# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### FSOC with WDM and SCM Techniques
In near-future communications networks, all the connections of the Earth's surface, terrestrial static or mobile links, and space satellite networks must be linked to the opto-transporter to have a high-data-rate transmission system. To increase the transmission rate in DWDM, advanced modulating techniques must be used. The project of DWDM full-optical FSO links is critical in the current scenario for the transmission of data from numerous facility foundations, such as 5G/6G cellular signals, terrestrial digital television, and WLAN and revolutionary wireless services. Figure 23 depicts the suggested combination DWDM/multi-beam FSO network's basic design [119]. An FSO access point, a free-space laser channel, a composite DWDM and multi-beam FSO system, a node for optical dissemination, and many FSO end-users invite the integration of SCM with FSO. A top-level schematic of an SCM with an FSO scheme is shown in Figure 24. The transmitted section is designed with data sources, OFDM circuits, RF combiners, MZMs, laser diodes, DWDM modules, and FSO Tx optics, while on the other hand is FSO Rx optics, an optical demultiplexing module, optical detectors, RF de-combiners and coherent decoders, an optical combiner, a detection lens, and a demultiplexer. In some applications, using optical fiber links, optical demultiplexed signals can be directly taken to the end-users' devices, where the conversion and data distributions take place within the end devices. There is an increasing need for optical capacity that is reliant on spectrum efficiency. Techniques that allow for the most efficient use of limited wavelength resources should be used in such scenarios [125]. An SCM-based single/multiple wavelength technique offers high capacity that greatly reduces the cost per bit [126]. In SCM, several RF signals are multiplexed and transmitted via an optical signal in the RF domain [127], as shown in Figure 24. Non-linearity of the optical source makes the SCM more sensitive to the effects of noise and distortions. An increased temporal and spatial bandwidth can be obtained from the combination of the SCM technique and an FSO communication system [128]. The combined M channels of an RF signal are modulated with the M number of optical  
Figure 23: The DWDM-supported next-generation FSO network. carrier signals generated by the laser diodes using optical MZM. The modulated optical signals are multiplexed, emitted to the FSOC channel with a transmission optics, and its reverse operations are then performed to receive the data at the output terminal of each channel. Various RF channels are thus transmitted to their destinations over the FSO-SCM channel [129]. By using the digital SCM technique, coherent electrical detection is possible. Due to the possibility of a high degree of optical beam coherency, it has become possible to make optimal use of the complete spectral bandwidth of the SOTA FSO communication system. The typical applications of SCM technology are particularly in optical wireless communications, economic local-area optical connectivity, cable television (CATV), and label switch routing [130]. The deployment of high RF power typically reduces the entire performance of an FSO communication system [131]; thus, the selection of optimum power in an SCM-based FSO system becomes significant. The effect of inter-channel cross-talk is also a concern in SCM-WDM-FSO communication. Inter-channel cross-talk is investigated in [132] for an all-optical TDM/WDM-PON by OLT burst-mode receiver. The impact of turbulence-accentuated inter-channel interference on system stability was discussed in [133]. Inter-channel cross-talk, scintillation, and spontaneous optical amplification emission noise were investigated in [134]. In 2008, a report was presented on the FSO-SCM transmission technique with ASK electrical modulation [135], and in the same year, someone applied for a patent for the apparatus design of an FSO SCM system used in airborne communication [136].