# A Survey of Vehicular VLC Methodologies
## 3 Vehicular VLC Overview
### Vehicular VLC Architecture  
Figure 5 shows the basic architecture of the vehicular VLC system. The vehicular VLC system contains the transmitter side of the signal processor (modulator) and the front end of the transmitter (e.g., headlight and taillight LEDs). On the receiver side, it consists of optical elements (which can optionally be used in the receiver) and signal processing (a device attached to the receiver that works on decoding and demodulation), and finally, the optical channel, which is the actual distance between the transmitter and the receiver. The LED driving component merges the modulated signal with the bias current required to power high-power LEDs [14] instead of the low-power LEDs used in indoor VLC systems. This manner modulates the transmitter front-end information on the light intensity [88]. Therefore, the design of the driving circuit for VLC systems is essential, as it must provide a correct bias so that the LEDs are lit at optimal operating points and ensure that there is no signal cut-off before the modulation process, because the low operating end of the LED leads to the component may interrupt the negative parts of the signal and furthermore may exceed the high operating point of the LEDs of the linear region. This leads to distortions in the movement or may damage the LED, as shown in [89; 90], where some applications are presented using an open-circuit driving source. The light emitted by the LEDs interacts with the optical elements inside the lighting unit, which control the shape of the beam through automotive regulations [23]. The resulting signal depends on the optical factors, which have a substantial effect. For example, lenses are used, through which the light is focused on a specific point in the distance; using different LED lamps, the matrix lights work on the spatial separation concerning the multiple transmissions of that point in different (but overlapping) directions [91]. Moreover, when the light signal propagates in the optical channel, it will undergo different propagation phenomena. The optical signal may weaken due to path loss. Additionally, outdoor channel disturbances cause an increase  
in its degradation. If not attenuated by the receiver sensitivity threshold, the signal reaches the receiver directly through a link or reflection.  
Optical filters can be used on the receiver side, placed in the receiver front end, which in turn helps improve the system and thus increases the signal-to-noise (SNR) because it filters unwanted optical channels (which contain noise). In addition, lenses (optical elements) collect light and focus on the receiver aperture. Then, the signal is amplified through optical pre-amplification, and thus the strength of the received signal is improved [8; 17; 33; 77; 92]. A camera image sensor or PD is used at the receiver side to convert optical signals into electric currents. The generated photocurrent is converted into voltage using a transversal circuit [93]. For camera image sensors, the generated photocurrent is converted into voltage via a reading circuit [94]. The signal processing block is the last step in the vehicular VLC system in the receiver. Moreover, receivers have fundamentally different architectures in image sensor-based and PD-based sensors, and the signal processing methods and steps also differ; for example, VLC-based systems do not require PDs to refer to image processing as in an image sensor. The analog current signal during signal processing is converted into a digital signal again, and then the information bits are decoded.