## 2 System Model  
We consider an uplink CRN that consists of a cognitive base station (CBS), a primary base station (PBS), multiple primary users, and \(2M\) cognitive users as illustrated in Figure 1. Each CU is outfittedwith a single antenna to transmit data to the CBS, and each is equipped with an energy-harvesting component (i.e., solar panels). The PBS and PUs have the license to use the primary channel at will. However, they do not always have data to transmit on the primary channel. Meanwhile, the CBS and the CUs can opportunistically utilize the primary channel by adopting a hybrid NOMA/OMA technique when the channel is sensed as free. To this end, the CBS divides the set of CUs into pairs according to Algorithm 1 where the CU having the highest channel gain will be coupled with the CU having the lowest channel gain, and one of available channels will be assigned to these pairs. More specifically, the CUs are arranged into \(M\) NOMA groups, and the primary channel is divided into multiple subchannels to apply hybrid NOMA/OMA for the transmissions between the CUs and the CBS, with \(\mathcal{G}=\left\{G_{1},G_{2},G_{3},...,G_{M}\right\}\) denoting the set of NOMA groups. Additionally, \(M\) NOMA groups are assigned to \(M\) orthogonal subchannels, \(\mathcal{SC}=\left\{SC_{1},SC_{2},SC_{3},...,SC_{M}\right\}\), of the primary channel such that the CUs in each NOMA group can transmit on the same subchannel and will not interfere with the other groups. In this paper, successive interference cancellation (SIC) [34] is applied at the CBS for decoding received signals, which are transmitted from the CUs. Moreover, we assume that the CUs always have data to transmit, and the CBS has complete channel state information (CSI) of all the CUs. ```
1:Input: channel gain, number of groups, \(M\), number of CUs, \(2M\). 2: Sort the channel gain of all CUs in decending order: \(g_{1}\geq g_{2}\geq...\geq g_{2M}\)
3: Define set of channel gains \(U=\left\{g_{1},g_{2},...,g_{2M}\right\}\)
4:for\(j=1:M\)
5:\(G_{j}=\left\{\varnothing\right\}\)
6:\(G_{\max}=\max\left\{U\right\},G_{\min}=\min\left\{U\right\}\)
7:\(G_{j}=G_{j}\cup G_{\max}\cup G_{\min}\)
8:\(U=U\backslash G_{\max}\backslash G_{\min}\)
9:endfor
10:Output: Set of CU pairs. ```  
**Algorithm 1** User-pairing Algorithm  
The network system operation is illustrated in Figure 2. In particular, at the beginning of a time slot, with duration \(\tau_{ss}\), all CUs concurrently perform spectrum sensing and report their local results to the CBS. Based on these sensing results, the CBS first decides the global sensing result as to whether  
Figure 1: System model of the proposed scheme. the primary channel is busy or not following the combination rule [35; 36], and then allocates power and bandwidth to all CUs for uplink data transmission. As a consequence, according to the allocated power and bandwidth of the NOMA groups, the CUs in each NOMA group can transmit their data to the CBS through the same subchannel without causing interference with other groups within duration \(\tau_{Tr}=T_{tot}-\tau_{ss}\), where \(T_{tot}\) is the total time slot duration. Information regarding the remaining energy in all the CUs is updated to the CBS at the end of each time slot. Each data transmission session of the CUs may take place in more than one time slot until all their data have been transmitted successfully.