## 4 Enabling Technologies for FSO Communication
### Error Correction Codes and Modulated Signal Optimization
#### 4.7.2 Coded Modulation and Error Correction Codes  
As previously mentioned, the performance of an FSO system regarding availability and reliability is greatly influenced by unpredictable variations resulting from AT, which is the primary factor hindering transmission performance [300]. Higher laser powers are essential to extend transmission distances. However, the free-space, optical IM/DD channel presents modem designers with unique challenges. Most practical FSO systems utilize LEDs or LDs as Txs, along with PIN PDs or APDs as Rxs, creating a relatively straightforward communication system. These devices are designed to modulate and detect only the intensity of the carrier wave, rather than its phase, meaning that all transmitted signal intensities must be non-negative. Additionally, considerations for biological safety (specifically eye safety) restrict the average optical power emitted, which in turn restricts the average signal amplitude. The effects of multipath distortion in signal propagation, along with the limited response times of optoelectronic components, impose significant restrictions on the channel bandwidth. Standard signal-space models and CM techniques for electrical channels cannot be directly applied to this scenario because they do not account for the constraints on signal amplitude [301; 302]. Consequently, there is a need for power-efficient modulation schemes [148]. As highlighted in Section 4.3, modulation schemes like PAM and QAM, often employed in wireless communications, offer high SE but lack power efficiency, making them less suitable for certain applications. In contrast, \(Q\)-ary PPM presents a compelling power-efficient alternative for FSO links [244; 265; 303; 304]. The significant bandwidth of FSO links, when contrasted with RF links, mitigates concerns regarding the low SE of PPM [302]. Additionally, as the number of slots in \(Q\)-ary PPM increases, the impact of background ra diation diminishes, largely due to the shorter PD integration interval [244; 265]. To enhance the performance of FSO systems utilizing PPM, various FEC strategies have been introduced, such as convolutional codes, Reed-Solomon (RS) codes, turbo codes, and trellis-CM (TCM) [302; 305; 306]. While RS and convolutional codes provide adequate coding gains in conditions of weak turbulence and without fog, their effectiveness declines under strong turbulence or in deep fog scenarios. In such cases, more sophisticated FEC solutions, including turbo and LDPC codes, become necessary [302; 307]. A power-efficient CM scheme employing multilevel coding with LDPC codes as component codes has been proposed for implementation in repetition MIMO FSO systems with \(Q\)-ary PPM [148]. Moreover, FEC techniques, such as polar codes and LDPC codes, are extensively utilized in FSO to counteract turbulence, achieving substantial coding gains under varying turbulence conditions [164; 186; 307; 308]. For instance, LDPC-coded optical OFDM has been shown to vastly outperform LDPC-coded OOK in AT channels in terms of both SE and coding gain. In strong turbulence conditions at a BER of \(10^{-5}\), the enhancement in coding gain of the LDPC-coded single-side band unclipped-OFDM system with 64 subcarriers exceeds that of the LDPC-coded OOK system by 20.2 dB for QPSK and by 23.4 dB for BPSK [186]. Additionally, the combination of FEC and spatial diversity has been demonstrated to mitigate fading [186; 213; 285]. The LDPC-coded MIMO FSO communication system, which employs LDPC codes and spatial diversity reception to combat turbulence fading when sub-channels are independent, has been examined [186]. In practice, deterioration due to spatially correlated fading is considerable, especially in moderate to strong turbulence channels [290]. Polar codes, a novel FEC technique, are the only ones proven to achieve the Shannon limit with low encoding and decoding complexity and have been selected for the 5G channel coding standard because of their distinctive benefits [309; 310].