# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Optical Camera Communication (OCC)
In this instance, the commonly utilized peak-SNR (PSNR) in image processing should be applied as [379; 380]  
\[PSNR=10\log_{10}\left(\frac{I_{p}^{2}}{MSE}\right), \tag{46}\]  
Figure 36: Top level schematic of a typical OCC system. where \(I_{p}\) denotes the measured frame's maximum intensity, and \(MSE\) is the mean squared error, which is given as  
\[MSE=\frac{\sum_{m,n}[I_{Tx}(m,n)-I_{Rx}(m,n)]}{N_{column}\times N_{row}}, \tag{47}\]  
where \(N_{column}\) and \(N_{row}\) are the number of columns and rows of the images, and \(I_{Tx}\) (\(m,n\)) and \(I_{Rx}\) (\(m,n\)) are the intensity levels within the region of interest of the transmitted and received images, respectively. Note that PSNR and SNR is only equal when the original clean signal is steady and has a maximum amplitude. In PSNR, Ip is the most important metric when measuring the signal's bandwidth or the number of bits used to describe it. The following are some examples of noise in OCC: (i) spatial noise--also known as fixed pattern noise (FPN)--which is caused by dark current non-uniformity in dark environments; and (ii) temporal noise, which includes photocurrent shot noise, flicker noise, and thermal noise. Note that, in cameras, \(T_{exp}\), ISO, and the aperture control the brightness level in the picture. Prior to exposing the image sensors to light, each pixel must be reset, i.e., rest duration \(T_{res}\). The PD is exposed to the light with accumulating photoelectrons in the period between when the pixel is reset and when it is ready to read-out (i.e., \(T_{exp}\)). Thus, the read-out circuit at time \(t=kT_{fp}+T_{res}+T_{exp}\) reads and samples the signal, where k is the pixel row, and \(T_{fp}\) is the frame duration. At the locations where OCC is installed, users may use the OCC system to locate themselves and collect product details, creating a two-tiered network within the large shopping malls. For an integrated OCC/Li-Fi system, a hybrid encoding scheme can be developed using a single multilevel-LED for transmitting both OCC and Li-Fi data. The PD receives high-frequency Li-Fi data signals, which the camera records and decrypts as binary sequences with two distinct power levels. Rate shift keying (RSK) is one of the better methods to modulate OCC data. Furthermore, the hybrid architecture (OCC and Li-Fi) increases (in the order of Mbps) the data transmission rate. However, to install a reliable WOC system, hybrid wireless networks must be created by combining two or more wireless technologies, resulting in enhanced capabilities and the elimination of individual technology flaws [381; 382]. Hybrid RF/optical wireless systems involve the integration or combination of RF and WOC networks [383]. Individual network efficiency can be improved by implementing these hybrid wireless systems in terms of load balancing, reliability, energy consumption, and throughput [384]. We see  
Figure 37: (**a**) CCD and CMOS image sensor; and (**b**) global and rolling shutter mechanisms. the advent of new hybrid WOC systems as a potential trend nowadays [385]. The quality of communication can be improved by network coding, multi-route use, simultaneous transmission between RF and WOC or Ethernet networks, and cooperation between wired and WOC networks [386]. Thus, any type of WOC system can be converged with RF/Wi-Fi systems to form ever more interpreted hybrid networks [387]. At the physical layer level, one of the most critical aspects of WOC is light modulation. When focusing on digital modulation schemes, many techniques are applied widely for all WOC applications [388]. The design of WOC Txs must include flicker mitigation, dimming control, and color quality. There are many modulation schemes which exist and are being used widely in WOC [389]. We classified the more popular modulation techniques in Figure 38.  
RF-optical wireless mixed systems can thus be deployed in a variety of ways, depending on the requirements, environment, and applications.