# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Visible Light Communication (VLC)
Figure 34a illustrates a top-level schematic of a typical VLC system. VLC systems, as shown, use LEDs as transmitters, with transistors controlling the current arriving to the LED, resulting in the high-frequency modulation of light signals emitted by the LEDs, making the transmission invisible to human eyes [273]. LEDs have several advantages such as low power consumption, low heat generation, high bandwidth, and high energy conversion efficiency. Micro-LEDs, multi-chip LEDs (MC-LEDs), phosphor-converted LEDs (PC-LEDs), and organic LEDs (OLEDs) are commercially available in various types/structures [274]. Photodiodes (PDs) are often used in VLC systems as receivers and are responsible for collecting and transforming light into electric currents. PDs are overly sensitive, detecting infrared and ultraviolet waves that are not visible light. When exposed to sunlight, PDs saturate quickly and, hence, data will not be received due to high interference. Modern LEDs and mobile camera image sensors can also be used to detect light. Since LEDs have photo-sensing features, they can also be used as receivers but for low data rates due to their low-speed sensitivity [261]. Although the sensitivity of an LED is lower than that of a PIN photodiode due to the absence of an intrinsic layer, it can be used as a photodetector in simple low-cost, short-range, and low-data-rate applications [275; 276]. Using LEDs as transmitters (Txs) as well as receivers is emerging nowadays due to their significant benefits such as being low-cost devices and their increased security. LED-to-LED communication provides a bidirectional VLC link without adding to the system's  
Figure 34: (a) Typical VLC system block diagram; and (b) indoor visible light propagation model. cost, complexity, space, or energy demands. It lowers the system's cost by replacing the more expensive photodetector and upgrading the VLC system's simplex channel to a time-multiplexed half-duplex channel, lowering the total number of transceiver devices [277; 278]. Figure 34b depicts a general model for the indoor propagation of a VLC system, which includes \(LoS\) connections and first-/high-order reflections [273; 279]. Based on [280], the average optical power collected from all links/paths has been reflected multiple times and may be insignificant. As shown in Figure 34b, a VLC Tx on a roof transmits data to a VLC receiver. Irradiance angles are denoted as \(\phi_{r}\) and \(\phi_{d}\), in the first reflection and the \(LoS\) links, respectively, and the corresponding angles of incidence in every link are represented as \(\psi_{r}\) and \(\psi_{d}\), respectively. The entire \(LoS\) optical channel DC attenuation, \(h_{d}\) [\(u\), \(a\)], from a VLC Tx to a receiver is given as [281; 282]  
\[h_{d}[u,a]= \begin{cases}\frac{(w+1)A_{pd}}{2\pi d_{d,n}^{2}}cos^{w}(\phi_{d}) T_{s}(\psi_{d})g(\psi_{d})cos(\psi_{d}),\psi_{d}\leq\psi_{F}\\ \\ 0,\psi_{d}>\psi_{F}\end{cases}  \tag{33}\]  
where \(w=-\ln\) (\(2\))/\(\ln\)(\(cos\)\(\phi_{1/2}\)), which is the Lambertian emission order; \(A_{pd}\) denotes the PD physical area; and \(\phi 1/2\) is the angle of radiation where the intensity is half of it in the main beam's direction and \(\psi_{F}\) is half of the receiver's FoV. When both the Tx and the receiver focus upward, the irradiance and incident angles are equal in a straight \(LoS\) direction, i.e., \(\phi_{d}=\psi_{d}\). \(g(\psi_{d})\) and \(T_{s}(\psi_{d})\) represent the optical concentration and the gain from the optical filter, respectively. Then, \(g(\psi_{d})\) can be stated as [281]  
\[g(\psi_{d})= \begin{cases}\frac{n_{r}^{2}}{sin^{2}\psi_{d}},0\leq\psi_{d}\leq \psi_{F},\\ \hskip 14.226378 pt0,\psi_{d}>\psi_{F}\end{cases}  \tag{34}\]  
where \(n_{r}\) is lens's refractive index at PD.