Generic blind spectrum sensing scheme for all optical-wavelength multi-user free space optical communications

Abstract

Spectrum sensing is a fundamental component in multi-user free space optical communication systems. In order to avoid interference among the users, it is required to periodically sense the optical frequency spectrum to detect if the channel is occupied by active transceiver pairs. Spectrum sensing allows the user to continuously sense the optical spectrum within their operating range to detect the channels. In this paper, we propose a generic novel blind spectrum sensing scheme for an unknown optical signal over strong atmospheric turbulence channel. By generic, we mean that the proposed scheme is applicable to all optical wavelengths, i.e. ultraviolet (UV), visible light, and infrared. The proposed blind spectrum sensing scheme is based on the estimated signal-to-noise ratio (SNR) and the noise power, for which we have derived expressions from a statistical ratio of the received signal. The atmospheric turbulence is the main reason behind the signal fading in free space optical links as it causes fluctuations in the received signal intensity. To mitigate this adverse effect, the proposed scheme is equipped with a spatial diversity scheme in the form of switch-and-stay combining. We use Gamma–Gamma distribution to model both small-scale and large-scale turbulence fadings. From the estimated SNR and noise power expressions, we formulate the novel closed-form expressions for the probabilities of detection and false alarm mathematically. Numerical and simulation results based on randomly generated optical signals are presented to verify the effectiveness of the proposed scheme.