# Optical-OFDM VLC System: Peak-to-Average Power Ratio Enhancement and Performance Evaluation
## 4 PAPR Reduction Methodology and Proposed Model
### Proposed PAPR Reduction System
It achieved this by avoiding utilising a DC bias level for clipping operations and transmitting the real unipolar signal within a single frame, unlike ASCO-OFDM, which relied on the concatenation of two frames. As mentioned earlier, DCO-OFDM demonstrated superior BER performance across various DC bias levels compared to ADO-OFDM. This performance advantage is attributed to the data arrangement within DCO-OFDM, which employs a single FFT/IFFT operation in both the transmitter and receiver. In contrast, ADO-OFDM employs dual FFT/IFFT operations in the transmitter, introducing noise from odd data subcarriers affecting the even subcarriers due to clipping effects. Additionally, the even data subcarriers in ADO-OFDM are impacted by clipping distortion caused by utilising the DC bias level in its process. DCO and ADO-OFDM delivered improved performance when operated at 10 dB and 13 dB DC bias levels compared to when operated at 7 dB. This enhancement is attributed to reduced clipping distortion as the DC bias level increases. The purpose of the clipping operation is to convert data into unipolar form. However, it is important to note that increasing the DC bias level beyond 13 dB may introduce upper clipping, resulting in signal distortion. At a BER of \(10^{-4}\), the tabulated numerical results of this evaluation, comparing different modulation orders to attain an equivalent spectral efficiency for various O-OFDM approaches, both with and without the establishment of the VLM methodology, are presented in Table 8.

Figure 21: BER comparison between traditional O-OFDM approaches and proposed VLM-O-OFDM approaches at 5.9883 (bits/sec/Hz) spectral efficiency for (**a**) BER and (**b**) PAPR. Figure 21b demonstrates the efficacy of the VLM methodology across various O-OFDM approaches and constellation sizes. At a CCDF of \(10^{-3}\), ACO and FLIP-OFDM exhibited notably higher PAPR0 reduction values following the implementation of the VLM methodology compared to other O-OFDM approaches. Specifically, ACO-OFDM showed a PAPR0 reduction of 2.29 dB, while FLIP-OFDM achieved a reduction of 2.768 dB. In terms of \(\text{E}_{\text{b}}/\text{N}_{\text{0}\text{difference}}\), the ACO-OFDM approach demonstrated minimal change, almost zero, while the FLIP-OFDM approach slightly enhanced the \(\text{E}_{\text{b}}/\text{N}_{\text{0}}\) performance by 0.107 dB at a BER = \(10^{-4}\). The LACO-OFDM approach, after applying the VLM methodology, revealed a PAPR0 reduction of 1.79 dB at a CCDF of \(10^{-3}\). However, there was a degradation in \(\text{E}_{\text{b}}/\text{N}_{\text{0}}\) by 0.56 dB at a BER of \(10^{-4}\). Table 8 presents the numerical results of the evaluation at a CCDF of \(10^{-3}\), comparing different modulation orders to achieve an equivalent spectral efficiency across various O-OFDM approaches. It outlines the results both with and without the implementation of the VLM methodology. ## 6 Comparison with Related Studies

This section offers an in-depth comparative analysis between this study and the relevant literature, outlined in Table 9. The essential criteria for assessment and their respective values are elaborated upon in the previous sections. However, this study marks the initial exploration of the PAPR concept across various O-OFDM approaches. Table 9 highlights that this research presents a distinctive advantage by achieving the highest computed PAPR reduction value while maintaining the primary objective of minimizing the \(\text{E}_{\text{b}}/\text{N}_{\text{0}\text{difference}}\) in all PAPR reduction-focused literature.