  Two emerging technologies, cognitive radio (CR) and free-space optical (FSO)
communication, have created much interest both in academia and industry
recently as they can fully utilize the spectrum while providing cost-efficient
secure communication. In this article, motivated by the mounting interest in CR
and FSO systems and by their ability to be rapidly deployed for civil and
military applications, particularly in emergency situations, we propose a CR
enabled radio frequency (RF)/FSO communication model for an aerial relay
network. In the proposed model, CR enabled RF communication is employed for a
ground-to-air channel to exploit the advantages of CR, including spectrum
efficiency, multi-user connectivity, and spatial diversity. For an air-to-air
channel, FSO communication is used, since the air-to-air path can provide
perfect line-of-sight connectivity, which is vital for FSO systems. Finally,
for an air-to-ground channel, a hybrid RF/FSO communication system is employed,
where the RF communication functions as a backup for the FSO communication in
the presence of adverse weather conditions. The proposed communication model is
shown to be capable of fully utilizing the frequency spectrum, while
effectively dealing with RF network problems of spectrum mobility and
underutilization, especially for emergency conditions when multiple unmanned
aerial vehicles (UAVs) are deployed.
