# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Optical Camera Communication (OCC)
In ITS, there are a few use cases that can be classified under eight categories: (i) localization--where location information is transmitted to vehicles via road-side units (RSUs); (ii) electronic parking management; (iii) traffic signal control management--where RSUs are used to collect traffic information for traffic management; (iv) traffic information--where vehicles can receive several pieces of information such as obstruction/barricades, road quality/conditions/works, road curve levels, height/width clearance, speed limit and traffic signal assistance, and parking facility/space availability via RSUs; (v) safety information--such as avoiding vehicle intersections/collisions, platooning, crashing, vehicle asynchronized flow, and inaccurate collision warnings, which can be transmitted between vehicles; (vi) emergency applications--where real-time video transmission takes place from DSRC devices to a control center; (vii) kiosk-related services--to deal with mechanical workshops such as automotive repair centers and firmware/software update centers; and (viii) other applications and services--when approaching a crossroads or crossing a road, pedestrians and cyclists who utilize DSRC devices can alert drivers. The vehicle's onboard unit (OBU) can also be used, in addition to above applications, for turning (left or right) assistance and road track/lane changing/warning information. However, there are a few drawbacks in RF-based ITS (or VANET) including (i) interference--due to sharing the RF carrier among the other systems/services [361]; (ii) high installation costs--due to the installation of a DSRC link with the mounting of an OBU and RSU on every vehicle and roadside supporting post/station, respectively; (iii) incompatibility with systems/services of future revolutions; (iv) packet collision/broadcast storm--due to the simultaneous transmission of data from multiple vehicles [362]; and (v) environmental/human hazards--due to the nearest high RF power radiation from/to vehicles [363]. To overcome these issue, one possible option would be to adopt the VLC-OCC technology [364], where LED (or laser diode)-based front, back, and sides lights with a camera can be used to detect an object/people, warning of possible collisions, enabling driver safety, estimating the range and data communications among vehicles, service stations, and vehicles to/from service stations [348; 358]. Interference is much lower in VLC-OCC systems than the PD-based VLC systems, and hence, it is very much useful in a scenario where communication has to happen between the huge number of mobile points and stationary optical service/Tx-Rx stations. In addition, (i) due to the advantages of low sampling frequency and high-quality ICs, VLC-OCC provides a relatively increased SNR compared to the equivalent Li-Fi system; (ii) there is parallel capturing of 2D image data with colors; (iii) since, in the OCC, received data are processed as an image, it is possible to distinguish shapes and distance based on the image depth perception in the camera's image frames; and (iv) due to vehicle head/tail lights, the receiver is biased with a certain amount of DC voltage (upper limit), and thus, the range and data rate in a PD-based VLC system is reduced, unlike the VLC-OCC, where the power requirement is just controlled by varying the exposure time, aperture of the camera, and ISO. These features are highly desirable for longer range (few tens of meters) vehicle communications. In a VLC-OCC system, however, the data rate is determined by the camera's frame rate, and it can be increased when using high-speed (30 fps, and 60 fps) frame cameras or any specialized ultra-speed (120-240 fps, 1-1.2 kfps or greater) cameras [365]. VLC-OCC with LED and PD arrays supports establishing massive MIMO-capable systems to have the increased transmission rates suitable for IoT applications both in outdoor and indoor environments [366; 367; 368]. Recent studies on MIMO-OCC with rolling-shutter-based camera can be found in [369].