Rfidcover: a coverage Planning Tool for rfid networks with Mobile Readers


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RFIDcover: A Coverage Planning Tool for RFID Networks with Mobile Readers

  • MTP Thesis Presentation by
  • S. Anusha
  • Guide: Prof. Sridhar Iyer

RFID System

  • Basics
    • Radio Frequency IDentification: use of radio waves
    • RF Tag: a low functionality microchip with an antenna
      • Passive: derives power from readers’ transmission, computationally thin, limits interrogation range.
      • Active: has its own battery power
    • Reader: a device that can read/write information from tags
  • Applications
    • Identification and Tracking of objects, Access Control

Problem Statement

  • Completely Covering an Area
    • At all time – Eg. intrusion detection
    • Periodically, within every T seconds – Eg. inventory check
      • Can use mobile readers
      • Cost-effective
      • Challenge: to determine the number of readers, their movement, their velocity etc.

The Coverage Problem: Using Fixed Readers

  • Fixed size circles covering rectangle
  • Non-overlapping cover
    • Optimal coverage - 90.69%
  • Overlapping cover
      • Optimal density - 1.209
      • Fopt = 2√3XY/9r2
      • where
      • X, Y : dimensions of area
      • r : interrogation range
  • Example
    • 10mx10m and r=2m, Fopt = 10
    • 50mx50m and r=2m, Fopt = 241

The Coverage Problem: Using Mobile Readers

  • Ellipse-like shape covering rectangle
  • Non-overlapping cover
    • Coverage =(2rvT+πr2)/(vT+2r)2r
      • where
      • r : interrogation range
      • v : velocity
      • T : period T
  • Overlapping cover
      • Density = 1+(πr2/2rvT)
      • Msufficient-bound = XY/2rvT
      • where
      • X, Y : dimensions of area

RFIDcover

  • Purpose
    • 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
    • Has an extendible architecture
    • Permits user to tune additional constraints online
  • Use
    • Supermarkets, Warehouse, Libraries ... any place where periodic inventory is needed.

RFIDcover Architecture

RFIDcover Operation

  • Three Phased Operation
    • Selection Phase
      • The mobility model, the MAC mechanism and an appropriate heuristic for layout generation is selected.
    • Generation Phase
      • 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
      • 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

  • 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

  • Graphs
    • Summarizing all layouts conforming to the constraintsTRT variation, NMR variation, TRT Vs NMR, Optimizing Objective Function
  • Best Layout
    • 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

  • Application Scenario
    • Retail Inventory - Supermarket
  • Mobility Model
    • Zig-Zag Mobility Model
  • Layout Generating Heuristic
    • LGH1 Heuristic
  • MAC Mechanism
    • Static Coloring MAC Mechanism
  • Optimizing Objective Function
    • Least Square Sum Optimizing Function

RFIDcover Design

Zig-Zag Mobility Model

LGH1 Heuristic

  • Definition:
  • l = length of the aisle; d = inter aisle distance; X, Y = dimensions of the area to be covered.
  • 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

  • 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
    • To use minimum number of readers
    • Read as often as possible i.e., TRT be as small as possible
  • Hence
    • Least Square Sum is used
    • Applied on TRT and NMR

Screen Shots: Input

Demonstration

  • Demo of RFIDcover

Screen Shots: Output Layout1

Screen Shots: Output Layout2

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

  • Retail Inventory Tracking Application Variant
    • To-and-Fro Mobility Model, LGH2 Layout Generating Heuristic, Static Coloring MAC Mechanism

RFIDcover Extensions

  • Dynamic Coloring MAC Mechanism
    • Starts with Min-Color-Mode and goes into General-Color-Mode 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

  • Covering 3-Dimensional Space
    • To-and-Fro Mobility Model, LGH2 Layout Generating Heuristic, Static Coloring MAC Mechanism
  • Limitations due to Assumptions
    • No Environmental Effects
    • Circular Range
    • Homogeneous System

Conclusions

  • 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

  • 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
  • My sincere thanks to

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