Proposal for RFID Tag Anti Collision Using MCCDMA Technique

8/6/2019 Proposal for RFID Tag Anti Collision Using MCCDMA Technique

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RFID Tag Anticollision using MC-CDMA TechniquePrepared and introduced by

Montadar Abas Taher

B.Sc. in Electronics and Communications Engineering,

M.Sc. in Satellite Engineering.

Assistant Lecturer in Diala University/ Collage of

Engineering/ Department of Communications

Engineering.

Page 2 to 25 is a comprehensivestudy about the RFID system.

Page 26 to 35 is theLiteratureSurvey

Page 35 to 38 is the proposal

Page 39 to 40 is theReferences


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1. Introduction: [1]

Radio frequency identification (RFID) or auto-identification (ID) is a contactless datatransmission and reception technique between the data carrying device, called a

transmitter responder (transponder) or an RFID tag, and an interrogator, which is alsoknown as an RFID reader. A more abstract approach to defining Auto-ID reveals thatAuto-ID involves the automated extraction of the identity of an object. The contactless IDsystem relies on data transmission via radio frequency electromagnetic (EM) signals, andconsequently, the whole operation is line-of-sight and weather independent. Theseadvantages overcome the limitations of optical barcodes, which are line-of-sight andweather dependent and need manual operation.

Atypical RFID system, as shown in Figure 1, comprises 1) an RFIDtransponder, which carries the ID data; 2) an RFID interrogator/reader, whichinterrogates the tag and extracts the data from it; and 3) application software acting as an

interface between the user and the RFID system. In brief, RFID technology is based onradio waves in order to transmit data by the reader to the tag, and in return, it receivesmodulated returned echoes from the tag via the reader. The transponder modulates theEM wave and transmits the data back to the reader, where it is processed for real-time ID,asset tracking, security surveillance and many other authentication and managementpurposes.

Figure 1. RFID system block diagram.

The omnipresent barcode that triggered a revolution in ID systems is showinginadequacies in many applications. Though barcodes are cheap to manufacture, they lack

the ability to be programmed and reprogrammed and have low data storage capacity. Dueto these limitations large corporations have been experiencing significant financial loss.However, any novel competing technology will inevitably face harsh criticism andskepticism before large-scale implementation. RFID technology is currently experiencingthis phenomenon.

Organizations are demanding a good return of investment (ROI) before approaching thisnew technology. Complete implementation of RFID technology in manufacturing


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processes, supply chain management, inventory control in warehouses, counterfeitprevention in bank notes and secured documents is not a trivial task. The crucial issues ofcost, reliability, security, and standards must be delineated and solved before beingimplemented in the mainstream of businesses, government services, and many otherareas.

Researchers and engineers throughout the world have been developing low-cost, small,reliable, and efficient RFID tags. A vast number of active and passive RFID systems canbe found in the market today. As development of RFID technology advances, so do thedesign and operating principles of RFID transponders. Hence, a wide range of RFIDtransponders have been designed and are available in the commercial market.

2. RFID Transponder System Architecture: [1]

Data carrying devices that are fixed on the items to be identified in an RFID system arecalled transponders or tags. The main purpose of a transponder is to carry ID information

of the object. The block diagram of a passive RFID transponder is shown in Figure 2. It iscomposed of 1) an antenna; 2) a radio frequency (RF) transceiver section; 3) an analogdetection and/or rectification section, which detects, and in passive tags, rectifies RFpower into an equivalent dc voltage; and 4) a digital control section that is either amicroprocessor or some other digital system.

Figure 2. Block diagram of a passive RFID transponder

2.1 Detection Section: [1]

The detection/rectification section converts RF energy received by the transponders

antenna and converts it to a baseband signal or an equivalent dc voltage. In a fullypassive tag, a voltage doubler or quadrupler circuit is used to pump up the voltage forefficient operation of the digital section. Therefore, the RF section receives the signalfrom the reader, which is used to provide dc power supply for the control section andmodulate and transmit the RF signal so that the ID data can be retransmitted to the RFIDreader.


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Depending on the tags ability to be read only (RO) or read/write (R/W), the RF sectioncan be modified in such a way that it also extracts the interrogation code from the reader.The extracted code is transformed from an RF signal to a baseband signal usingdemodulation circuitry and sent to the control section for further processing. It isimportant to note that the RF signal from the reader has to provide sufficient power to the

tag for efficient communication between the two entities. Passive and semipassive RFIDsystems that use amplitude shift-keying (ASK) modulations for reader-tagcommunication are especially affected by this RF signals power budget. In this regard,the RF rectifier circuit plays a vital role. The rectifier circuit can be realized in variousways depending on the technology used for the transponder design. In the rectifier circuit,there is a limiter and voltage pump circuit for either limiting or increasing the dc power.Obviously, the closer the transponder is to the RFID reader, the more power it willextract.

2.2 Digital Control Section: [1]

The block diagram of the transponder control section is shown in Figure 3. The digitalsection has both analog and digital signal processing subsections. Todays trend is tointegrate the digital control section into a single integrated circuit (IC) so that minimumspacing and package dimensions are achieved. Additionally efficient power budgeting ispossible when the functional blocks are in close proximity to each other. That is why verylow-powered RFID ICs are available in the market. After the RF signal is received anddemodulated in the RF section, the demodulated signal is sent to an analog to- digitalconverter (ADC), which converts the RF signal to a low frequency baseband signal. Thebaseband signal is then converted to a digital signal and processed further in the protocoldetection circuit and decrypted. The signal is processed by the microcontroller, whichgenerates a response signal. The response signal is sent to the encryption circuitry and

then converted to an analog signal by the digital- to-analog converter (DAC). This signalis then modulated by an RF carrier signal and propagated back to the reader via the tagantenna. This operation is done in the RF section of the tag and the antenna.

Figure 3. Block diagram of the transponder control section.


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4.2.3 Passive Tags: [1]

Passive Tags do not possess an on-board power supply and therefore rely only on thepower emitted from the reader for both data processing and transmission. Passive tagsmay or may not contain an IC, memory block, or application specific IC. This means that

some passive tags perform data processing, but others do not. These tags are usually inthe form of electronic article surveillance (EAS) transponders commonly found in retailshops for security purposes or surface acoustic wave (SAW) tags. Most passive tags havelow power consumption and low cost due to the nature of their design. Because they relysolely on the readers emitted energy to cull its operating energy, all passive transpondersmust have an RF front end, an analog circuit, and depending on their data processingtechniques, a digital circuit.

The RF front end of the passive RFID tag consists of the antenna and the impedancematching circuit in order to minimize signal reflection between the antenna andtransponder circuit. The analog part of the passive tag may comprise an LC tuning circuit

and a rectifier. The rectifier supplies the required dc voltage to the digital circuit. Thedigital circuit of the RFID passive tag is completely optional and may have an IC, ASIC,or just a memory block of a few kilobits. Most passive tags have precisely designedmicrochips and/or ICs that contain digital logic sectors, which process data rapidly.Passive tags from Alien Technology and Intermec are shown in Figures 5 and 6. PassiveRFID transponders can be made using printing techniques. There have been tremendousefforts and interests in direct printing of RFID tags on plastic, fiber, and other low-costlaminates to compete with the ultra-low-cost optical barcodes. Also, all ink-jet-depositedprocesses capable of creating high quality passive devices for RFID applications havebeen envisaged and are being developed. Due to the absence of on-board power supplies,passive RFID tags have a much shorter reading range (up to 2m). They are more

vulnerable to environmental effects and have poorer or no data processing abilities at alland hence cant be easily reprogrammed.

The advantages of passive RFID systems are low cost and low maintenance. Due to thesesalient features, passive tags are used in a wide range of applications such as medical,supply chain management, and wireless sensing.

Figure 5. Alien Technology passive RFID transponders. (Courtesy of Alien Technology Corporation.)


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Figure 6. Intermec passive RFID transponder. (Courtesy of Intermec Corporation.)

4.3 Data Processing: [1]

The next embodiment of the classification is the data processing techniques of RFID tags.RFID transponders can process data using ICs or exploiting physical effects (chipless).

4.3.1 RFID IC: [1]

RFID transponders that contain ICs (Figure 6) usually comprise digital circuits operatingas memory blocks and microprocessors or microprocessor systems, or ASICs. Some ofthese transponders are active, and some are passive. RFID chip manufacturers designhighly power efficient, low-cost RFID transponders that are direct competitors of barcodes. Liu, Yang and Zhang [14] report a 900 MHz passive transponder. This

transponder consists of an RF antenna and a matching circuit, an analog section, a digitalsection, and a memory block, as shown in Figure 7.

Figure 7. Block diagram of a RFID tag consisting of a RF front end, analog and digital section and

data storage memory block.


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The RF antenna and matching circuit represent the RF front end of the tag that has thefunction of receiving and transmitting the RF signal from and back toward the readerdevice. The analog circuit can have various functions depending on the architecture andoperating principles of the tag but must rectify the induced voltage from the readerscarrier signal in order to support the operations of the digital section and memory block.

The digital section usually consists of sequential networks and acts like a state machine.The memory block can be electrically erasable programmable RO memory (EEPROM),static random access memory (SRAM) or ferroelectric random access memory (FRAM).EEPROM has high power consumption during writing operations and a limited writecycle. It is used in a wide range of applications due to its low manufacturing cost andhigh number of possible reprogramming cycles. The FRAM chips are used due to theirlow read power consumption in comparison to the EEPROM as well as their significantlylower write time. Manufacturing difficulties have hindered widespread usage of FRAM.

RFID transponders with microprocessor units are able to implement complex functionssuch asanticollision

protocols and authentication using dedicated central processor units

(CPUs). Transponders with microprocessors will become increasingly common inapplications using contactless smart cards in the near future due to their excellentprocessing capabilities.

Some RFID transponders have been developed with embedded digital signal processorsfor efficient and high data rate transfer. Although microprocessors tend to have highpower consumption, great efforts have been made to lower their power consumption sothat they can be used in RFID transponders for complex signal and data processingapplications. RFID transponders containing microprocessors usually use interrogatordriven procedures for communication due to their ability to be interrogated and answerupon a request or command. A typical operating sequence of this type of RFIDtransponders is comprised of three sections: receiving and decoding the request from thereader, data processing, and finally, data encryption and transmission back towards thereader.

In order to be enabled with such powerful features the RFID transponder is usuallydeveloped using the architecture shown in Figure 8. The transponder module comprisesthe antenna, high frequency (HF) interface, microprocessor (CPU) with internal randomaccess memory (RAM) and encryption coprocessor, and RO memory (ROM) and RAMblocks for data storage. The antenna and HF interface enable effective data transmissionbetween the reader and the transponder and thus establish the link between the RF anddigital circuitry in the RFID transponder architecture.

The HF interface consists mainly of the feeding and impedance matching network,modem circuit and ac/dc conversion circuit for supplying power to the digital section.The CPU consists of the microprocessor, internal registers, the encryption coprocessor,and the microprocessors internal RAM. Complex systems like these demand an efficientoperating system. The transponders operating system is implemented in the externalROM and thus cannot be deleted after the loss of the power supply (upon leaving theinterrogation zone of the reader or loss of battery power). The operating system consists


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of software drivers and applications that manipulate hardware for anticollisionprocedures, data processing, and authentication. Encryption and authenticationprocedures and protocols are needed since these types of transponders can process andretrieve data that has a dollar value [bank account numbers, personal ID numbers (PINs),etc]. The application data is stored in an EEPROM and can be changed depending on

which types of services the RFID transponder and RFID system are providing. Thesetypes of transponders are usually found in the form of smart cards and are at a higher costthan chipless transponders or those with simple memory functions.

Figure 8. Block diagram of a RFID transponder with a microprocessor.

4.3.2 Application Specific Integrated Circuit: [1]

The use of ASIC in RFID transponder design in recent years has acceleratedtremendously. Researchers and engineers around the world have been working on waysto minimize the physical dimensions of RFID transponder ICs, lower powerconsumption, and finally, have produced a low-cost, efficient RFID transponder IC.Using high operating frequencies for RFID, designers minimize the physical dimensionsof passive components, which have geometrically dependent parameters andperformances. Furthermore designers and engineers have developed ASICs that canperform complex data processing using efficient engineering solutions and smartplanning. Nowadays, almost all RFID ASICs are mode from complimentary metal-oxide

semiconductor (CMOS) technology, which is passive and consumes very little power. Atypical RFID transponder IC (ASIC) is composed of an RF front end, rectifier,demodulator, modulator, low-power digital logic and EEPROM, as shown in Figure 9.Most RFID transponders like the one shown in Figure 10 operate in two phases: chargeup phase and data transmission phase. During the charge up phase, the storage circuitrectifies energy from the readers EM waves. The equivalent dc power is used to supportthe operations of the digital sectors of the ASIC. In data transmission phase, thetransponder starts to transmit data to the reader. In this case, the modulation block is used


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to power the modulation of the carrier signal received from the reader in order to sendvalid information. The demodulation block is used to decode any commands from thereader toward the tag. The reader usually modulates its signal using ASK

because itrequires a simple demodulating circuit (single diode as an envelope detector).

Figure 9. Transponder ASIC block diagram.

Figure 10. Circuit structure of RFID embedded microchip.

As for the tag binary phase shift keying (BPSK) or ASK is preferred. Backscatteringmodulation is used by varying the RF input impedance of the transponder and influencingthe scatter aperture (described in the Semipassive Transponder Design section). Thelow-power digital logic performs three tasks during the data transmission phase:

Generates the clock signal for the EEPROM Relocates data from the EEPROM into the shift register Generates modulation signals to perform data transmission by means of

modulation toward the reader.The EEPROM contains stored data and allows the incoming data to be stored andpreserved after the power supply is cut off. AR/W transponder supported by EEPROMallows greater flexibility and broadens the RFID systems potential applications.Transponder ASICs have gone a step further by developing embedded RF antennas.


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Hitachi has disclosed this invention by presenting their embedded RFID microchip in2004. Embedding an antenna on an RFID chip is an ideal method of reducing the area ofRFID devices and their cost. The shape of the RFID antenna is similar to a coil antenna,and the received EM energy by the antenna is used to power up the RFID chip. An ultrasmall RFID chip structure is shown in Figure 10.

As shown in Figure 10, the microchip is completely passive and draws power from theRF carrier signal received from the reader. The internal voltage of the chip increases inproportion to the EM field energy and can possibly exceed the breakdown voltage of theCMOS circuits. Therefore, a voltage limiter circuit is used to prevent the breakdown ofthe IC. This circuit activates when the input voltage exceeds a certain threshold voltagelevel in order to protect the IC. As for most batteryless transponders, the EM wave isrectified by a power rectifier at the front end of the chip. This circuit consists of acombined capacitor and diode, which are similar to conventional back-bias circuits ofdynamic random access memory large scale integration (DRAM) LSI. This structure isbased on the principle that an RFID chip needs at least one port (also called a terminal

pair) to connect to the antenna terminals and is shown in Figures 11 and 12.

Figure 11. The rectifier circuit of the microchip for one port antenna terminal.

Figure 12. Rectifier and limitation circuit for passive RFID transponder IC.


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These small microchip transponders can be effectively put on products and are suitable

for paper media and even small products due to their extremely small dimensions (0.4

0.4 mm). The development of new methods and principles of designing low-cost RFIDASIC transponders is at its peak today. A whole variety of new design methodologies forachieving reader-transponder data transmission and new applications for transponder ICs

are enhanced. Due to their IC architecture, packaging of RFID transponders at microwavefrequencies has been developed as well. RFID transponder packaging requires uniquematerials and innovative methods in order to provide functional, reliable and inexpensivetransponders. Packaging methods as chip-on-board (COB) and chip-in-board (CIB) allowdesigners to utilize the existing semiconductor infrastructure to achieve a reliablemicrowave transponder and provide high volume manufacturing capabilities.

Techniques for direct chip attachment (DCA) include COB, chip-on-flex (COF), chip-on-glass (COG), multichip module (MCM), and tape-automated bonding (TAB). COB hasby far become one of the most widely used high-density assembly technologies. Onceconsidered useable only for inexpensive throwaway applications, COB has made a

comeback as a reliable way of producing complex high volume RFID transponders.

Using this type of packaging can provide cost effective designs and lower themanufacturing and implementation cost of RFID transponders. Most recently, researchershave been working on printing RFID transponders onto organic substrates, such as paper.The Department of Defense and large retailers such as Wal-Mart have encouraged thetremendous drop in price by showing significant interest in RFID technology. Rida,Yang, and Tentzeris present the design and characterization of novel paperbased ink-jetprinted UHF antennas and transponders. They report the use of two methods for printingRFID tags onto paper. The first method was an ink-jet printing where a low-costDimatrix printer system was used with a specially formulated conductive silver ink. The

second method was based on conventional lamination and copper etching chemistries.The integration of sensors with RFID transponders is also reported in order to allowlarge-scale production of wireless sensing systems using ink-jet printing systems. Theoutcome in the near future would be to design a flexible three-dimensional package withembedded actives and passives and thin film battery in paper substrates that is expectedto be the cheapest solution for wireless sensing with RFID transponders for large volumeapplications.

4.3.3 Chipless RFID Transponders: [1]

Chipless RFID transponders exploit the physical effects of the transponders design. Allchipless RFID tags are exclusively passive and they differ fundamentally fromtransponders containing an IC in terms of operational principles and power consumption.Chipless RFID transponders do not need a power supply because they do not contain anyadditional data carrying device/IC. The transponder itself generates data due to itsphysical architecture and design, and therefore, represents a unique ID device. There arefour types of chipless RFID tags known today, and only one of these four types has beenmade available commerciallythe LC resonant chipless tag.


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4.3.4.3 Thin-Film Transistor Circuits

Thin-film transistor circuits (TFTCs) are the third type of chipless RFID transpondersknown to exist today. They are being highly developed and around forty companies areworking on TFTC tags. Most of them are capable of being printed at high speed and with

low-cost plastic film. TFTCs can have the same electronic circuit as in a silicon RFIDchip, so subject to limitations in the materials used; they can employ the samefrequencies and standards as chip-based RFID. The ability to operate at 13.56 MHz isextremely important as a majority of tags operate at this frequency. It is the preferredfrequency for cards, tickets, libraries, laundry, pharmaceutical, and postal items. TFTCtags offer advantages over active and passive chip-based transponders due to their smallsize and low power consumption. They require more power than other chipless tags butoffer more functionality. However low-cost manufacturing processes for TFTC tags havenot been developed yet. Another issue is the frequency of operation, which imposes agreat challenge on RFID researchers and designers. The frequency limitation up to 13.56MHz will not allow TFTC technology to be deployed in areas where a higher data

transfer rate is required.

4.3.4.4 LC Resonant

LC Resonant chipless tags comprise a simple coil that is resonant at a particularfrequency. The resonance of the tag causes an EM disturbance that identifies its presencewhen scanned by the reader. Hence these transponders are considered 1-b RFIDtransponders. Due to the fact that these transponders operate at a distance of severalcentimeters, the operating principle is based on the magnetic coupling between the readerantenna and the LC resonant tag. The reader constantly performs a frequency sweepsearching for transponders. Whenever the swept frequency corresponds to the

transponders resonant frequency, the transponder will start to oscillate producing avoltage dip across the readers antenna ports. The advantage of these tags is their priceand simple structure (single resonant coil), but they are very restricted in operating range(few centimeters), information storage (1 b), operating bandwidth and multiple tagcollision. These transponders are mainly used for EAS in many supermarkets and retailstores.

Chipless RFIDs are still in the development stage, and many products havent left theprototyping cycle in laboratories. That is why chipless tags only occupied 0.4% of the

RFID tag market in 2006, with cumulative sales of 100 million chipless tags produced

to date compared to 2,322 million chipped RFID. Ultimately, the largest achievement

of RFID will be the replacement of barcodes. It will mean RFID is implemented in thesame way as the optical barcodes are being used today. The salient features of chipless

RFIDs are: 1) they operate over ten meters in range, 2) carry up to 256 b of data, 3)

they cost one tenth of their silicon chip equivalents and 4) they have a greater physicalperformance. Chipless RFID technology is addressing mainstream RFID applications

and will rapidly grow causing market price reductions of one to two orders of

magnitude.


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4.4 Programmability: [1]

Another classification of RFID transponders can be based on their programmability.Hence, we can classify RFID transponders as

RO

Write once read many (WORM) R/W.

RO transponders can be programmed just once in a lifetime. The data is imprinted intothe tag during the manufacturing process and cannot be removed or changed in anyway.Such tags are called factory programmed tags. This type of tag is good for smallapplications only, but is impractical when data needs to be customized based on theapplication. This tag type is used today in small pilot programs and business applications.

WORM tags can be written once; this is generally done by the tag user, not by themanufacturer. In practice however, WORM tags can be reprogrammed around a hundredtimes. If a tag is reprogrammed more than the defined times, it is permanently damagedand useless. AWORM tag is also called field programmable. This type of tag iscommonly used in businesses today.

R/Wtags can be programmed and rewritten a large number of times. Usually a RW tagcontains an EEPROM or Flash memory so it can be reprogrammed over and over again.The number of reprogramming cycles can go up to 100.000. Data security is a challengefor R/W transponders and they are more expensive to produce. Hence, they are notwidely used in todays applications but researchers are working on lowering the cost ofthese tags and enabling their use in a large number of applications.

4.5 Communication Range: [1]

RFID transponders can be classified using other criteria, one of which may be the mostimportant classification of RFID transpondersrange. We can distinguish between threetypes of transponders depending on their range.

Close coupled Remote coupled

Long-range systems.

Close coupled transponders operate at a small range, up to 1cm. For operation thetransponder must be inserted into the reader or positioned in a predefined place to enablereading purpose. This type of transponder is passive and is powered by a strong magneticfield emitted by the reader. The close coupling systems allows significant powertransmission between the reader and transponder and thus powerful microprocessors maybe used on the tag. Close coupled systems are used in security systems where large rangeis not required. Close coupling systems are used as ID-1 format contactless smart cards


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(ISO 10536). The role of close coupled RFID transponders and systems are becomingless important on the market due to the development of low-cost chipless RFID systems.

Remote coupledtransponders operate at read ranges up to 1m. Almost all of the remotecoupled transponders operate at HF frequencies using inductive coupling. This means

that power supply is supported from the reader using the magnetic field. These RFID tagsare also known as inductive radio transponders. There is a few numbers of transpondersthat operate using the field strength of the electric field and they are called capacitivecoupling. At least 90% of all remote coupled RFID transponders are inductive.

Long-range RFID transponders have reading ranges significantly above 1m. There aresystems that operate up to 30m using battery powered active transponders with activetransmitters. All long-range systems operate using UHF or microwave frequencies anduse backscattered modulation to communicate with the reader. The operating frequenciesare 868 MHz (Europe), 915 MHz (USA), 2.45 GHz and 5.8 GHz. Typical ranges of 3 mcan be achieved using passive transponders and ranges up to 30 m can be achieved using

active transponders. The power from the battery is never used for data transmission, butonly to power the tags chip (data carrying device). The power of the EM field receivedfrom the reader is the only power used for data transmission.

4.6 Communication Protocol: [1]

RFID Transponders can communicate with the interrogator by initiating thecommunication upon detecting an interrogator field or by responding to the interrogatorsquery. Hence, we can classify two types of tag communication protocols:

Transponder driven (tag talk first); and

Interrogator driven (interrogator talk first).Transponder driven protocols are asynchronous procedures since the reader does notcontrol or initiate the data transfer. Once the transponder is located in the interrogatorsradiation field it starts transmitting data to the reader. Transponder driven protocols areconsidered very slow and inflexible and are usually implemented in active and somesemi/active RFID systems. An example of a transponder driven protocol is the ALOHAprotocol. This procedure is used exclusively with RO transponders. The data transfer issent to the reader in a cyclical sequence and represents only a fraction of the repetitiontime so there are relatively long pauses between data transmission. The procedure relieson the low probability that two transponders will not transmit their data packages atthe same time causing a collision.

Interrogator driven procedures are controlled by the reader/interrogator as the masterdevice. The nature of these procedures is considered synchronous since all thetransponders are interrogated by the reader simultaneously. The communication linkbetween the reader and transponder is established by selecting an individual transponderfrom a large group via authentication. Once the data transfer is complete, thecommunication link with the transponder is terminated and another transponder is


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selected. The authentication of transponders is done either by polling or by a binarysearch and hence each transponder must have a unique ID code to avoid multipletransponders responding.

4.7 Antenna Configuration: [1]

RFID transponders can be differentiated on the number of antennas for data reception andtransmission as single antenna tags or multiple antenna tags. Single antenna tags use thesingle antenna mode for backscatter modulation and for extracting power from thereaders EM field. The transmitting and receiving circuit of the transponders is usuallyconnected to the transponder via switch in order to create data transmission and datareception mode. In data transmission mode the tag sends data back to the reader viabackscatter modulation. In data reception mode the transponder receives data from thereader and extracts power from the EM field for operation. The advantage of the singleantenna tag is that it has smaller dimensions but passive transponders with singleantennas cannot be in transmission mode very long or they will lose power and reset thedigital circuitry.

Multiple antenna tags are not as present on the market as single antenna tags due to thefact that they have larger dimensions and hence are more expensive. These types oftransponders use multiple antennas that operate in a single mode such as datatransmission or data reception. For example, if we have a multiple antenna tag like theone shown in Figure 14 one of its antennas will be used for receiving data from the readerand the other antenna will be used for sending data to the reader. In this way thetransponder will constantly be in data transmission and data reception mode and hencewill not be subjected to potential power loss due to long transmission modes.

Figure 14. Texas Instruments passive RFID transponder.

Courtesy of Texas Instruments (www.ti.com).


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5. Readers or Interrogators: [2]

A software application that is designed to read data from a contactless data carrier(transponder) or write data to a contactless data carrier requires a contactless reader as aninterface. From the point of view of the application software, access to the data carrier

should be as transparent as possible. In other words, the read and write operations shoulddiffer as little as possible from the process of accessing comparable data carriers (smartcard with contacts, serial EEPROM).

Write and read operations involving a contactless data carrier are performed on the basisof the masterslave principle (Figure 15). This means that all reader and transponderactivities are initiated by the application software. In a hierarchical system structure theapplication software represents the master, while the reader, as the slave, is only activatedwhen write/read commands are received from the application software.

Figure 15: Masterslave principle between application software (application), reader andtransponder

To execute a command from the application software, the reader first enters intocommunication with a transponder. The reader now plays the role of the master inrelation to the transponder. The transponder therefore only responds to commands fromthe reader and is never active independently (except for the simplest read-onlytransponders).

A simple read command from the application software to the reader can initiate a seriesof communication steps between the reader and a transponder. A read command first

leads to the activation of a transponder, followed by the execution of the authenticationsequence and finally the transmission of the requested data.

The readers main functions are therefore to activate the data carrier (transponder),structure the communication sequence with the data carrier, and transfer data between theapplication software and a contactless data carrier. All features of the contactless communication, i.e. making the connection, and performing anticollision and

authentication procedures, are handled entirely by the reader.


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5.1Reader and Tag communication: [3]

In a typical system, tags are attached to objects. Each tag has certain amount of internalmemory in which it stores information about the object, in particular an object identifiersuch as the Electronic Product Code (EPC). When these tags pass through a reader, they

transmit information back to the reader, thereby identifying the object. The data then isfiltered and routed to the backend IT systems. The RFID front-end and the IT backend. Areader emits a signal at the selected frequency band, such as 860 - 960MHz for UHF or13.56MHz for HF. Any corresponding tag in the vicinity of the reader will detect thesignal and use the energy from it to wake up and supply operating power to its internalcircuits. Once the Tag has decoded the signal as valid, it replies to the reader, andindicates its presence.

If' many tags are present then they will all reply at the same time, which at the reader

end is seen as a signal collision and an indication of multiple tags. The reader

manages this problem by using an anti-collision algorithm designed to allow tags to be

sorted and individually selected

. Once a tag is selected, the reader is able to perform anumber of operations such as read the tags identifier number, or write information to it

(in the case of a read/write tag).

5.2RFID Interrogator Components and their Function: [4]

Figure 16 shows the components of a basic interrogator. They include

A receiver that holds an amplifier and a demodulator

A transmitter that holds a modulator and a power amplifier An oscillator A controller/processor An input/output port to an antenna

5.2.1 Receiving: [4]

The amplifier expands the signal received from the tag through the interrogatorsantenna for processing, and the demodulator extracts the information from the signal. Thecontroller/processor performs the data processing functions and manages thecommunications with the external network.

5.2.2 Transmitting

The oscillator provides the carrier signal to the modulator and a reference signal to thedemodulator circuits. The modulator adds information to the signal to be transmitted to atag. Then the power amplifier amplifies the modulated signal and routes it to the antenna.The antenna radiates the signal to a tag.


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These interrogators can come in different forms

(for instance mobile phone, PDA, or vehicle

mounted).

Differ from hand-held interrogators in size and formfactor Usually are powered from their own powersource(battery) or by the vehicle they are mounted onTypically have wireless connectivity

Mobile phones and PDAs Are very small in size but have fairly advancedfunctionality

Are a proven technologyAre attractive to industrial applications

Vehicle mounted interrogators Are typically mounted on forklifts or clamp trucksAre built to withstand environmental extremesAre placed for minimal contact with material beinghandled

5.6Interrogation Zone Considerations: [4]

Special considerations should be addressed when setting up an RFID system with

multiple interrogators that have overlapping interrogation zones. You can deal with

these types of situations in several ways, such as using dense interrogator mode,interrogator synchronization, arbitration, and anticollision protocols. Some of these

features are only available with Generation 2 devices.

5.6.1 Dense Interrogator Mode: [4]

Dense interrogator mode, also called dense reader mode, provides each interrogator

the capability to operate at a slightly different frequency, which helps reduce the radiointerference between interrogators

. Other techniques are used as well, such as ListenBefore Talk (LBT), frequency hopping, or a combination of the two.

5.6.1.1 Listen Before Talk

Using the LBT technique, an interrogator tries to listen or hear whether anotherinterrogator is using a channel. If it learns that another interrogator operates on thatchannel, it rolls to another channel to avoid interfering with the other interrogator.

5.6.1.2 Frequency Hopping

Interrogation signals hop between channels within a certain frequency spectrum. In theUnited States, they can hop between 902 MHz and 928 MHz, and they can be required tolisten for a signal before using a channel.

5.6.2 Interrogator Synchronization: [4]

In certain applications that require multiple interrogators operating at the same time andin the same proximity, it is necessary to coordinate their transmitting and receivingfunctions. The radio transmissions from the interrogators antennas may interfere withother interrogators, so much so that the tags are unable to completely understand the


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signal by the same pseudo random sequence that is combined with the desired data.Assuming the incoming waveform is centered on zero and has peaks that range fromnegative one, doing this operation would force the modulating sequence to converge toone for the desired wave, thus only leaving behind the low frequency data wave. Whenthis unique pseudo random sequence is multiplied with the other waves comprising the

incoming waveform received by the interrogator, other high frequency waves are created.Therefore the only low frequency wave left behind is the desired original data. Once themultiplication is completed, the original data can be recovered by passing the resultingwaveform through a low pass filter. All the high frequency waves generated from othertags will be removed.

Michael A. Hughes and Richard M. Pratt, in the US patent No 7009495 B2, Mar. 7,2006, [7] implement another method to resolve the collision problem which can beexplained clearly by a flow chart as shown below:

Star

Reader transmits a session ID and issues acommand of enter tag discovery mode

Reader starts monitoring all discrete I.F. freq.for presence of on-off keyed modulated RF.

Tag randomly selects a timeslot and IF freq.

Reader issues a sequence of timing pulses

Respective tags present a tone or simple modulation ofIF during selected timeslot

Reader continues to issue timing pulses and to provideillumination until the final timeslot

Reader identifies the timeslots and IF freq.s where a tone wasdetected. Reader sends discovered you message identifyingthe time slot and IF freq. for which tags tones were discovered

Discovered tags respond with a found me messagewhich contains the tags ID number

Reader issues a youre discovered message to causethe discovered tag(s) to leave discovery mode

Have alltags beenidentified?

Stop

Reader transmitsanother enter

discovery modewith the same

session ID


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When received 'Query' command from Interrogator, every Tag works as diagram shownbelow.

Interrogato

uer

ID1 RN Nm1/GC1

IDn RN Nmn/GCn

IDx, RN Nmx0/GC in collision

ACK ACK Rea

Data of tag 1/GC1

Data of tag n/GCn

Data of tag x/GCx1

ACK Rea

Interro ato

ueryRep

IDx, RN Nmx1/GCx1

IDz, RN Nmz1/GC in collision

Data of ta

ACK Rea

Interro ato

ueryRep

IDy, RN Nmy2/GCy2

IDz, RN Nmz2/GCz2 Data of ta z/GCz2

ACK

Initialstatus

Query command

Set Q value and chooseGC generator pick up

ID_Nm from ID.Generate RN_Nm

form GC

Miller modulation ofremaining bits of ID

and RN_Nmiller,GC modulation and

the signal transmitting

Waitingfor

ACK

Waitingfor

Read

ACK command received containing the tag ID

Regeneration ofRN_Nm and new GC

uer Re command

QueryAdjust received

Query command

Read command received

Miller modulation andGold Code modulation

of the tag, the signaltransmitting


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Joongheon Kim, Wonjun Lee, Jieun Yu, Jihoon Myung, Eunkyo Kim and Choonhwa Lee, 2005 [17], proposes an adaptive and dynamic localized scheme unique tohierarchical clustering in RFID networks, while reducing the overlapping areas ofclusters and consequently reducing collisions among RFID readers. The scheme adjustscluster coverage to minimize energy consumption, Low-Energy Localized Clustering for

RFID networks (LLCR) addresses RFID reader anti-collision problem. LLCR is a RFIDreader anti-collision algorithm that minimizes collisions by minimizing overlapping areasof clusters that each RFID reader covers. LLCR takes into account each RFID readersenergy state as well as RFID reader collisions. For the energy state factor, we distinguishhomogeneous RFID networks from heterogeneous ones according to computing power ofeach RFID reader. The simulation-based performance evaluation shows that LLCRminimizes energy consumption and overlapping areas of clusters of RFID readers.

Kin Seong Leong, Mun Leng Ng (Member of IEEE), Alfio R. Grasso and Peter H.Cole, 2005 [18], they have identified synchronization of RFID readers as a mechanism toassist in RFID readers deployment in dense reader environments. Several fine-tuning

methods are also proposed in optimizing the performance of a synchronized RFIDsystem. As compared to conventional unsynchronized RFID systems, a synchronizedRFID system can offer more coverage, less reader collision or interference, while strictlyfollowing the European regulations and the EPC C1G2 recommendation. However, thesebenefits require the use of more complex hardware and hence can marginally increasedeployment costs.

Bogdan Carbunar, Murali Krishna Ramanathan, Mehmet Koyutrk, SureshJagannathan and Ananth Gramab, 2008 [19], they have been addressed two importantproblems in wireless RFID systems. The first problem that of accurately detecting thetags covered by each reader, is made difficult by reader collisions occurring at remotetags, the second problem relates to extending the lifetime of the reader network bydetecting and temporarily disabling the wireless interfaces of redundant readers. Theydefined redundancy in terms of discrete sets of points, tags, and proved that theoptimization version of the problem is NP-complete. For both problems, they presentdistributed and localized algorithms, based on a randomized querying technique thatensures the accurate receipt of reader queries by tags. Also they provide a probabilisticanalysis of the algorithms. Their extensive simulations showed the impact of readercollisions on the accuracy of a tree walking algorithm (TWA). Moreover, they show thattheir solution achieves high accuracy at the expense of slightly increased traffic overhead(on average 4 messages per query).

9. My proposal:It is clear from the above survey that the most important technique is the CDMA which isvery useful in the RFID field, however, this still has a problem due to the pseudo codeused in its construction such as the Gold code and the Walsh code, in other words it stillhas limitations in the tag capacity due to the used code as stated above in ref. [9] and[10]. To overcome this problem, we can use the Orthogonal Frequency DivisionMultiplex (OFDM). The use of OFDM is a promising technology and it has useful


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Many spread spectrum techniques are currently available. For example, there is directsequence spread spectrum (DSSS), frequency-hopping spread spectrum (FHSS), time-hopping spread spectrum (THSS), and hybrid techniques, which are combinations of thefirst three.

Multicarrier CDMA is the combination of OFDM signaling and CDMA scheme has onemajor advantage that it can lower the symbol rate in each subcarrier so that longersymbol duration makes it easier to quasi-synchronize the transmissions.

The MC-CDMA transmitter spreads the original data stream over different subcarriersusing a given spreading code in the frequency domain. In other words, a fraction of thesymbol corresponding to a chip of the spreading code is transmitted through a differentsubcarrier. Spreading code like the Hadamard Walsh code has been shown to be optimumin maintaining ortogonality between subcarriers, because here does not pay attention tothe auto-correlation characteristics of the spreading code. Figure 20 shows the basic MC-CDMA system of the j

thuser, where GMC denotes the processing gain, NC the number of

subcarriers, and C

j

(t)=[C1j

C2j

..CGMCj

] the spreading code of the j

th

user. In this figurethe MC-CDMA scheme is discussed assuming that the number of subcarriers and theprocessing gain are all the same. Therefore, in this figure the number of subcarriers isequal to the processing gain (GMC=NC

).

Data

NC=GMC

Cj

GMC

C2j

C1j

DataStream

Co ier

Parallel

ToSerial

Converter

C

H

A

N

N

EL

Serial

ToParallel

QjGMC

Q2j

Q1j

Parallel

To

Figure 20. Basic MC-CDMA system when NC=GMC

NC=GMC

tfCos GMC2

tfCos 22

tfCos 12

tfCosGMC

2

tfCos 22

tfCos 12


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10.References:

1. Stevan Preradovic, Nemai C. Karmakar, and Isaac Balbin RFID Transponders,IEEE microwave magazine, p-p 90-103, 2008.

2. Klaus Finkenzeller, RFID Handbook, second edition, Copyright 2003 JohnWiley & Sons Ltd.

3. Yan Chen, Integrating Radio Frequency Identification (RFID) data with Electronic Data Interchange (EDI) business processes, Thesis of Master ofEngineering in Logistics at the Massachusetts Institute of Technology, June 2005.

4. OTA Training, RFID+ Exam Cram, Copyright 2006, by Exam Cram. Part ofthe Exam Cram 2 series.

5. Donald L. Black and Dale Yones, Method for Resolving Signal Collisions

Between Multiple RFID Transponders in a Field, US Patent No. 62656963 B1,Jul. 24, 2001.

6. Anil Rohatgi, RFID Anti-Collision System Using the Spread SpectrumTechnique propagation group document ID: PG-TR-050426-AR, 26 April 2005.

7. Michael A. Hughes, Pasco and Richard M. Pratt, SYSTEM AND METHOD TO IDENTIFY MULTIPLE RFID TAGS, US Patent No. 7009495 B2, Mar. 7,2006.

8. Cheng-Hao Quan, Won-Kee Hong, and Hie-Cheol Kim, Performance Analysis

of Tag Anti-collision Algorithms for RFID Systems, IFIP, EUC Workshops2006, LNCS 4097, pp. 382391, 2006.

9. Ping Wang, Aiqun Hu and Wenjiang Pei, The Design of Anti-collision Mechanism of UHF RFID System based on CDMA, IEEE, APCCAS, p-p1703-1708, 2006.

10.Maurizio A. Bonuccelli, Francesca Lonetti and Francesca Martelli, Instant collision resolution for tag identification in RFID networks, ScienceDirect,M.A. Bonuccelli et al. / Ad Hoc Networks 5, p-p 12201232, 2007 ElsevierInc.

11.Joshua Y. Mainaa, Marlin H. Mickle, Michael R. Lovell and Laura A. Schaefer, Application of CDMA for anti-collision and increased read efficiency ofmultiple RFID tags, ScienceDirect, Journal of Manufacturing Systems, p-p 3743, 26 2007 Elsevier Inc.
http://www.informit.com/authors/bio.aspx?a=60b0af82-bb2f-42e3-980f-6bbaf09b6ec1http://www.examcram.com/http://www.informit.com/imprint/series_detail.aspx?ser=335493http://www.informit.com/imprint/series_detail.aspx?ser=335493http://www.examcram.com/http://www.informit.com/authors/bio.aspx?a=60b0af82-bb2f-42e3-980f-6bbaf09b6ec1


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12.Piotr Jankowski-Mihulowicz, Wlodzimierz Kalita and Bartosz Pawlowicz,Problem of dynamic change of tags location in anticollision RFID systems,ScienceDirect, Microelectronics Reliability, p-p 911-918, 48 2008 Elsevier Inc.

13.Yuan-Cheng Lai and Chih-Chung Lin, A Pair-Resolution Blocking Algorithm

on Adaptive Binary Splitting for RFID Tag Identification, IEEECOMMUNICATIONS LETTERS, VOL. 12, NO. 6, pp 432-434, JUNE 2008.

14.Jeong Geun Kim, Member, IEEE, A Divide-and-Conquer Technique forThroughput Enhancement of RFID Anti-collision Protocol, IEEECOMMUNICATIONS LETTERS, VOL. 12, NO. 6, pp 474-476, JUNE 2008.

15.You-Chang Ko, Sumit Roy, Joshua R. Smith, Hyong-Woo Lee, and Choong-HoCho, RFID MAC Performance Evaluation Based on ISO/IEC 18000-6 TypeC, IEEE COMMUNICATIONS LETTERS, VOL. 12, NO. 6, pp 426-428, JUNE2008.

16.Shailesh M. Birari and Sridhar Iyer, Mitigating the Reader Collision Problem inRFID Networks with Mobile Readers, IEEE, 1-4244-0000-7/05/$20.00, pp 463-468, 2005.

17.Joongheon Kim, Wonjun Lee, Jieun Yu, Jihoon Myung, Eunkyo Kim andChoonhwa Lee, Effect of Localized Optimal Clustering for Reader Anti-Collision in RFID Networks: Fairness Aspects to the Readers, IEEE, 0-7803-9428-3/05/$20.00, pp 497-502, 2005.

18.Kin Seong Leong, Mun Leng Ng, Member, IEEE, Alfio R. Grasso and Peter H.Cole, Synchronization of RFID Readers for Dense RFID ReaderEnvironments, Proceedings of the International Symposium on Applications andthe Internet Workshops (SAINTW06), 0-7695-2510-5/05 $20.00, 2005 IEEE.

19.Bogdan Carbunar, Murali Krishna Ramanathan, Mehmet Koyutrk, SureshJagannathan, Ananth Grama, Efficient tag detection in RFID systems,ScienceDirect, J. Parallel Distrib. Comput., 2008 Elsevier Inc.

20.Eric Phillip LAWREY BE, Adaptive Techniques for Multiuser OFDM, thesisof Doctor of Philosophy in Electrical and Computer Engineering, School ofEngineering, James Cook University, December 2001.

21.Shinsuke Hara and Ramjee Prasad, Overview of Multicarrier CDMA, IEEECommunications Magazine, 0163-6804/97/$1000, pp 126-133, December 1997.

Documents

Proposal for RFID Tag Anti Collision Using MCCDMA Technique