# Achievable Rate of NOMA-Based Cooperative Spectrum-Sharing CRN over Nakagami-\(m\) Channels
## 1 Introduction  
The Internet of Things (IoT) is considered to be a useful component of future smart cities and provides several new applications for human safety and comfort. Physical devices are embedded with small sensors that help collect data from the devices itself as well as their surroundings. These data are transmitted to remote servers for analyses and feedback. So, the major benefit of IoT in future networks will be in the form of providing connected physical devices [1].  
The efficient sharing of an available spectrum is considered to be a challenging task in IoT networks. The lack of spectrum and power resources in IoT networks limits their performance. To overcome this performance limitation, the introduction of collaborative communication with the support of cognitive radio technologies is believed to be essential [2, 3]. The allocation of spectrum bands to IoT devices is regarded as a non-trivial task since the number of IoT devices is predicted to increase significantly in future. Thus, it is important to design spectrum-sharing techniques for future IoT networks.  
In addition to cooperative spectrum sharing among networks, efficient multiple access techniques are also essential for achieving effective spectrum utilization. In this regard, it is expected that the role of non-orthogonal multiple access (NOMA) will be important as it can help improve spectral efficiency gains compared to conventional orthogonal multiple access (OMA) schemes [4, 5, 6]. More precisely, NOMA allows users to use the same time/frequency resource but with different power levels. The users employ successive interference cancellation (SIC) techniques to get rid of co-channel interferences and obtain the desired message. It has been shown in the literature that when NOMA is integratedwith cognitive radio networks, the network's performance improves in terms of spectral efficiency and connectivity [7].  
In IoT-based future communication networks, there will be a massive number of devices that will compete among each other for the spectrum resource. In order to meet their spectrum requirements, IoT devices may have to collaborate with the devices of the surrounding networks. In this study, we consider a cognitive radio spectrum-sharing network, where two set of users are present: primary users (PUs) and secondary users (SUs). The PUs are the licensed owners of the frequency band, whereas the SUs are the unlicensed users (IoT devices). By allocating the licensed spectrum of the PU to the unlicensed SU, cognitive radios can increase the spectral efficiency of a communication system [8]. A cognitive radio can operate in three different ways, namely, interweave, overlay, and underlay spectrum-sharing paradigms [8]. NOMA can be used in spectrum-sharing cognitive radio networks to meet the higher spectral efficiency and high connectivity requirements of future wireless networks [7]. More precisely, NOMA-based cooperative spectrum-sharing networks can improve the reception's reliability (i.e., lower outage probability) compared to their non-cooperative counterparts [4].