Every Bit Counts – Fast and Scalable RFID Estimation

Muhammad Shahzad and Alex X. LiuDept. of Computer Science and Engineering

Michigan State UniversityEast Lansing, Michigan, 48824

USA

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Radio Frequency Identification

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Chip

Antenna

ActivePassive

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Radio Frequency Identification

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RFID Estimation

Exact IDs can not be read due to privacy requirements

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Exact IDs are not required but only a count

Identification protocols can use the count to speed up identification process

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Problem Statement Input

─ Confidence interval β ∈ (0,1]─ Required Reliability α ∈ [0,1)

Output─ An estimate te of tag population size t such that

● 1-β ≤ te / t ≤ 1+β

● P{ 1-β ≤ te / t ≤ 1+β } ≥ α

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Additional Requirements Single Reader environment Multiple reader environment with overlapping

regions C1G2 standard compliant tags Active tags and Passive tags Scalable

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Why do we need a new protocol? Non compliance with C1G2 standard Non-scalable Inability to achieve required reliability Room for improvement in speed

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Communication Protocol Overview

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0 1 1 C 0 1 1

frame size f =73 2 6 4 47

Faster to distinguish between empty and non-empty slots Slower to distinguish between empty, singleton, and collision Singleton and collision » non-empty At the end of frame, reader gets a sequence of 0s and 1s

─ 011C011 becomes 0111011

1 2 3 4 5 6 7

0 1 1 C 0 1 1

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Estimation Any measure which is a

monotonous function of t can be used for estimation─ Number of 1s in a frame─ Number of 0s in a frame

Any measure which is a monotonous function of t can be used for estimation─ Number of runs of 1s─ Number of runs of 0s

Any measure which is a monotonous function of t can be used for estimation─ Average run size of 1s─ Average run size of 0s

1 13 25 37 49 61 73 85 97 1091211331451571691811930

2

4

6

8

10

12

Number of Tags

Ave

rage

ru

n s

ize

of 0

s

1 13 25 37 49 61 73 85 97 1091211331451571691811930

2

4

6

8

10

12

14

16

Number of Tags

Nu

mb

er o

f 0s

1 13 25 37 49 61 73 85 97 1091211331451571691811930

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Number of Tags

Nu

mb

er o

f ru

ns

of 0

s

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1 13 25 37 49 61 73 85 97 1091211331451571691811930

2

4

6

8

10

12

14

16

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Number of Tags

Ave

rage

ru

n s

ize

of 1

s

1 13 25 37 49 61 73 85 97 1091211331451571691811930

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Number of Tags

Nu

mb

er o

f ru

ns

of 1

s

1 13 25 37 49 61 73 85 97 1091211331451571691811930

2

4

6

8

10

12

14

16

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Number of Tags

Nu

mb

er o

f 1s

011100 0 111 00

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Useable Measures

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Average run size of 1s

0 50 100 1500

5

10

15

20

Number of tags t

Aver

age

size

of ru

ns1s0s

101

102

10�1

100

101

102

103

Number of tags t

Varia

nce

Size of first run of 0sTotal 0sTotal 1sRuns of 1sRuns of 0sAvg. run size

Number of 1s Number of 0s Number of runs of 1s Number of runs of 0s Average run size of 1s Average run size of 0s

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ART Protocol

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0 1 1 1 0 1 1

frame size f = 73 2 6 4 47

1 2 3 4 5 6 7

0 1 1 1 0 1 1

Repeat frames n times

Calculate avg. run size of 1s from n frames

Number of Tags

Ave

rage

ru

n s

ize

of 1

s

Obtain the estimate

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Scalability Problem

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0 0 01 1 1 1C C C CC C CC

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Scalability Problem Addressed

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0 01 C

Use persistence probability p

frame size f = 4/p = 16

= 0.25

8 3 16 1211

2 5 3 12 79

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Obtain the estimate using information from this frame

Tags follow a uniform distribution Extrapolate with the factor of p

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10 20 30 40 50400

500

600

700

800

Frame size f

f n

Optimization The expression for number of rounds n depends on

─ Confidence interval β─ Required Reliability α─ Frame size f

n = func(α, β, f )

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Estimation time ∝ f × n─ d/df (f ×n ) = 0

Two equations1. n = func(α, β, f )2. d/df (f ×n ) = 0

Two unknowns1. Number of rounds n 2. Frame size f

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Multiple Readers Environment First proposed by Kodialam et. al. in “Anonymous

tracking using RFID tags”

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frame size f = 4

f =4

R

f =4

R

2

2

3

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1 0 1 01 1 1 0

Seed RSeed R

Logical

OR1 0 1 01 1 1 0 1 1 1 0

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Advantages of ART over prior art Speed:

─ 7 times faster than fastest ● β = 0.1%, α = 99.9%

Deployability─ Does NOT require modifications to

● tags ● communication protocol

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Performance Evaluation

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104

105

1060

50

100

150

200

250

Number of tags t

Estim

atio

n tim

e (s

ec)

FNEBMLEEZBUPEART

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Performance Evaluation

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0 0.02 0.04 0.06 0.08 0.1

100

102

104

Confidence Interval

Estim

atio

n tim

e (s

ec)

FNEBMLEEZBUPEART

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Performance Evaluation

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0.9 0.92 0.94 0.96 0.98 10

50

100

150

Required reliability

Estim

atio

n tim

e (s

ec)

FNEBMLEEZBUPEART

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Conclusion New estimator: the average run size of 1s Faster than existing estimation schemes

─ smaller variance Single and multiple reader environment C1G2 standard compliant tags Active tags and Passive tags Scalable: independent of tag population size

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Questions?

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