## 3 Optimal Power Allocation (PA), Cooperative Relaying and Channel Models in PD-NOMA
### Cooperative Relaying and Channel Models in PD-NOMA
In this context, the largest SNR difference in DBPA scheme indicates that powers are not distributed fairly, whereas the minimal SNR level in the FIS-based system establishes justice between users in terms of PA. Similarly, another attempt was made to test and compare the proposed FIS-based methodology with the DBPA scheme in Figure 11, for distinct input signals. Here in DBPA scheme, U\({}_{1}\) with \(d_{1}=40\) m is assigned with \(P_{1}=0.32\) and U\({}_{2}\) with \(d_{2}=80\) m is allocated with \(P_{2}=0.68\). As a result, U\({}_{2}\) obtains an SNR of 5.4 dB and U\({}_{1}\) obtains an SNR of 9 dB. So, the obtained SNR difference between the users is 3.6 dB. On the other hand, the FIS-based system uses (\(d_{1}\)=40 m, \(d_{f1}=5\) m, and \(SNR_{1}=15\) dB) and (\(d_{2}\)=80 m, \(d_{f2}=10\) m, and \(SNR_{2}=10\) dB) for U\({}_{1}\) and U\({}_{2}\), subsequently. Here, the FIS system results in \(P_{1}=0.44\) and \(P_{2}=0.56\). The obtained SNR level is 8 dB for U\({}_{1}\); 6.3 dB and 6.6 dB for U\({}_{2}\) with D&F cooperation and only PD-NOMA, respectively. Here, the highest SNR difference is 1.7 dB, which indicates that power is fairly distributed among users and hence user fairness is established. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Carrier frequency (\(f_{c}\)) & 28 GHz \\ \hline gNodeB radius & 200 m \\ \hline gNodeB and U\({}_{i}\) antenna & (SISO) 1,1 \\ \hline gNodeB height (\(h_{i}\)) & 10 m \\ \hline \(\beta\) & 0.0019 (Collective) \\ \hline Users (U\({}_{i}\)) height (\(h_{r}\)) & 1 m \\ \hline Path Loss exponent (\(n\)) & 2 \\ \hline Modulation schemes & BPSK \\ \hline Phase shifts & \(\pi/2,0\) \\ \hline \(X_{\sigma}\) & Mean 0, variance 2.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters used in implementation of System Model. Furthermore, simulation is carried out for the proposed FIS-based model in the Rician and Rayleigh channels. The SUI-3 and SUI-6 models are utilized as standard parameter values for Rician and Rayleigh fading channels, respectively. The BER performance for the proposed FIS-based system is compared with the DBPA in Figures 12 and 13 under the Rician and Rayleigh fading channels, respectively. Similar to previous results from NYUSIM models, the proposed technique again outperforms under these two channels in terms of user-fairness. In the Rician channel, the proposed FIS-based system achieves a threshold BER of \(10^{-3}\) at an SNR of 11.4 dB for U\({}_{1}\) (\(P_{1}\) = 0.34); 8.2 dB and 8.4 dB for U\({}_{2}\) (\(P_{2}\) = 0.66) with D&F cooperation and only PD-NOMA, respectively. While utilizing the DBPA scheme, the threshold BER is achieved at SNRs of 13.2 dB for U\({}_{1}\) and 8 dB for U\({}_{2}\), respectively. The DBPA allocates \(P_{1}\) = 0.23 to U\({}_{1}\) and \(P_{2}\) = 0.77 to U\({}_{2}\), respectively. Here, again, we can see the difference in the achieved SNRs between U\({}_{1}\) and U\({}_{2}\) is less in the case of the proposed FIS-based than the DBPA. As we can see in Figure 12 the SNR difference in FIS-based is 3.2 dB while it is 5.2 dB for DBPA. Similarly, Figure 13 also ensures that an FIS-based system outperforms the DBPA schemes in terms of optimal power allocation and user-fairness. The lower the difference between the achieved SNR levels, the larger the user-fairness and vice versa. User-fairness is the key capability of the NOMA technique that differentiates it from other OMA techniques. In comparison with the power allocation scheme of DBPA, the proposed FIS-based allocates optimal power factors with equity and fair-play and hence ensures user-fairness. The BER performance when utilizing the Rician channel is comparatively better than that of the Rayleigh channel. This is because the Rician channel is more sensitive to the channel's power allocation, which is more sensitive to the channel's power allocation. Figure 11: Comparisons of user fairness, in DBPA scheme [18] and the proposed FIS based model for two users in terms of the difference between obtained SNR levels.