# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Light-Fidelity (Li-Fi)
In travel by flight, passengers are often told to turn off their phones and electronic devices since they can interfere with the sensitive communication equipment  
Figure 32: (**a**) Li-Fi downlink configuration; and (**b**) indoor Li-Fi downlink propagation geometry; (**c**) Indoor Li-Fi uplink propagation geometry. upon a pilot's deck. Airlines can have a Li-Fi device installed to deliver internet access while flying. Airline companies can easily provide pre-flight safety guidance, data connection, movies, online games, flight movement details, and establish contact between the pilots, flight crew, and passengers, etc., with a Li-Fi network [252]. Figure 32b depicts indoor Li-Fi downlink geometry. Each user interface is presumed to have a PD-mounted vertically positioned (towards the ceiling-access point) wireless optical dongle. The Li-Fi AP \(i\) and user \(u\) are in the \(LoS\) direction and the corresponding Euclidean distance is \(d_{i,u}\); parameters \(\varphi_{i,u}\) and \(\psi_{i,u}\) are the irradiance and incidence angles associated with the \(LoS\) directions of APi and PDu, respectively, as marked in Figure 32b. Then, the Li-Fi \(LoS\) channel gain is defined as as [242]  
\[H_{LoS}^{i,u}=\frac{(m+1)A_{pd}}{2\pi d_{i,u}^{2}}cos^{m}(\varphi_{i,u})g_{f}g_ {c}(\psi_{i,u})cos(\psi_{i,u}), \tag{27}\]  
where \(g_{c}(\psi_{i,u})\) denotes the optical concentrator gain, \(g_{f}\) denotes the optical filter gain, \(A_{pd}\) is the PD physical area, and \(m\) denotes the Lambertian emission order \(m=-\ln(2)/\ln(cos\Phi_{1/2})\), where \(\Phi_{1/2}\) is the radiation angle at which the intensity is half that of the main beam direction. The optical concentrator gain is expressed as  
\[g_{c}(\psi_{i,u})=\left\{ \begin{aligned} &\frac{n^{2}}{sin^{2}(\Psi_{ max})},0\leq\psi_{i,u}\leq\Psi_{max}\\ & 0,\psi_{i,u}>\Psi_{max}\end{aligned} \right., \tag{28}\]  
where n represents the refractive index and \(\Psi_{max}\) represents the PD's FoV semi-angle. To reduce complexity, only first-order reflections are considered in the NLoS path that consists of two parts: from the AP to small area \(w\) on the wall and from \(w\) to the user. These segments are designated as \(d_{i,w}\) and \(d_{w,u}\), respectively, by Euclidean distances. The first segment's radiance and incidence angles are \(\phi_{i,w}\) and \(v_{i,w}\), respectively, while the second segment's angles are \(v_{w,u}\) and \(\psi_{w,u}\), respectively. The delay between various paths may be ignored, as the paths for indoor propagation are relatively short. Thus, the signals reaching the PD are from different directions, but they are at the same time. The Li-Fi \(NLoS\) channel gain is calculated as [242]  
\[H_{NLoS}^{i,u}=\int_{A_{w}}\frac{(m+1)A_{pd}}{2(\pi d_{i,w}d_{w,u})}\rho_{w} cos^{m}(\phi_{i,w})g\&fg_{c}(\psi_{w,u})cos(\psi_{w,u})cos( \vartheta_{i,w}), \tag{29}\]  
where \(\theta_{w}\) represents the diffuse reflectivity of the wall and Aw is the wall's small reflective area. When Equations (26) and (27) are combined, the channel gain for user u from the Li-Fi AP is  
\[H_{Li-Fi}^{i,u}=H_{LOS}^{i,u}+H_{NLOS}^{i,u}, \tag{30}\]  
In a receiver, the PD captures photons and converts them into electric current, which can be expressed using the equation  
\[I_{elec}=\frac{R_{pd}H_{Li-Fi}^{i,u}P_{opt}}{k}, \tag{31}\]  
where \(\kappa\) is the optical to electrical power conversion coefficient, \(P_{opt}\) is transmitted optical power and Rpd is the responsivity of the detector. The optical signal power is represented by the coefficient \(P_{opt}/\kappa\). The desired signals SNR received by the user is expressed as  
\[SNR_{Li-Fi}^{i,u}=\frac{\left(R_{pd}H_{Li-Fi}^{i,u}P_{opt}/k\right)^{2}}{ \mathcal{N}_{Li-Fi}B_{Li-Fi}+\sum\limits_{j\
eq i}\left(R_{pd}H_{Li-Fi}^{i,u}P_ {opt}/k\right)^{2}}, \tag{32}\]where \(B_{Li\text{-}Fi}\) is the Li-Fi AP's system bandwidth and \(N_{Li\text{-}Fi}\) is the noise PSD at the PD. Li-Fi encompasses a wider range of network systems, such as bidirectional, multiuser, broadcast, and multicast networking [248].