# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Light-Fidelity (Li-Fi)
The transmitter module modulates the Li-Fi carrier, i.e., the LED light with respect to the router data, and then it transmits the modulated light within its optical divergence regime. Mainly, the Li-Fi installation system consists of a transmitter, optical wireless Li-Fi channel, and receiver. Receivers collect optical signals, convert them to electrical pulses (using PD), and format the data for downward communication [242]. The same is performed in the reverse direction during the uplink transmission, as shown in simple diagram in Figure 32c, and explained in detail in [243]. In a simple modulation scheme of Li-Fi, the baseband signal modulates the intensity of the optical signal denoted as IM/DD. Relatively, the devices of Li-Fi installation systems are low cost, compact in size, light weight (thus suitable for dense integration), havelong life, high reliability, low power consumption, highly directional beams (thus suitable for spatial safety/reuse), and also give the advantage of being modulated with high-speed data as well as being operable in a broadband unlicensed light spectrum [236; 237; 238]. Today's RF wireless communication technology is vulnerable to many security risks because it is susceptible to data breaches or data losses. Li-Fi has been recognized as alternative having a lot of capabilities particularly in defense sectors, due to its inherent ability to protect data [244]. The installation of Li-Fi in the defense sector would further improve the security of the communication/connection to a person, clusters, and/or devices. An indoor positioning system (IPS) includes personal tracking and material tracking (in supermarkets, shopping malls, airports, warehouses, logistics terminals, and automotive centers, etc. ), and provides location-based services and advertisements to users, etc. A street lamppost equipped with a Li-Fi system can provide internet access in urban areas and parks [245; 246]. By offering a full coverage area, Li-Fi will reduce the high-tower installation and land costs (or rent) of RF base stations. Full street-lamppost-installed Li-Fi coverage would also assist accident management systems, traffic information transfer, remote road condition monitoring, the management of route/site emergency situations, disaster management communications, weather data transmission, etc. [247; 248]. Li-Fi will allow netizens to benefit from high data access volumes in areas such as living spaces, function halls, meeting rooms, and lobbies/hallways. Li-Fi would be a safer choice (since RF propagation needs to be prohibited) where large volumes of data are needed to be transferred in dangerous environments, such as industrial areas, hospitals, research laboratories, and some commercial/government buildings. When using Li-Fi, these places are much more secure and reliable, since Li-Fi systems need less energy and produce less heat during the data transmission [249]. A Li-Fi network can be incorporated on school premises for academic and administrative enhancements with high-speed internet connectivity for online meetings, online study/office material transfer, webinar lectures, attendance systems, library book movement tracking, indoor positioning, on-campus monitoring, public/private announcements, research, etc. [250]. The healthcare industry does not allow the use of electronic equipment that emits RF signals. Most operation theaters/patient monitoring equipment are highly sensitive to RF radiation and interference from RF devices such as cellular phones and Wi-Fi-enabled devices. Li-Fi systems are less prone to interference/risk, more reliable, less expensive, and more efficient in communication between staff and patients and with equipment. Thus, Li-Fi finds its applications in hospital security, patient monitoring, emergency situations needing quick data transmission, and the storage/transfer of patient records [251].