## 4 Performance Evaluation
### Simulation Results and Discussion
#### 4.2.1 Performance Comparison and Analysis of Bit Error Rate (BER) between SISO-NOMA and OMA  
To study the effect of varying path loss exponent (\(\eta\)) with fixed PA coefficient values, the performance comparison of BER is observed and measured for the SISO-NOMA and OMA schemes using two-user and three-user scenarios, with the following cases: \(a\), \(b\), \(c\) and \(d\). The PA coefficients for Users 1, 2 and 3 are denoted as \(\alpha_{1}\), \(\alpha_{2}\) and \(\alpha_{3}\), respectively. The varying \(\eta\) allows system behaviour analysis under different environmental conditions. When \(\eta\) increases, the system represents the received signal's faster decay with distance. Case (_a_): Considering a two-user scenario with \(\eta=2\) and fixed PA coefficient pairs as (\(\alpha_{1}=0.70\) & \(\alpha_{2}=0.30\)) and (\(\alpha_{1}=0.80\) & \(\alpha_{2}=0.20\)). The BER performance trend is observed in Figure 2(a) against a varying Signal-to-Noise Ratio (SNR) in decibels (dB) for the SISO-NOMA and OMA schemes, having a two-user scenario with \(\eta=2\) and fixed PA coefficient pairs as (\(\alpha_{1}=0.70\) & \(\alpha_{2}=0.30\)) and (\(\alpha_{1}=0.80\) & \(\alpha_{2}=0.20\)) for Users 1 and 2. User 1 (weak user) is located far away from the BS, whereas User 2 (strong user) is located near the BS. It is observed that User 1, with \(\alpha_{1}\) having 70%, has incurred more BER than User 1, with \(\alpha_{1}\) having 80%, at an \(\text{SNR}=80\) dB. This performance trend is because more power has been allocated to User 1, with 80%, than User 1, with 70%. In contrast, even though higher power is allocated to User 2, with 30%, it shows a higher BER than User 2, with 20%. This is mainly due to cross-user interference between User 1, with 70%, and User 2, with 30%. Thus, there is less interference between Users 1, with 80%, and 2, with 20%. Keeping \(\eta=2\) constant, when the BER performance trend with the SISO-NOMA scheme is compared with the OMA scheme, it is seen that lower BER is noted for the user pairs (with 70% and 30%) and (with 80% and 20%). In the SISO-NOMA scheme, the extra signal processing complexity is added at the receiver end, i.e., decoding and performing SIC of received signals, thus leading to higher BER than in the OMA scheme. Case (_b_): Considering a two-user scenario with \(\eta=4\) and fixed PA coefficient pairs as (\(\alpha_{1}=0.70\) & \(\alpha_{2}=0.30\)) and (\(\alpha_{1}=0.80\) & \(\alpha_{2}=0.20\)). On similar lines, the BER performance trend is observed in Figure 2(b) with the same fixed PA coefficient pairs as (\(\alpha_{1}=0.70\) & \(\alpha_{2}=0.30\)) and (\(\alpha_{1}=0.80\) & \(\alpha_{2}=0.20\)) for Users 1 and 2, but changing \(\eta\) from 2 to 4. For this simulation case, the effect of increasing \(\eta\) is noted for BER performance for Users 1 and 2. In Figure 2(b), we observe that a slightly lower BER is encountered for User 1, with 80%, than for User 1, with 70%, at \(\text{SNR}=150\) dB. In contrast, a similar trend is observed for Case (a), followed by User 2, which has a lower BER of 20%, than User 2, which has 30%. Moreover, in the OMA scheme, smaller BER is observed than in the proposed SISO-NOMA scheme. Considering a constant \(\eta=4\), as the SNR increases BER tends to improve as the received signal becomes more distinguishable than the noise signal. Hence, improvement in BER performance is observed when \(\eta\) is increased from 2 to 4 and the PA coefficient pairs are kept fixed. Consequently, a trade-off is observed in BER performance to strike a balance between \(\eta\) and PA coefficients to optimise the communication performance.