# Best Relay Selection Strategy in Cooperative Spectrum Sharing Framework with Mobile-Based End User  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] Relay Selection Strategy in Cooperative Spectrum Sharing Framework with Mobile-Based End User  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON] Relay Selection Strategy in Cooperative Spectrum Sharing Framework with Mobile-Based End User  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON]. Another smart technique that has attracted significant research attention to meet the heavy data traffic in mobile communications is termed as cognitive radio (CR). This paradigm of wireless communication can alter its operational parameters according to the environment and the user needs, enabling better spectrum utilisation and improving link reliability [14; 15; 16]. In particular, it shares the licensed spectrum between various users in an opportunistic manner, allowing simultaneous communications over a given spectrum band. The primary users (PUs) are the licensed owners of the frequency band (i.e., they have higher priority for transmission), whereas the secondary users (SUs) are the non-licensed owners (i.e., they have lower priority for transmission) [17; 18]. Interweave, underlay, and overlay are considered the main spectrum sharing paradigms in CR networks [19]. Specifically, the SU is able to access the licensed bands in a dynamic manner in an interweave technique [20]. On the other hand, the underlay and overlay techniques allow transmissions from the PU and SU simultaneously. In particular, the PU and SU can simultaneously use the same spectrum band with interference limitations at the PU receiver in the underlay technique. In contrast, the overlay technique can access the licensed spectrum band in time, frequency, or spatial domain [21; 22]. Different levels of cognition are required for each paradigm, leading to different challenges. Particularly, the underlay CR is based on channel state information (CSI) statistics, providing a less complex solution for the SU to access the licensed spectrum [23]. Detecting the PU presence over a licensed spectrum band and the limitation of the unfavorable effects of fading channels are mainly the key challenges of CR networks. Therefore, cooperative communication has been recently utilized to be merged with CR networks, leading to a new paradigm known as cognitive relay networks (CRNs) [24; 25]. Cooperative communication is considered one of the fundamental strategies in wireless networks that mitigates the impact of channel fading by using cooperative nodes (relays) [26]. The use of this paradigm can utilize the multi-input multi-output (MIMO) schemes to overcome the drawbacks of point-to-point systems [27; 28; 29; 30; 31]. The main approaches used in cooperative communication are mainly amplify-and-forward (AF), decode-and-forward (DF), and compress-and-forward (CF) relaying protocols [32]. In an AF scheme, the relay amplifies the incoming message and then re-transmits it to the end user [33; 34]. In contrast, in the DF scheme, the signal is decoded and re-transmitted towards the destination [35]. On the other hand, a coding scheme is used in the CF scheme at the relay to compress the signal and re-transmit it to the final destination [36]. Overall, cognitive relay networks (CRNs) have been proposed to overcome the various challenges of CR networks for further improvements in terms of link reliability, coverage area, and interference level [37; 38; 39]. The performance of CRNs was investigated in several works in the literature. The secrecy outage performance was investigated in [40] for threshold-based CRNs using DF relaying under different combining schemes. The obtained results showed that several factors affect the system secrecy such as the interference power and the diversity scheme. The secrecy outage probability was obtained in [41] for multi-hop CRNs under spectrum constraint from the PU. In this work, the SU channels experienced a Rayleigh fading model and the impact of the PU transceivers number was investigated.