# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### FSOC for WiMAX  
WiMAX is for global cooperation for microwave access and is a wireless technique that enables the provision of wireless fast broadband connections that are an alternative to a conventional digital subscriber line (DSL) [142]. This standard offers fast information rates [143], quality of service (QoS) [144], and wide coverage compared to WLAN [145; 146]. WiMAX was launched in 1999 to provide the conditions for wideband wireless metropolitan area networks [147]. In 2001, 802.16 received the first nod, and then three more improving standards: 802.16a, 802.16b, and 802.16c, followed it [148; 149; 150]. IEEE 802.16d-2004 standards were published to support static and roaming access in 2004 [151]. An enhancement to 802.16d, and IEEE 802.16e (previously known as IEEE 802.16e), in 2005, achieved targeting  
Figure 24: Schematic of SCM-based FSO system. portability. The key characteristic of 802.16e-2005 is support for security, QoS, and the use of modifiable OFDM access, which is referred to as mobile WiMAX. A few standards are available for WiMAX, from 802.16 to 802.16 m [152]. Even though its use is still underutilized, WiMAX was evidenced as the major type of web correspondence alternative to DSL [153]. A WiMAX broadcast system has two main parts: (i) tower--this is very similar to a cellular tower and it is capable of delivering coverage over a huge area of about 7500 square kilometers; and (ii) receiver--this is typically a small card/box-like arrangement which is attached to a laptop/system just as with the Wi-Fi receiver [153]. The tower station of a WiMAX can be directly connected to a wired network (backbone), having a wide bandwidth, as shown in Figure 25. The fixed backhaul-connected WiMAX base station provides relatively high-speed internet connectivity (data access) to customers wirelessly (via WiMAX microwave) connected, for example in cafes, stations, offices, houses, and business spots [154]. The construction of similar neighboring towers taking the fixed backhaul connection from the internet backbone network will increase connectivity. A WiMAX tower can also be connected to another WiMAX tower with a directional microwave beam; thus, WiMAX is able to cover a very wide area. WiMAX was devised to enable a variety of developments to have wireless broadband access, whether in competition with existing wired systems or alone in unpopulated locations [155]. In addition, an internet connection to WLAN hotspots can be set up via WiMAX [156]. With the use of WiMAX, people would be able to shift away from the internet connection they currently have and access the internet at high speed from anywhere. The main issue identified with building internet-enabled zones in an underdeveloped country is finances, which plays a role in the motivation behind the use of WiMAX [157]. Regardless of the cost of the initial set up, it is modest thereafter. A WiMAX network incorporates a base station and some other components such as cellular systems. A base station operates with a unidirectional radio wire to provide a cheaper operation than that of a divisional receiver [158]. Although a unidirectional radio wire transfers EM waves 360\({}^{\circ}\), it does not provide a larger area of coverage than a division receiver. Either way, a 10 km sweep back-up by unidirectional receivers is enough to cover most localities. Greater inclusion may be achieved through a segment receiver base station or by several base stations in place of a single base station. The other important cost is the range, which is estimated depending on the area to be covered by the organization. Conversely, the overhead could be restricted or kept proportionate because WiMAX uses bandwidth in the range of 1.28 MHz, 2.56 MHz, 5 MHz, 10 MHz, and 20 MHz for data transmission [159; 160]. This implies that a lower data transmission rate is used for a territory having fewer customers, which increases the coverage area. System span also has a direct impact on operating costs, meaning that costs can increase as the system evolves and vice versa [160].