## 4 Enabling Technologies for FSO Communication
### Modulation Schemes
#### 4.3.2 Advanced Modulation Techniques
However, the effectiveness of POLSK in standard fibers is limited due to fiber birefringence. The polarization state changes unpredictably, making it difficult to track at the Rx. Conversely, POLSK does not encounter these issues in FSO communication systems. Theoretically, AT causes minimal depolarization, and there is negligible crosstalk between orthogonal polarization states during transmission. The polarization state of an optical wave is well preserved over distances of several kilometers, allowing reliable detection at the Rx [179]. Therefore, since turbulence only causes intensity distortion, while the polarization state remains stable in FSO channels [160; 179], the use of polarization modulation (PolM) is both feasible and attractive for enhancing system performance and reducing turbulence-induced fading in FSO systems [167]. Additionally, subtracting the two signals reduces common mode noise. PolM-DD FSO communication systems for OOK signals have been extensively studied and shown to achieve an approximate 3 dB sensitivity gain compared to IM [155; 160; 180]. Turbulence-induced scintillation can be significantly mitigated by utilizing redundancy offered by multiple Txs and/or Rxs. Consequently, POLSK is also employed in MIMO FSO systems, as demonstrated in [181; 182; 183]. These systems typically investigate coherent BPOLSK or coherent multilevel POLSK (MPOLSK). However, the non-coherent BPOLSK system described in [178] has not been taken into account in a MIMO architecture. To address this, an innovative non-coherent MIMO optical modulation technique, termed PPM-BPOLSK, has been introduced. This approach integrates the implementation simplicity and performance benefits of non-coherent BPOLSK with the power efficiency of PPM. It has been shown that the proposed method offers enhanced error performance in AT channels compared to systems that rely solely on BPOLSK or PPM, establishing it as an effective alternative to other IM/DD techniques regarding power efficiency [167]. Also, assuming bandwidth is virtually unlimited, the main design emphasis in FSO systems has been on power efficiency instead of SE [167; 184]. Consequently, MIMO FSO systems are typically designed using Space-Time Coding (STC) or repetition coding (RC) on the transmission side (see Section 4.6), along with diversity combining or aperture averaging techniques on the reception side to address scintillation effects. Additionally, utilizing multiple Txs and Rxs helps prevent beam blockage and enables coverage over greater distances [184]. OFDM is an extensively utilized method for broadband wireless communications, renowned for its inherent advantages such as enhanced resilience to frequency-selective fading, low cost, efficient implementation, narrow-band interference, and high spectral and power efficiency. As a result, OFDM has been incorporated into various high-speed digital communication standards and is being regarded as a potential solution for future mobile wireless systems [98; 185; 186; 187]. Despite its disadvantages, such as sensitivity to phase noise and a high PAPR, OFDM can be optimized to maximize its benefits and mitigate its drawbacks for FSO channels through careful design [186; 188]. Coherent detection is another promising technique for FSO channels, significantly enhancing Rx sensitivity [189]. Therefore, combining OFDM with coherent detection presents considerable potential for future FSO communication systems [186; 188]. OFDM is a type of multicarrier transmission where high data rate streams are divided into lower rate streams and transmitted simultaneously over multiple narrow-band subcarriers. These subcarriers are modulated using PSK or QAM and are carried on a high-frequency carrier [98; 186; 187]. For example, in OFDM-based coherent FSO systems using multilevel PSK (MPSK) or \(M\)-ary (multilevel) QAM (M-QAM), it is necessary to employ additional pilot tones or training sequences for channel estimation and equalization.