Channel gain modeling in fiber-coupled Free Space Optics

Free Space Optics (FSO) has met a renovated interest for 6G applications. The large availability of unlicensed bandwidth, and the lack of mutual interference between the apparata, make FSO a promising technology for next-generation transport networks. However, propagation impairments severely affect FSO, like clear air turbulence, which induces irradiance and angle-of-arrival (AOA) fluctuations across the wavefront. Receiving diversity can be applied to mitigate turbulence fading. Recently, all-optical processing has emerged as the paradigm for coherent FSO systems, where the received field, coupled inside waveguides, e.g., single-mode fiber, is processed optically before detection. Coupling over-the-air propagation to fiber introduces limitations on the angular acceptance of the receiver, hence, the interaction of the AOA with the numerical aperture of the optical antenna might induce an increase of fading, on top of the irradiance. Appropriate modeling of the latter is necessary for predicting the performance of FSO systems. In this paper, we propose a channel model for fiber-coupled FSO in turbulence, with the aim of fully characterizing the fading, including angle and irradiance fluctuations, and we derive approximate expressions for the Bit-Error Rate (BER) with OOK modulation. When compared with the classical results obtained ignoring the AOA dispersion, we evaluate several remarkable differences.