# Optical-OFDM VLC System: Peak-to-Average Power Ratio Enhancement and Performance Evaluation
## 4 PAPR Reduction Methodology and Proposed Model  
This section provides a comprehensive exploration of the PAPR reduction technique and outlines the proposed model methodology employed for implementing this reduction technique within the context of the various O-OFDM configurations discussed earlier. To some extent, the conventional PAPR reduction methods may apply to both traditional OFDM and O-OFDM systems. Still, there are specific considerations and challenges unique to O-OFDM that warrant clarification. The transmitted signal of conventional OFDM is complex and bipolar, but in O-OFDM, the transmitted signal must be real and unipolar. The simulation results in Section 5.1 illustrate the change in BER performance of the bipolar DCO-OFDM signal before DC-biasing and zero clipping. With the unipolar DCO-OFDM signal with different DC-biasing, the performance after DC-biasing and zero clipping is decreased. The same behaviour is achieved in Section 5.2 for the ACO-OFDM. This illustrates the PAPR reduction methods that can be used in both OFDM systems, but with different PAPR and BER performance evaluations. However, sometimes, we must modify the PAPR reduction technique to meet the VLC transmission requirement while operating at different reduction parameter values. To the best of the authors' knowledge, we can categorize the PAPR mitigation approaches for MCM techniques as adding signal techniques (AST), multiple signal representation (MSR) techniques, and coding techniques (CT)/precoding techniques (PCT) [49]. AST reduces PAPR through three primary techniques: signal clipping, the compression of large peaks using non-linear companding transform (NCT), and the application of peak reduction signals/stretching the constellation. These methods introduce distortion noises into the broadcast signal that cannot be eliminated. Among these techniques, NCT stands as the most widely used and promising one. It involves adding a companding function to the initial transmitted signal, enhancing small signal amplitudes, and compressing high signal amplitudes. The companding function maintains the signal's average power constant. Various types of NCT include the A-law and \(\upmu\)-law techniques [24]. These two techniques can be easily used for conventional OFDM without any modification for O-OFDM systems but at different companding values to achieve better performance in both PAPR and BER. MSR approaches generate alternative signals from the same source signal by altering phases, amplitudes, or data positions. The two prevalent types of MSR techniques are partial transmit sequence (PTS) and selective mapping (SLM) [50]. Yet, this approach carriessubstantial downsides, like increased computational complexity and the need for extra data transmission to the receiver, which reduces overall bandwidth efficiency. VLC prioritizes lower computational complexity, which is crucial for real-world applications. However, VLC with OOFDM exhibits lower spectral efficiency than conventional OFDM. Consequently, MSR is not the favoured option for mitigating PAPR in VLC systems. CT can lead to increased side information, computational complexity, or may only function with a limited number of subcarriers, making it inefficient for high-speed VLC-OOFDM communication. To address these drawbacks, PCT emerges as a solution. PCT techniques can be easily used for both conventional and optical OFDM. These techniques use orthogonal matrix multiplication with the complex modulated symbols output from the mapper process. The output signal from this multiplication is applied to the IFFT process in conventional, but in the optical OFDM, it is applied to the Hermitian symmetry and then applied to the IFFT process. The inverse orthogonal matrix is applied to the receiver side before the received signal is applied to the de-mapper process.