# Mono Camera-Based Optical Vehicular Communication for an Advanced Driver Assistance System
## 3 Proposed Scheme
### Transmitter  
In general, LEDs that transmit visible light are primarily used as vehicle headlights or backlights, which are mainly utilized as the transmitter in OCC. An LED array might be used to increase the data transmission rate, but as the distance increases, it becomes difficult for the camera to recognize and classify each LED. Therefore, an LED array can be considered for short distance data transmission and it can be segmented into several groups for medium distances [23]. For long distances, the LED array can be considered as a single LED. Two types of vehicular data, i.e., the vehicular ID and multimedia data, were encoded as the low-rate and high-rate data streams and simultaneously transmitted using a single LED. The vehicular ID was used to distinguish between different vehicles, whereas the multimedia data were transmitted after establishing a connection with a specific vehicle. To modulate both data streams in a single LED, we proposed BPPM utilizing multiple power levels of the LED. Data decoding using different intensity levels is challenging, so we considered three intensity levels, i.e., high, intermediate, and low, for feasibility. According to the PHY IV mode of IEEE 802.15.7-2018, a modulation technique for vehicular communication should offer detection and tracking of several transmitters, as well as communication data. The OCC transmitter is designed in such a way that it can transmit the high-rate and low-rate data streams together as a hybrid waveform for the successful establishment of a communication link. Existing schemes standardized in IEEE 802.15.7-2018, i.e., twinkle variable pulse position modulation (Twinkle-VPPM) and hybrid spatial phase-shift keying (HS-PSK) are designed so that they transmit undersampled frequency-shift on-off keying (UFSOOK) and spatial 2-PSK (S2-PSK) as low-rate data streams, and VPPM, and dimmable spatial 8-PSK (DS8-PSK) as high-rate streams, respectively. However, HS-PSK and Twinkle-VPPM require two cameras to receive the data streams. In Twinkle-VPPM, a low-rate UFSOOK stream is generated by varying the pulse width of the high-rate VPPM stream. On the other hand, in HS-PSK, S2-PSK is generated by varying the intensity of DS8-PSK among seven possible dimming levels. The low-rate and high-rate streams are decoded using a low-speed and a high-speed camera, respectively. For DS8-PSK, a reference LED group consisting of eight LEDs is required to decode every 3 bits of data [13]. Consequently, the number of bits transmitted per image frame is lower than the number of LEDs in the LED array. Additionally, one of the rear LEDs of the vehicle can go out of the camera's field of view or become non-LOS owing to blockage. These challenges are overcome in the proposed BPPM scheme by transmitting same hybrid waveform from each of the rear LEDs. Figure 3: Proposed OCC architecture for vehicle localization and communication. As shown in Figure 4, each of the rear LEDs transmitted the same information concurrently in our proposed scheme. High-speed data streams were modulated with a PPM scheme, and 1 and 0 bits were represented by a pulse of the same amplitude at the opposite end of the bit period. Low-speed data streams were transmitted by modulating the pulse amplitudes of the high-speed streams using two different intensity levels. For Level 1, the amplitudes of the pulses were assigned to the highest level, and for Level 0, the amplitudes of the pulses were assigned to an intermediate level between the high and low levels. Two threshold values, i.e., \(Th_{L}\) and \(Th_{H}\), were used to decode the low-rate and high-rate streams, respectively. To remediate the flickering effect observed by the human eye, the chosen modulation frequency was above 100 Hz. It is worth noting that the human eye perceives the average level of intensity transmitted by an LED. Therefore, the average intensity level must be kept constant for comfortable driving. Consequently, we increased the amplitude of the PPM signal and reduced the duty cycle to keep the average intensity level constant.