# 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  
The need for broadband is increasing almost every day due to the huge usages of massive data applications, and the availability of various multimedia services, which results in spectrum congestion. In addition, there is a significant rise in network density due to the rapid increase in device connectivity, which requires the need for high-speed wireless links. One of the studies reported by Cisco predicts that by 2023, almost 2/3 of the world's inhabitants will have internet connectivity. The amount of IP-connected mobile device users will be at least three times more than the global population in 2023 [91]. Due to the high demand, alternative solutions have been developed that can offer a higher data rate compared to the RF networks. Using optical fibers, the high bandwidth requirement can be fulfilled for now, but it is still a wired technology [92]. Therefore, it is logical to deploy an FSO system which offers the same bandwidth as optical fibers, for last-mile access networks and beyond. FSO can give the same bandwidth as optical fibers, but it provides a wireless connection. FSO is considered an appropriate alternative, particularly in areas where fiber-optical cable deployment is not practicable [93]. High transmission protection, license-free operation, full duplex transmission, and protocol clarity are the main advantages that enable FSO communication integrating the WDM and DWDM [94; 95]. Nowadays, FSO systems employ a range of modulation multiplexing schemes including WDM and DWDM that are being used in many applications needing high transmission capacity [96; 97; 98; 99]. Several successful milestones have enabled the development of viable WDM transport systems for international, national, regional, and metropolitan telecommunications networks. The inventions of lasers in 1960 [95]; dielectric waveguides as a possible means of information transmission in 1966 [96]; low-loss optical fiber (\(\approx\)20 dB/km) in 1970 [98]; an InGaAsP diode laser with a 1300 nm wavelength in 1976 [99]; a single-mode low-loss fiber (about 0.2 dB/km) in 1978 [100]; experimental WDM systems in 1978 [101]; and EDFA in 1987 [102], resulted in the development of commercial WDM systems in the year 1995. The development of advanced electro-optic components such as 1550 nm lasers, SMFs, and effective photo detectors have followed these inventions. An FSO link was developed between satellites and ground stations in New Mexico in the 1970s. In the year 1980, a WDM system was demonstrated between aircraft and the ground at data rate of 1 Gbps. These distinct wavelengths were then multiplexed by a WDM filter at the transmitter and de-multiplexed using the same kind of filter at the receiver, or by coherent detection involving a tunable optical local oscillator (LO). The transport ability of FSO communication can be increased by using WDM by increasing the available bandwidth. Therefore, it can facilitate the reduction of the data traffic of today's network. FSO and WDM systems have been investigated for different user case scenarios [103; 104]. Several architectures of WDM-FSOC have been suggested and demonstrated in the literature  
Figure 21: Design of an integrated RF–FSO antenna. including (i) Tbps WDM-FSO [104] and (ii) WDM-PONS for FSO for multiple users [103]. Since FSO and fiber-optic communications use almost the same type of opto-electronic devices and subsystems, FSO can be used as an intermediate link in short-/long-range communications. Figure 22 illustrates a top-level schematic diagram of WDM-FSO communications system. Pseudorandom binary sequence data signals following line coding (i.e., RZ or NRZ) are applied to MZMs which are injected with an optical beam at different wavelengths (i.e., \(\lambda_{1}\) through \(\lambda_{\text{N}}\)). The output of MZMs is applied to the multiplexer to generate the WDM signal for transmission over a free-space channel. The reverse process is carried out at the receiver in order to regenerate the estimated input data sequences.