Anti-tag collision algorithms of RFID

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Anti-tag collision algorithms of RFID

• Chun-Fu Lin

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• Agenda
• Taxonomy of tag collision protocols
• Bi-slotted tree based anti-collision protocols
• Adaptive binary splitting
• I-code

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Reader collision
Tag collision
Centralized Distributed
Probability-based Deterministic-based
(Prefix-based)
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• RFID research topics
• Hardware
• circuit, antenna design, etc.
• Collision
• Reader collision
• Tag collision
• Security privacy
• Application
• CSI/CTPAT/WCO SAFE
• Supply chain management, medical care, etc.

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Taxonomy of tag anti-collision protocols by
Dong-Her Shih et. al., published in Computer
Communications, 2006
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• SDMA (Space Division Multiple Access)
• Reuse a certain resource, such as channel
  capacity in spatially separated area.
• Reduce the reading range of readers and forms as
  an array in space.
• Electronically controlled directional antenna
• Various tags can be distinguished by their
  angular positions.

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• FDMA (Frequency Division Multiple Access)
• Several transmission channels on various carrier
  frequencies are simultaneously available.
• Tags respond on one of several frequencies.

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• CDMA (Code Division Multiple Access)
• Too complicate and too computationally intense
  for RFID tags as well

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• TDMA (Time Division Multiple Access)
• The largest group of RFID anti-collision
  protocols
• Tag driven (tag talk first, TTF)
• Tag transmits as it is ready
• Aloha
• SuperTag
• Tags keep quiet and retransmit until reader
  acknowledges
• Reader driven (reader talk first, RTF)
• Polling, splitting, I-code, contactless

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• Polling
• Reader must have the complete knowledge
  (database) of tags
• Reader interrogates the RFID tags by polling
  whose serial number starts with a 1 in the
  first position?
• Those tags meet this test reply yes while
  others remain
• Slow, inflexible

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• Splitting method
• Tree algorithm
• Based on binary search tree algorithm
• Each collided tag generates a random number by
  flipping an unbiased B-sided coin
• B 2, each collided tag would generate a number
  0 or 1
• The reader always sends a feedback informing the
  tags whether 0 packet, 1 packet, or more than 1
  packet is transmitted in the previous slot.
• Each tag needs to keep track of its position in
  the binary tree according to the readers
  feedback

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R set responds first
L set generates 1 R set generates 0 S single
reply Z zero reply C collision
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• Query Tree
• Prefix based
• Tags match the prefix respond

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• I-Code
• stochastic passive tag identification protocol
  based on the framed-slotted Aloha concept.
• Each tag transmits its information in a slot that
  it chooses randomly based on the seed sent by the
  reader.
• The reader can vary the frame size N, the actual
  size of a slot is chosen according to the amount
  of data requested

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• Approximation of N
• The reader detects the number of slots by a
  triple of numbers c (c0, c1, ck), where c0
  stands for the number of slots in the read cycle
  in which 0 tags have transmitted, c1 denotes the
  number of slots in which a single tag transmitted
  and ck stands for the number of slots in which
  multiple tags are transmitted.
• Lower bound method
• Minimum Distance method distance between read
  result c and the expected value vector of n

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Various N values corresponding to specific ranges
have been found from experiments and tabulated
If n ? 17, 27, both 32 and 64 are appropriate
choices for N
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• Contact-less
• Is based on the tree splitting methodology to
  identify one bit of the ID in every arbitration
  step
• The tag uses the modulation scheme which
  identifies 0 in the specified bit position with
  00ZZ (Z stands for no modulation) and 1 as
  ZZ00. In this way, the reader can recognize the
  responses from all the tags and divide the
  unidentified tags into 2 groups.

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1
1
Identified 1101
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• Related papers published in IEEE communications
  letters (I/F 1.196) from 2006 now
• MARCH 2006, Adaptive Binary Splitting for
  Efficient RFID Tag Anti-Collision, Jihoon Myung,
  Student Member, IEEE, Wonjun Lee, Senior Member,
  IEEE, and Jaideep Srivastava, Fellow, IEEE
• APRIL 2006, Enhanced Binary Search with
  Cut-Through Operation for Anti-Collision in RFID
  Systems, Tsan-Pin Wang
• DECEMBER 2006, Bi-Slotted Tree based
  Anti-Collision Protocols for Fast Tag
  Identification in RFID Systems, Ji Hwan Choi,
  Student Member, IEEE, Dongwook Lee, and Hyuckjae
  Lee, Member, IEEE
• JANUARY 2007, Optimized Transmission Power
  Control of Interrogators for Collision
  Arbitration in UHF RFID Systems, Joongheon Kim,
  Member, IEEE, Wonjun Lee, Senior Member, IEEE,
  Eunkyo Kim, Dongshin Kim, Student Member, IEEE,
  and Kyoungwon Suh, Student Member, IEEE
• JANUARY 2007, Query Tree-Based Reservation for
  Efficient RFID Tag Anti-Collision, Ji Hwan Choi,
  Student Member, IEEE, Dongwook Lee, and Hyuckjae
  Lee, Member, IEEE

Tag collision
Reader collision
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• Ji-Hwan Choi, Dongwook Lee, Hyuckjae Lee,
  Bi-slotted tree based anti-collision protocols
  for fast tag identification in RFID systems,
  IEEE communication letter, December 2006

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• BSQTA
• Reader sends n-1 bits prefix.
• Tag that match the prefix will
• Send ID from n1 bit to end bit if its n-th bit
  is 0.
• Wait for LENGTH-n bit duration if its n-th bit is
  1.
• If reader detects any collision occurs, it pushes
  the prefixes (prefix0, prefix1) into stack and
  repeats the exploring process.
• BSCTTA (variation from BSQTA, tags send their ID
  from n1 bit to the time that ACK signal
  received)

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P101
Reader
0
1
0
Tag1 1010010
0
1
0
Tag2 1011010
Wait N-prefix bits
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• Simulation
• One reader
• Number of tags increases from 2 to 65536
• Tag IDs are randomly generated
• ID length is not clear (believe to be 250)
• Compared to QTA and CTTA

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• In another way of thinking
• The tag collision problem can be referred to as a
  distributed systems problem how to reach a
  consensus agreement (transmission slot) for every
  distributed node (tag) without interaction to
  each other?
• The constraints will be
• Tag is unable to detect collision
• Tag has limited or no calculation capability
• Tag has limited or no memory space
• Tags can be moved in and out dynamically
• Reader has no information of the number of tags
• There could be not just one reader

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• Jihoon Myung, Wonjun Lee, Jaideep Srivastava,
  Adaptive binary splitting for efficient RFID tag
  anti-collision, IEEE communication letter, March
  2006

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• Every tag maintains two local variables Pc and
  Ac(i).
• Pc, progressed-slot counter number of tags
  recognized by the reader so far.
• Ac(i), allocated-slot counter, time slot for tag
  is transmission.
• Reader sends feedback (readable, idle, collision)
  to tags
• According to readers feedback, each tag decides
  its transmission time of slot.
• If feedbackreadable, tag adds 1 to Pc.
• If feedbackidle and Pc lt Ac(i), tag decreases
  Ac(i) by 1
• If feedbackcollision and Pc lt Ac(i), tag
  generates a random number of 0 and 1 and adds to
  Ac(i).

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• Simulation results
• Identification delay
• Number of time slots required for all tags
• Tag communication overhead
• Average number of bits transmitted by a tag for
  identification
• n number of tags recognized in the last process
• ? number of staying tags (ts?Sr,inSr,i-1)
• a number of arriving tags (ta?Sr,i1-Sr,i)
• ß number of leaving tags (tl?Sr,i-Sr,i1)
• w given, w?/n, a/nß/n1-w
• Binary tree protocol and query tree protocol are
  compared

Sr,i the set of all the tags recognized by
reader r in the ith Identification process
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Stable for static tags
w0, No staying tags, arriving tagsleaving
tags w1, static tags, no arriving and leaving
tags
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• Harald Vogt, Efficient Object Identification
  with Passive RFID Tags, Proceedings of the
  International Conference on Pervasive Computing,
  April 2002, pp.98-113.

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• I-Code
• Reader is attached to the serial interface of a
  host (PC)
• Communication and power transmission between the
  readers and tags takes places by inductive
  coupling
• All tags within reading range will answer
  requests from the reader
• An I-Code tag provides 64 bytes memory
• It employs a variant of slotted Aloha for access
  to the shared communication medium

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• Tag reading cycle
• I what data is requested in memory
• Rnd random value ? 0, 31 for tags function sT
  of randomization
• N frame size ? 1, 4, 8, 16, 32, 64, 128, 256
• ?s tag sends in slot s, 0sN
• The result of a read cycle can be viewed as a
  triple of numbers ltc0, c1, ckgt

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• Mathematical Preliminaries
• Occupancy
• Given N slots and n tags, the number r of tags in
  one slot is
• The expected value of the number of slots with
  occupancy number r is given by

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• µr the number of slots being filled with exactly
  r tags
• Remaining arrangement

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• Tags reading as a Markov process

0
n
3
2
1
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• The matrix Q is used to compute a lower bound of
  the number of reading steps necessary to identify
  all tags with a given probability.

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• How to estimate n?
• Based on the results of read cycles cltc0, c1,
  ckgt, and the current value of N, the function
  that compute estimations of n is
• Error function sums up the weighted errors over
  all possible outcomes of the read cycle

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• An problem to optimal value for the number of
  cycles
• Small frame size ? high collisions
• Large frame size ? high response time
• Stochastic nature of reading process (frame
  slotted Aloha) can not guarantee 100 probability
  of identifying all tags
• Compute the time to achieve a given assurance
  level a values were obtained by performing read
  cycles for 1 min. and computing the average
  consumed time

tN cycle time, also depends on the connection
speed between reader and host
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• TN is nearly linear in N
• For a fixed frame size N, the time Ta required to
  achieve an assurance level a is
• S satisfies
• If the optimal frame size is known, e.g. if n can
  be estimated correctly, then the identification
  time that meet the threshold a increase linearly
  with the number of tags

Min number of read cycles
Probability of identify k tags after s read
cycles, K 1, 2, , n. Choose its nth component
and Compare it to a
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• Two estimation functions
• Lower bound ? a collision involves at least 2
  different tags
• Distance between read result c and the expected
  value vector of n

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Lower-bound is accurate for small n but grows
fast with larger n. e-dist is more steady
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• Due to the inaccuracy of the estimation functions
  and the jitter as shown in Fig.3, it is free to
  choose the actual frame size for a given
  estimate ex. if n?17,27, both 32 and 64 are
  appropriate choice for N.

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Thank you