## 4 Performance Evaluation
### Simulation Results and Discussion
#### 4.2.5 Performance Comparison and Analysis of Latency  
The performance comparison in terms of latency is conducted and observed with a fixed \(\eta\) in the following cases: \(a\) and \(b\). In our simulation, the transmission and processing delays are the major contributors to the latency for the users, whereas queuing and propagation delays are considered negligible. For the queuing delay to be negligible, the simulation does not consider multiple users sending traffic simultaneously. Considering 2- and 3-user scenarios, the likelihood of traffic congestion is quite low without queueing the packets on the BS side, thus creating less traffic demand at the BS. Since the propagation delay  
Figure 8: Achievable sum rate comparison between fair and fixed PAs. Figure 7: Outage probability of fair PA with a two-user scenario. (**a**) Outage probability of fair PA. (**b**) Improved outage probability of fair PA.  
depends on the distance between the BS and the user, the simulation scenario considers user distances from the BS, such as 1000 m, 500 m and 200 m. Hence, the propagation delay will be in the order of \(\upmu\)s, thus treating it as negligible. Case (_a_): Considering a three-user scenario and comparing the latency trend for the SISO-NOMA and OMA schemes. The latency trend is observed in Figure 9 as a function of SNR for the SISO-NOMA and OMA schemes having a three-user scenario with \(\eta=2\) and fixed PA coefficients (\(\alpha_{1}=0.70\), \(\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)). The latency is plotted for each user in both cases, i.e., SISO-NOMA and OMA. This figure represents how the latency varied with varying SNR (signal quality) having three different user distances from the BS. We observe that the latency decreases with increasing SNR. At low SNR, the latencies in SISO-NOMA show higher latency than in OMA as the system consumes more time to multiplex (superposition coding) and decode (SIC) the signal. However, as the signal improves with increasing SNR and negligible noise, the rapid reduction of latencies in SISO-NOMA is observed around 5 dB-10 dB due to appropriate PA and simultaneous access to RB. As User 1 is located farthest from the BS and most likely goes to an outage condition, more power (70%) is allocated to ensure efficient sharing of resources, thus contributing to the higher latency than User 2 (20%) and User 3 (10%) in SISO-NOMA. The signal quality is poor for OMA at low SNR, leading to higher BER, as observed in Figure 4a. Hence, frequent retransmissions are required, increasing each user's latencies. Figure 9 shows that, at low SNR, the latencies for each user are higher. However, as SNR improves, the decreasing trend of latencies for each user is observed as SNR increases from 15 dB. Moreover, the latencies depend on users' respective distances from the BS, as a similar trend is observed for SISO-NOMA users' latency. To compare the performance in terms of latency, SISO-NOMA outperforms OMA as SNR increases. Case (_b_): Considering a two-user scenario and comparing the latency trend for fair and fixed PAs in SISO-NOMA. Figure 10 represents the latency trend for fair (derived in Equations (19) and (20)) and fixed (\(\alpha_{1}=0.80\) and \(\alpha_{2}=0.20\)) PAs as a function of the transmitted power in SISO-NOMA. The latency trends for fair and fixed PA cases decrease with increasing transmitted power. However, the decrease in latency for fair PA is more pronounced than in fixed PA, as fair PA dynamically adjusts the power allocated to SISO-NOMA users based on channel conditions. Such fair PA adjustments most likely provide better latency performance because of the efficient utilisation of resources. On the contrary, for fixed PA, users are allocated a fixed proportion of power independent of channel conditions, thus leading to inefficient use of  
Figure 9: Latency comparison between SISO-NOMA and OMA with \(\eta=2\) and fixed PA coefficient (\(\alpha_{1}=0.70,\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)). the available power.