Earth-to-HAP FSO Communication With Spatial Diversity and Channel Correlation

In this paper, we investigate an Earth-to-air Free-Space Optical (FSO) system with spatial diversity, considering the influence of atmospheric turbulence, pointing error, and angle-of-arrival (AoA) fluctuations. Our study employs a High Altitude Platform (HAP) as the aerial platform in the stratosphere. We begin by proposing a comprehensive mathematical channel model for the Earth-to-HAP FSO system with multiple apertures, accounting for both correlated and uncorrelated channel conditions. To verify the model's accuracy, we conduct Monte-Carlo simulations and successfully match the results. Next, we evaluate the system's performance using outage analysis, average bit error rate (BER), and asymptotic BER analysis. To gain insight into the system's behavior, we derive simplified expressions for diversity order and coding gain using asymptotic BER analysis. Through our analysis, we demonstrate the efficacy of spatial diversity in mitigating the impact of AoA fluctuations in a ground-to-HAP FSO system. Additionally, we conduct a comparative analysis to assess the effect of AoA fluctuations in correlated and uncorrelated channels. Moreover, we derive the optimum receiver field of view required to design a practical ground-to-HAP FSO system capable of achieving the maximum diversity under specific AoA fluctuation conditions. Our study offers valuable insights into the potential use of spatial diversity to enhance the reliability and performance of Earth-to-HAP FSO systems, providing useful guidelines for system design and optimization.