TSAI_LAYOUT_Layout 12/3/13 1:49 PM Page 50 VISIBLE LIGHT COMMUNICATIONS: THE ROAD TO STANDARDIZATION AND COMMERCIALIZATION Smart Automotive Lighting for Vehicle Safety Shun-Hsiang Yu, Oliver Shih, and Hsin-Mu Tsai, Intel-NTU Connected Context Computing Center and National Taiwan University Nawaporn Wisitpongphan, King Mongkut’s University of Technology North Bangkok Richard D. Roberts, Intel Labs ABSTRACT current status such as positions and speed, but also what they observe of neighboring objects at It is believed that vehicle-to-vehicle (V2V) different locations. The advantages of the new communications and accurate positioning with paradigm include: sub-meter error could bring vehicle safety to a •Wireless communications and networks can different level. However, to this date it is still penetrate or route around (i.e., utilizing unclear whether the envisioned V2V standard, multihop communications) obstacles block- dedicated short-range communications, can ing line-of-sight (LOS), effectively extend- become available in commercially available vehi- ing the sensor coverage to include all of cle products, while widely available consumer- those neighboring vehicles. grade GPS receivers do not provide the required •Cooperation among vehicles can reduce accuracy for many safety applications. In this uncertainty of measurements [1], such as article, combining visible light communications the position and the speed of a certain vehi- and visible light positioning, we propose the use cle, in multiple ways: of smart automotive lighting in vehicle safety –Self-information of a vehicle is more accu- systems. These lights would be able to provide rate than observations from neighboring the functions of illumination and signaling, reli- vehicles. able communications, and accurate positioning –Observations from a nearby vehicle are in a single solution. The proposed solution has more accurate than observations from a dis- low complexity, and is shown to be scalable in tant vehicle.1 high vehicle density and fast topology changing –Combining observations of the same object scenarios. In this article, we also present several from multiple vehicles can reduce the noise design guidelines for such a system, based on the in the measurements. results of our analytic and empirical studies. •The system can have lower cost as a direct Finally, evaluation of our prototype provides evi- result of the lower required number of sen- dence that the system can indeed detect poten- sors in each vehicle — information does not tial risks in advance and provide early warnings have to be only from on-board sensors, but to the driver in real-world scenarios, lowering also from sensors in neighboring vehicles. the probability of traffic accidents. THE COOPERATIVE PARADIGM REQUIRES INTRODUCTION RELIABLE V2V COMMUNICATIONS The conventional paradigm for vehicle safety has Dedicated short-range communications (DSRC) been an “independent” approach — to incorpo- is often considered the most promising V2V rate a large number of sensors in each vehicle so technology [2]. The technology incorporates a that sufficient coverage can be provided to iden- set of physical, data link, and higher layer stan- tify neighboring objects that could cause acci- dards that use the 5.9 GHz radio spectrum to dents. The main downside of such an approach enable V2V and vehicle-to-infrastructure (V2I) is the high cost of the system as a result of the communications. Although the standards in large number of sensors. Recent advances of DSRC have been drafted or ratified for more communication and networking technologies than a few years, to this date there is still no have enabled a new cooperative paradigm for commercial adoption of the technology. The 1 For example, when esti- vehicle safety systems, in which, via direct vehi- future of DSRC is not clear as of today. For mating the distance to a cle-to-vehicle (V2V) communication links, vehi- example, the U.S. government will make a deci- vehicle using a camera. cles not only share information about their sion regarding the technology at the end of 2013, 50 0163-6804/13/$25.00 © 2013 IEEE IEEE Communications Magazine • December 2013 TSAI_LAYOUT_Layout 12/3/13 1:49 PM Page 51 based on real-world tests carried out in the past The unique properties of optical propagation two years [3]. and VLC present many advantages over conven- The implication of The main reason that V2V technology has tional radio frequency (RF) communications for the 10 percent not yet seen its adoption is that there is very low safety applications: incentive for early adopters to purchase a new Low complexity and low cost: The design of minimum market vehicle with the technology. Many envisioned the VLC transmitter and receiver is much less penetration is that, applications that use multihop communications complex than a comparable RF transceiver, due require a minimum market penetration rate of to a much less severe multipath effect. Com- even if every new 10 percent or more [4]. bined with simple modulation schemes and the vehicle were The implication of the 10 percent minimum fact that the main transmitting component, equipped with the market penetration is that even if every new LED, already exists in vehicles, this minimizes vehicle were equipped with a new V2V radio the additional cost to realize smart automotive new V2V radio start- starting from today, it will still take more than lighting. ing from today, it two years before these applications can be func- Scalability: In scenarios with high vehicle tional on the road.2 As a result, early adopters density, conventional V2V communications will still take more do not gain any benefit in purchasing a V2V- using RF typically experience longer delay and than two years equipped vehicle, which has a higher price tag. lower packet reception rate due to the large before these applica- To jumpstart the initial market penetration rate, number of nodes that participate in channel con- we believe the key directions include: tention. An adaptive transmission power scheme tions can be func- •To discover applications that can immedi- can be used to mitigate the problem, but requires tional on the road. ately provide benefits to the driver on the high overhead to accurately estimate the number day they obtain the new vehicle of nodes in the neighborhood, which could also •To lower the complexity of the technology reduce the reliability. On the other hand, in the so that the added cost to the vehicle prod- case of VLC, only a small number of nodes with- uct is minimized, and we do not rely on a in direct LOS of the receiver are in the same large sale quantity to bring down the costs contention domain, and those nodes are of the The latter implies that the communications tech- most importance since they are the vehicles most nology should be designed to fulfill the require- likely to collide with the receiving vehicles. This ments of only a small set of key applications. filtering mechanism depends on the optical Visible light communications (VLC) utilize propagation property, does not require any over- modulated optical radiation in the visible light head, and hence is highly scalable. spectrum to carry digital information in free Positioning capability: Due to the high direc- space. A VLC system usually uses an LED as tivity of visible light propagation, it is possible to the transmitting component and a photodiode as perform accurate relative positioning between the receiving component. Over the past five vehicles [8]. Compared to a typical positioning years, LED has become very common in auto- error of up to 10 m with GPS [9], the positioning motive lighting due to its long service life, high error of visible light positioning (VLP) is on the resistance to vibration, and better safety perfor- order of only tens of centimeters, much more suit- mance due to its short rise time. Today, LEDs able for vehicle safety applications. can be found in center high mount stop lamps Security: For a malicious attacker to send fal- (third brake lights), brake lights, turn signals, sified information or jam a VLC receiver, due to and headlamps in many commercial vehicles. As the closer operation range and the LOS-only a result, VLC becomes a new and attractive propagation property of VLC, the attacker has solution to realize V2V communications [5–7] in to be in visual range of the victim vehicle for an a completely different way. extended period of time, which renders the In this article, we propose the use of VLC in action less likely compared to when using con- vehicles for several key safety applications, creat- ventional RF communications. In addition, the ing smart automotive lighting that combines the positioning capability can be used to provide an functions of illumination and signaling, commu- additional layer of security by verifying whether nications, and positioning. As shown in Fig. 1, in the received message is sent from a correct or the case of cars, a VLC transmitter is connected possible position. to each of the two headlamps and two taillights VLC also presents a few challenges when to create four VLC-capable lights transmitting used in outdoor environments, most of which the same signal; a photodetector is placed next have been investigated in existing literature [10]. to each of these four lights and connected to a Severe weather conditions: Heavy fog, rain, VLC receiver. Additional photodetectors can be or snow could create additional attenuation to installed on the rear facing side of each of the the transmitted optical signal, shortening the two side rearview mirrors to provide communi- operation range. However, existing work [11] has cation coverage to vehicles on the side. In the shown that a VLC receiver has better sensitivity case of motorcycles or scooters, both the head- than human eyes; this implies that, in these dras- lamp and the taillight are connected to the VLC tic weather conditions, VLC will be able to transmitters, creating two VLC-capable lights receive a message before human eyes can per- transmitting the same signal, and a photodetec- ceive the light from the same transmitting light, tor is installed right next to each of these two proving the usefulness of VLC.