# Optical-OFDM VLC System: Peak-to-Average Power Ratio Enhancement and Performance Evaluation
## 4 PAPR Reduction Methodology and Proposed Model
### Proposed PAPR Reduction System
Consequently, it is advisable to utilize a 7 dB DC bias level when dealing with small constellation sizes, while a 13 dB DC bias level is more suitable for high constellation sizes like 1024-QAM. The DCO-OFDM system operating at 7 dB with a 16-QAM modulation scheme provided nearly the same BER performance when working at 10 dB with 4-QAM. Furthermore, the DCO-OFDM system working at 10 dB with a 256-QAM modulation scheme exhibited slightly better BER performance when operating at 13 dB with the same 256-QAM scheme. Finally, the numerical results for this approach are illustrated in Table 6 at a BER equal to \(10^{-4}\) for various modulation schemes. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline OOFDM symbols & 1000 \\ \hline FFT/IFFT size & 1024 \\ \hline Modulation technique & QAM \\ \hline Constellation order & 4, 16, 64, 256, 1024, 4096 \\ \hline DC-bias level & 7,10,13 \\ \hline Cyclic prefix & \(1024/4=256\) \\ \hline Channel model & AWGN \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Simulation parameters. Also, Figure 11a illustrates the effectiveness of changing the DC-biasing with different modulation orders in the DCO-OFDM system performance. For example, when employing high modulation orders, such as 512-QAM, and using low DC-biasing levels, such as 7 and 10 dB, the system's performance was notably subpar, with a high bit error rate of \(10^{-3}\) even for high SNR levels. However, the system exhibited superior performance when employing the same high modulation order but with a higher DC shift of 13 dB. However, lower modulation orders, such as 4-QAM, and low DC-biasing levels, such as 7 dB, performed better than 10 and 13 dB. This means that higher DC-biasing is needed for higher modulation orders to reduce the effect of zero clipping distortion because, in higher modulation orders, the Euclidean distance between any two adjacent symbols is lower than in lower modulation orders. After evaluating the influence of DC biasing and modulation order on the system, we determined whether the PAPR was affected by both DC biasing and the selected modulation order. The CCDF of the DCO-OFDM system was assessed, as depicted in Figure 11b. At \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**MCM**} & \multicolumn{2}{c}{**4-QAM**} & \multicolumn{2}{c}{**16-QAM**} & \multicolumn{2}{c}{**64-QAM**} & \multicolumn{2}{c}{**256-QAM**} & \multicolumn{2}{c}{**1024-QAM**} \\ \cline{2-10}  & **BER** & **PAPR0** & **BER** & **PAPR0** & **BER** & **PAPR0** & **BER** & **PAPR0** & **BER** & **PAPR0** \\ \hline DCO\_7 dB & 15.9 & 9.625 & 21.65 & 9.58 & NA & 9.53 & NA & 9.455 & NA & 9.36 \\ \hline DCO\_10 dB & 18.4 & 7.962 & 22.25 & 7.71 & 26.8 & 7.948 & 33.6 & 7.8 & NA & 7.75 \\ \hline DCO\_13 dB & 21.32 & 5.91 & 25.13 & 6.02 & 29.45 & 6.07 & 34.18 & 6.03 & 39.15 & 6.14 \\ \hline ACO & 11.44 & 16.86 & 15.23 & 16.44 & 19.48 & 16.3 & 24.28 & 16.57 & 29.17 & 16.23 \\ \hline FLIP & 11.42 & 16.59 & 15.2 & 16.58 & 19.53 & 16.81 & 24.18 & 16.77 & 29.11 & 16.69 \\ \hline ASCO & 13.51 & 15.15 & 17.37 & 15.03 & 21.71 & 15.52 & 26.35 & 15.51 & 31.3 & 15.32 \\ \hline ADO\_7 dB & 19.4 & 10.5 & 23.22 & 10.59 & NA & 10.15 & NA & 10.13 & NA & 10.02 \\ \hline ADO\_10 dB & 22.13 & 8.44 & 25.9 & 8.5 & 30.15 & 8.59 & 35 & 8.353 & NA & 8.47 \\ \hline ADO\_13 dB & 24.81 & 6.94 & 28.5 & 7.14 & 32.84 & 6.97 & 37.55 & 7.13 & 42.46 & 6.98 \\ \hline
**LACO\({}_{2}\)** & 11.76 & 15.22 & 15.62 & 15.03 & 19.99 & 15.4 & 24.65 & 15.38 & 29.66 & 15.57 \\ \hline
**LACO\({}_{3}\)** & 12.82 & 14.43 & 16.78 & 14.595 & 21.16 & 14.59 & 25.9 & 14.22 & 30.86 & 14.63 \\ \hline
**LACO\({}_{4}\)** & 13.67 & 13.77 & 17.52 & 13.96 & 22.06 & 13.67 & 26.71 & 13.83 & 31.68 & 13.837 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The \(\rm E_{b}/N_{O}\) (dB) at a BER of \(10^{-4}\) and PAPR0 (dB) at a CCDF of \(10^{-3}\) numerical outcomes for different O-OFDM approaches across different constellation sizes. Figure 11: Performance of DCO-OFDM for (**a**) BER and (**b**) PAPR.