Deployment of Cognitive RF for Performance Enhancement of FSO systems: A FSO/Cognitive RF Hybrid Approach

Cognitive Radio has been proposed as an ideal solution for identifying and utilizing the spectral holes in the licensed RF spectrum. Due to FSO's (Free Space Optical) susceptibility to extreme weather conditions like fog, snow etc., a RF link as a backup for the FSO system is a largely accepted solution. Since licensed RF spectrum is scarce, thus, allocating a dedicated RF resource for each FSO link may not be viable. Cognitive Radio can overcome this hurdle. This work investigates the possible role of Cognitive RF as a supporting link for a FSO system. A hybrid FSO/Cognitive RF system is considered in which the FSO system is equipped with a RF transceiver but does not have a dedicated licensed RF resource. Energy detection is used to detect the status of a nearby licensed RF spectrum. Depending on the detected status, the licensed RF is used on a Cognitive basis in `Interweave' mode to form a Hybrid FSO/Cognitive RF system. Performance analysis is presented and closed form expressions are derived for outage probability and RF utilization. The analysis assumes a Gamma-Gamma fading channel for the FSO, Rayleigh fading for the RF sensing channel and Nakagami-m fading for the secondary RF channel. Impact of primary channel occupancy, probability of detection, probability of false alarm and thresholds (optical and RF) have been analyzed. Results show considerable improvement in outage probability over the FSO system with better utilization of the licensed RF resource.