Dynamic Clustering for Acoustic Target Tracking in Wireless Sensor Networks Wei-Peng Chen*, Jennifer C. Hou and Lui Sha Department of Computer Science University of Illinois at Urbana-Champaign IEEE Transactions on Mobile Computing 2004

Outlines  Problem Formulation  Background knowledge  Protocol skeleton  Simulation results  Conclusions

Problems for Static Clusters  Not robust in terms of fault tolerance If a CH dies of power depletion, all the sensors in the cluster render useless. One or more sensors die, a cluster may not have sufficient sensors to carry out its tracking task.  Cannot share data in different clusters incurs large estimation error as target is on several cluster boundaries

REQ Dynamic Clustering – Follow the Target High-end sensor (Cluster head) Low-end sensor

REQ Dynamic Clustering – Follow the Target High-end sensor (Cluster head) Low-end sensor

Cluster Architecture for Acoustic target Tracking  Hierarchical sensor network – A cluster consists of: A cluster head (CH): high-end sensor Low-end sensor  Tracking steps within a cluster 1. CH: sound (energy) detection 2. CH: classification 3. CH: broadcast REQ (energy + signature) 4. Sensors: matching and reply energy 5. CH: localization 6. CH: report the results to a sink

Background knowledge(1/) Energy Based Acoustic Localization  Magnitude of signal decays with propagation distance exponentially log of magnitude log of distance (inch)

Goals  (I1) Only one CH (preferably closest to the target) is active  (I2) Only a sufficient number of sensors respond to determine the target location when receiving REQ  (I3) REQ, REP and report packets do not incur collisions

Protocol Skeleton 1. Distance Calibration and Tabulation 2. Cluster Head Volunteering 3. Sensor Replying

Voronoi diagram d1d1 d2d2 d3d3 d4d4.... d High-end sensor Back-off time Max & Min back-off time Uniform distribution High-end sensor (Cluster head) Cluster Head Volunteering CH i Response table d: estimate of the distance from the target to itself 

d (x,y)

Voronoi diagram Sensor Replying SjSj Low-end sensor High-end sensor.... Broadcast Energy ( r i ) + Signature packet When the time D’ expires, the sensor reply only if (1)The signal strength it detected is larger than that overheard reply packets (2)It is one of the Voronoi neighbors of the sensor that reports the largest signal stregth Response table

Construction of Response Table for Sensors  Sensor S j determines Pr(j|r i  j ) : prob. of target closer to S j than any other sensors or CHs  Given r i  j, the ratio of energy from CH i to its own energy, the possible location for the target is a circle  Use the same algorithm described above to determine Pr(j|r i  j )

Two phase broadcast mechanism  1st: Energy(short) 2nd: Signature(long)  During the back-off period, if a CH overhears a energy packet with larger energy or a signature packet, it cancels its transmission; otherwise, ignores the overheard packet  To reduce as many potential competitor as possible in the first phase  To avoid long signature packet get corrupted

Simulation Scenario  Using SensorSim from UCLA (built on ns-2)  36 CHs and 288 sensors in 180*180m 2 field  One sink is at (0,0)  Performance comparison: Static cluster Full-fledged version of proposed approach  2 phases broadcast mechanism  Back-off based on response table V.1 : One phase and no response table V.2 : No response table

Simulation Results-Random error(m) noise =40 (SNR=10),speed=20 delay(s) Target speed fullv.2v.1 error(m)delay(s) Static cluster