# Performance Analysis of Soft-Switching FSO/THz-RF Dual-Hop AF-NOMA Link Based on Cognitive Radio
Thus far, there exist scholarly researches that have explored the combination of CR with dual-hop links or NOMA. In CR-based RF-FSO dual-hop links, [PERSON] et al. investigated amplify-and-forward (AF) in the presence of imperfect channel state information (CSI) [20]. Based on this, the case of multiple antennas at the relay (R), PT, and primary receiver (PR) was studied [21]. In addition, the multi-relay case introduced feedback links to determine and select the link with the best channel gain [22]. Notably, the fixed and CSI-assisted AF cases were examined to study single and two power-constrained points [23]. Shifting the focus to the fusion of CR and NOMA, CR-enabled AF-NOMA links in a two-user scenario were reviewed, considering the scaling coefficient of additive white gaussian noise (AWGN) [24]. Additionally, a CR-NOMA bidirectional cooperative relay scheme was proposed [25]. Lastly, a CR-based DF-NOMA in five modes was analyzed, considering the maximum transmit power of the ST and the maximum tolerable interference power of the PN [26]. Through the above survey, it is found that most of the investigations only focus on a single problem, as CR with FSO/THz solves the problem of scarce spectrum resources and CR-NOMA improves SE; thus, it can be seen that the combination of both of them is essential and innovative, which arouses a great deal of research interest. In this article, research gaps were identified, so FSO/THz-RF dual-hop links, and NOMA and CR were combined for their unique and fascinating characteristics. Consequently, a novel soft-switching FSO/THz-RF dual-hop AF-NOMA link based on CR is proposed. The link entails a soft-switching FSO/THz link serving as the first hop from the ST to the R, while the second hop exploits the potential of CR to effectively allocate the spectrum resources of the PN from the R to the secondary user (SU). The signals are transmitted via the AF-NOMA scheme, traversing an RF link to reach two users. In short, a soft-switching FSO/THz-RF dual-hop AF-NOMA link based on CR can use CR to address lower spectrum usage, NOMA can improve SE, the soft-switching FSO/THz pairing can reduce the impact of different atmospheric conditions, and RF can traverse obstacles to communicate with users. The rest of the paper is organized as follows. Section 2 derives the probability density function (PDF) and cumulative distribution function (CDF) for FSO following the Weibull distribution, and the PDF and CDF for THz following the \(\alpha-\mu\) distribution and considering pointing errors, and sets two FSO thresholds and a single THz threshold to form a soft-switching FSO/THz link. In Section 3, we first analyze the characteristics of the underlay CR and derive the relay transmission power with the maximum tolerable interference power in PN and the maximum relay transmission power. Subsequently, we analyze the properties of the NOMA system and derive the link OP by combining CR and NOMA. We deliver meaningful scenarios into the system to obtain beneficial numerical results in Section 4, and Section 5 concludes the paper.