24 PART | I ITS technology enablers
The message set dictionary of SAE J2735 contains 15 messages, 72 data
frames, 146 data elements, and 11 external data entries. The most important
type of message is safety-related and is the message that informs surrounding
vehicles and the infrastructure about the existence of a vehicle. This “heartbeat”
message increases the amount of information of nearby vehicles and assists their
autonomous driving mechanisms to respond better. This basic type of message
contains temporary identity information and a timestamp, 3D position informa-
tion and the associated accuracy, speed, heading, and acceleration information,
as well as the steering wheel angle, the brakes’ status (on/off), and information
about the vehicle size.
Apart from the heartbeat message, there exist other message types such as:
• A template-based (a la carte) message, which is composed of message ele-
ments that the sender chooses. This type of messages allows fast and flexible
data exchange, with a small degree of customization.
• Emergency vehicle approaching message that alerts surrounding cars and
the infrastructure that an emergency vehicle is approaching the area.
• Generic transfer message, which is the main interface for V2I messages.
• Probe-vehicle data message, which contains vehicle-mobility information
that can be used for applications that analyze traffic conditions.
• Common safety request message that is used for exchanging basic vehicle
information for car-safety applications.
2.7 LED-enabled visible light communications (IEEE TG
802.15.7)
Light-emitting diodes (LEDs) are quite popular choices for display and illumina-
tion applications. The offer high-brightness, low-power, and heat-dissipation, and
live longer than conventional lamps. Finally, they are used widely for road illu-
mination, traffic signs, and the headlights of vehicles. Another advantage of LED
technology is that diodes can be modulated at high speeds thus allowing them
to be used for data transmission tasks. Visible Light Communication (VLC) on
top of LED offers increased reliability, at a low-cost and with a reduced-energy
footprint. The transmission is free of interference, whereas the LED technology
is easy to integrate and interoperable (Bouchet et al., 2010; Komine, 2001)
It is important to notice that the national roadmaps for transportation in
many countries assume that road infrastructure elements, such as traffic signs
and traffic lights will be soon equipped with sensors, actuators, and embed-
ded processors and the same elements employ or are soon planned to use LED
technology for display or illumination purposes. On the other side, vehicles are
also equipped with LED (e.g., LED head lights), which makes LED technology
and VLC ideal candidates for V2X communications (Wook, Haruyama, & Nak-
agawa, 2006). In this scenario, LED lights in vehicles can be used to transmit
data from the vehicle and the infrastructure or other vehicles in a fast, reliable,
and energy-efficient way.