheric turbulence and generalized PE particularly impaired by BWAN. Second, we formulate the closed-form expressions for outage probability, where we also derive an asymptotic expression at high signal-to-noise ratios (SNR)s. Third, we derive the optimum beamwidth at the UAV leading to the instantaneous minimum outage probability under generalized PE and BWAN. Finally, we address the optimal flight altitude of the UAV to avoid strong turbulence in order to ensure efficient data transmission. Our simulation results validate both the effectiveness and the accuracy of the derived asymptotic outage probability and the adequate choice of beamwidth and flight altitude to minimize the average outage performance. For instance, our results show that at an SNR of 20 dBm, the link availability is increased from 20% (with nominal beamwidth) to more than 70% by adopting the proposed beamwidth optimization.