# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Visible Light Communication (VLC)
Prior to the noise filtering at the receiver, SNR is determined by shot noise and thermal noise of the PD circuitry as  
\[SNR=\frac{P_{R_{E}}^{2}}{\left(\sigma_{shot}\right)^{2}+\left(\sigma_{thermal }\right)^{2}}, \tag{43}\]  
where \(\sigma_{thermal}\) and \(\sigma_{shot}\) denote the standard deviation (std) of thermal and shot noise, respectively. The shot noise, i.e., number of photons received by the PD, naturally fluctuates statistically. A Poisson distribution follows when counting photons, meaning that if x is the mean number of photons absorbed by the PD in a unit time, then \(\surd{\rm x}\) is the statistical standard deviation. As a result, the photoelectrons generated by PD will exhibit Poisson distributed variation, and then the shot noise variance can be estimated as [280; 285]  
\[\left(\sigma_{shot}\right)^{2}=2 qP_{R_{E}}B+2 qI_{B}I_{2}B \tag{44}\]  
The thermal noise variance can be expressed as [263; 269]  
\[\left(\sigma_{thermal}\right)^{2}=\frac{8\pi kT_{k}}{G_{ol}}C_{pd}AI_{2}B^{2}+ \frac{16\pi^{2}kT_{k}\eta}{g_{m}}C_{pd}{}^{2}A^{2}I_{3}B^{3} \tag{45}\]  
where \(g_{m}\) is the FET transconductance, \(IB\) is the background-radiation-induced photocurrent, \(I_{2}=0.562\) and \(I_{3}=0.0868\) are the noise-bandwidth factors, \(C_{pd}\) is the PD capacitance per unit area, \(T_{k}\) is the absolute temperature, \(B\) is the photodetector's bandwidth, \(\kappa\) denotes the Boltzmann's constant, \(G_{dl}\) is the open-loop voltage gain, and \(\eta\) is the FET channel noise factor. Room temperature, ambient light, and other variables have an impact on thermal noise and shot noise [280]. With many Txs and receivers operating at the same time, multiple VLC access is conceivable. More than one user may be connected to a VLC access point in an indoor context, and a corporate/residential building. Mechanisms for controlling the access, managing the number of devices, and allowing mobility must be established when multiple devices need to be linked [286; 287]. The main multiple access mechanisms used in VLC include the conventional orthogonal multiple accesses such as code division multiple access (CDMA), time division multiple access (TDMA), carrier sense multiple access (CSMA), orthogonal frequency division multiple access (OFDMA), and NOMA [288]. The main issues that need to be considered while designing a MIMO VLC network are coverage, mobility, uplink, multipath interference, dimming, blockage and flicker. To control the illumination, usually referred to as dimming, one must of course change the current and/or voltage of the source of electric light. However, this comes at an extra expense for the hardware. When deploying LED in a VLC system, the light intensity has a significant impact on the communication signal strength. Lowering the light intensity narrows the data transmission rate [289]. Dimming is an important function in many places, providing benefits such as energy savings and relaxing environments. The IEEE 802.15.7 standard specifies a set of mechanisms for configuring modulations so that dimming can be regulated. VLC communication dimming is well tolerated by modulation techniques such as OOK, VPPM, and CSK [290; 291; 292]. Flickering is the fluctuation of a human's perceptible light brightness that occurs because of light-wave phase/intensity modulation which can cause discomfort and health risks. Therefore, the on/off status or strength of light must be modulated such that the smallest of its on/off frequencies is greater than the human's perceived frequency, i.e., \(\approx\)3 KHz. Low-frequency communications have a duality: lower data rates and flickering [293; 294]. The short-range VLC protocol IEEE 802.15.7 specifies, mainly, two methods of flickering as intraframe and interframe, i.e., flickering between frames and flickering between adjacent transmissions, respectively. The intraframe flickering problem is solved by the OOK with Manchester encoding, since 0 and 1 symbols are often present to denote a signal in this technique.