A Novel Blind Spectrum Sensing Technique for Multi-User Ultraviolet Communications in Atmospheric Turbulence Channel

One of the primary challenges in multi-user free space optical wireless systems is to periodically sense the optical frequency spectrum at the receiver to identify whether the channel is occupied by other transceiver pairs in order to avoid interference. Spectrum sensing enables the user to continuously monitor the channel prior to its access to the channel. In this paper, we propose a novel blind spectrum sensing scheme for an unknown ultraviolet signal over strong atmospheric turbulence channel, based on the estimated signal-to-noise ratio (SNR) and the noise power. We derive this estimated SNR and the noise power from a statistical ratio of the received signal. The turbulence effect causes fluctuations in the received signal intensity, resulting in power loss at the receiver. Taking this negative effect into account, the blind SNR-based spectrum sensing scheme is equipped with spatial diversity scheme in the form of switch-and-stay combining. The intensity fluctuations are modeled by a gamma-gamma distribution. Based on the estimated SNR and noise power expressions, novel closed-form expressions for the probability of detection and the probability of false alarm are derived. It is also found that sensing capability is enhanced with the diversity scheme.