# Computational-Intelligence-Based Spectrum-Sharing Scheme for NOMA-Based Cognitive Radio Networks
## 1 Introduction
### Related Work  
The overlay scenario of NOMA-based CRNs presents a promising combination for improving spectrum utilization efficiency, in which a SU relays the PU's signal and as a reward transmits its own signal simultaneously using the NOMA principle. In [22], the authors investigated a cooperative spectrum-sharing scheme in which a secondary transmitter assisted the primary transmission as a relay. In exchange for cooperation, the secondary transmitter employed the NOMA transmission policy and was able to transmit simultaneously with the PU over the same subchannel. Another NOMA-assisted cooperative transmission scheme was proposed for CRNs [23]. It was shown that by applying cooperative transmission in a downlink overlay scenario, diversity gains were achieved for both PU and SU. [PERSON] proposed a NOMA-based overlay spectrum-sharing protocol over independent Rayleigh fading channels [24]. In order to enhance spectrum utilization efficiency and system capacity, the proposed scheme involved coordinated direct and relayed transmission through a secondary transmitter acting as a relay to assist a weak PU in its communication. Performance analysis in terms of ergodic sum capacity and outage probability (OP) showed superior performance of the proposed cooperative spectrum-sharing-based NOMA transmission over conventional NOMA-based communication. In [25], a multiple-user NOMA-based CRN is investigated that has an overlay scenario in which a single best SU is selected based on signal-to-noise ratio (SNR) to assist the primary communication. The performance of the proposed system is investigated through OP and capacity analysis for both primary and secondary networks. Yet another cooperative spectrum-sharing strategy exploiting NOMA in the second time slot was proposed by [PERSON] et al. in reference [26], in which a secondary transmitter relays the primary information concurrently with its own transmission. In another NOMA-based spectrum-sharing protocol for overlay CRNs [27], spatial modulation at the secondary relay is incorporated with antenna selection to enable simultaneous transmission to primary and secondary receivers. The authors showed through simulations that NOMA with spatial modulation significantly improved spectrum utilization efficiency with reduced detection complexity. In another NOMA-aided spectrum-sharing scheme for multicast overlay CRNs [28], the communication process is completed in three phases. In the first phase, a common base station serves both primary and secondary networks using the NOMA principle, while SUs relay the PU's signal in the second phase to overcome fading impairments. As an incentive, the third phase is dedicated to supporting the multicast of the SUs over the PU's licensed spectrum. The derived analytical expressions and simulation results showed improved outage performance of SUs while securing the primary user's QoS requirements. [PERSON] proposed a CR NOMA system for the performance enhancement of both primary and secondary networks with a primary direct link (DL) and relay selection [29]. The performance of the primary network is improved by combining technologies, whereas channel-state-information (CSI)-based relay selection is performed on the secondary network to offer spatial diversity. [PERSON] et al. proved that cooperative NOMA-based overlay cognitive terrestrial networks show superior performance to traditional OMA networks [30]. All these research contributions highlight that combining overlay CRNs with cooperative NOMA under the Rayleigh channel model holds tremendous potential for improving spectrum utilization efficiency. To further elaborate on the contributions of this article, the features of existing contributions on NOMA-based CRNs for Rayleigh fading channels are summarized in Table 1, while highlighting the benefits of this study.