CMPE 257 Spring CMPE 257: Wireless and Mobile Networking Spring 2005 Location management

CMPE 257 Spring Announcements Homework #2 due tomorrow by midnight. Stay tuned for homework #3. Class evaluations on Tuesday, Need campus volunteer. Final exam on Thu, June 2. In class, closed books/notes. Project presentations on June 9 th, 4-7pm.

CMPE 257 Spring Today Finish reliable multipoint e2e. Location management.

CMPE 257 Spring Location Management

CMPE 257 Spring Why is location management needed? In wired networks, hosts don’t move. Constant association between host (id, address) and its location. In mobile wireless networks, hosts can move. Host id/address no longer provides location information. Need location tracking mechanism to deliver information destined to host.

CMPE 257 Spring Location for the Active Office [Ward97] Indoor sensor system that tracks location of: people (active badge), equipment (equipment tags), etc. Requirements: accurate (within 15cm), 3 dimensions, scalable (number of objects locatable, area covered), cost. RF communication.

CMPE 257 Spring System components Transmitters attached to every locatable object. Matrix of receiver elements in all rooms where objects are to be tracked. Controller which polls one mobile object at a time.

CMPE 257 Spring Operation Periodically, mobile node is polled. Polled mobile broadcasts signal. Controller synchronizes receivers, who listen for some time to detect the peak of mobile’s transmission. Controller polls receivers for the measured time interval between the sync signal and the signal peak (if any).

CMPE 257 Spring Distance computation Time measured by receiver composed of: time to transmit the polling signal (from controller to mobile)+time to transmit pulse (function of distance being calculated)+processing time. Distance between mobile and receiver calculated. Empirically computed speed of sound in the room and service times.

CMPE 257 Spring Position calculation Triangulation using 4 receivers to determine a point in 3 dimensional space as estimate of position. In this particular set up, since all receivers are in the ceiling, only 3 distances required. Extra reported distances can be used for higher accuracy.

CMPE 257 Spring Evaluation Experiments with prototype show 95% of readings within 14cm accuracy. Even better accuracy for averaged readings. Addresses limit number of trackable objects. Large number of receivers and ultrasound nature of transmission from mobile proved to pay off regarding accuracy. Power savings mode minimizes maintenance. Low interference levels from office equipment.

CMPE 257 Spring Testbed Single floor (10500 sq. ft.) with 50+ rooms. 3 base stations covering entire floor. Lucent WaveLAN RF technology. 2 Mbps. 1-2 ms one-way delay. 200m and 25m range (open/close environments).

CMPE 257 Spring RADAR [Bahl et al.] Similar to the [Ward97] paper. Provide indoor location service. RF. Use received signal strength & triangulation. Low cost. Off-the-shelf hardware.

CMPE 257 Spring Testbed Single floor (10500 sq. ft.) with 50+ rooms. 3 base stations covering entire floor. Lucent WaveLAN RF technology. 2 Mbps. 1-2 ms one-way delay. 200m and 25m range (open/close environments).

CMPE 257 Spring Operation Off-line and real-time functions. Off-line: derive and validate accurate signal propagation models. Real-time: user location.

CMPE 257 Spring What is being collected? Signal strength (in dBm). s (Watts) = 10*log 10 (s/.001) (dBm) Signal-to-noise ratio (SNR) (in dB). SNR (dB) = 10*log 10 (s/n) (dB). For each received packet, SS and SNR are recorded.

CMPE 257 Spring Data collection process Mobile broadcasts beacons periodically. Base stations record SS and SNR. Different than the ORL system. Scalability? Path asymmetry.

CMPE 257 Spring More on data collection All clocks synchronized. Mobile broadcasts packets (4 pkt/sec). BS records (t, bs, ss). Off-line: mobile also provides its location by using a floor map. Orientation is important (LoS, obstruction, etc.). In off-line phase, collected SS in all 4 directions at 70 different floor locations. For each (x, y, d), 20 ss samples. d is direction: N, S, E, W.

CMPE 257 Spring Processing data Off-line data used to build signal propagation model. Validation of assumption that from signal strength location can be inferred. How is location determined? Signal strengths from 3 BSs. Compare to floor layout/energy map. Pick location that minimizes (Euclidian) distance between measured and recorded set of SS’s.

CMPE 257 Spring Results “Empirical” method performs better than random and strongest BS. Error approx. size of a room… Taking “k” nearest neighbors shows some improvement. Analysis of impact orientation, number of data points, and number of samples. User tracking.

CMPE 257 Spring Radio propagation model Model of indoor signal propagation. No need for empirical data. Indoor propagation: Reflection, diffraction, scattering. Multipath effect. Receiver gets signal from multiple paths. Distorted signal. Challenges: dependency on layout, material, obstacles (number and type), etc.

CMPE 257 Spring Radio propagation model (cont’d) Adaptation of existing model to single floor. Consider effects of walls. Signal strength varies with distance AND number (and type) of obstacles. Empirical characterization of wall attenuation. Use (corrected) empirical data and linear regression to determine other parameters. Similar values for different BSs (location, surroundings, etc.) Less accurate results than empirical model, but more practical.

CMPE 257 Spring Localization in Sensor Networks [Bulusu01]

CMPE 257 Spring What are sensor networks? Large number of small, low-power devices (wirelessly) connected. Applications: Monitoring, surveillance, tracking, etc. Typically ad-hoc deployable, unattended operation. Data-centric (instead of node-centric).

CMPE 257 Spring Localization Estimation of physical position (coordinates). Localization. Why is this important? Data usually identified by location (temperature of a given area, target tracking, signal processing applications). No a priori knowledge of location. GPS?

CMPE 257 Spring Approaches Multilateration: nodes measure enough pair- wise distance estimates. Combination of radio (for time reference) and acoustic (time of flight for distance) signals. Proximity-based: “beacon” nodes periodically broadcast position; nearby nodes then estimate their position. Iterative multilateration: beacon information propagated multi-hop. Beacon density sparse in some areas.

CMPE 257 Spring Self-configuring localized algorithms Adjust to current conditions (load, environment, etc). Localized algorithms: distributed computation where communication is restricted to given neighborhood. Node density. Multiple modalities. Environmental adaptation.

CMPE 257 Spring Density Trade-off: sparse vs. dense networks. Controlling density: transmit power. Higher power makes networks more dense. Multiple power levels for tiered structure. Problem: right balance between number of beacons (for coverage) and good localization. Power conservation. Interference.

CMPE 257 Spring Sensor modalities Use different modalities (acoustic sensors, cameras, etc) to overcome environmental unpredictability. Example: acoustic sensors and acoustic/visual sensors. Acoustic sensing prefers LoS. Cameras can help by determining LoS sensors.

CMPE 257 Spring Adapting to the environment Not only to dynamics but also to fixed characteristics (e.g., obstructions, terrain, etc.). Example: boundary beacon can extend its lifetime by cutting down its duty cycle. Example: adapting to the dynamics of wireless channel using learning algorithms.