Muiti-Access methods
in RFID System
Student :Yi-Shyuan WU
Adviser : Kai-Wei Ke
Date
: 2006.12.5
1
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
2
RFID System



Interrogator(Reader)
Transponder(Tag)
Application Software
Data
RFID Reader
Timing
Energy
Contactless data
carriar
transponder
Coupling element
(coil, microwave antenna)
Application
3
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
4
Multi-Access Procedures —
broadcast and multi-access
Transponder
1
Transponder
3
Transponder
2
Reader
Transponder
4
Transponder
6
Transponder
5
Transponder
1
Transponder
3
Transponder
2
Reader
Transponder
4
Transponder
6
Transponder
5
5
Multi-Access Procedures
(Anticollision)
Muti-access/
Anticollision procedures
Space
(SDMA)
Time
(TDMA)
Frequency
(FDMA)
Code
(CDMA)
Other
(Eq.Spred-spectrum)
6
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.
7
Multi-Access Procedures —
FDMA used for RFID applications
 FDMA


For RFID systems is to used various
independent 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.
8
Multi-Access Procedures —
TDMA used for RFID applications
9
Multi-Access Procedures
(Anticollision)
 Unfortunately, all these methods can not be used in a
RFID 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
not generally prepared to publish the anticollision
precedures that they use.
10
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
11
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.
12
ALOHA algorithm
 Tag transmits upon data
ready
 Detect success or collision
 Tag retransmits after
random backoff time
following collision
 Tag’s can’t detect/sense
carrier.
Collision is:


Determined by listening
for Reader’s (N)ACK
…undetected
13
ALOHA algorithm extensions
 Switch-off

If Tag response successfully
decoded, Tag automatically
enters Quiet state
 Slow-down
compromise 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
14
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)
15
Slotted ALOHA algorithm
16
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 „NextSlot“ command on no
responses received
17
Framed Slotted ALOHA algorithm
 Further discretisation of time:



Medium access grouped
into 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
18
Framed Slotted ALOHA
algorithm extensions
 Adaptive

Reader can temporarily
expand / 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“
19
Perspective
20
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
21
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.
22
Binary search algorithm
23
Binary search algorithm
 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.
24
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.
25
Dynamic binary search
algorithm
26
Improved Dynamic Binary search
algorithm
 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.
27
Improved Dynamic Binary
search algorithm
28
Comparison Binary search
algorithm
 Binary search algorithm



L : The average number of iterations
N : The number of transponders in the
interrogation zone of the reader
L(N) = log(N) / log(2) +1
 Improved Dynamic Binary search
algorithm

N-bit successive collisions are detected,
2 n tags can be recognized at best with
only 2n 1 +1 commands transmitted.
29
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
30
Conclusion
 Qualitative description of collision-
resolution algorithms.
 Comparisons difficult, meaningful?
 Different application used different
multi-
access procedures .
31
Conclusion
32
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
33
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.
34