## 4 Enabling Technologies for FSO Communication
### Soft-Switching Configurations  
One solution to address the challenges of hard-switching is to synchronize data transmission across both links using channel coding, as illustrated in Figure 15b. In this method, both the FSO and alternative links are utilized simultaneously for data transmission. One approach involves sending identical data concurrently over both links and employing diversity combining techniques for the received signals from each link [435]. This method, however, limits the system's data rate to that of the slower link. Alternatively, the data stream can be jointly coded and divided between the two links, potentially leading to a significant enhancement in overall system capacity [431]. In this context, data are encoded using a single LDPC code, with part of the codeword allocated to both FSO and alternative links, and the rate is modified through puncturing based on the current channel conditions [429; 432]. While this approach offers improvements over hard-switching, it necessitates knowledge of channel conditions at both the Tx and Rx, as well as complex soft decoding, which poses challenges at FSO data rates. Recently, Raptor codes have also been explored for hybrid FSO/RF links [214; 434; 436; 437]. These codes operate without the need for channel knowledge at the Tx and automatically adjust the rate between RF and FSO links based on channel conditions, utilizing just a single bit of feedback per message [432]. Generally, hard-switching schemes tend to result in some rate loss compared to soft-switching schemes. Both approaches require that the FSO and alternative links remain continuously active, even during periods of poor quality. Consequently, both links must be operational even when the FSO link has good quality and can independently meet the required BER. In such situations, these solutions are inefficient regarding bandwidth utilization and resource usage across both channels [429]. For instance, the power used for alternative link transmission is wasted, leading to unnecessary interference with the environment [428; 430]. To tackle these issues of frequent switching, unnecessary RF interference, and RF power wastage, a new hybrid FSO/RF transmission scheme with adaptive combining is proposed in [430]. In this approach, the FSO link is utilized for data transmission as long as its quality remains acceptable, while the RF link is placed in standby mode. When the quality of the FSO link deteriorates, the system activates the RF link and employs MRC for signals received from both the FSO and RF links. The complementary characteristics of FSO and RF channels have prompted several proposals for hybrid RF/FSO systems. A combination of high-rate FSO with low-rate RF systems operating in the 2.4 GHz frequency band is suggested in [438] and experimentally validated in [433; 439]. However, the maximum data rate achievable in such hybrid systems is constrained by the limited RF bandwidth available in the 2.4 GHz band. This limitation can be addressed by utilizing RF systems in the mmWave frequency band, which supports data rates comparable to those of FSO [440; 441; 442]. When instantaneous CSI is available at the Tx, a straightforward CSI-dependent switching mechanism between the FSO and RF systems can be implemented [438]. Additionally, much of the prior research on hybrid FSO/RF systems has concentrated on the joint design of coding schemes for both FSO and RF links, facilitating soft-switching between the two links [429; 434; 443]. For instance, to enhance the performance of hybrid FSO/RF systems, ref. [443] employs hybrid channel codes that not only optimally achieve the combined capacity of the FSO and RF channels but also ensure carrier-grade reliability (99.999%) for the FSO link. The coding scheme for the hybrid channel is based on non-uniform punctured LDPC codes. This scheme relies on knowledge of the instantaneous channel conditions at the Tx and the appropriate adjustment of code rates for RF and FSO transmission [428].