Beam Size Optimization for High-Altitude Platforms to Ground Links in FSO Communications

Abstract

Free-space optical (FSO) communication has been used in practice mainly for short-distance transmission because it requires light of sight (LoS) between the transmitter and receiver. For long-distance communication, to avoid terrestrial obstacles, high-altitude platforms (HAPs) flying at stratosphere are used to carry intermediate FSO transceivers which relay data through several hops from the source to the destination stations. A HAP can communicate with a large ground area if its FSO transceiver projects a wide beam onto the ground. However, an excessively large beam makes the FSO transceiver consume a lot of energy. This study investigates the problem of finding individual optimal beam sizes for FSO transceivers on HAPs so that the total cost of the HAP network, including the amortization, energy, and maintenance costs, is minimized. An optimization algorithm was proposed and implemented. The simulation results show the network designed by the algorithm achieves a nearly optimal number of HAPs, leading to a low network cost.