# Performance Analysis of Dual-Hop Hybrid RF-UOWC NOMA Systems
## 6 Results and Discussion
Furthermore, Figure 5 depicts the temperature gradient (\(TG\)) and air bubbles level effect on the OPs performance. This figure investigated three different scenarios. We set \(BL=2.4\) and \(TG=0.05\) in case1, \(BL=2.4\) and \(TG=0.15\) in case2, and \(BL=4.7\) and \(TG=0.1\) in case3. It is clear that the higher the level of the air bubbles and/or the temperature gradient, the stronger the turbulence, leading to a OPs performance deterioration. Figure 6 demonstrates the influence of the power allocation factor \(a_{1}=b_{1}\), which varies from 0.5 to 0.99, on the OPs performance with \(\rho=40\) dB in two varying air bubble  
Figure 4: OPs versus SNR for thermally uniform salty UOWC links at \(BL=2.4\) L/min for varying values of \(\eta\). Figure 5: The effect of temperature gradient and air bubbles level on OPs performance. levels of \(BL=2.4\) and \(BL=4.7\) L/min. We can observe that the \(OP_{1}\) enhances with the increase in \(a_{1}\) due to the increase of its own message power. On the other hand, the \(OP_{2}\) witnesses an improvement at first with \(a_{1}\) increase as \(D_{2}\) needs to decode \(x_{1}\) first before decoding its own message \(x_{2}\). However, with the continuous increase in \(a_{1}\), an inflection point is reached since increasing \(a_{1}\) means decreasing the allocated power for \(D_{2}\) message (\(a_{2}=1-a_{1}\)) that degrades the \(OP_{2}\). Finally, the \(OP_{sys}\) follows the same trend as \(OP_{2}\) with a bit increase. Additionally, this figure graphically proves the convexity of the optimization problem in (29). Figure 7 illustrates the influence of the residual power factor of imperfect SIC on ECs performance of the proposed system under uniform thermally salty water at \(BL=2.4\) L/min where \(\eta=0.01\), and \(0.05\). We can see that the \(EC_{x_{2}}\) and ESC performance degrades by increasing \(\eta\). This is due to the fact that an increase in \(\eta\) leads to a higher interference level at the decoding process of \(x_{2}\). On the other hand, the \(EC_{x_{1}}\) performance is not affected by changing \(\eta\). The figure also shows a perfect agreement between the simulation and the obtained analytical results at high SNR with a small deviation at low SNR. This deviation is due to the usage of the tight approximated expression for the CDF of the EGG distributions at high SNR. Figure 6: OPs over the entire range of power allocation factor at SNR = 40 dB. Figure 8 illustrates the ECs for the proposed system under uniform temperature salty water with two air bubble levels of \(BL=2.4\) and \(BL=4.7\) L/min. It is clear that the increase in the level of air bubbles leads to a deterioration in the ECs performance; this is due to the increase in water turbulence. Moreover, Figure 9 shows the effect of \(TG\) on the ECs performance in salty water under the air bubbles level \(BL=2.4\) L/min. The figure investigated two different values of \(TG=0.05,0.15\). It is obvious that the higher the level of the temperature gradient, the stronger the turbulence, leading to a ECs performance degradation. From Figures 8 and 9, we can conclude that the effect of the variation in water turbulence (\(BL\), \(TG\)) is negligible at the high SNR regime. Figure 8: ECs versus SNR for thermally uniform UOWC links for varying air bubbles levels. Figure 7: ECs versus SNR for thermally uniform salty UOWC links at \(BL=2.4\) L/min for varying values of \(\eta\). Figure 10 demonstrates the influence of the power allocation factor \(a_{1}\), which varies from 0.5 to 0.99, on the ECs performance to gain insight into the effectiveness and the fairness with \(\rho=50\) dB, under uniform temperature salty water with \(BL=2.4\). We can see that \(EC_{x_{1}}\) increases as \(a_{1}\) increases because the higher power allocation factor means a higher SINRs \(\gamma_{R}^{1}\), \(\gamma_{D1}^{1}\), and \(\gamma_{D2}^{1}\), but \(EC_{x_{2}}\) drops as power allocation factor increases because the SINRs \(\gamma_{R}^{2}\) and \(\gamma_{D2}^{2}\) degrade.