## 2 State of the Art in Optical Wireless Communications  
### The Contribution of Optical Camera Communications in the Processes of Information Transmission  
Current studies and developments in the presented direction have an extremely rapid succession, but a detailed and comprehensive study on OCC technology can be found in [30]. This section presents the most notable developments both in the direction of OCC systems and their use in the implementation of complex solutions. OCC can also include LEDs and image sensors that can be used for both transmission and reception. We emphasize again that almost any application uses LEDs for interior lighting, street lighting, signaling, road lights and car headlights [31]. Fast switching between On/Off states allows extremely viable and much more efficient transmission in such applications. Using this property, LED light can be modulated using high frequencies and several types of modulation. Figure 3 shows the block diagram of an OCC system, which uses both the camera and the front of the phone for communications [32; 33]. Therefore, the coding process of the optical channel is performed to detect and correct the error element while the transmission process through that channel takes place, having noise and interference that are induced by ambient light sources for the interior. When we use the same process for the external environment, things are much more delicate because parasitic light and disturbing sources are present everywhere. The first solution used by most research groups to solve the stated problem was the Manchester coding which helps  
Figure 3: Modulation and demodulation scheme for VLC: mobile communications. in the process of eliminating the flicker of light and reducing losses [34; 35]. Although its use reduces certain aspects, sometimes the changes in the light of an LED are seen by the human eye when the luminescence exceeds the threshold of 200 Hz, a threshold considered safe [36]. Another aspect is that data are encoded and used to modulate the LED light at much higher frequencies than a camera could detect; thus there is the option of using the application that uses the front area where there is a photoreceptor [37; 38; 39]. The responses in the case of photoreceptor pulses are indirect measurements of high-intensity LEDs in order to generate flashes with a duration between 0.05 and 1.0 ms, these are mediated at 100-500 pulses, with a repetition rate of 1-2.5 Hz. When the peak wavelengths of the LEDs are located in the immediate vicinity of the UV threshold (360 nm, at half the width of 15 nm) or blue (470 nm, representing 22 nm) in the spectrum. Thus, we have a maximum spectral sensitivity of photoreceptors on the blue and green areas that tend to be 420 nm and 550 nm, at 480 nm relative sensitivity with about 0.30 and 0.5 [40; 41]. This spectral sensitivity at the peak of the receptors sometimes reaches 348 nm, with a relative sensitivity of 360 nm. The aspect of source attenuation must be taken into account, as well as the attempt to find a way to effectively calibrate and capture quantum light for quality information processing when talking about individual photoreceptors, such as smartphones. It was observed during the studies that the noise in the photoreceptor area indicates individual quantum events, and considering the environment, there is the possibility to operate in a reliable and viable way after it is mediated with the environmental conditions. Adaptation in the case of low ambient light facilitates transmission, observing an increase in noise in the dark every few seconds, causing flashes at 10 ms (medium intensity), a factor that attenuates the amplitude limitation of the peak response to <10 mV. Therefore, we can deduce that the use of both the front area of the mobile phones and the camera is absolutely necessary to achieve compliant and stable communication [42; 43]. Thus, the data encoded by the front of the smartphone use LED light at a much higher frequency than a camera can detect.