D. and F. , These approaches manifest lower PAPR than their O-OFDM counterparts, however, it is determined that the Hermitian symmetry (HS) requirement to achieve a real-valued TD signal bounds the level of PAPR that can be reduced. This entails that same level of PAPR cannot be achieved by precoding in IM-DD based OWC systems as can be achieved in RF systems, as Fourier transform precoded (FTP)

, subchannel is used for O-SCFDMA approaches) are modulated which results in a half-wave symmetric TD complex signal, from which a real-valued TD signal can be attained forthrightly, A novel O-SCFDMA approach which averts the use of HS termed as Hermitian symmetry free OSCFDMA

, This results in inefficient utilization of the bandwidth. In state-of-theart O-OFDM approaches, at least half of the subcarriers are sacrificed for HS to attain real-valued TD signal, while for approaches, such as ACO-OFDM further subcarriers have to be relinquished to achieve a non-negative TD signal. This signifies that ACO-OFDM has half the SE of DCO-OFDM. The SE of ACO-OFDM can be enhanced by using LACO-OFDM, however, LACO-OFDM has following practical limitations: (i) HS is required, so, half of the subcarriers go un-used; (ii) LACO-OFDM also manifest high PAPR; and (iii) the computational complexity is significantly higher. To overcome the dilemma of PAPR, performance of DFT and DHT precoding is analyzed for LACO-OFDM because PAPR reduction methods (only a few are available) require high computational resources. The corresponding approaches are termed as FTP-LACO-OFDM and HTP-LACO-OFDM. In FTP-LACO-OFDM, quadratureamplitude modulation (QAM) modulation alphabets and DFT/IDFT (for both precoding/decoding and multiplexing/demultiplexing) are used, while in HTP-LACO-OFDM, pulse-amplitude modulation (PAM) alphabets with real-valued DHT/IDHT are used. In HTP-LACO-OFDM, the HS requirement is averted and virtually all the subchannels can be modulated, thus, providing an efficient utilization of bandwidth. Nevertheless, the SE cannot be increased from that of DCO-OFDM because of use of PAM alphabets. In terms of PAPR, HTP-LACO-OFDM manifests the lowest PAPR among the three approaches. Though the PAPR of FTP-LACO-OFDM is less than LACO-OFDM, however, it is more than HTP-LACO-OFDM because of HS requirement, The non-negativity is achieved by using bias as in DCO-OFDM. It is accentuated that the impact of bias on HSFO-SCFDMA is substantially less than DCO-OFDM because the TD HSFO-SCFDMA signal manifests lower PAPR, hence the bias required to achieve non-negativity is also less resulting in a lesser penalty on optical power consumption than DCO-OFDM or FTP-DCO-OFDM. Moreover, HSFOSCFDMA has the same spectral efficiency (SE) as that of DCO-OFDM

, Fiber based IM-DD system is used because of unavailability of OWC prototype. The experimental results are in accordance with simulation results. From the experimental measurements for TDCSR/FDCDR, it is discerned that the both approaches are capable for recovering the information lost because of clipping, Finally, experimental demonstrations have been presented for TDCSR/FDCDR and HSFO-SCFDMA using fiber based IM-DD system

, This comportment is in complete contradiction with linear modulations where the energy efficiency diminishes with an increase in alphabet size

, another class of M-ary orthogonal modulations for IM-DD based OWC system, i.e., M-ary frequency-shift keying (FSK) is explored

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