## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Modulation Scheme** & **Metrics** & **Comments** & **Ref.** \\ \hline \multicolumn{3}{p{56.9 pt}}{Numerical simulation} \\ \hline PPM-MSK-SIM & BER & The BER performance of PPM-MSK-SIM surpasses that of both PPM and BPSK-SIM. & [146] \\ \hline DOSM-PAM & ASER, APEP & A DOSM scheme that eliminates the need for CSIR results in a slight performance degradation compared to the conventional scheme with perfect CSIR. & [147] \\ \hline \(Q\)-ary PPM & BER & The suggested \(Q\)-ary PPM FEC-based approach offers outstanding coding gain. & [148] \\ \hline OOK, DPSK, DQPSK & BER & DPSK offers modulation gains of 3.2 dB over the OOK format in strong turbulence and 4.5 dB in weak turbulence. In moderate and strong turbulence, DPSK and DQPSK formats exhibit nearly identical BER performance. & [149] \\ \hline MPAM & BER & The adaptive LDPC-coded modulation scheme can tolerate deep fades of 30 dB and higher in strong turbulence conditions. & [150] \\ \hline MPPM & BER & Simulative evidence indicates that RL-MLC MPPM, when used with standard LDPC codes, can surpass the performance of any PPM scheme under identical transmission constraints. & [151] \\ \hline OSM & ABEP & OSM can provide performance on par with conventional coherent FSO systems using spatial diversity, while also surpassing them in spectral efficiency and hardware simplicity. & [152] \\ \hline SIM/SM & BER & The SIM/SM scheme can surpass the performance of conventional SIM in both AWGN and outdoor channels across different spectral efficiencies. & [153] \\ \hline SCPPM/SCMPPM & BER & In comparison to the SCPPM, the SCMPPM offers improved BER performance under a peak power constraint and is capable of achieving higher data rates. & [154] \\ \hline CPolsK & BER & The CPolSK system requires \(\approx\) 3 dB less SNR compared to the OOK system. & [155] \\ \hline OOK, M-QAM-OFDM & BER & System utilizing hybrid WDM-PON-FSO technology has the potential to achieve high data rates. & [156] \\ \hline OSSK & BER, EC & AT and PE have an insignificant impact on the performance of the OSSK-based system. & [157] \\ \hline COIQSM & BER & COIQSM provides an efficient way to improve the reliability and resilience of FSO systems against atmospheric turbulence. & [158] \\ \hline \multicolumn{3}{p{56.9 pt}}{Experimental demonstration} \\ \hline CAP & BER & The proposed CAP and STBC scheme is minimally affected by air turbulence. It can be scaled up for higher speeds and extended for a greater interconnection range. & [159] \\ \hline PAM-4-PoM-DD & BER & The PoM-DD system offers greater reliability compared to the conventional IM-DD FSO system. & [160] \\ \hline OOK, BPSK, PPM & SER, \(Q\)-factor & PPM delivers superior performance in weak turbulence environments compared to OOK and BPSK. & [161] \\ \hline SCOM, PM & BER & SCOM reduces RF power fading and interference. PM scheme with remotely injection-locked DFB LD has high resilience to noise and distortion. & [162] \\ \hline M-QAM-OFDM, WDM & BE, BER & HH, PTS, and AC algorithms provide more than a 21\% improvement in capacity. WDM can further enhance this capacity. & [163] \\ \hline M-QAM-DMT & \(Q^{2}\) factor, BER & In under weak turbulence conditions, the \(Q^{2}\) factor of the M-QAM DMT signals with FEC encoding schemes maintains its maximum value and is improved compared to the regular scheme. & [164] \\ \hline \hline \end{tabular}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & OOK: On–Off Keying, PM: Phase Modulation, PSK: Phase-Shift Keying, DPSK: Differential PSK, DQPSK: Differential Quadrature PSK, PAM: Pulse Amplitude Modulation, PoM: Polarization Modulation, PPM: Multiplex PPM, SCPPM: Serially Concatenated PPM, SCPPM: Serially Concatenated MPPM, CPolsK: Circle Polarization Shift-Keyning, M-QAM: \(M\)-ary Quadrature Amplitude Modulation, OSSK: Optical Space Shift Keying, COIQSM: Coded-Optical Improved Quadrature Spatial Modulation, CAP: Carrierellis-Amplitude-Phase Modulated, DMT: Discrete Multi-Tore, SCOM: Single-Carrier Optical Modulation, SIM: Subcarrier Intensity-Modulation, SM: Spatial Modulation, DD: Direct detection, M-DD: Intensity Modulation and Direct Detection, OSM: Optical Spatial Modulation, DOSM: Differential OSM, FEC: Forward Error Correction, LDPC: Low-Density-Parity-Check, MLC: Multilevel Coding, RL-MLC: Reduced-Layer MLC, CSIR: Channel State Information at Receiver, BE: Bit Efficiency, BER: Bit-Error Rate, ASER: Average Symbol Error Rate, APEP: Average Pairwise Error Probability, ABEP: Average Bit Error Probability, EC: Ergodic Capacity, DFB LD: Distributed Feedback Laser Diode, AWGN: Additive White Gaussian Noise, AT: Atmospheric Turbulence PE: Pointing Errors, HH: Hughes-Hartops, PTS: Partial Transmit Sequence, AC: Arithmetic Coding. & [165] \\ \hline \hline \end{tabular}
\end{table}