Optimal multiple FSO transceiver configuration for using on High-altitude platforms

Free-space optical (FSO) communication requires light of sight (LoS) between the transmitter and the receiver. For long-distance communication, many research projects have been conducted towards using a network composed of high-altitude platforms (HAPs) flying at an elevation of 20 km to carry intermediate FSO transceivers that forward data between ground stations. The clear environment at high elevations prevents terrestrial obstacles from cutting the LoS between the transceivers. An FSO transceiver on a HAP can communicate with ground stations within a small area owing to its limited beam size. We suggest using multiple FSO transceivers on a HAP to extend its ground coverage. However, the use of too many FSO transceivers may quickly exhaust the onboard energy of the HAP. As a result, HAP must be lowered to recharge frequently. In this study, we first propose a configuration of multiple FSO transceivers to widen the ground coverage of a HAP. We then propose a set of closed-form expressions to calculate the extended coverage. Finally, to implement a HAP network using multiple FSO transceivers, we seek the optimal configuration of multiple FSO transceivers that minimizes the total cost of the HAP network, including amortization, energy, and maintenance costs. The simulation results show that the proposed multiple FSO transceiver configuration clearly increases the ground coverage of a HAP and significantly reduces the cost of the HAP network.