Without wireless communication, there would be no intelligent transport systems – that speaks for itself. But what technologies are being used in traffic? And what characteristics, advantages and limitations do they have? This article probes the ‘hidden force’ that enables traffic engineers to use the road network in a safer and more efficient way.
Article from NM-Magazine, March 2014 – also available in PDF (Dutch only)
In 1886, Rudolf Hertz discovered waves in the form of electromagnetic radiation with wavelengths, also called radio waves. Like so many scientists of his era, Hertz couldn’t see any practical application for his discovery. “It serves no purpose at all”, he is reported to have said. “It’s just an experiment that proves that Maestro Maxwell was right.” When one of his students asked him: “What happens now?”, he answered: “Nothing, I think.” But history was to take a different turn. In 1894 the young Italian engineer Guglielmo Marconi began to experiment with “Hertz waves”. In 1895 he was the first to establish a radio connection across several kilometers – which made him the father of wireless communication.
Foundation underlying ITS
We could also call Marconi the father of intelligent transport systems. Because, to put it simply: without wireless communication there would be no ITS. ITS requires large numbers of ‘connecting lines’ between control centers and vehicles, between roadside systems and vehicles and between vehicles themselves. It is about time, therefore, to take a closer look at the way in which radio waves are being applied to make smarter traffic possible. First: which wireless applications are being used in our field? And which radio technologies underlie these applications?
Mapping our own surroundings
Smart cars that have the capability to think along with their drivers, at the very least need to be able to form a picture of their surroundings. They can do this using radar, or Radio Detection and Ranging. Radars emit radio waves and, on the basis of the reflected waves, determine the distance, speed and direction of the detected object in relation to their own position, speed and direction. Because radars use radio waves, they are relatively immune to weather conditions, more so than for instance video cameras or infrared systems. A new system is Lidar, Light Detection and Ranging or Laser Imaging Detection and Ranging, which uses laser pulses instead of radar waves.
Broadly speaking, two types of radar are used for ITS applications. The first, automotive radar sensors, detects objects around the vehicle with an approximate range of 40 meters. These sensors provide the detection data for driver support functions such as collision avoidance, stop-and-go and lane change support. The second type detects objects in the lane in front of the vehicle up to a distance of 150 meters. The detection data which the vehicle collects in this way is important for autonomous cruise control for instance.
Ad hoc networking in traffic
Observing what goes on around you is essential, but ITS really becomes interesting once vehicles are able to communicate with their immediate surroundings. ITS G5, or 802.11p, was made to enable this application: a radio system specifically developed for vehicle-vehicle (V2v) and vehicle-to-infrastructure (V2I) communication. ‘Specifically’ means that a frequency band has been especially reserved for it (5.9 GHz) and that a lot of attention has been given to the speed and reliability of the communication. ITS G5 builds on the technology used for the popular wifi application.
ITS G5 makes it possible to set up ad hoc communication networks around vehicles with other (passing) vehicles and around roadside stations. ITS G5 uses a broadcast mechanism: it emits messages to the surrounding environment that can be received by anyone present there.
Setting up one-on-one connections
It is also important for many ITS services to have continuous one-on-one connections, for instance with the service provider. Cellular communication systems are suitable for this purpose. These systems – which we also use for mobile telephony – use a network of transmitters, with each transmitter covering part of the country. There is a distinction between macro cells (range of 1 km to 30 km), micro cells (200 m to 2 km) and pico cells (up to 200 m). The mobile device – a smartphone, navigation system or in-car system in the case of ITS – can set up a network with the computers of the service provider or traffic control center within a cell, using the transmitter and the underlying central telephony system. As soon as the mobile device moves, it will be handed over from one transmitter to the next as it passes along.
Cellular communication systems exist in different generations: 2G (GSM), 2.5G (GPRS), 3G (UMTS) and, the latest generation, 4G (LTE). Each generation has more bandwidth than the previous one, as well as lower latency in the communication. The extra bandwidth can be used to transmit larger data sets. The lower latency leads to a higher and more reliable response speed.
Broadcasting information to anyone who is ‘listening’
It is useful for road managers and service providers to be able to transmit information to a large group of vehicles at once. The good old radio is very useful for this one-way communication: a radio transmitter can easily cover an area of dozens of square kilometers.
In addition to the trusty analog radio, digital radio is in the process of becoming a serious successor. A digital channel has already been added to the analog radio signal, in the form of RDS, radio data signal. In Europe, RDS has been used for a number of years now to transmit traffic updates through the traffic message channel (TMC). Most radio and navigation systems can receive and use RDS/TMC messages. Fully digital radio, DAB or digital audio broadcast, is a newcomer. DAB can be supplemented with TPEG, a standard for language-independent communication of traffic information. TPEG is (in part) a further development of TMC. It supports traffic information, public transport information and location information. In addition, a number of further developments are being undertaken to add things such as parking information, traffic congestion, travel time and weather information. In order to be able to receive DAB/TPEG, the vehicle has to have a DAB receiver. Cradles and power cords with DAB receivers are also currently available on the market. DAB/TPEG is already being used for instance in the United Kingdom and in Germany.
All of the ‘typical’ applications per communication technology have been discussed in the above. Figure 1 (see PDF) shows the different applications once again, but ranked this time in relation to the required speed of communication and the required precision of the positioning. It also indicates which radio system is best suited for which function. The Figure shows clearly that there is no ‘universal’ system: the different technologies complement each other and are certainly not mutually exclusive. All of them can be found in smart vehicles.
Figure 2 gives an interesting overview (see PDF), showing the various uses arranged according to flow and safety and according to whether they are vehicle-specific or traveler-specific. Thanks to mobile internet on our smartphones, which uses 2.5G, 3G or 4G, we can stay connected with our ITS service provider even outside our car – see the text box in the PDF.
Positioning vehicles in time
As was already evident from Figure 1, the practical benefits of a radio system depend partially on the possibility of correctly determining the position of a vehicle or traveler on the road network or road lane. With radar, this is implicit in the technology itself. Traffic information and information about traffic congestion only serves its purpose if the receiver can link this information to a specific location within the road network and preferably even with a specific road lane. The added value of an ad hoc network constructed via ITS G5 is enhanced if it can be determined in which road lane vehicles in the network are positioned. Actually it is not the position itself that is valuable, it is the position at a particular point in time. The link between space and time is a hard one. There are a number of techniques that use radio waves to determine the position lines or the position of a moving object. A well-known way of doing this is triangulation, in combination with mobile communication systems such as GSM, UMTS and LTE. If at least three base stations register the time of arrival of the signal of a mobile device, it is possible, using triangulation, to localize the device – provided that the clock signals of the base stations in question have been synchronized. A limitation of this way of determining position is that the precision achieved varies by geographical area: it coincides with the configuration of cellular communication networks. And communication cells are smaller in urban than in rural areas, in order to be able to deal with the higher number of mobile telephones. This results in a higher precision in cities than in the countryside.
Satellite navigation is an alternative technique that has better and more consistent precision in determining the position of objects. Well-known satellite navigation systems with global coverage are GPS (USA), Galileo (EU) and GLONASS (Russia). These systems are also known as GNSS or Global Navigation Satellite Systems. Positions on highways can be determined with a margin of 10 to 15 meters. It is somewhat more complicated in cities: high buildings can affect reception and there is the multipath effect, where GPS receivers unduly receive signals that have been reflected by buildings.
In any case, as Figure 1 shows, even a margin of 10 to 15 meters is not enough. What we really need is to be able to determine the position of vehicles on road lane level, i.e. with a precision of 1 meter. To realize this, scientists are working on improving satellite technology. In the short term, there are systems that correct GPS signals received on earth for – among other things – ionospheric disturbances, variations in satellite orbits or clock errors: EGNOS (EU), WAAS (USA) and MSAS (Japan). A new version is Precise Point Positioning, which has been developed by knowledge institutions, including Delft University of Technology. It is this system that Delft University of Technology, TomTom and Technolution are trialing in the Brabant In-car III project ‘Dynamic Lane Guidance’. In the long run, greater precision in positioning cars and travelers can be achieved by having double sets of satellite signals. Thus each GPS satellite for the civil market will transmit at two different frequencies. GPS receivers will then be able themselves to correct the signals received for ionospheric disturbances, variations in satellite orbit or clock errors. In addition, there are a number of new techniques that are becoming available for correcting the multipath effect.
It may be true that the focus in ITS is primarily on nice in-car applications, but this article has demonstrated that underlying ITS, there is a sound foundation of various, partially overlapping communication technologies. These technologies – which are all developments of Marconi’s first radio – make it possible to provide smart services designed specifically for their task. They are the ‘hidden force’ that enables traffic engineers to make safer and more efficient use of the road network.
Coen Bresser, former Business Developer of Technolution