Free-space optical (FSO) links are considered as cost-effective, noninvasive alternative to fiber optic cables for 5G cellular backhaul networking. For FSO-based backhaul networks, we propose an additional millimeter-wavelength (MMW) radio-frequency (RF)-FSO link, used as a backup. Uninterrupted and reliable network connection is possible by switching between primary FSO link and the secondary RF-FSO link; when the primary link is under atmospheric turbulence, the secondary link maintains connectivity as the MMW RF link exhibits complementary characteristics to atmospheric effects. In order to analytically assess the improvement, we also derive concise mathematical expressions for different performance metrics, such as outage probability, average bit error rate (BER), and capacity. Our results demonstrate that the FSO/RF-FSO topology performs better than a single FSO link in terms of outage probability and BER. The dual-hop mixed RF-FSO link is realized with an amplify and forward (AF) relay that adapts an average power scaling strategy. The irradiance fluctuations in the FSO links are modeled by gamma-gamma distribution, assuming strong atmospheric turbulence while it is assumed that the RF link experiences multipath Rayleigh fading. For switching between links, a single FSO threshold is considered first, followed by a dual FSO threshold to prevent unnecessary switching.

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Atmospheric turbulence and pointing errors are major hindrances behind the proliferation of free-space-optics (FSO) technology in next-generation fronthaul networks. This paper investigates the effect of a decode-and-forward relay in overcoming these impairments and how an additional source-to-relay millimeter-wave (mmWave) radio-frequency (RF) backup link improves the performance of the relay-aided FSO system further; i.e., we study the performance of a hybrid FSO/(FSO-FSO)/(RF-FSO) system. Generalized Málaga distribution is adopted to model atmospheric turbulence in FSO links, whereas the fading statistics of the mmWave RF link is modeled by Nakagami- m distribution. The performance of the hybrid system is studied on the basis of analytical expressions derived for metrics such as outage probability, bit error rate, and ergodic capacity, and the results show the supremacy of the proposed hybrid FSO system. The improvement is, however, dependent on factors such as turbulence location and signal-to-noise ratio per bit, and not all performance metrics exhibit equal improvement.