RFIDcover:
A Coverage Planning Tool
for RFID Networks with
Mobile Readers
MTP Thesis Presentation by
S. Anusha
Guide: Prof. Sridhar Iyer
RFID System
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Basics
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Radio Frequency IDentification: use of radio waves
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RF Tag: a low functionality microchip with an antenna
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Passive: derives power from readers’ transmission, computationally thin,
limits interrogation range.
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Active: has its own battery power
Reader: a device that can read/write information from tags
Applications
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Identification and Tracking of objects, Access Control
Problem Statement
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Completely Covering an Area
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At all time – Eg. intrusion detection
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Placing sufficient number of fixed readers
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High deployment costs
Periodically, within every T seconds – Eg. inventory check
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Can use mobile readers
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Cost-effective
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Challenge: to determine the number of readers, their movement, their
velocity etc.
The Coverage Problem:
Using Fixed Readers
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Fixed size circles covering
rectangle
Non-overlapping cover
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Optimal coverage - 90.69%
Overlapping cover
Optimal density - 1.209
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Fopt = 2√3XY/9r2
where
X, Y : dimensions of area
r : interrogation range
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Example
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10mx10m and r=2m, Fopt = 10
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50mx50m and r=2m, Fopt = 241
The Coverage Problem:
Using Mobile Readers
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Ellipse-like shape covering
rectangle
Non-overlapping cover
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Coverage =(2rvT+πr2)/(vT+2r)2r
where
r : interrogation range
v : velocity
T : period T
Overlapping cover
Density = 1+(πr2/2rvT)
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Msufficient-bound = XY/2rvT
where
X, Y : dimensions of area
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RFIDcover
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Purpose
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Given an application scenario and reader specifications, RFIDcover
automatically determines the number of readers required, their
placement and movement pattern to guarantee complete coverage of
an area within the specified period T.
Features
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Has an extendible architecture
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Permits user to tune additional constraints online
Use
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Supermarkets, Warehouse, Libraries ... any place where periodic
inventory is needed.
RFIDcover Architecture
RFIDcover Operation
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Three Phased Operation
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Selection Phase
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Generation Phase
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The mobility model, the MAC mechanism and an appropriate heuristic for
layout generation is selected.
A set of possible layouts, each conforming to the input constraints and
completely covering the given area is generated. Cost of deployment (as a
function of the number of readers), and the TRT (total time taken to read all
the tags in the entire area) are computed for each layout.
Optimization Phase
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An appropriate objective function for optimization is chosen and applied to
the set of layouts generated and the best layout amongst them is selected
and recommended to the user.
RFIDcover Inputs
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Application Scenario & parameters – say Supermarket with
aisle length and inter aisle distance
Reader Specification - interrogation range, interference range,
tag reading speed (TRS), unit cost, maximum speed (if mobile)
Topology Specification – dimensions of the min-area bounding
rectangle, tag distribution with parameters
Constraints – number of fixed readers, number of mobile readers,
maximum tag reading time (TRT), maximum cost, maximum
number of slots
RFIDcover Outputs
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Graphs
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Summarizing all layouts conforming to the constraints – TRT
variation, NMR variation, TRT Vs NMR, Optimizing Objective Function
Best Layout
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The details of the “best” layout - Number of readers, Placement of
readers, Mobility pattern of mobile readers, Velocity of mobile readers,
Tag Reading Time (TRT), Cost, Number of slots
RFIDcover Implementation
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Application Scenario
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Mobility Model
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LGH1 Heuristic
MAC Mechanism
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Zig-Zag Mobility Model
Layout Generating Heuristic
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Retail Inventory - Supermarket
Static Coloring MAC Mechanism
Optimizing Objective Function
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Least Square Sum Optimizing Function
Zig-Zag Mobility Model
LGH1 Heuristic
Definition:
l = length of the aisle; d = inter aisle distance;
of the area to be covered.
X, Y = dimensions
Assumption: The length of the aisle is along X.
generateLayout Function:
for ( d1 = l+d; d1 <= X; d1 = d1+l+d ) {
for ( d2 = d; d2 <= Y; d2 = d2+d ) {
• Form a column of readers by placing them d2 distance
apart, along Y.
• Place a copy of the column formed d1 distance apart, along
X.
• Within each d1xd2 rectangle, place as many mobile
readers as needed for completely covering the area within
specified time.
• This forms one layout.
}
}
Static Coloring MAC Mechanism
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A TDMA mechanism
Models the reader network as a graph G(R) = (V,E), with the set
of vertices V representing the readers, and the set of edges E
representing interference between readers.
Assignment of slots to readers equivalent to the problem of
coloring this graph.
Considers all possible scenarios, assigns and operates with as
many colors as needed in the worst case.
Simple and easy to implement for specific mobility models and
layouts of readers.
May be inefficient.
Least Square Sum OOF
Requirement
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To use minimum number of
readers
Read as often as possible i.e.,
TRT be as small as possible
Hence
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Least Square Sum is used
20
Example
18
16
14
12
TRT
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10
8
6
4
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Applied on TRT and NMR
2
0
0
5
NMR
10
15
Screen Shots: Input
Demonstration
Demo of RFIDcover
Screen Shots: Output Graphs
RFIDcover Evaluation
The Primary Example
Reader Specification
Topology
Specification
Application
Interrogation Range: 2m
Dimension X: 24m
Supermarket
Interference Range: 2.5m
Dimension Y: 15m
Scenario
Tag Reading Speed (TRS):
70tags/s
Tag Distribution:
Uniform
Aisle Length: 5m
Unit Cost: 1
Tag Density: 5/m2
Inter Aisle Distance: 3m
Maximum Speed: 5m/s
Aisle Length Along X
The Other Example - Same as above except:
Dimension X: 48m
Aisle Length: 10m
Dimension Y: 30m
Inter Aisle Distance: 6m
Mobile Vs Fixed Readers
Zig-zag Mobility Model
RFIDcover Extensions
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Retail Inventory Tracking Application Variant
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To-and-Fro Mobility Model, LGH2 Layout Generating Heuristic, Static
Coloring MAC Mechanism
RFIDcover Extensions
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Dynamic Coloring MAC
Mechanism
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Starts with Min-Color-Mode
and goes into General-ColorMode when collisions occurs
Number of slots given by 1*P1 +
2*P2 + ... + m*Pm
where
Pi =
Pr(i-Color-Mode)&
m=
total readers
Considerable overhead
RFIDcover Extensions
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Covering 3-Dimensional Space
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To-and-Fro Mobility Model, LGH2 Layout Generating Heuristic, Static
Coloring MAC Mechanism
Limitations due to Assumptions
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No Environmental Effects
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Circular Range
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Homogeneous System
Conclusions
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Providing complete coverage of an area is an important
requirement in an RFID system.
Using mobile readers is cost-effective for providing complete
coverage periodically, within every T seconds, even for small
values of T.
Deriving sufficient bound for number of mobile readers is
theoretically useful.
The Zig-Zag mobility model and LGH1 layout generating
heuristic result in layouts with number of readers close to the
sufficient bound.
RFIDcover architecture and design is easily extendible,
making it a useful RFID deployment tool.
References
[1] Klaus Finkenzeller. RFID Handbook : Fundamentals and Applications in Contactless Smart Cards
and Identification. Chichester : John Wiley, Leipzig, dritte edition, 2003.
[2] Radio Frequency Identification - A Basic Primer. White Paper, AIM Inc WP-98/002R2, August 2001
http://www.aimglobal.org/
[3] http://mathworld.wolfram.com/CirclePacking.html
[4] Richard Kershner. The Number of Circles Covering a Set. In American Journal of Mathematics,
volume 61, page 665, July 1939.
[5] Yi Guo and Zhihua Qu. Coverage Control for a Mobile Robot Patrolling a Dynamic and Uncertain
Environment. Proceedings of World Congress on Intelligent Control and Automation, June 2004.
[6] Daniel W. Engels. The Reader Collision Problem. Technical report, EPC Global, 2002.
http://www.epcglobal.org/
[7] J. Waldrop, D. W. Engels, and S. E. Sarma. Colorwave: An anticollison algorithm for the reader
collision problem. In IEEE Wireless Communications and Networking Conference (WCNC), 2003.
[8] Draft paper on Characteristics of RFID-systems. White Paper, AIM Inc WP-98/002R2, July 2000.
[9] A Basic Introduction to RFID Technology and its use in Supply Chain. Technical report, Laran
Technologies, January 2004.
Acknowledgement
My sincere thanks to
Prof. Sridhar Iyer
Research Scholars and members of the Mobile Computing
Research Group at KReSIT
My batchmates B. Nagaprabhanjan, Charu Tiwari and Shailesh M.
Birari
Questions?