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 idPositionTimeRangeWeightDistanceVote A(x A,y A )tntn RARA wAwA (d u,A - e, d u,A + e)Positive C(x C,y C )tktk RCRC wCwC (R C,  )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 position Peers A, B, and C have positioned themselves

Parameters G m ： the grid that a host maintains during a run R n ： the wireless range of host n. w n ： the voting weight ： the estimated distance interval between host m and n v (x,y) ： the value of the cell. P m,n ： the set of cells in G m 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

CLS Voting Each peer performs the following steps ： 1. Initialize the cells of the grid G m (value of each cell is 0) 2. For each CLS entry about a host (eg. host n ) with known/computed location (1) Compute P m,n (2) accumulate votes ： 3. Set of cells with maximal values defines possible position 4. 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

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 u Corresponds to the terrain A cell is a possible position The higher the value, the more hosts it is likely position of the host RcRc Host u with unknown position Peers 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% 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 for networks with low degree of connectivity or few landmarks

Extension of CLS using signal map of the environment Take advantage of the IEEE 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 s i : measured signal value

no-CLS: only landmarks vote no voting from non-landmarks Extended CLS with two APs-landmarks

Extended CLS with two APs

Extended CLS: Impact of ranging error with 2 AP and 3 landmarks

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