6.964 Pervasive Computing ContextAware Networking

1
6.964 Pervasive ComputingContext-Aware Networking

• Stephen J. Garland
• October 11, 2001
• MIT Laboratory for Computer Science
• Networks Mobile Systems
• http//nms.lcs.mit.edu/

2
Cricket Indoor Location System

• Support for mobile, indoor applications
• Location-aware scenarios
• Active maps
• Resource discovery and interaction
• Way-finding and navigation
• Stream redirection

3
Where am I?(Active Map)
4
Whats near me? Find me a (Resource Discovery)
Location by intent Print map on a color
printer. System response Locates nearby free
color printer Sends data there Tells you
where it is
5
Whats over there?(Interaction)
Viewfinder Point-and-use interface
6
How do I get to Haris office?How do I get to
Compaqs booth at Comdex?(Navigation)
7
Cricket Functionality

• What space am I in?
• Application-dependent notion
• Room 510, NE-43, MIT, Compaqs booth,
• Basic Cricket
• What are my (x,y,z) coordinates?
• Relative to arbitrary inertial frame
• Cricket GPS
• Which way am I pointing?
• Relative to fixed point in inertial frame
• Cricket compass

8
Design Goals for Cricket

• Operates well indoors
• Detects boundaries well
• Preserves user privacy (doesnt track users)
• Facilitates innovation in applications
• Operates with low energy
• Easy to deploy and administer

9
Determining Distance
Beacon
Ultrasound (pulse)
Listener

• Beacon transmits simultaneous RF, ultrasound
• RF carries location data, ultrasound is narrow
  pulse

• Listener measures time between reception
• Velocity of ultrasound 1/ms ltlt velocity of RF

10
ProblemDetermining Space from Distance
Room A
Room B
I am at B
11
Solution Beacon Placement
Room A
Room B
x
x
I am at A

• Position beacons to detect the boundary
• Multiple beacons per space are possible

12
Problem Multiple Beacons
Beacon A
Beacon B
Incorrect distance
t
Listener
RF B
RF A
US B
US A

• Hard to correlate RF and ultrasound signals
• Beacon transmissions are uncoordinated
• Ultrasonic signals reflect heavily
• Ultrasonic signals are pulses (no data)
• Can lead to incorrect distance estimates

13
Solutions Avoiding Interference

• Limit stray signal interference
• Envelop all ultrasonic signals with RF
• Carrier-sense, randomized transmission
• Reduce chances of concurrent beaconing
• Listener inference algorithm
• Use distance samples to estimate location

14
Limiting Stray Signal Interference

RF B
US B
RF A
US A
t

• Envelop ultrasound by RF
• Stray ultrasound causes RF signals to collide
• Listener does a block parity error check
• To detect collision and discard reading

15
Reducing Concurrent Beaconing

• Randomize beacon transmissions
• do while (true)
• pick r UniformT1, T2
• delay(r)
• xmit_beacon(RF,US)
• Optimal T1, T2 can be calculated analytically
• Trade-off latency against collision probability
• Erroneous estimates do not repeat

16
Estimation AlgorithmWindowed MinMode
A
Frequency
B
5
Distance (feet)
5
10
17
Determining Coordinates
Beacons on ceiling at known coordinates
(x,y,z)
Four equations, four unknowns Velocity of sound
varies with temperature, humidity Can be
eliminated (or calculated!)
18
Determining Orientation
B
Beacons on ceiling
Orientation relative to B on horizontal plane
?
Cricket listener with compass hardware
Mobile device (parallel to horizontal plane)
19
Trigonometry 101
Beacon
Idea Use multiple ultrasonic sensors and
estimate differential distances
sin ? (d2 - d1) / sqrt (1 - z2/d2) where d
(d1d2)/2
Two terms need to be estimated 1. d2 d1 2.
z/d
Heading
20
Differential Distance Estimation

• Problem for reasonable values of parameters (d,
  z), (d2 - d1) must have 5mm accuracy
• Well beyond all current technologies!

Estimate phase difference between ultrasonic
waveforms!
21
Making This Idea Work
Beacon
d2
d3
d1
L23
L12
t
3l/2
4l/2
Estimate 2 phase differences to estimate
d2-d1 Can do this when L12 and L23 are relatively
prime multiples of l/2
22
System Administration

• Authentication (password) for configuration
• Currently, coordinates entered manually
• algorithm to deduce them from other beacons
• Database with Web front-end
• Centrally managed relational DBMS
• Challenge queries that dont divulge device
  location, but yet are powerful

23
Cricket v1 Prototype
RF module (rcv)
RF module (xmit)
Ultrasonic sensor
Ultrasonic sensor
RF antenna
Listener
Beacon
Atmel processor
RS232 i/f
Host software libraries in Java Linux daemon
(in C) for Oxygen BackPaq handhelds Several apps
24
Deployment Beacon Placement Considerations

• Must support correct inference of space
• Boundaries between spaces need to be detected
• Must support coordinate estimation
• No 4 beacons on same circle on a ceiling
• At least one beacon must have ? lt 40 degrees
• sin ? (d2 - d1) / sqrt (1 - z2/d2), so Dq
  goes as tan q

25
Deployment
26
Some Results

• Linear distances to within 6cm precision
• Spatial resolution of about 30cm
• Coordinate estimation to within 6cm in each
  dimension
• Orientation to within 3-5 degrees
• when angle to some beacon lt 45 degrees
• Several applications (built, or being built)
• Stream redirection, active maps, Viewfinder,
  Wayfinder
• People-locater, smart meeting notifier,
• Probably no single killer app, but a whole suite
  of apps that might change the way we do things