## 4 Performance Evaluation
### Simulation Results and Discussion
#### 4.2.1 Performance Comparison and Analysis of Bit Error Rate (BER) between SISO-NOMA and OMA
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Users 1, 2 and 3 distance from BS & 1000, 500 and 200 metres, respectively \\ \hline For a two-user scenario, PA coefficients (\(\alpha_{1}\) & \(\alpha_{2}\)) & (70\% & 30\%) and (80\% & 20\%), respectively \\ \hline For a three-user scenario, PA coefficients (\(\alpha_{1}\), \(\alpha_{2}\) & \(\alpha_{3}\)) & (70\%, 20\% & 10\%) and (76\%, 16\% and 8\%), respectively \\ \hline Modulation scheme & Binary phase shift keying (BPSK) \\ \hline Path loss exponent (\(\eta\)) & 2 and 4 \\ \hline Channel & Rayleigh Fading \\ \hline Number of OFDM subcarriers & 128 \\ \hline Packet size & 128 bytes \\ \hline Noise & AWGN \\ \hline System bandwidth & \(1\,\mathrm{GHz}\) \\ \hline Power transmitted from BS & 40 dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters used in the simulation. Case (\(c\)): Considering a three-user scenario with \(\eta=2\) and fixed PA 3-tuple coefficients as (\(\alpha_{1}=0.70\), \(\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)) and (\(\alpha_{1}=0.76\), \(\alpha_{2}=0.16\) & \(\alpha_{3}=0.08\)). The BER performance trend is observed in Figure 3(a) against varying Signal-to-Noise Ratio (SNR) in decibels (dB) for the SISO-NOMA and OMA schemes having a three-user scenario with \(\eta=2\) and fixed PA 3-tuple coefficients as (\(\alpha_{1}=0.70\), \(\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)) and (\(\alpha_{1}=0.76\), \(\alpha_{2}=0.16\) & \(\alpha_{3}=0.08\)) for Users 1, 2 and 3. In this three-user scenario, User 1 is located far away from the BS, whereas User 3 is located near the BS. User 2 is located in the middle of User 1 and User 3. We observe that User 1, with 70%, has incurred higher BER than User 1, with 76%, at \(\text{SNR}=90\,\text{dB}\). This is because less power (70%) is allocated to User 1 than to User 1, with 76%. Contrarily, lower BER is observed for User 2, with 16%, than User 2, with 20%, even though higher power is allocated to User 2, with 20%. In both cases, cross-user interference is observed between the User 2 PA coefficients, which are 16% and 20%, respectively. For User 3 (with 10% and 8%), almost the same BER is noted. As \(\eta=2\) is also kept constant for OMA scheme users, it can be clearly perceived that the BER performance trend is relatively less than SISO-NOMA scheme users, as OMA users do not have to undergo decoding and SIC operations for received signals. Case (\(d\)): Considering a three-user scenario with \(\eta=4\) and fixed PA 3-tuple coefficients as (\(\alpha_{1}=0.70\), \(\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)) and (\(\alpha_{1}=0.76\), \(\alpha_{2}=0.16\) & \(\alpha_{3}=0.08\)). On the same note, Figure 3(b) shows the BER performance trend for \(\eta=4\) (instead of 2) and the same fixed PA 3-tuple coefficients as (\(\alpha_{1}=0.70\), \(\alpha_{2}=0.20\) & \(\alpha_{3}=0.10\)) and (\(\alpha_{1}=0.76\), \(\alpha_{2}=0.16\) & \(\alpha_{3}=0.08\)). The same BER performance is observed for Case (\(c\)) with User 1 (with 70% and 76%) at an \(\text{SNR}=140\,\text{dB}\). However, User 2, with 20%, has shown higher BER than User 2, with 16%. User 2, with 20%, has experienced more cross-user interference with User 1, with 70%, while User, 2, with 16%, has experienced less cross-user interference with User 1, with 76%. Approximately the same BER is observed for User 3 (with 10% and 8%). When OMA scheme users are compared to SISO-NOMA users, lower BER is noted for far, middle and near users with higher SNR values. It can be inferred that when \(\eta\) is increased from 2 to 4 in a three-user scenario, the degradation (increment) of BER performance is observed with \(\eta=4\) at higher SNR when compared with \(\eta=2\) due to increased interference. Figure 3: BER comparison between SISO-NOMA and OMA with \(\eta=2\) and 4, and fixed PA coefficient pairs as (\(\alpha_{1}=0.70\) and \(\alpha_{2}=0.30\)) and (\(\alpha_{1}=0.80\) and \(\alpha_{2}=0.20\)). (a) BER comparison between SISO-NOMA and OMA with \(\eta\) as 2. (b) BER comparison between SISO-NOMA and OMA with \(\eta\) as 4.