Wireless Systems Design

Keep An Eye On Vehicular Communications Systems

The 5.850-To-5.925-GHz Telematics Band Will Capture The Industry’s Attention By Using Low-Cost 802.11a-Based Systems.

Ever since the first radiomatic garage-door opener was developed in 1926, wireless technology has been used to make car-related activities easier, more enjoyable, and convenient. Throughout the history of vehicular-communications or telematics systems, however, one rule has always held true: Everyone has his or her own idea about how to do things. OnStar has its system. Tele Aid has its technology. Wingcast does its own thing. Shell Stations use one type of fob for gasoline payment, while Exxon Mobil uses another. Each accomplishes similar goals, but uses different equipment and protocols.

As a result, the variety of RF tags and active transponders used for things like electronic toll-collection systems and parking garages outnumbers the amount of models that car manufacturers currently build. This situation is of no help to third parties wanting to build applications supported by a variety of automotive equipment. It is even more frustrating for public agencies and private companies wanting to take advantage of different vehicles' built-in systems. The myriad of proprietary technologies limits the development and adoption of telematics applications.

In 1999, the FCC realized this problem and allocated 75 MHz of spectrum—from 5.850 to 5.925 GHz—for use by Dedicated Short Range Communications (DSRC)systems in the service of Intelligent Transportation Systems (ITSs). The allocation held that services within the band would use non-voice radio techniques to transfer data over short distances, such as between roadside and mobile radio units, between mobile radio units, and between portable and mobile units. Services in this band would include operations related to the improvement of traffic flow, traffic safety, and other ITS applications in a variety of public and commercial environments. This allocation of transportation-specific spectrum is comparable in size to the 83.5 MHz of spectrum available for all unlicensed services in the 2.4-GHz band. Because it's part of the 5-GHz band, however, the higher-performance technology designed for this upper band can be used in the new spectrum. This technology was known provisionally as 802.11a R/A (road access), and is now the ASTM E2213-02 telecommunications standard. It is a derivative of the IEEE 802.11a standard used for wireless networks from 5.150 to 5.850 GHz. Because it's based on an existing open standard, it has the advantage of being technically proven. It also benefits from existing research and development efforts.

One key variation necessary for the E2213-02 spec is the definition and channelization of the protocol for the 5.9-GHz spectrum. Under the FCC Part 15 rules for unlicensed wireless operations, the core IEEE 802.11a spec defines 20-MHz channels in the spectrum from 5.150 to 5.825 GHz. 802.11a R/A instead defines 10-MHz channels in the spectrum from 5.850 to 5.925 GHz under the Part 90 rules for operation.

Seven independent, 10-MHz-wide channels are available in the United States from 5.850 to 5.925 GHz(see figure). Because of the availability of 5.825 to 5.850 GHz, three independent channels also are available in Canada. These channels are short range (less than 100 m typical). They support data rates of up to 27 Mbps, and may be used simultaneously by many vehicles and applications.

802.11a R/A is a component of licensed services operating in the DSRC band. As a result, it has less restrictive emission-mask and guard-band rules than IEEE 802.11a products operating in the Unlicensed National Information Infrastructure (U-NII) and Industrial, Scientific, and Medical (ISM) bands. Less overall spectrum is available (75 to 100 MHz) in the DSRC band than in the U-NII and ISM bands (350 MHz), however. This prompted the decision to halve the clock of the protocol, but run the channels to the edge of the band as permitted by the Part 90 emission-mask rules. Thanks to this change, each E2213-02 channel consumes only half as much spectrum as IEEE 802.11a channels at the expense of delivering half the maximum data rate (27 Mbps instead of 54 Mbps).

Fundamentally, the protocol remains IEEE 802.11a. It just does so at a half-clock rate while operating in the DSRC spectrum. To accommodate the needs of the vehicular environment, it also has undergone slight modifications to parameters like RSSI sensitivity and adjacent-channel rejection. The keeper of this standard for telematics applications is the American Society for Testing and Materials (ASTM) subgroup E17.51.

To target the vehicular environment, RF designers also must adjust timing, calibration, and other parameters in their 802.11a systems (see table). According to field trials and simulation testing, these modifications allow for applications at a relative speed of up to 120 mph. They also permit up to 255 simultaneous users on a single channel.

E2213-02 is not a universal telematics technology. It is optimized for short-range or local communications. It complements technologies like GPS, automated emergency communications systems, and cellular phones. Other technologies offer different methods for delivering information/ services from a remote location. Yet, E2213-02 provides a universal method for exchanging information in a vehicle's local area.

This universal local-communications capability forms the basis for public/private usage of the 5.9-GHz band. Under the original FCC allocation, the spectrum's primary applications support public safety in transportation. These applications involve new methods of communicating with vehicles and the people inside them. There is a very real need for these applications. Sirens and flashing lights are no match for today's hermetically sealed cars and their very large speaker systems. According to the National Safety Board, 250 accidents and 10 deaths occur every week from collisions with emergency vehicles.

Unfortunately, it's difficult to convince consumers and automobile manufacturers to incorporate sophisticated radios just to warn of approaching emergency vehicles, construction areas, or dangerous intersections. Yet a solution may be found in private applications. For consumers, the latest entertainment sells. So do local news and traffic information. Commercial operations have their own desirable applications in tracking and logistics. These private applications can all be licensed on the same bands as the public-safety applications, which only need to communicate on an occasional basis. The private applications carry both the rationale and the marketing to make E2213-02 successful. Private applications will drive the technology, but public applications will take advantage of its ubiquity.

E2213-02 also can easily be marketed to the design world. It just has to build on its IEEE 802.11a roots. E2213-02 and 802.11a operate similarly over the air. They therefore appear identical to higher-level applications. An enormous base of hardware and software developers has worked with the IEEE 802.11 standards family. They have the experience and tools to build vehicular applications using 802.11a technology. Because 802.11a products are delivered in high volume to the computer and consumer-electronics markets, costs for R/A-capable hardware will quickly go down. Furthermore, anyone can build products to this open ASTM standard on a fair and equitable basis.

The telematics services supported by this technology are unusual in that they focus on exchanging localized information. Driving is an activity that must be tailored to what's happening in the immediate vicinity. E2213-02 provides the information necessary to make driving safer and more enjoyable. Beacons, for example, can broadcast the congestion status of approaching roads. Emergency vehicles and dangerous intersections can signal their presence, and road signs can provide detailed information about upcoming points of interest.

Performance was the focus of most of the evaluation and testing that led to E2213-02 being chosen as the 5.9-GHz DSRC standard. It showed that it could provide high data rates for large and small data payloads. In the mobile environment, both types of payloads must be transmitted very quickly (in less than 2 ms).

Robustness also was the focus of evaluation and testing. 802.11a technology had to have the ability to tolerate the presence of severe multipath and other vehicular conditions. Outside Washington D.C., communications between approaching vehicles was tested at highway speeds. The tests were performed at up to 1000 ft. through multiple lanes of heavy traffic. Toll applications also were tested in government-sponsored research at speeds of up to 120 mph. 802.11a technology passed 100% of all tests.

For any specific application, there could be a more effective wireless solution than E2213-02. In electronic toll taking, for example, certain low-cost, battery-operated wireless toll tags already exist. But these application-specific technologies don't meet the need for a single, high-performance, low-cost technology to accommodate multiple, simultaneous public/private applications. As a result, the momentum is building behind the use of low-cost, interoperable technology for telematics applications. It has the performance, the robustness, and the track record to meet a new generation of vehicular needs.

Sheung Li
Product Line Manager, Atheros Communications, 529 Almanor Ave., Sunnyvale, CA 94041; (408) 773-5300, [email protected], www.atheros.com.

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