Advances in digital technology over the past two or three decades have made wireless communications an attractive option for road-based ITS in situations, such as control of VMS, car park counters, traffic signal communications, remote monitoring and CCTV. Specialised ITS applications also rely on a variety of wireless radio services for communication – principally:
Standardisation of Wireless Communications for ITS in Europe
In Europe efforts to standardise wireless communications for ITS are directed towards the creation of a continuous long-range / medium-range continuous air interface using a variety of communication media, including cellular, 5 GHz, 63 GHz microwave and infra-red links. The initiative is known as CALM – which stands for Continuous Air-interface Long and Medium (CALM). CALM will provide the communications platform for a range of applications, including vehicle safety and information, as well as entertainment for driver and passengers.
Some of the more common wireless communications media that are used for ITS include:
Point-to-point microwave communication uses ground-based transmitters and receivers resembling satellite dishes to provided dedicated backhaul links where landlines would impractical or prohibitively expenses to connect roadside networks to a control center. They are usually in the low-gigahertz range and limited to line of sight. Repeater stations can be spaced at approximately 48 km intervals to cover greater distances.
Wi-Fi has become a very popular technology for the exchange of data wirelessly using radio waves over a computer network. Wi-Fi can support non-critical ITS applications because it avoids delay – but does not have bandwidth guarantees. Wi-Fi operates using unlicensed frequencies – so are more susceptible to interference. The technology has potential for use in ITS as a means for connecting field devices to a Traffic Control Centre – for example, where a wired communications solution would be too expensive. In this case, a secure Wi-Fi connection, such as the wifi-mesh network shown in the figure below would need to be provided. This shows a multipoint to multipoint WiFi Mesh network suitable for VMS, car park counters, traffic signals, remote monitoring and non-enforcement CCTV.
Wi-Fi is based on the Institute of Electrical and Electronics Engineers’ (IEEE) (IEEE) Standard 802.11. It is designed to provide local network access over relatively short distances (between 50 – 100 meters) with speeds of up to 54 Mbits/second.
(Figure courtesy of Barry Moore)
Bluetooth technology also supports data exchange over short distances. It uses short-wavelength ultra-high frequency (UHF) radio waves. Bluetooth technologies are used in many different applications such as smart phones, headsets, tablets and laptop computers. Recently, there has been increased interest in building on the presence, on-board vehicles, of Bluetooth devices in “discoverable” modes, to track and monitor traffic. These applications use roadside Bluetooth detectors to discover Bluetooth devices on-board the vehicles and detect their unique identifier. By tracking the same identifier through different location points, important traffic parameters can be determined such as travel time, speed, vehicle origin and destination.
Another group of communications technologies widely used in ITS is dedicated short-range communications (DSRC). DSRC was developed specifically for vehicular communications and is likely to witness a dramatic increase in use with the introduction of Connected Vehicle technologies. The technologies are used in a number of ITS applications including:
In the USA, DSRC generally refers to communications on a dedicated 5.9GHz frequency band reserved specifically for Wireless Access in Vehicular Environment (WAVE) protocols – defined in the IEEE 1609 standard and its subsidiary parts. These protocols are based on the widely-used IEEE 802.11 standard for Wi-Fi wireless networking.
Several DSRC technologies are used in transportation. These include:
Passive tags do not have an internal power supply. Instead, they use the very small electrical current induced in the antenna by the incoming radio frequency signal, to transmit a response. For this reason, the antenna has to be designed not only to collect power from the incoming signal, but also to transmit the outbound backscatter signal. The main advantage of passive microwave tags is that they can be quite small and have an unlimited life. Passive microwave was used in ITS for early types of electronic toll collection systems. Innovations in their use continue.
Active tags have their own internal power source which can generate the outgoing signal. Compared to passive tags, they may have a longer range and can store additional information sent by the transceiver. Active microwave is employed in many electronic toll collection systems. More expensive on-board units have batteries which need replacing.
Infra-red DSRC uses infra-red technology, as opposed to radio spectrum or microwave, for short-range communications. Infra-red DSRC can be used in ITS where it is difficult to secure a frequency spectrum license. The technology is also appropriate when the weather is generally rainy – but not foggy. Infra-red DSRC is less susceptible to security intercepts.
Bluetooth is a wireless technology designed to allow data exchange over short distances (a maximum of about 10 meters). Most cell phones on the market today have Bluetooth technology. They also have Wi-Fi wireless technology, which uses radio waves for connections for distances to a Wi-Fi base station of up to 90 meters. In recent years, several automotive manufacturers have been embedding Bluetooth technology into their vehicles to allow drivers to connect their phones or music devices to in-vehicle audio systems.
When activated, Bluetooth and Wi-Fi transceivers continuously broadcast “discovery” messages to allow other devices to find and connect with them. The discovery messages include a unique identifier that can be used for vehicle detection and tracking. Essentially, all that is needed is a Bluetooth or Wi-Fi sensor installed close to the roadway. These sensors record the time at which a a vehicle equipped with an on-board Bluetooth or Wi-Fi device drives past them. By utilising the unique identifiers recorded at successive monitoring points, information on travel times along a road segment – or the pattern of Origin-Destination flows through a network – can be derived.
The use of Bluetooth or Wi-Fi is ideal for crowd sourcing but the results have to be calibrated as:
not all vehicles report an identifier – since some will not be equipped with the technology or the equipment may be turned off – leading, in both cases, to no count being registered
or a single vehicle may have several active devices – leading to multiple counts.
When using this vehicle sampling technique a key challenge is to ensure that a sufficiently high proportion of vehicles are equipped with Bluetooth and Wi-Fi devices. In urban areas, this may not be a major concern, but in other regions low market penetration may limit the application of these detection technologies.
Communications over a wide area are often required in Network Operations – particularly in rural areas where the options for voice and data communications and the transmission of CCTV images are more limited.
WiMax stands for Worldwide Interoperability for Microwave Access. WiMax is designed to provide much higher bandwidth, compared to Wi-Fi, and at a much extended range. Recent years have seen increased interest from the ITS industry in integrating WiMax and Wi-Fi as an alternative communications medium to wired communications.
Under ideal conditions, WiMax could have a range of more than 40 kilometres, and offer speeds of up to 70 Mbits/seconds. It implements the IEEE 802.16 Standard, with newer standards designed for speeds of up to 1 Gbits/second. A typical WiMax network would consist of a base station connected to several client radios (Customer Premise Equipment or CPE).
Cellular networks were established primarily for voice communications, but there is steadily growing increased interest in their use for data communications as well. Two voice communication cellular technologies have evolved in this way:
Global System for Mobiles (GSM)
Code Division Multiple Access (CDMA)
They differ in the way they transfer data. GSM divides the frequency band into multiple channels for use by different users. CDMA digitises calls and unpacks them at the back end. Both GSM and CDMA have been refined and enhanced over the years to allow for increased speed. For example GPRS (for packet data communications) uses the GSM cellular network at speeds suitable for transmitting commands to VMS and car park counters.
The latest cellular data technology is Long Term Evolution (LTE) (also known as 4G LTE). Unlike GSM and CDMA, LTE is designed primarily for data communications, with voice as an override. It offers high bandwidth, low latency, and supports full data rates while travelling at high speeds (a feature which is important in the ITS environment with fast-moving vehicles). Cellular data communications can be used in ITS where wireline communications are not available or are cost-prohibitive.
Digital Radio Data (DRT) refers to the practice of transmitting digitised and compressed data over FM radio. This allows small amounts of digital information to be embedded in conventional FM radio broadcasts. A good example of a DRT application in ITS is the Radio Data System-Traffic Message Channel (RDS-TMC), where digitally encoded traffic information is made available for in-vehicle navigation devices. RDS-TMC is an early form of digital data transmission. It was developed in Europe to exploit the Radio Data System used by some broadcasting authorities. Travel information is transmitted digitally over FM radio frequencies – and a decoder, built into the car radio or navigation device, interprets the digital code for text or graphic display.
Spread spectrum radio is a radio network that has a “line-of-sight” requirement (unobstructed line of path between a subject and an object). In this network, one radio serves as the master and the other as slave. An example of the use of spread spectrum radio in ITS is the connection between a set of traffic controllers at signalised intersections and the Traffic Management Centre (TMC) needed for monitoring and signal timing purposes.
In some cases, it might not be possible to directly link the two radios because of distance or interference – in which case another radio (called a repeater) would need to be installed in-between the two radios. These networks are most commonly used to allow a number of traffic controllers to communicate with one another, or to communicate with a traffic operations centre. They can broadcast over the unlicensed frequencies of 900MHz, 2.4 GHz and 5.8 GHz. The 5.8 GHz frequency provides the highest bandwidth at about 54 Mbits/second, but is very susceptible to line-of-sight problems.
Whilst theoretically, spread spectrum radio can provide a communication range of up to 60 miles, in practice the range is much shorter because of line of sight attenuation or obstructions.