  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.
