1 muiti-access methods in rfid system student :yi-shyuan wu adviser : kai-wei ke date : 2006.12.5

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Muiti-Access methods in RFID System

Student :Yi-Shyuan WU

Adviser : Kai-Wei Ke

Date : 2006.12.5

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Overview

Multi-Access Procedures (Anticollision) Anticollision Algorithms in RFID System

ALOHA algorithm (Pure) ALOHA Slotted-ALOHA Frame-Slotted ALOHA

Binary search algorithm Binary search algorithm Dynamic binary search algorithm

Improved Dynamic Binary search algorithm

Conclusion References

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RFID System

Interrogator(Reader) Transponder(Tag) Application Software

Application

Timing

Data

Energy

Contactless data carriar

transponderRFID Reader

Coupling element(coil, microwave antenna)

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Multi-Access Procedures

The operation of RFID systems often involves a situation in which numberous transponders are present in the interrogation zone of a single reader at the same time.

Can differentiate between two main forms of communication. Broadcast mode Multi-access to a reader

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Multi-Access Procedures —broadcast and multi-access

Reader

Transponder 1

Transponder 2

Transponder 5

Transponder 3

Transponder 4

Transponder 6

Reader

Transponder 1

Transponder 2

Transponder 5

Transponder 3

Transponder 4

Transponder 6

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Multi-Access Procedures (Anticollision)

Muti-access/Anticollision procedures

Space(SDMA)

Time(TDMA)

Frequency(FDMA)

Code(CDMA)

Other(Eq.Spred-spectrum)

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Multi-Access Procedures — SDMA used for RFID applications

SDMA These consist of several dipole antennas,

and therefore adaptive SDMA can only be used for RFID application at frequencies above 850MHz(typical 2.45 GHz) as result of size of the antennas.

A disadvantage of the SDMA technique is the complicated antenna system. The use of this type of anti-collision procedure is therefore restricted to a few specialised applications.

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Multi-Access Procedures — FDMA used for RFID applications

FDMA For RFID systems is to used various indep

endent subcarrier frequencies for the data transmission from the transponders to the reader.

A disadvantage of the FDMA technique is the relatively high cost of the readers, since a dedicated receiver must be provided for every reception channel. This anti-collision procedure, too, remains limited to a few specialised applications.

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Multi-Access Procedures — TDMA used for RFID applications

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Multi-Access Procedures (Anticollision) Unfortunately, all these methods can not be used in a RF

ID system directly because they are much too complicated. The first limiting factor for RFID systems is the constraint

on memory and computation capabilities. Secondly, several regulatory bodies restrict the readers’

maximum operating field strength . The inability to sense the medium preventing tags to be

aware of each others’ presence and transmissions. For reasons of competition, system manufacturers are no

t generally prepared to publish the anticollision precedures that they use.

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Overview






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ALOHA algorithm

Transponder-driven stochastic TDMA procedure (Tag-Talks-First).

The procedure is used exclusively with read-only transponders.

The implicit start of the exchange between the tags and reader, with the tags automatically sending their IDs upon entering a powering field.

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ALOHA algorithm

Tag transmits upon data ready

Detect success or collision Tag retransmits after rando

m backoff time following collision

Tag’s can’t detect/sense carrier. Collision is: Determined by listening fo

r Reader’s (N)ACK …undetected

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ALOHA algorithm extensions

Switch-off If Tag response successfully d

ecoded, Tag automatically enters Quiet state

Slow-downcompromise between Aloha and Switch-off

Reader overwhelmed by responses

„Slow-down“ command sent, Tag adapts its (random) backoff algorithm

Goal is to diminish Tags‘ reply frequency

„Carrier Sense“ MUTE signal to all Tags when

start of transmission is detect

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Slotted ALOHA algorithm

Interrogator-driven stochastic TDMA procedure (Reader-Talks-First).

Transponders may only begin to transmit data packets at defined, synchronous points in time (slots).

Packet either collide completely or do not collide at all

Synchronization overhead: Reader SOF(start of frame), EOF(end of frame)

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Slotted ALOHA algorithm

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Slotted ALOHA algorithm extensions

„Terminating“ If Tag response successfully

decoded, Tag automatically enters Quiet state

Avoids collisions due to Tags replying indefinitely

Tags re-enter Active state upon next “Wake-up” from Reader

Failure to recognize “Wake-up” a problem:Tags time-out of sleep mode automatically

Also called “Muting” „Early End“

Slot delimited by Reader SOF, EOF

Reader issues „Next-Slot“ command on no responses received

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Framed Slotted ALOHA algorithm

Further discretisation of time: Medium access grouped in

to Frames,with N slots per frame

Tags transmit at most once in a randomly selected slot, within maximum N

Little extra synchronization overhead:

Reader SOF, EOF for slots

maximum slot number N set in Tag as default

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Framed Slotted ALOHA algorithm extensions

Adaptive Reader can temporarily e

xpand / contract number of slots for upcoming round

Number of slots in a round varies with number of Tags in field

Previous extensions also applicable:

Terminating / Muting (slotted) „Early End“

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Perspective

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Overview






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Binary search algorithm

The implementation of a binary search algorithm requires that the precise bit position of a data collision is recognised in the reader.

Manchester code is used in order to recognize the bit where there is a collision.

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After the completion of the read/write operations, transponder 2 can be fully deactivated by an UNSELECT command, so that is no longer responds to the next REQUEST command.

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Dynamic binary search algorithm In the binary search procedure both the search

criterion and the serial numbers of the transponders are always transmitted at their full length.

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Dynamic binary search algorithm

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Has two differences compared with Dynamic Binary search algorithm If there is only one collided bit no matter

where it is, the reader does not need to sent REQUEST command again and can automatically identify two tags once.

After successive collisions have been detected, ever correlative bit but the last in REQUEST command will be set to zero.

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Comparison Binary search algorithm

Binary search algorithm L : The average number of iterations N : The number of transponders in the inte

rrogation zone of the reader L(N) = log(N) / log(2) +1

Improved Dynamic Binary search algorithm N-bit successive collisions are detected,

tags can be recognized at best with only +1 commands transmitted.

n212 n

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Overview






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Conclusion

Qualitative description of collision-resolution algorithms.

Comparisons difficult, meaningful?

Different application used different multi-access procedures .

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Conclusion

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Overview






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References

RFID Handbook : Fundamentals an Applications in Contactless Smart Cards and Identification , Second Edition, Klaus Finkenzeller.

Luc André Burdet, "RFID Multiple Access Methods," ETH Zürich, Summer semester 2004, Seminar "Smart Environments".

Leian Liu, Zhenhua Xie, Jingtian Xi and Shengli Lai, "An improved anti-collision algorithm in RFID system," 2005 2nd International Conference of Mobile Technology, Applications and Systems.

Jae-Ryong Cha and Jae-Hyun Kim, " Novel anti-collision algorithms for fast object identification in RFID system," 11th International Conference of Parallel and Distributed Systems, 2005.

1 muiti-access methods in rfid system student :yi-shyuan wu adviser : kai-wei ke date : 2006.12.5

Documents

multiaccess slide

aloha algorithm tag

reader slide

multiaccess procedures

aloha algorithm transponder

multiaccess procedures

multiaccess procedures

rfid applications sdma