## 1 Introduction
This increases the energy usage and the employed chip size of the transceiver and imposes computational complexity and hardware cost constraints [15]. Moreover, increasing the subcarrier number leads to a cumulative effect of the PAPR [16]. The related literature has studied several unipolar OFDM schemes. These schemes were designed according to various frequency domain constraints and further expanded by specific time domain processes. Some popular, substandard examples of these schemes are DC-bias optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) [17; 18; 19]. DCO-OFDM employs DC biasing to append a positive amount of DC to the signal, thereby elevating the nonpositive polarity of the dipolar signal [19]. DCO-OFDM is extensively used in VLC [20; 21]. However, it experiences depressed power efficiency because it relies on DC bias. A significant DCO-OFDM defect is represented by the high-level peak-average power ratio, which escalates with an increase in the input data. A considerable DC bias must be increased in the signal to counteract the high negative peak amplitudes. This extensive DC bias results in reduced power efficiency and aggravates fiber nonlinearity [22]. Many approaches to reducing the PAPR have been discussed in the literature to decrease this defect. The pilot-assisted (PA) technique, introduced in [23], is regarded as an effective method for reducing the PAPR in DCO-OFDM-based systems. Conversely, cutting the nonpositive signal polarity in ACO-OFDM converts it into a unipolar form, introducing clipping noise. Although this method decreases spectral efficiency by half, the signal can still be identified similarly to any OFDM signal by eliminating the non-odd carriers that bear the cutting noise [24]. On the other hand, Flip-OFDM is a nondipolar signaling system that saves the nonpositive signal values by negating the polarity. This method considers a technique to offset the loss in spectrum efficiency compared to ACO-OFDM [17; 22; 24]. Flip-OFDM divides the two components of the signal (negative; positive) from a real OFDM bipolar symbol to keep the Hermitian symmetry of the data symbols to be transmitted. The negative components' polarity is squared to transmit the positive and negative components, one OFDM symbol after another. The fact that the signal transmitted is always positive allows Flip-OFDM to be classified as a unipolar OFDM system applicable to unipolar communications [22; 25]. However, Flip-OFDM and ACO-OFDM experience diminished spectral capacity because they utilize a sole half of the subcarriers compared to DCO-OFDM. To enhance OFDM, ADO-OFDM was introduced, combining the strengths of DCO-OFDM and ACO-OFDM [26]. Nevertheless, a common drawback of these OFDM tactics is the out-of-band (OOB) emanations, which can lead to significant interference with adjacent frequency bands. A hybrid DCO-OFDM-PTM scheme was introduced in [27], which integrates DCO-OFDM-PWM and DCO-OFDM-PPM under varying channel conditions, demonstrating reduced PAPR and enhanced BER performance compared to ACO-OFDM and ACO-OFDM-PTM. Moreover, in [28], two OFDM-based QG-MIMO schemes, TD-QGSM and TD-QGSMP, are introduced for a VLC system to enhance spectral efficiency and bit error rate performance. The simulation results indicate that performing IQ decomposition of constellation signals before OFDM modulation significantly improves the system's spectral efficiency. Many flipped OFDM formats based on HS have been suggested in the literature to boost the efficiency of the system. Nonetheless, because of the HS restriction, just one-half of the subcarriers will be able to transmit information, as mentioned previously, leading to a doubled IFFT size in the transmitter. In [29], the efficacy of Flip-OFDM and ACO-OFDM was compared, and Flip-OFDM was considered to have a 50% reduction in receiver equipment intricacy in comparison with ACO-OFDM. At the same time, the performance of both methods was equivalent.