A Novel Companding-Based Hermitian Symmetry Free ACO-UFMC Scheme for IM/DD in Low-Power IoT-Centric Future Wireless Communications

For optical communication, the transmitted signal must be real-valued that can be achieved by employing Hermitian symmetry (HS). But the HS results in increased power consumption and decreased spectral efficiency (SE). Therefore, a HS free asymmetrically clipped optical (HSF ACO) - universal filtered multicarrier (UFMC) system, which increases SE and reduces power consumption, is proposed. In the proposed scheme, a real valued UFMC signal is attained by juxtaposing the real and imaginary components of the conventional complex UFMC signal. The combination of UFMC with visible light communication technology provides higher data rates but results in a high peak to average power ratio (PAPR) due to the multi carrier waveform. Hence, it is essential to address the PAPR in the proposed HSF ACO-UFMC system. A new two-level rooting companding transform (TL-RCT), which not only reduces PAPR but also provides a higher degree of freedom and greater design flexibility, is proposed. In TL-RCT, a single threshold level and two companding parameters are utilized to meet the desired requirements. However, it increases the average power of the HSF ACO-UFMC signal. To overcome this, an enhanced TL-RCT is proposed, ensuring companding without raising the signal's average power. Simulation results demonstrate that the proposed HSF ACO-UFMC with the enhanced TL-RCT achieves superior PAPR reduction, maintains a constant average power, and exhibits better bit error rate performance compared to the uncompanded HSF ACO-UFMC signal and other companding techniques. Moreover, the proposed TL-RCT outperforms other companding functions in power spectrum efficiency. Additionally, it delivers a PAPR reduction of 6.1 dB and enhances the net gain by 6.9 dB.