WiFi Plus LiFi

From headlight to smart communication unit: optical data transmission from vehicle to vehicle

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Innovations, such as autonomous driving or platooning, are inconceivable without connected vehicles. So far, only wireless-based technology, such as WiFi, has been used for car-2-X data communication. Visible and infrared light are an attractive addition. The optical connection of vehicles provides high data rates, short time lags and a high level of robustness against manipulation and unauthorized reading. IAV has been working on this topic since 2014 and is pushing forward its use in volume production in cooperation with partners like headlight manufacturer ZKW.

State of the art in car-2-car connectivity is the use of WiFi in the frequency range between 5.85 and 5.925 GHz (IEEE 802.11p). The technology is tried and proven, permits high data rates, involves no visual contact to set up the connection and can be used on virtually all vehicle platforms. But it also has weaknesses. Besides electromagnetic compatibility, the very narrow frequency range, manipulated signals and transmission errors can lead to problems. For this reason, alternative transmission paths are being sought that can be used alongside WiFi communication.

This is where optical data transmission comes in: Instead of using radio waves for communication, optical data transmission involves modulated visible or infrared light which is sent from a transmitter to a receiver. The most familiar example of this is sending messages in Morse code using light signals, which is still used today for communication between ships. “In the connected vehicle of the future, a laser or an LED as the light source could serve as emitter with a photodiode or phototransistor receiving the signals”, explains Dr. Timo Denker, development engineer at IAV in the Vehicle Integrated Functions division.

Optical drawbar permits platooning

It is handy that vehicles are already equipped with numerous light emitters: headlights and fog lamps, taillights and turn indicators. “This means that optical communication could be realized without any major intervention in the existing vehicle structure”, says Tino Pöhlandt, who also works as a development engineer in the same IAV division. With data transmission directed to the front or rear, a vehicle could identify road users traveling ahead or following up from behind, determine mutual distance and relative speed, exchange traffic information or communicate specific driver requests, such as the wish to pass the vehicle in front, change lane or adjust speed. For platooning – automated driving in convoys of vehicles spaced at very short distance – the new technology could provide an “optoelectronic drawbar”. All vehicles always keep the same distance by using data communication like a mechanical drawbar connects a truck with its trailer.

Communication in the visible or infrared range has significant benefits in terms of safety. Due to the fact that radio antennas are omnidirectional emitters, WiFi signals can be received in a large radius around the vehicle and are vulnerable to manipulation. In contrast to this, light signals can only be received within a very limited beamwidth of approx. 25 degrees. Thus, the light source and receiver of a potential attacker must be within this beamwidth of the LiFi module to feed in manipulated signals, otherwise they are filtered out as interferrng light pulses and ignored.

However, optical data transmission between vehicles not only lends itself to communicate information that is particularly critical in terms of security. It can also be used as a redundant channel for transmitting the same data as sent in parallel from vehicle to vehicle by WiFi. In this scenario, the optical and the radio receiver would compare the incoming bit flows and, in the event of any differences, give an alarm. Also conceivable is a job split between WiFi and light transmission. This way, communication partners could be authenticated very quickly on the optical channel with the actual information being sent via radio signal.

IR communication currently less susceptible to interference

However, transmission by light signals also leads to new challenges. For instance, reliable transmission must also be ensured even when the sun is low, in rain, snow and fog. At the present day, interferences from visible ambient influences are still too big. A further reduction by making improvements to transmitters, receivers and modulation methods is highly probable in near future. Communication using infrared light such as the Fraunhofer Institute for Photonic Microsystems (IPMS) in Dresden has developed is way less vulnerable, providing data rates of up to 12.5 gigabits per second. Their purpose is to interconnect production robots in industrial facilities.

IAV has been investigating the potential of optical communication since 2014. The aim is to develop a module for transmitting data between vehicles and test it in practice. IAV’s experts are working on this in cooperation with Austrian headlight manufacturer ZKW. They are examining the feasibility of a truck platoon (SAE level 4) on highway-like roads. The long-term goal is to transform standard lighting sources into smart communication units on the basis of visible and non-visible light, enabling vehicle lighting to establish as a new channel for short-range car-2-X communication. “Wireless Fidelity” (WiFi) would be joined by “Light Fidelity” (LiFi) as a further data transmission channel.

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