## 2 Modeling the VLC Networks: Architecture, System Model and Scenarios
### VLC Architecture
#### 2.1.1 Transmitter  
The optical APs based on LED technology are typically deployed on the ceiling of rooms, corridors or another area in order to avoid shadowed zones and to generate a uniform illumination. For high data rate VLC, the LEDs must provide a large modulation bandwidth while ensuring satisfactory lighting [50]. In this sense, there are two alternatives for generating white light illumination; using blue emitters with a phosphor layer or multicolor red-green-blue (RGB) integrated circuits so that the combination of colors results in white lighting. Although multicolor LEDs provide larger bandwidth, phosphor LEDs are widely proposed for VLC, because of their lower cost, lower complexity and illumination properties. Moreover, \(\mu\)LEDs are currently an alternative technology for VLC transmitters subject to providing lower illumination than phosphor LEDs [51].  
The electrical equivalent of an LED is analyzed in [52]. Specifically, the modulation bandwidth depends on the bias capacitance of the LED electrical equivalent. In this sense, in [52], it is shown that the modulation of a phosphor white LED OSRAM Ostar [53] is greater than 40 MHz. This way, the illumination LEDs Citizen Citised considered in Figure 1 and Figure 2 achieve up to 1 MHz bandwidth for industrial applications [54], which allowsus to provide connectivity in hostile environments, such as the construction of tunnels, although it cannot be considered as a high data rate application. Focusing on high data rate VLC, in [55], it is shown that using equalization techniques a modulation bandwidth of up to 416 MHz can be achieved. In this context, the maximum data rates achieved by RGB LEDs, phosphor LEDs and \(\mu\)LEDs are equal to 15.73 Gb/s [56], 3 Gb/s [57] and 7.91 Gb/s [58], respectively. However, these rates are subject to be surpassed in the future.  
In contrast to RF systems, transmission in VLC is based on intensity modulation and direct detection (IM/DD). That is, the intensity of the signal at the input of the optical transmitter is modulated and the received energy is collected by a PD, which converts the optical power into an electrical signal. This condition yields the following constraint; the transmitted signal must be real and non-negative. As a consequence, imposing IM/DD involves relevant differences between VLC and RF systems, e.g., it is unfeasible to apply coherent detection to VLC systems, which is usually considered in RF transmission. As illustrated in Figure 5, optical APs provide illumination over a limited drive current range between \(I_{L}\) and \(I_{H}\), where \(I_{L}\) is typically zero and \(I_{H}\) corresponds to the saturation current. Within this range, the APs provide an optical power between zero and \(P_{\text{max}}\). Moreover, for IM/DD, the input current of the optical transmitter varies according to a modulation criterion. Recall that, in addition to data transmission, it is necessary to guarantee a constant illumination avoiding flickering [59]. In this sense, the resulting illumination is given by the average of the transmitted signal, i.e., the average output optical power denoted by \(P_{\text{avg}}\).  
Data transmission for VLC can be categorized as single and multi carrier schemes. Moreover, several modulation schemes can be applied to each of these categories. In the following, a brief description of the modulation schemes available in the state of the art is presented.