cooperative location- sensing for wireless networks authors ： haris fretzagias maria papadopouli...
TRANSCRIPT
Cooperative Location-Sensing for Wireless
Networks
Authors ： Haris Fretzagias
Maria Papadopouli
Presented by cychen
IEEE International Conference on Pervasive Computing and Communications (PERCOM ’04)
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Localization in WSN
Location information is very useful to routing and other applications.
Global ID, local ID, no ID? Position type
Absolute Relative Symbolic
Major Phases of Localization
1. Initialization Phase To facilitate operations in the localization phase Range or angle measurement
RSSI,TOA,TDOA,AOA
2. Localization Phase Calculate the locations of sensor nodes. Settlement of coordinate system Spread the locations.
3. Refinement (optional)
Design characteristics
Robust to tolerate network failures, disconnections, delays due to host mobility
No need for extensive training and specialized hardware
Scalable Computationally inexpensive Suitable for indoor and outdoor environments
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Overview(1/2)
Grid-representation of the terrain Communication protocol - disseminates
positioning and distance estimates among hosts in the network.
Voting process - to accumulate and assesses the received positioning information
Overview(2/2)
A collaborative location-sensing scheme. Each host
estimates its distance from neighboring peers refines its estimations iteratively as it receives new
positioning information from peers Only the computationally powerful hosts run the
voting process and compute their location.
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Grid-representation
Grid-based representation of the terrain. Each Host initializes its grid at the beginning
of a CLS run. The grid size doesn’t need to be the same .
Coordinate System
Global coordinate system All hosts in the terrain share a common (global)
coordinate system used to represent their position. Local coordinate system
A cell is represented using the local grid coordinate system.
A host transforms the global coordinates of all the (acquired) position information to coordinates of its local grid.
A host transforms its local coordination to the global coordination after the CLS operations.
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Some Definitions
Active host ： A host computes its own location
Passive host ： A host does not compute its own location
CLS server ： A sever computes the locations of their local passive hosts.
CLS communication protocol
Neighbor discovery protocol with single-hop broadcast beacons.
Respond to beacons with positioning information Distance estimation using these beacons Building CLS entry for that neighbor
Aggregate new positioning entries to a single message. Makes active hosts learn about the host are more than one-
hop away Controlled-dissemination of CLS entries
Active hosts only rebroadcast the updated or new CLS entries based on the position field and Time-To-Live (TTL) value.
CLS beacon & update messages
Peer idPositio
nTim
eRang
eWeig
htDistance Vote
A (xA,yA) tn RA wA
(du,A- e , du,A+ e)
Positive
C (xC,yC) tk RC wC (RC, ) Negative
CLS table of host u with entries for peers A and C
A ： within the range of the reference host u. C ： u learned through updates (from its neighbors)
CLS table
This table is initialized at the beginning of a run and updated when the local host gathers a non-stale CLS update.
An active host maintains a table with all the received CLS entries.
A CLS server maintains a similar table for each host that it tries to position
A passive host forwards new CLS entries to their server.
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Voting Process
Takes place as the local host (or CLS server) receives CLS update messages and builds the table.
Accumulating and assessing the received positioning information to estimate the location of a passive nodes.
The Weights
Each host is configured with a voting weight a constant depends on the confidence of the host about its
position estimation.
Landmarks have higher voting weight than hosts that compute their position using CLS.
The higher the value of a cell is, the more hosts agree that this is a likely position of that host.
x
Accumulation of votes from peers
1. Host A votes
2. Host B votes
x
3. Host C votes
Most likely position
x
x
Host u with unknown positionPeers A, B, and C have positioned themselves
Parameters Gm ： the grid that a host maintains during a run
Rn ： the wireless range of host n.
wn ： the voting weight ： the estimated distance interval between host m and
n v (x ,y) ： the value of the cell. Pm,n ： the set of cells in Gm that are likely positions of host m given
its CLS entry about host n . Host m,n are one-hop neighbors.
Host m learns about n indirectly
),( ,,u
nml
nm dd
CLS Voting Each peer performs the following steps ：1. Initialize the cells of the grid Gm (value of each cell is 0)
2. For each CLS entry about a host (eg. host n) with known/computed location
(1) Compute Pm,n
(2) accumulate votes ：
3. Set of cells with maximal values defines possible position4. If there are enough votes and the precision is acceptable
• Report the centroid of the set as the host position• The host transforms the coordinates of this centroid to coordinates of
the global coordinate system.5. Otherwise go to step 2
0),(,),( yxvGyx m
knm wyxvyxvPyx ),(),(,),( ,
Two Thresholds
ST (voting threshold ) ： The number of votes in each cell of the potential solution must be above ST.
LECT (local error control threshold ) ： The size of the grid region that contains the potential solution (i.e. number of cells with maximal value) must be below LECT.
Grid for host uCorresponds to the terrain
A cell is a possible position
The higher the value, the more hosts it is likely position of the host
Rc
Host u with unknown positionPeers A, B, and C have positioned themselves
Host A positive votes
Example of voting process (at host u)
Host B positive votesHost C negative votes
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Simulation Testbed
100x100 square units in size Randomly placed nodes in the terrain Location & range errors as percentages of
the transmission range
Impact of range error
10% of hosts landmarks, average connectivity degree 10
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
Location error (%R)
Perc
enta
ge o
f hos
ts w
ith c
ompu
ted
loca
tion
10% Range Error
20% Range Error
30% Range Error
Impact of range error10% landmarks and average connectivity degree of 12
CLS (“Voting scheme”) vs. “Hop-TERRAIN” and “with Refinement” for various connectivity degrees and landmark %. The range error is 5%.
CLS performs worse fornetworks with low degree of connectivity or few landmarks
Extension of CLS using signal map of the environment
Take advantage of the IEEE 802.11 infrastructure APs act as landmarks Training and measurement phase Each position c in the terrain is associated with mean (SM[c].avg),
max (SM[c].max), min (SM[c].min) signal strength received from APs
For each cell, cell c accumulates a vote from AP i, if
si: measured signal value
1.0SMi[c].min - SMi[c].max
[c].avgSM– s ii
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Location error (m)
Pe
rce
nta
ge
of
ho
sts
wit
h c
om
pu
ted
lo
ca
tio
n
50x50 no CLS
100x100 no CLS
200x200 no CLS
100x100 with CLS (20% range error)
no-CLS: only landmarks vote no voting from non-landmarks
Extended CLS with two APs-landmarks
Extended CLS with two APsImpact of range error
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Location error (m)
Perc
en
tag
e o
f h
osts
wit
h c
om
pu
ted
locati
on
10%
20%
30%
50%
Impact of size grid size
0
10
20
30
40
50
60
70
0 1 2 3 4
Location error (m)
Pe
rce
nta
ge
of
ho
sts
wit
h c
om
pu
ted
loc
ati
on
50x50
100x100
200x200
Extended CLS: Impact of ranging error with 2 AP and 3 landmarks
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Location error (m)
Pe
rce
nta
ge
of
ho
sts
wit
h c
om
pu
ted
lo
ca
tio
n
10%
20%
30%
50%
Outline
Introduction CLS (Cooperative Location-Sensing) System
Grid-representation Communication protocol Voting process
Performance analysis CLS Extension of CLS using signal map
Conclusions
Conclusions (1/2)
Voting from peers and signal map have substantial impact
When signal map is available, a few additional landmarks do not have dramatic impact
100x100 grid size is sufficient CLS thresholds should be tuned based on
density of hosts and landmarks, and range error
Conclusions(2/2)
Nice scaling properties Robust to tolerate network failures,
disconnections, delays due to host mobility Distributed and centralized architecture No need for extensive training and specialized
hardware