# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Light-Fidelity (Li-Fi)  
Short-range communication is at its peak in terms of technology, users, and applications [226]. For almost everything nowadays, people need a method for secure, high-bandwidth, low-congestion communication with low data transmission latency and high-speed, for voice/video conference calls, SMS, MMS, music/video transmissions, HDTV program watching, office work, banking, gaming services, etc. [227; 228; 229]. Smart phones, tablets, wearables, smart televisions, smart hybrid vehicles, wireless gaming devices, and electrical appliances with sensors require an internet connection in today's smart home. Nearly 80% of such devices require internet connections with short-range coverage within a home or office environment [230]. Wi-Fi is a popular wireless networking technology that is being widely used nowadays. Mostly, Wi-Fi and other such wireless communications work in the RF bands, in which interference, insecurity, congestion, and bandwidth limitation are the major issues [231]. As a result, it becomes overcrowded, giving low throughput, and has high data latency. With such issues, Li-Fi is the best alternative and attractive method that overcomes the problems associated with RF technology [226; 223]. A new era has begun in light-based communication, specifically for indoor OwC, with the invention of LEDs in the 1960s. Today's LEDs are reliable, avoid the flickering effect, maximize communication capacity, control color contrast, vary light quality, and increase energy efficiency [233]. Li-Fi is a new full-duplex WOC technology that enables us to connect to the Internet by using LEDs. Li-Fi communication can be established for point-to-multipoint, multipoint-to-point, and to/from multiusers [227; 228]. Several studies were conducted in the 1970s demonstrating the potential of infrared-based OwC technology in an indoor environment in terms of bandwidth, data transfer rate, communication link range, etc. [229]. During the 1990s, the data transmission rate over a short range, in an indoor environment, was increased to 50 Mbps using the OOK modulation method with decision feedback equalization [230]. Harald Haas from the University of Edinburgh presented Li-Fi in a TED talk in 2011. The Li-Fi consortium was established in Oslo, Norway, in the same year [234]. Today, the Li-Fi system has a data transfer rate of several Gbps which is very much higher than Wi-Fi and LTE access technologies [235]. Recently, Prof. Harald Haas has demonstrated data rates of 11 Gbps with Li-Fi, using a single LED source [234]. M/s Pure Li-Fi internet-accessing systems (Li-Fi-X and Li-Flame) are now commercially available. With a data rate of 40 Mbps, for both up and downlinks, Li-Fi-X offers wireless connectivity for mobility, security, and multi-user handling applications [236; 237]. Few commercial Li-Fi systems support specific applications such as internet access, 5G/6G communication and beyond, classroom access, smart facilities, healthcare, disaster operations, military applications, etc. [233; 234; 235]. With Li-Fi, the surrounding environment such as homes, cars, offices, and streets can also be connected by creating a wireless network combining several LEDs [238]. The key benefit of the installation of Li-Fi is setting-up atto-cell (small cells) wireless infrastructures with a full-duplex very-high-speed data transfer rate that eliminates the data rate crunch of the Wi-Fi spectrum [239]. This kind of atto-cell Li-Fi installation is completely energy-efficient, free from health hazards, ensures internet-enabled smart device connectivity, and is secured with the usage of low-powered devices [240]. The top-level architecture of a Li-Fi system installation is shown in Figure 32a. The internet connection either from the server or from the backbone network must be connected to the router circuit that selects the access point and switches the connection to the addressed AP [241].