High-altitude platform for free-space optical communication: Performance evaluation and reliability analysis

High-altitude platforms (HAPs) have been proposed previously for transmitting radio frequency signals from ground stations over large distances of several hundred kilometers. These HAP architectures suffer from the limitations of data rate. The availability of the system has also not been evaluated for these architectures. In this paper, we propose a high-capacity and highly reliable optical wireless multi-hop HAP network configuration with free-space optical links from the ground stations and between the serial HAPs in the stratosphere. In the proposed scheme, communication services for long distance requiring very high bandwidth are transmitted optically from the ground station to the HAP station in the stratosphere, where it can cover distances of several hundreds of kilometers on the free-space optical links with serial multiple hops and be transmitted back optically to the distant ground receiver. In order to increase reliability and availability, particularly with reference to fading of vertical links due to atmospheric conditions, alternate slant links are added as redundant paths. The closed-form expressions for the average bit error rate and average channel capacity are derived assuming the composite channel model, which includes atmospheric attenuation, turbulence, and pointing errors. The gamma-gamma distribution with beam wander effect is used to model the atmospheric turbulence. Beam optimization is also done using the Nelder-Mead simplex algorithm. The effects of beam wandering and random pointing error are mitigated by using multi-hop HAP architecture. Link availability and system downtime of the complete ground-to-ground FSO network and reconfigured architecture has been calculated for component failure and atmospheric turbulence, moderate fog, rain, and snow. The proposed reconfigured architecture has high availability, meeting telecom requirements, and high mean time between failures and lower system downtime.