## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
#### 4.3.2 Advanced Modulation Techniques
This is to counteract the amplitude and phase variations resulting from turbulence in the channel and laser phase noise. Consequently, this increases system complexity and reduces SE. Conversely, multilevel DPSK (MDPSK) shows great potential for OFDM-based coherent FSO systems due to its strong resilience to amplitude fading and phase distortion. MDPSK can mitigate the disadvantages of OFDM's sensitivity to phase noise and ensure good system performance without the need for equalization [188; 190]. In OFDM, the spacing between adjacent subcarriers is meticulously chosen so that each subcarrier aligns with the spectral nulls of all the others, allowing them to be packed as closely as possible. This careful arrangement ensures spectral orthogonality, meaning each subcarrier is orthogonal to the others. This orthogonality enables the subcarriers to be separated at the Rx using correlation techniques, thereby eliminating intersymbol interference between channels. The orthogonal carrier set is generated by applying the Inverse Fast Fourier Transform (IFFT) at the Tx and the Fast Fourier Transform (FFT) at the Rx, thus negating the need for equalization. Figure 9 illustrates the fundamental setup of OFDM signal transmission over an FSO link [98; 186; 187]. The transmission of OFDM through an optical link is a specialized form of multiple-subcarrier modulation (MSM). In this method, multiple independent bit streams are modulated onto subcarriers at different frequencies, combined in the RF domain, and transmitted via high-capacity optical links employing IM/DD. The MSM signal undergoes minimal distortion and does not require equalization at the Rx. Each subcarrier functions as an individual narrow-band signal with a reduced symbol rate [98]. The baseband OFDM signal is inherently complex and bipolar, but an IM/DD optical link requires a real and positive RF signal to drive the LD. To create a real OFDM signal, Hermitian symmetry is applied to the input vector of the Tx's IFFT block. To convert the OFDM signal to unipolar, a DC bias is added (DC-OFDM), making the signal positive. This DC bias must be sufficiently large to avert clipping and distortion in the optical domain. However, the primary drawbacks of IM/DD MSM systems are the inefficiency in average optical power owing to the large DC bias and the distortions caused by the nonlinearities of the LD and optical channel. This issue holds particular significance for IM/DD DC-OFDM systems, where a significant number of subcarriers result in high PAPR [188]. Additionally, the LD's nonlinearity induces interference between subcarriers and broadens the overall signal spectrum. These factors create a complex scenario and impose strict demands on the linearity of optical devices to minimize distortion and ensure satisfactory performance [98]. Furthermore, one way to enhance SE is by using amplitude modulation techniques, such as PAM and QAM, with higher-order constellations [184; 167]. However, these multilevel modulation methods come with increased hardware costs, as higher modulation levels lead to greater PAPR and increased sensitivity to noise and device nonlinearities [83]. High PAPR values can push Txs beyond their linear operating range, making large PAM constellations less desirable [184]. Moreover, systems utilizing subcarrier intensity modulation (SIM) can enhance SE [83; 167; 184; 191]. Consequently, the performance of subcarrier binary PSK (BPSK) IM in FSO systems has been thoroughly examined [92; 193; 194; 195]. When compared to conventional SIM in FSO communication, BPSK-SIM demonstrates superior BER performance relative to other modulation schemes such as QPSK, MPSK, DPSK, and QAM [196; 192; 83; 146]. To further improve the BER performance of FSO systems utilizing BPSK-SIM, a hybrid modulation scheme called PPM-MSK-SIM, which integrates PPM and MSK SIM, has been proposed [146]. However, the complexities of both the Tx and Rx in SIM systems are considerably higher than those found in pulse-based optical communications.