Tom Chao Zhou, CUHK 1Tom Chao Zhou, CUHK 1

Wireless Sensor Network

Speaker: Tom Chao Zhou

Feb, 23rd@3DLBS Study Group

Subtopic: Sensor Technology

Tom Chao Zhou, CUHK 2Tom Chao Zhou, CUHK 2

Outline

• Definition• Overview• Related Research

– Applications– Network Services

• Case Study: Multi-hop Relative Location Estimation (A Googler’s paper)

Tom Chao Zhou, CUHK 3Tom Chao Zhou, CUHK 3

Definition

• Wireless sensor networks (WSNs)– Sensor nodes: sense, measure and gather information;

transmit the data to the user• E.g. biological, chemical, optical, and magnetic sensors

– Potential applications• Military target tracking, natural disaster relief, etc

– Resource constraints• Energy, short communication range, low bandwidth, limited

processing and storage

Personal opinion: Energy is especially important if we consider each node is a mobile phone.

Tom Chao Zhou, CUHK 4Tom Chao Zhou, CUHK 4

Overview

Tom Chao Zhou, CUHK 5Tom Chao Zhou, CUHK 5

Applications

• Connection-less sensor-based tracking system using witness (CenWits)– Mobile sensors worn by people– Use the GPS receivers and location points to determine

current location– Use of witnesses to convey a subject’s movement and

location information to the outside world– Huang et al. 2005, Sensys

Tom Chao Zhou, CUHK 6Tom Chao Zhou, CUHK 6

Application

• ZebraNet– Mobile wireless sensor network used to track animal

migrations– Positional readings are taking using the GPS– Sent multi-hop across zebras to the base station– Zhang et al. 2004, Sensys

Tom Chao Zhou, CUHK 7Tom Chao Zhou, CUHK 7

Network Services

• Localization– Problem of determining the node’s location

• Synchronization– Time synchronization in a wireless sensor network is

important for routing and power conservation– Apply time synchronization methods in data

synchronization? E.g. p2p data

Tom Chao Zhou, CUHK 8Tom Chao Zhou, CUHK 8

Localization

• Existing methods– Global positioning system (GPS)

• GPS receiver• Pros: simple• Cons: not work if there are obstructions

– Beacon (or anchor) nodes• Beacon nodes, which know their own position• Cons: does not scale well in large networks (may not be true in

3DLBS, wifi spots)

– Proximity-based localization• Use neighbor nodes to determine their location; act as beacons

Tom Chao Zhou, CUHK 9Tom Chao Zhou, CUHK 9

Localization

• Moore’s algorithm (Moore et al., 2004, Sensys)– Distributed localization algorithm, use of a robust

quadrilateral.

• Secure localization (Srinivasan et al., 2007)– Securing the localization process, rely on beacon

information.

Tom Chao Zhou, CUHK 10Tom Chao Zhou, CUHK 10

Synchronization

• Energy is conserved when there are less collisions and re-transmissions

• Energy is saved when nodes are duty-cycled– Periodically turn its radio off to save energy and on to

participate in network communication

Tom Chao Zhou, CUHK 11Tom Chao Zhou, CUHK 11

Synchronization

• Uncertainty-driven approach (Ganeriwal et al. 2005)– Modeling long-term clock drifts between nodes to

minimize duty-cycling overheads

• Lucarelli’s algorithm (Lucarelli et al. 2004)– Bi-directional nearest-neighbor coupling

• Clock-sampling mutual network synchronization– Rentel et al. 2005– Distributed and autonomous network synchronization– Non-hierarchical

Tom Chao Zhou, CUHK 12Tom Chao Zhou, CUHK 12

Case Study

• On the Accuracy of Multi-hop Relative Location Estimation in Wireless Sensor Networks

• Youssef et al. 2007, IWCMC• Goal

– Analyze the accuracy of multi-hop relative location estimation and study the different sources of errors

– Present the Multi-hop Relative Location Estimation (MRLE) algorithm which avoids reflection error

Tom Chao Zhou, CUHK 13Tom Chao Zhou, CUHK 13

Relative Coordinate System (RCS)

• Estimate nodes’ positions relative to a coordinate system established by a reference group of nodes– R0,R1,R2

• A RCS can be transformed to absolute coordinate system by using only three anchor nodes in 2D (or four anchors in 3D)

Note: we only need a proximity measure between nodes

Tom Chao Zhou, CUHK 14Tom Chao Zhou, CUHK 14

Multi-hop Relative Position Estimation

• A node u knows distances to three non-colinear identified neighbors.

u2 is the reflection of u1 across the base line

R2 is the reflection resolver

Tom Chao Zhou, CUHK 15Tom Chao Zhou, CUHK 15

Reflection Error

• Reflection propagation error

Measured distances contain error.

Tom Chao Zhou, CUHK 16Tom Chao Zhou, CUHK 16

Reflection Error

• Skinny triangles

• Have the RCS near the center of the network

Tom Chao Zhou, CUHK 17Tom Chao Zhou, CUHK 17

MRLE

• Adding R0, R1, R2 to the set of identified nodes.• Iteratively estimate an unidentified node u

– Select base line as far as possible from u– Resolver node, the neighbor to u with the highest altitude

from the base line

Tom Chao Zhou, CUHK 18Tom Chao Zhou, CUHK 18

Open Research Issues (Personal)

• This approach assumes wireless sensor nodes are static, how to handle dynamic cases if we consider people who have mobiles moving around?

• This approach assumes there is only one gateway, how about if there are multiple gateways? (In a shopping mall, we have multiple wi-fi routers (gateways))

Tom Chao Zhou, CUHK 19Tom Chao Zhou, CUHK 19

References

• Wireless sensor network survey• On the Accuracy of Multi-hop Relative Location

Estimation in Wireless Sensor Networks• Localization and Tracking in Sensor Systems

Tom Chao Zhou, CUHK 20Tom Chao Zhou, CUHK 20

• Thanks!• Q & A