# Performance Analysis of Soft-Switching FSO/THz-RF Dual-Hop AF-NOMA Link Based on Cognitive Radio  
[PERSON]  
1 Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Guangzhou510006, China; [EMAIL_ADDRESS]  
[PERSON]  
1 Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Guangzhou510006, China; [EMAIL_ADDRESS]  
[PERSON]  
1 dongyuan Wang  
be activated, if it is higher than the threshold of both at the same time, the corresponding link can be activated according to the preset situation, and if they are both lower than the threshold of both, the hybrid link will be outage. For soft-switching, FSO requires two thresholds to be set, while THz requires only one threshold. Therefore, THz links work the same as when hard-switching, and for two threshold FSO links, the relationship with the high and low thresholds is judged. Similarly, if both are outage, the hybrid link is outage. The proposed soft-switching hybrid link increases the complexity of the link, but it greatly improves the problem of frequent switching damage to the link by hard-switching [9]. The proposal of using non-orthogonal multiple access (NOMA) is a revolutionary solution to the issue of low SE [10]. By enabling multiple users to share frequency bands or time bands, NOMA increases the communication capacity in the new generation of wireless networks [11]. While NOMA can undoubtedly enhance SE, it still requires the allocation of a specific frequency band. This becomes particularly adverse in an age where spectrum usage remains disappointingly low. For instance, in 2005, the spectrum usage in Chicago for the 30-3000 MHz range stood at 17.4% [12]. In 2008, the spectrum usage for the 80-5850 MHz range in Singapore was 4.54% [13]. Similarly, in 2012, the spectrum usage for the 30-3000 MHz range in Beijing was 7.63% [14]. In order to solve the abovementioned problem of low spectrum usage, cognitive radio (CR)-NOMA has been proposed. There are three ways of spectrum sharing based on CR: underlay, interweave, and overlay [15]. In the case of underlay CR, the secondary transmitter (ST) and the primary transmitter (PT) may transmit simultaneously using the original frequency bands of the primary network (PN), ensuring that the interference from the secondary network (SN) to the PN is below a tolerance level. This way needs to protect the communication quality of the PN, and it has strict requirements for spectrum sharing. In interweave CR, the cognitive engine first determines the frequency bands that the PN has a license to use, and when the PN does not detect activity in these bands, the SN uses these licensed bands. The determination of these spectrum holes by the cognitive engine is called spectrum sensing. This way requires the flexibility to detect the spectrum usage of the PN at any time. In the case of overlay CR, the ST is transmitted simultaneously with the PT, but the SN needs to be used as a relay to forward the signals from the primary user (PU) to the target receiver to compensate for the interference caused to the PN. This way requires the SN to bear part of the signals' transmission in the PN. This increases the complexity of the SN. The fusion of underlay CR and NOMA has received particular attention in the last few years due to the fact that underlay CR, compared to interweave CR, can be transmitted simultaneously with the PU, increasing the flexibility of the SU, and not being responsible for the signals of the PU reduces the burden on the SN, compared to overlay CR [16]. By incorporating CR into the SN, underutilized spectrum bands in the PN can be effectively utilized, which not only improves SE and optimizes spectrum usage [17] but also contributes to a more efficient allocation of limited spectrum resources [18]. Leveraging CR, the SN can access and utilize the underutilized spectrum of the PN, an innovative approach with great potential to improve SE and optimize the use of available frequency resources [19].