# 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
Experimental work has shown the viability of WDM-FSO networks. In Ref. [105], Jinno, M. et al. demonstrated, for the first time in 1997, eight 10 Gbps evenly spaced WDM channels with repeater-less transmission over a 160 km dispersion shifted fiber. In Ref. [106], a 4\(\lambda\)\(\times\) 10 Gbps silicon photonics CWDM link was demonstrated using all-optical-component-electronics-packaging technology. In Ref. [107], 10 Gbps OOK was demonstrated with a bidirectional WDM-FSO communications architecture using 5 m FSO and 25 Km fiber links. A WDM-FSO fiber access network over a 51 km link range was demonstrated in [108]. Therein, symmetric 4 \(\times\) 10 Gbps OOK downstream/upstream WDM-FSO keys were transferred. In a WDM-FSO system, the reliability depends on the received power being above a defined minimum threshold level. Ren et al. experimentally showed that the capability and the system performance of FSO can be improved by minimizing the cross-talk [109], by multiplexing different independent OAM beams and combining them with WDM signals at a high data rate of 100.8 Tbps [110]. A 200 Gbps WDM-FSO system using 4-PAM was demonstrated to be suitable for 5G/6G wireless networks in [111], where a 16 \(\times\) 16 Si-photonics cyclic frequency arrayed waveguide grating router system for O-band routing applications in DWDM was used [112]. To overcome inter-channel cross-talk and improve BER performance and system quality, the combination of N-hybrid spatially modulating diversity using M-ary PPM was investigated in [113]. Using the Gamma-Gamma model [114], the WDM-FSO efficiency based on the parallel multi-light beam selection method was studied. In Ref. [115], the authors demonstrated a WDM-FSO high-speed hybrid system in which a single beam was used to convey a hybrid fast-paced signal coupled with four-modulated wavelengths at data rates of 10, 25, 28 and 32 Gbps. In the WDM-FSO scheme, a recent 2D-PDA and four-combined WDM filters were implemented. During the demonstration of a 4\(\lambda\)\(\times\) 25 Gbps FSO communication system, the 2D-PDA receiver performance was analyzed [116]. A dual-band two orthogonal polarization state and wavelength-interleaving scheme based on integrated FSO/PON and Ro-FSO for a DWDM system was proposed [117], as shown in Figure 23. A 100 Gbps bandwidth capability combined system is shown in WDM-PAM4 FSO-UWOC [117]. Figure 22: Schematic of WDM in FSOC system. Multiplexing multiple (m) channels in the FSO link obviously reduces the actual capability of every channel which leads into the estimation of an effective high data rate. When multiplexing four channels, if each channel is capable of 250 Mbps, the effective information speed will be 1 Gbps and the practical data rate will be close to this. Furthermore, the inclusion of WDM networks allows data transportation that improves data access and resilience. In WDM-FSO networks, there is a lot of scope to improve network optimization, link alignment, and service protections. WDM can also be combined with a different combination of convergent systems with respect to ground, terrestrial, aerial, and space to increase the entire device's capability. FSO technology with a multi-beam can only function in point-to-point mode [118]. A fusion DWDM FSO network has been tested as a solution in [119]. Similarly, in [120; 121; 122], the authors investigated the advantages of using solitary and multi-light ray FSO systems with DWDM. DWDM technology has various advantages, including a passive operating principle, an acceptable power budget, a large enough margin to allow for a high-capacity setup, the ability to multiplex across various systems without interference, etc. [123]. Even though FSO is a good connectivity system, the new determination to integrate it through DWDM has given communication applications a recent boost, expanding the possibility for high-speed broadband [124].