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Electronic Toll Collection Essay

A 2. 45 GHz RFID System  for Electronic Toll  Collection B. Tech. Project by Praneeth Tammiraju  04007032 Index 1. Introduction 2. RFID in ETC a. RFID – a review b. The actual role c. Technicalities in brief 3. The Reader Design a. The transceiver design b. Circuit description c. The PCB of the reader d. Programmability of the reader 4. The Transponder Design a. Broad Overview b. Redesigning rectifier 5. Communication Protocol 6. Conclusion 7. References 2 3 3 3 4 6 6 7 9 9 11 11 11 13 14 15 Introduction Tolling on roads and highways today is done mostly on a manual basis i. e. vehicles stop at a check gate and the toll is collected manually by authorized personnel. This poses a lot of problems such as traffic congestion, inefficient use of human resources, vast amount fuel and time wastage apart from, of course seeming very primitive in today’s world of automation. There are already methods in practice that attempt to automate the whole process.

Systems that use installed video cameras that scan the license plates of the vehicles to identify the vehicles and initiate the tolling process are a viable solution but with their own problems in execution – the most important being the logistical diversity in the nature of vehicles which makes a universal protocol impossible. In this context, RFID puts forward a very efficient solution to the automation problem in the case of electronic toll collection. This document discusses the use of RFID in Electronic Toll Collection. It discusses the requirements of such an pplication and how RFID systems should be customized to meet these needs. It describes a circuit for the transceiver at frequency 2. 45 GHz and its interfacing with a computer. A brief note about the specifications of the antenna used and an introduction to the transponder design are made. 2 RFID in ETC RFID – a review The basic working of an RFID is well described in Fig. 1 below. Fig. 1: Schematic of an RFID system [1] The actual role Given this background of RFID and its working, its application in toll collection becomes obvious.

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The use of RFID in tolling can be explained simply with the analogy of pre-paid mobile phone users. Every mobile phone user pays a certain amount of credit amount and gets a calling card. Now, whenever he makes a call, the stipulated amount of call charges is deducted from his credit amount. This is the basic idea of electronic toll collection. Vehicle owners pay some credit amount and get an RFID transponder or tag on which information as to the vehicle identity and credit amount is coded. This tag is stuck on the vehicle itself.

Whenever the vehicle crosses a highway toll gate where RFID transceivers or readers are installed, the vehicle is identified with the help of communication between the readers and the tag and the information is passed onto the centralized network and database from where the toll amount is deducted. Ideally, this is a perfect remedy for manual toll collection where complete automation is possible. Still for purpose of policing and legal issues, video cameras can be installed for video confirmation of the vehicle and some personnel can be appointed to supervise the processes. 3 Fig. : Electronic Toll Collection [2] An improvement in this system could be if this installation is made in two stages i. e. if two gateways are installed some distance apart so that when a vehicle passes through the first gateway the tolling process is initiated and the toll deduction is confirmed at the second gateway whereby a check gate can be opened thereby enabling law enforcement and automation. Technicalities in brief The technical issues involved in building the above described system are [3]: The RFID reader at the gateway The RFID tag on the vehicle Proper communication between the above The network connectivity

The RFID reader The RFID reader must be equipped enough to perform functions such as initiating information exchange with the in-vehicle tags and with the central database network simultaneously. This typically asks for an RF transceiver connected to microcontroller which is in turn connected to a computer interface. The detailed discussion on its design features is made ahead in this document. The RFID tag The RFID tag that is present inside the vehicle will preferably be an active tag i. e. it carries an on board power source like a battery.

It also carries enough memory on it to store the identification information of the vehicle. Also, read-write capabilities must both be present on the tag. 4 Communication between the reader and the tag Electronic Toll Collection has as its primary objective the unobstructed passage of vehicles through the gateway. This implies the information exchange that needs to take place between the reader and the tag must occur even the vehicle speeds are of the order of nearly 80 kmph especially when the vehicles pass from a good distance of nearly 5 m – 8 m from the reader.

This necessitates fast data transfer between the reader and the tag, typically of the order of 1 – 2 Mbps. At the same time, the open spaces usually encountered on highways minimize the trouble of obstructions in the line of sight. Under these conditions, a system that operates in the microwave region seems most conducive. The ISM band around 2. 45 GHz serves this purpose. Thus, the system to be discussed in this document is designed to be operational in the range of 2. 45 GHz. There is no standard protocol that is used universally in RFID applications in the range of 2. 5 GHz. Still, on studying the various automatic toll collection systems in place, it is understood that the IEEE standard 802. 11 could be used for the information exchange between the reader and the tag. The network connectivity The reader collects the data from the tag and it must then coordinate with the centralized database through a perpetual network so that toll deduction procedure is completed without glitches. This requires credible and fast network systems that offer similar data transfer rates so as to enable the passage of the vehicle without delay.

The speeds of the network can, of course, be traded off with the distance between the two stages of the toll gateways. In cases of network failure, a backup buffer memory for the transactions must be present at the reader itself. Among the four critical technical issues mentioned here, this document describes in detail the RFID reader design. 5 The Reader Design The reader design can be broadly divided into two parts – the actual transceiver design and its interfacing with the computer through a serial port like MAX32 protocol.

Here, only the transceiver design is concentrated upon while a diagram describing the interfacing with the computer is provided. The transceiver design This document describes the design of the transceiver made by the use of the industry-range chip CC2511f8 – an RF transceiver chip from Chipcon (now from Texas Instruments). This chip is chosen so as to enable RF communication at 2. 45 GHz and also because it suits our needs of data transfer speeds and power consumption. Fig. 3: Transceiver circuit using CC2511f8 [4] 6 Circuit description [4]

The three most important parts of the circuit to be described are the biasing resistor, the RF impedance matching circuit and the USB connectivity circuit. The biasing resistor The resistor R271 is used to setup a biasing current accurately. The RF impedance matching As seen from the pins RF_N and RF_P, the impedance of the antenna circuitry must be Zout = 80 + j74 ? This is taken care of by the balun circuitry. The USB connection The resistors R262 and R263 near the pins DP and DM are used for impedance matching and the resistor R264 is used as a pull-up resistor.

This resistor must be tied to the voltage provided by the USB bus itself. The typical values of the various elements used in the circuit are given below in Table 1. An important note should be made about the decoupling capacitors at the power source. Their values and placement very close to the supply is a critical issue in the performance of the reader. The antenna [5] The diagram shows an alternative solution for the antenna as a folded dipole PCB antenna as opposed to the shown 50 ? antenna.

The characteristics of that antenna are given in the tables and figures to follow. 7 Table 1: Values of the components used in the circuit [4] Fig. 4: Folded dipole antenna circuit [5] Table 2: Folded Dipole properties [5] Table 3: Dimensions of the antenna [5] 8 The PCB design of reader Given below is a simplified design of a PCB which is ready to be plugged directly into computer through a USB connection. There were quite some issues faced in the design of this PCB which ideally should have been done in many layers to reduce the size and ease the physical utility.

Considering experimental and laboratory utility, the physical dimensions of the PCB have been bloated and hence, the whole circuit has been accommodated in just two layers with the bottom layer being a complete ground plane. The commercial availability of the capacitors and inductors of the specified values and size was a major concern and hence, some minor modifications have been made while designing the PCB. Despite such extensive care taken to relatively ease this PCB’s manufacturability, physically making this PCB is still a tough job given the acutely narrow connections and dense packing of elements on the board.

Programmability of the reader The reader consisting of CC2511f8 has some unique programming features such as direct programmability from a computer via USB connection. However, it needs a programmer/evaluation kit and a software named SmartRF from TI. If once programmed initially, the given circuit can then be used as a dongle for all information exchange with a computer. The programming is rather simple given the on-board microcontroller being an adapted version of 8051. Also, the chip provides elaborate programmability thanks to the various components like ADC, USB connector, I/O devices etc. eing on-board and controllable. The reader could have well been made of CC2500 which doesn’t have an on-board microcontroller and hence can be programmed via an external microcontroller in a much simpler fashion. However, this typically increases the complication of the circuitry and necessitates the usage of external components. This greatly limits the functionality of the reader. The USB connectivity is also greatly handicapped in such systems. Most importantly, though the initial costs of the reader with CC2511f8 are high due to the ost of the programmer, in the long run or for manufacturing purposes, it proves to be much cheaper than a reader with CC2500 and an attached 8051 microcontroller. 9 Fig. 5: The 2-layer design of the USB connectable reader using CC2511f32 Fig. 6: PCB layout and dimensions of the antenna 10 The Transponder Design Broad overview The design of the transponder is dealt in brief over here concentrating on the essentials. Its basic architecture is described vividly in the figure given below: Fig. 7: Transponder architecture [6] Redesigning rectifier

Typically, it is expected to use an active tag for toll collection purposes as fast data rates and long ranges are mandatory. However, if we can have a good rectifier with high efficiency, passive tags can be a good option too as they would then make the whole system cost effective. An effort to throw light on one such possibility is made in this document. Consider the following rectifier design: Fig. 8: Rectifier design [6] 11 This rectifier has been tested for an efficiency of 37% [6] and thus, it makes it very much possible to use passive tags in the design of transponders for electronic toll collection.

This design has added advantages of providing the possibility to predict the input power in order drive a specific DC current at a specific DC voltage. There is a lot of scope for further work on this architecture to specify the exact design of the transponder in the later stages of this project. 12 Communication Protocol The electronic toll collection systems use the back-scattering modulation techniques to establish tag-reader communication. This is achieved essentially by varying the reflection coefficient at the rectifier input. The reflection coefficient can be varied both in its amplitude and in its phase.

Thus, both ASK and PSK are possible. In ASK the input impedance is switched between the matching and the reflecting values whereas in PSK the impedance reactive part it switched between two complex conjugate values [6]. Both ASK and PSK have their own advantages and disadvantages and their choice is a strict trade-off between the power used by the tag for its operations and the power reflected in back-scattering for communication with the interrogator. In order to accomplish multi-lane tolling, we need to ensure that anti-collision algorithms are in place.

Usually TDMA protocol with Slotted-Aloha procedure is effective in road-tolling systems. 13 Conclusion This document has discussed a design of the 2. 45 GHz RFID system for the use of toll collection. The complete design of the reader is now ready to be built and tested and for the transponder, a design for an active tag was studied and a new design using the mentioned rectifier for a passive tag is proposed. The prime challenge in taking this project ahead would be the programmability of the new chip that was discussed as it requires a completely new programmer tool different from its previous RFID semiconductor counterparts.

Alternate designs using the older chips can be accomplished but the cost and size reduction and the efficiency one can achieve with this CC2511f32 make the installation of the new programmer worthy for any scale of manufacture larger than laboratories. 14 References [1] O. Shoewu and O. Badejo, “Radio Frequency Identification Technology: Development, Application, and Security Issues” Pacific Journal of Science and Technology, Volume 7, Number 2, November 2006. www. akamaiuniversity. us/PJST7_2_144. pdf [2] www. quatech. com/applications [3] www. freepatentsonline. com/6639509-0-large. pg [4] Datasheet, “Low-Power SoC (System-on-Chip) with MCU, Memory, 2. 4 GHz RF Transceiver, and USB Controller”, CC2511f8, Texas Instruments. [5] Design Note, “Folded dipole for CC25xx”, DN004, Texas Instruments. [6] Jari-Pascal Curty, Norbert Joehl, Catherine Dehollain, Michel Declercq, “A 2. 45 GHz Remotely Powered RFID System”, Research in Microelectronics and Electronics, 2005 PhD, Volume 1, 25-28 July 2005 Page(s):153 – 156 vol. 1. [7] U. S. Patent: Woo et al. , Dual Mode Electronic Toll Collection Transponder, U. S. Appl. No. 11/409,897, filed Apr. 24, 2006. [8] U.

S. Patent: Tang, RF Transponder with Electromechanical Power, U. S. Appl. No. 11/054,520, filed Feb. 9, 2005. [9] U. S. Patent; Tang et al. , Method of Enabling Two-State Operation of Electronic Toll Collection System, U. S. Appl. No. 11/437,236, filed May 19, 2006. [10] Sabri Serkan Basat, “Design and Characterization of RFID modules in Multilayer Configurations”, etd. gatech. edu/theses/available/etd-11202006124610/unrestricted/basat_sabri_s_200612_mast. pdf [11] IEEE Standard 802. 11b-1999 R(2003) http://standards. ieee. org/getieee802/download/802. 11-1999. pdf 15

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