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  • pg 1
									The Cricket Location-Support
  N.B. Priantha, A. Chakraborty, H.
     Presented by Lewis Girod
• Cricket “listeners” use acoustic time-of-
  flight measurements to determine the
  nearest beacon
• Objectives:
  – Granularity of “a few square feet”
  – Different regions distinguishable
  – Low cost, both H/W and installation
• In general, the system seems to work
                  Related Work
• Niru’s RF beaconing
• “RADAR” from Microsoft Research
   – centralized computation, not clear how well it works
• Active Bat from ORL
• The Cricket work differs:
   – It does not attempt to calculate any kind of absolute
     position, but only the nearest beacon
   – Interest in demonstrating that nearest beacon is “good
     enough” for application requirements
      • Knowing what room you are in, which part of room
               Design Goals
• Privacy: listeners are passive
• Decentralized administration: owner of space
  installs and configures beacons as needed
• Network heterogeneity: Cricket not coupled to
  any networking technology
• Cost: under $10 per unit
• Granularity: spatial regions can be as small as a
  “few square feet”. This will really depend on
  beacon placement
       Metrics & Terminology
• Precision: how well can a listener detect a
  boundary (rate of correct detection)
• Granularity: The smallest possible size for a
  detectable geographic region
• Objective is near 100% precision with a
  granularity of a few square feet
         System Architecture
• Beacons placed near ceiling where they
  have better chances of LOS with listeners
• Beacons delimit boundaries:
  – On either side of doorway to delimit two
    adjacent rooms
  – On either side of non-physical “region
  – In corners and near walls (physical boundaries)
   Operation of Ranging System
• Time of flight measurement
   – From beginning of RF transmission to ultrasound (US)
   – RF transmission is as long as the maximum US
     propagation time; “matching” US pulse must arrive
     during RF transmission. Avoids need to code pulses.
• Beacons are not coordinated
   – Interference prevented using randomized delays
     between beaconing
           Interference Problems
• Mnemonics:                             RF-A
  – RF-A
  – US-A
  – US-RA                                       US-RA
    (reflection of US-A)
  – RF-I                        RF-I
    (interfering RF signal)   US-I
  – US-I
  – US-RI                     True TOF
         Case 1: RF-A / US-RA

• Standard NLOS problem:
   – the direct path is blocked, longer reflected path detected
• Cricket uses placement of beacons to avoid this.
   – Emitters are on ceiling, pointed downwards towards
     floor and “into” their region: towards the probable
     location of listeners in their region
   – Larger number of beacons also mitigates this problem:
     beacon pointing downwards will have LOS, and will be
     shorter path than a reflection from farther away.
Case 2: RF-A / US-I, RF-I / US-A
                               I Sends early

    2a.       RF-I
                                 A Sends later

• Results of RF interference
   – 2a. RF-A transmission drowns out earlier but farther-away RF-I;
     US-I detected earlier
   – 2b. RF-I transmission from beyond a solid partition drowns out
     RF-A, but US-A detected.
• Solutions:
   – Random inter-beacon delays reduce likelihood of persistent errors
   – If RF TX range is larger than US TX range, case 2a is less likely..
     more likely the RF messages will collide and neither will be
       Case 3: RF-A / US-RI
• Stray reflected US pulses may cause
  incorrect readings.
• Solution:
  – Limit the beacon density and rely on random
    inter-beacon delays to avoid collisions.
  – Currently, Cricket system is engineered to have
    at most 6 beacons within range of each other at
    any given point
       Overcoming Interference
• Statistical approaches for filtering out bad data
   – Majority: (strawman)
      • select the beacon heard most frequently regardless of distance
   – MinMean:
      • For each beacon, calculate the avg. distance, then select beacon
        with minimum value.
      • Problem: multipath causes modal behavior, this algorithm
        ignores that information, and averages across modes
   – MinMode:
      • For each beacon, calculate the mode of last n samples, then
        select beacon with minimum mode.
      • Robust to stray signals
                      Deployment issues

      Problem arising from directionality
                                            (top view) Using directionality to enable “virtual boundaries”

• Deployment critical to making Cricket work
   – US emitters are highly directional, which can easily
     lead to problems with reflections
   – Instead, Cricket exploits directionality by arranging
     emitters at boundary of region, facing in downwards
     from ceiling.
       • Each region must mark the border with one of its own beacons,
         separated by at least 4 feet
       • This requirement is needed because the beacons are on the
         ceiling; so the relative distances that must be distinguished are
         too close unless the beacons are separated.
        Performance Analysis
                                            4 feet

                                                     6 feet

• Boundary detection
  – Two beacons on ceiling, 4 feet apart
  – Listener moves outwards from center of two
    beacons; distances measured at 0.5 foot
  – Region membership is detected correctly after
    the listener is more than 1 foot into the region
     Robustness to Interference
• Static performance with 2 listeners and 5 beacons
   – Experimental configuration
      • One listener had two nearby pure-RF interferers
      • The other was nearby a boundary marked by 3 beacons
   – Interference detected in less than 1% of samples.
      • Caveat: the transmission rate was not given
          – This behavior is probably a direct result of the randomized
            transmission schedule
          – What is the tradeoff between beacon density, beaconing rate (and
            therefore response to dynamics), and interference rate?
   – Majority performed poorly; minMean and minMode
     performed perfectly, but indistinguishably
      • Very little interference for the statistics to deal with
       Tests Involving Mobility
• In this experiment, a listener is moved through the
   – Stop at the first time a new region is detected
   – Take a series of samples at that point
   – Continue moving
• Tests performed very near boundaries
   – Good test: ambiguity should be highest
   – Results show that if several samples are taken the
     correct answer is determined with high probability
• Cricket works quite well at what it does
• Location support, not location tracking: eliminates privacy
• Caveats:
   – Incompatible with ad-hoc beacon placement
   – Does not solve problem of fine-granularity location.. at best within
     2-4 foot radius
       • Tradeoff between granularity of location and interference from
         neighboring beacons (they had a max of 6 beacons in range)
       • May mean it works best indoors in confined spaces?
   – Does not attempt to interpolate coordinate system between
     beacons, simply presents name of closest beacon.

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