The Cricket Location-Support System N.B. Priantha, A

1
The Cricket Location-Support System

• N.B. Priantha, A. Chakraborty, H. Balakrishnan
• Presented by Lewis Girod

2
Overview

• 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

3
Related Work

• Nirus 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

4
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

5
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

6
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
  boundaries
• In corners and near walls (physical boundaries)

7
Operation of Ranging System

• Time of flight measurement
• From beginning of RF transmission to ultrasound
  (US) pulse.
• 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

8
Interference Problems

• Mnemonics
• RF-A
• US-A
• US-RA
• (reflection of US-A)
• RF-I
• (interfering RF signal)
• US-I
• US-RI

RF-A
US-A
US-RA
RF-I
US-I
True TOF
9
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.

10
Case 2 RF-A / US-I, RF-I / US-A
I Sends early
RF-A
I
2a.
2b.
US-A
RF-I
I
A Sends later
US-I

• 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 received.

11
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

12
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

13
Deployment issues

• 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.

14
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 intervals
• Region membership is detected correctly after the
  listener is more than 1 foot into the region

15
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

16
Tests Involving Mobility

• In this experiment, a listener is moved through
  the environment
• 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

17
Conclusions

• Cricket works quite well at what it does
• Location support, not location tracking
  eliminates privacy concerns
• 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.