energy-efficient rate-adaptive gps-based positioning for smartphones
DESCRIPTION
Energy-Efficient Rate-Adaptive GPS-based Positioning for Smartphones. Jeongyeup Paek , Joongheon Kim, Ramesh Govindan CENS Talk April 30, 2010. Problem. Many emerging smartphone applications require position information to provide location-based or context aware services. - PowerPoint PPT PresentationTRANSCRIPT
Energy-Efficient Rate-Adaptive GPS-based Positioning for Smartphones
Jeongyeup Paek, Joongheon Kim, Ramesh Govindan
CENS TalkApril 30, 2010
2
Problem
• Many emerging smartphone applications require position information to provide location-based or context aware services.
• GPS is often preferred over GSM/WiFi based methods.
• But, GPS is extremely power hungry!– Can drain phone battery in few hours.
WPS
GSM
GPS
Average ErrorGPS: 23.2mWPS: 36.8mGSM: 313.6m
WPSGPS
GSM
Average ErrorGPS: 8.8mWPS: 32.44mGSM: 176.7m
0 50 100 150 200 250 300 350 400 450 5000
0.2
0.4
0.6
0.8
1 poweravg
Time (Seconds)
Pow
er (W
att)
Off (0.06) On (0.44)
3
GPS (In)accuracy
Actual Path taken
Incorrect GPS Path
Never went here
A
B
C
GPS may provide less accurate positioning in urban areas, especially for pedestrian use
Then… can we sacrifice a little accuracy in exchange for significant reduction in energy usage?
Dis
tanc
e (m
eter
)
0
50
100
150
200
250
300 Average Distance ErrorMaximum Distance Error
Samples (70 locations)
No received GPS signals
Relatively less clear view of the sky
4
Periodic Duty-Cycling• There exist uncertainty
– Key challenge is to decide T
• Accuracy vs. Energy trade-off exists
• May introduce significant uncertainty
T
Uncertainty(distance)
Energy
….
5
RAPS : Rate-Adaptive Positioning System• An energy-efficient positioning system that adaptively duty-cycle GPS
only as often as necessary to achieve required accuracy based on user mobility and environment
• Design Goal– Reduce the amount of energy spent by the positioning system while still
providing sufficiently accurate position information– Trade-off position accuracy for reduced energy
• Challenge– Determine when and when not to turn on GPS efficiently using the sensors
and information available on a smartphone
6
RAPS Components• Movement Detection
– Use duty-cycled accelerometer with onset detection algorithm to efficiently measure the activity ratio of the user
• Velocity Estimation– Use space-time history of the past user movements along with
their associated activity ratio to estimate current user velocity
• Unavailability Detection– Use celltower-RSS blacklisting to detect GPS unavailability (e.g.
indoors) and avoid turning on GPS in these places
• Position Synchronization– Use Bluetooth-based position synchronization to reduce
position uncertainty among neighboring devices
When to turn on GPS
When NOT to turn on GPS
7
Activity Detection• Use accelerometer to detect user motion
– Binary sensor to detect non-movement
– Measure activity ratio• Onset detector for identifying activity
– Duty-cycle it for energy efficiency• 5 min accelerometer consumes
more energy than 1 min GPS0 50 100 150
0.5
1
1.5
Time (sec)
Acc
ele
ratio
n (
g)
Acceleration
Activity DetectedSignal Envelope
Activity
0 50 100 150 2000
0.1
0.2
0.3
0.4poweravg
Time (seconds)
Pow
er (W
att)
Off (0.062) On (0.141)
0 10 20 30 40 500
5
10
15
20
Duty Cycle (%)
Err
or
(%)
Operating point: 12.5%
8
Velocity Estimation• Use history of user positions
– Associate average velocity and activity ratio to particular space and time– Use these information to estimate current user velocity– Using this velocity, calculate uncertainty and decide when to turn on GPS
A
BD
C
9
Celltower Data for Movement Detection?• Celltower and RSS data cannot reliably measure user movement
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
Distance between cellID changeMax. distance within same Cel-lID
Distance (meters)
Cum
ulati
ve F
racti
on (%
)
0 5 10 15 20 25 300
100
200
300Max. DistanceAvg. DistanceMin. Distance
RSSI difference (dbm)
Dis
tanc
e (m
eter
)
10
Celltower-RSS Blacklisting
• However, it can detect GPS unavailability– Signatures exist for indoor places that you go often
70 75 80 85 90 95 100 105 110 115 1200
20
40
60
80
100 cell ID 1cell ID 2
WCMDA Celltower Signal Strength (-dbm)
GPS
Ava
ilabi
lity
(%)
Good BadVariable
Turn on GPS only when available!
11
Bluetooth Position Synchronization• Use Bluetooth to synchronize position
information with neighboring nodes– Cheaper than GPS– Little uncertainty
• Short communication range (~10m)
– Bluetooth is widely being used
Save energy by lowering overall uncertainty and reducing the number of GPS activations
12
Bluetooth Position Synchronization• Use Bluetooth to synchronize position
information with neighboring nodes– Cheaper than GPS– Little uncertainty
• Short communication range (~10m)
– Bluetooth is widely being used
• Saves energy by 43% in 2-node example– TX costs ~3.07 Joule, RX costs ~1.58 Joule– GPS activation for 60sec costs ~22 Joule– (22 * 1 + 3.07 + 1.58) / (22 * 2)
• More the merrier!– Energy cost is amortized over the number
of nodes in neighborhood– For 5 nodes, 74% reduction in energy
0 50 100 150 2000
0.2
0.4
0.6
0.8
1
poweravg
Time (seconds)Po
wer
(Watt
)
0 50 100 150 2000
0.2
0.4
0.6
0.8
1
poweravg
Time (seconds)
Pow
er (W
att)
Off
Off
Bluetooth Master
Bluetooth Slave
On
On Listen & connect
Device Discovery
RX
TX
Listen
Listen
Off
Off
13
Evaluation
• Benefits of RAPS– Energy savings achieved by RAPS – Contribution of individual components
• Comparison to periodic GPS strategy
• Flexibility – Integration with WPS
• Pervasiveness of GPS errors– GPS vs. AGPS– Different platforms
14
Benefits of RAPS
LabClass
LibraryLunch
Shopping
Home
USC
~3 miles
0.4 miles
Class
History Accel.Cell-RSS Blacklist
BPS
RAPS (2)
RAPS-B
RAPS-BC
RAPS-BCA
Always-On Periodic GPS with 20 seconds interval
• Energy savings achieved by RAPS • Contribution of individual components
• Methodology– 6 phones with 5 different schemes– around USC campus area (in & outside buildings)– 34 hours
15
Benefits of RAPS - Lifetime• RAPS’s lifetime is 3.87 times longer than that of Always-On
– Each of its components contribute to this saving
RAPS RAPS-B RAPS-BC RAPS-BCA Always-On (20sec)
0
5
10
15
20
25
30
35
4034:41
31:53
16:42 16:19
8:57Life
time
(hou
rs)
Tested Schemes
3.87 times longer lifetime!
BSP – 10.8%
Blacklist – 59.0%
Accel – 1.5%
History – 28.5%
16
Benefits of RAPS - Reasons
Avg. GPS Activation Interval Expected Avg. Power Usage
(20)
GPS
Inte
rval
(sec
onds
)
Tested Schemes
RAPS RAPS-B RAPS-BC RAPS-BCA Always-On (20sec)
0
100
200
300
400
500
600
700630.9
588.5
259.5
135.4
Estim
ated
Ave
rage
Pow
er (W
)Tested Schemes
RAPS RAPS-B RAPS-BC RAPS-BCA Always-On (20sec)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4 BluetoothAccelerometerGPS
17
Did BPS work?• Contributed 10.8% of the total RAPS lifetime savings
– Defers GPS activation– Bi-directional natural incentive for sharing
BPS EnabledNode 2
BPS EnabledNode 1
GPS Activation:
BPS Communication:
BPS Disabled
18
• Contributed 59% of the total lifetime increase– Significantly increase the average interval between GPS activations– For majority of cell towers, GPS position fixing never fails– For a smaller number of cell towers, mostly those that the user visits often,
GPS failures do occur
Did Celltower-RSS Blacklist work?
0
20
40
60
80
100
0
20
40
60
80
100
120
140
GPS failGPS successsuccess ratio
Observed Cell-towers
Succ
ess
Ratio
(%)
Observation Count
Do not turn on GPS when not available!
19
Comparison to Periodic GPS• RAPS consumes,
– 48% less power compared to periodic GPS with comparable average uncertainty– 22% less power compared to periodic GPS with comparable success ratio
30 90 150 210 270 330 390 450 510 5700
20
40
60
80
100
120
140
160
180
Dis
tanc
e (m
eter
)
Periodic Interval (Seconds)
60 120 180 240 300 360 420 480 540 6000
10
20
30
40
50
60
70
80
90
100
Periodic Interval (Seconds)
Succ
ess
Ratio
(%)
85.8 m
Average Distanceachieved by RAPS
72.2%
Success Ratioachieved by RAPS
1.47 times longer lifetime
1.14 times longer lifetime
20
Is RAPS flexible?• Integration with WPS, a WiFi Positioning System
– Energy cost include WiFi scanning and data communication with database server
– Known to be less accurate than GPS
• Yes!!– Consumes less energy
• Faster position fix and turn off time
– Lower accuracy
Pow
er (W
att)
Time (Seconds)0 20 40 60 80 100 120 140
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8poweravgAvg.
Positioning Interval
Avg. Power Usage
Avg. Uncertainty
RAPS - GPS 465.1 sec 0.064 W 85.8 m
RAPS - WPS 387.3 sec 0.035 W 122.9 m
21
Are GPS Errors Pervasive?
AGPS vs. GPS Four different types of phones
0
20
40
60
80
100
120
140
160
180 GPS with G1 (avg. error: 38.4m)A-GPS with G1 (avg. error: 36.1m)GPS with N95 (avg. error: 20.2m)A-GPS with N95 (avg. error: 19.1m)
Samples (35 Locations)
Dis
tanc
e (m
eter
)
0
20
40
60
80
100
120
140
160
180 G1 (avg. error: 36.1m)N95 (avg. error: 19.1m)MotoDroid (avg. error: 18.2m)WinMobile HTC (avg. error: 13.7m)
Samples (35 Locations)D
ista
nce
(met
er)
Relatively less clear view of the skyRelatively less clear view of the sky
Similar(except G1)
Negligible difference(difference only in FTTF)
22
Conclusion and Future Work
• RAPS is a rate-adaptive positioning system for smartphone applications– GPS is generally less accurate in urban areas, so it suffices to turn on GPS
only as often as necessary to achieve this accuracy– Uses collection of techniques to cleverly determine when to and when
not to turn on GPS– Increases lifetime by factor of 3.8 relative to Always-On GPS
• Future Work– Parameter settings (e.g. accelerometer duty cycling)– Privacy and security for position sharing– User study (variety, consistency, handling behavior, etc.)
Thank you