## 4 Enabling Technologies for FSO Communication
### Adaptive Transmission
#### 4.4.2 Adaptive Transmission Techniques  
Adaptive modulation and coding (AMC), which is already utilized in wireless channels, facilitates reliable and spectrally efficient transmission over channels that vary over time [217]. The core concept involves estimating the channel conditions at the Rx and relaying this estimate back to the Tx via an RF feedback channel, allowing the Tx to adjust according to these conditions. Recently, adaptive transmission techniques have been integrated into FSO systems, showing promise as solutions to counteract AT and enhance FSO link performance [150; 214]. A prevalent assumption in the existing literature is that open-loop implementations are used, where the Tx lacks knowledge of the channel conditions. Such open-loop designs are beneficial in time-varying channels, where obtaining feedback on channel estimates can be challenging. However, in the case of quasi-static channels, reliable feedback is attainable, and having available CSI at the Tx can enable the design of adaptive transmission schemes that significantly enhance performance. AT causes slow fading in FSO systems, with the coherence time of the optical channel ranging from 1 to 100 ms. This means that fading can remain constant over hundreds of thousands to millions of sequential bits at standard transmission rates, allowing the channel to be modeled as a slow fading process [20]. Consequently, adaptive transmission presents an attractive option for FSO systems. Figure 10 illustrates a typical block diagram of an integrated power control and channel coding system in FSO communication. The feedback information needed for adaptive transmission can be relatively easily implemented in FSO systems, as commercially available FSO units typically support bidirectional (full-duplex) communication. A small fraction of the extensive available bandwidth can be assigned for feedback without significantly impacting data rates. For instance, in some hybrid RF/FSO systems, the RF channel can be utilized for CSI feedback. This makes reliable CSI accessible in FSO systems, which is crucial for developing adaptive transmission schemes. As a result, to improve communication reliability and throughput, the Tx can adjust its transmission parameters--such as code rate, transmit power, and modulation size--based on the estimated current CSI [218; 219; 220; 221]. Thus, various varieties of adaptive transmission systems have been suggested in the existing research on FSO communication, demonstrating their feasibility and advantages. For example, in [223], a straightforward rate-adaptive transmission technique is introduced that utilizes variable silence intervals and OOK formats with memory. The study in [224] presents a statistical-delay quality of service (QoS)-aware adaptive modulation and power allocation method for a dual-channel coherent OWC system facing AT fading. The findings indicate that this adaptive scheme significantly outperforms conventional adaptive transmission methods under strict statistical-delay constraints and highlights the benefits of joint channel optimization in conditions of strong turbulence fading and tight statistical-delay requirements. Additionally, in [225], a joint relaying link selection and power allocation strategy that is statistical delay QoS-aware is proposed for the uplink of a multichannel coherent FSO communication-based fronthaul network. The analysis shows that to maximize the overall end-to-end (e2e) effective capacity, it is essential to account for both the statistical delay QoS needs of the transmitted traffic and the channel gains of the transmission links. The results further demonstra  
Figure 10: Block diagram of integrated power control and channel coding in an FSO communication system. enhances the statistical delay QoS-aware throughput despite the challenges posed by AT fading and PE.