# Best Relay Selection Strategy in Cooperative Spectrum Sharing Framework with Mobile-Based End User
Moreover, a novel decentralized technique for multi-user CRN was proposed in [42]. The outage probability (OP) was obtained for the considered system using AF and DF relaying techniques. The obtained results demonstrated that, for the considered system, the DF scheme outperformed the AF scheme in terms of outage probability performance. In [43], the bit error rate (BER) of CRNs was investigated considering DF incremental relaying technique. The presence of multiple non-identical interferers near the destination was considered in this work. In addition, all channels followed Rayleigh distribution and an MRC scheme was deployed at the destination, combining the signals from the direct and relaying paths. The OP expression was derived in [44] for the system model as well. Moreover, the same system model was investigated using Nakagami-\(m\) channels to derive the BER and OP expressions in [45] and [46], respectively. The obtained result showed that the system performance can be affected by several parameters such as the fading parameter \(m\). In addition, the co-channel interference and the number of interferers have a crucial impact in how well the system works. The performance of CRNs was investigated in [47] using a bidirectional DF relaying technique with the presence of an eavesdropper. In this work, both users (i.e., primary and secondary users) use a spatial modulation (SM), enhancing the spectral efficiency of the system. The OP and secrecy outage probability (SOP) expressions were obtained and the impacts of several system parameters were investigated. A partial relay selection scheme was considered in [48] to study the performance of DF CRNs to assist ultra-reliable and low-latency communications (URLLCs). No direct connection was available between the source and destination, bearing in mind that in the system model, the channels followed Rayleigh fading. Moreover, a power allocation strategy was considered to minimize the block error rate. Furthermore, non-orthogonal multiple access (NOMA) techniques were exploited with underlay CRNs for the sake of [49], improving the spectrum efficiency. The best relay selection (BRS) scheme was implemented in this work assuming Rayleigh fading model. The OP and bit error rate (BER) were derived for the considered system and the impact of the number of relays was investigated as well. A CRN with multiple users and AF relays was investigated in [50] considering Rayleigh fading channels. The OP and symbol error rate (SER) expressions were obtained for the considered system model, deploying a BRS scheme at the receiver. The obtained results demonstrated that the system performance can be enhanced using multiple relays. An optimal power allocation scheme using partial relay selection was considered in [51] for the sake of enhancing the performance of underlay CRNs. In this work, NOMA was used for transmission and one relay was selected among several DF relays. The OP expression was derived for the considered system model and the obtained results showed that the proposed power allocation scheme can increase the throughput compared with the random power allocation scheme. An optimal power allocation scheme was proposed in [52] to maximize the throughput delivered to mobile users under a NOMA relay-transmission scenario. Specifically, a key algorithm was proposed to compute the optimal power allocation for such system where NOMA was used by not only the BS to transmit the data toward the relays, but also by the relays to forward the data toward the mobile users. The obtained results showed that the proposed scheme can increase the throughput up to 30 percent compared with the time division multiple access (TDMA) scheme. The authors in [53] investigated the DF CRNs with power constraints from the PU and eavesdropper over Nakagami-\(m\) channels. A selection combining strategy was used at the destination to study the impact of changing several design parameters on the secrecy outage performance.