## 3 System Model  
### Framework for HNOMA  
This section outlines the pairing schemes used in HNOMA systems. As mentioned, NOMA is a prominent technology supporting dense connection networks. However, the SIC inherent to NOMA can complicate the system design and degrade overall performance. HNOMA has been introduced as a combination of OMA and NOMA to address this issue. This approach benefits from the simplicity of the OMA structure while achieving the massive connectivity offered by NOMA systems. The system combines time division multiple access (TDMA) with the NOMA framework, specifically designed for \(M\) UE systems. Figure 1 illustrates the \(i\) th sub-frame allocated to the \(p\)th pairing scheme for the IOS-aided hybrid cooperation NOMA (C-HNOMA) system. The total time frame is denoted as \(T\), which is divided into \(T_{i}=2T/M\) sub-frames in a \(M\)-user system. In the \(M\)-user system, the total time frame, \(T\), is divided into \(M/2\) sub-frames. During each sub-frame, pairs of devices are allowed to have DL transmissions from gNB. Within each sub-sub-frame, the paired UE uses a two-UE C-NOMA system. This arrangement results in a two-user IOS-aided C-HNOMA system within the sub-frames. A crucial aspect of this setup is the process of pairing the UE. This section elaborates on the various pairing schemes and the complexities associated with each. Figure 1 pertains to the \(i\)th sub-frame. During \(T_{i}/2\), there is a transmission from gNB to the SUE and WUE through IOSs. In the remaining \(T_{i}/2\), cooperative relaying occurs between the SUE and WUE through IOSs. The direct link between the SUE and WUE during the cooperative relaying is assumed to have significant blockage. Hence, the effect of the direct link is negligible. The following assumptions are established for this system: the gNB and all connected UE function using a single antenna architecture to simplify the overall design and signal processing. The total number of devices, represented as \(M\), is in the power of 2, allowing two pieces of UE in each pair. Additionally, the devices are ordered based on their channel gains, which are dependent on the distance from the gNB to the path-loss exponent; the UE closest to the gNB is identified as the strongest, benef  
Figure 1: The sub-frame allocation for the IOS-aided C-HNOMA system. quality, while the farthest UE is the weakest, experiencing the poorest signal reception. This structured ordering is essential for optimizing the PA and network's performance. **NOTE:** The manuscript states that the device with the higher channel gain in each pair is considered the SUE, while the other device is regarded as the WUE. For the description of pairing schemes, the pairing of eight UE/IoT devices with various pairing schemes is illustrated below. The assumption is that UE\({}_{1}\), UE\({}_{2}\), UE\({}_{3}\), and UE\({}_{4}\) are on the reflecting side of the IOS, and UE\({}_{5}\), UE\({}_{6}\), UE\({}_{7}\), and UE\({}_{8}\) are on the refracted side of the IOS. From Figures 2-6, a DL transmission from gNB to UE through the IOS is presented. The total time frame is equally divided into four sub-frames for eight pieces of UE, represented as \(T_{1}\), \(T_{2}\), \(T_{3}\), and \(T_{4}\), as shown in blue, red, yellow, and purple, respectively, in the figures. The transmissions from gNB to UE through the IOS in the direct transmissions in the sub-frames are represented as solid lines in the figures from Figures 2-6. As mentioned before, within each sub-frame, part of the time frame is allotted for direct transmission, while the remaining time frame is allocated for the SUE to relay the WUEs' message during cooperative relaying. The cooperative links between the SUE and WUE are represented as dotted lines in Figures 2-6. The paired UE in every sub-frame for various pairing schemes is listed in Table 2.  
* **Near-near and far-far (NN-FF) pairing [27]:** Figure 2 illustrates NN-FF pairing. In this configuration, adjacent pieces of UE are paired, resulting in similar channel gains.