  Free space optical (FSO) communication refers to the information transmission
technology based on the propagation of optical signals in space. FSO
communication requires that the transmitter and receiver directly see each
other. High-altitude platforms (HAPs) have been proposed for carrying FSO
transceivers in the stratosphere. A multihop HAP network with FSO links can
relay traffic between ground FSO nodes. In this study, we propose an end-to-end
switching model for forwarding traffic between massive pairs of ground FSO
nodes over a HAP network. A protection mechanism is employed for improving the
communication survivability in the presence of clouds, which may break the line
of sight (LoS) between HAPs and ground nodes. We propose an algorithm for
designing the topology of the survivable HAP network, given a set of ground FSO
nodes. The results demonstrate that, even though networks with survivable
capacity use more resources, they are not necessary much more expensive than
those without survivability in terms of equipment, i.e., HAPs and FSO devices,
and in terms of wavelength resource utilization.
