extending the reach of gps-assisted femtocell synchronization and localization through ... ·...
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Extending the Reach of GPS-assisted Femtocell
Synchronization and Localization through Tightly-
Coupled Opportunistic Navigation
Ken Pesyna, Kyle Wesson, Robert Heath Jr., Todd Humphreys
Presentation at Globecom 2011 | December 5, 2011
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Why Synchronization & Localization?
Ensure reliable handover
Reduce interference
Operate within licensed spectrum
Locate emergency calls
Synchronization and Localization are
key requirements for femtocells
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Challenges of Satellite-based Synchronization
Femtocells placed indoors
GPS signals weak: -155 dBW
Up to 45 dB penetration loss
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Time and Frequency Requirements
Minimum synchronization requirements [Rakon]:
CDMA2000 GSM
WiMAX
LTE WCDMA
TD-SCDMA
0
100
200
300
400
500
600
0 2 4 6 8 10 12
Fre
quency R
equir
em
ent
(ppb)
Time Requirement (µs)
Requirements by Cellular Standard
These are very stringent requirements!
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Spectrum licensing & operator control
Emergency caller location identification (E911)
Location Requirements
10m
20km
E911
Spectrum
Licensing
Challenging!
Big Issue: E911
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CDMA2000 GSM
WiMAX
LTE WCDMA
TD-SCDMA
0
100
200
300
400
500
600
0 2 4 6 8 10 12
Fre
qu
en
cy R
equ
ire
me
nt (p
pb
)
Time Requirement (us)
Prior Work
Precision time protocol (PTP)
[Eidson02]
Cellular network listen [Edwards08]
GPS-based solutions
[Lewandowski99, Smith09]
20km
50 - 300m
5m
1-20 km
10m
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Challenges of GPS
High signal sensitivity is required to
acquire GPS signals indoors
5 dB-Hz to cover 90% of residences
Sensitivity attainable today:
State-of-the art via EGPS/AGPS is 14
dB-Hz [RoDuJaGra08]
Goal: Close this 9 dB gap
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Closing the 9 dB gap: Requirements
Coherently integrate GPS signal over extended interval
1. Local oscillator drifts
This requires a more stable oscillator
2. # of time-frequency search cells explodes
Increases probability of false alarm
The search space must be reduced
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Use ambient radio “signals of opportunity”:
Stabilize femtocell’s local oscillator
Narrow the GPS search space
Proposed Solution: Tightly-Coupled
Opportunistic Navigation
Cellular HDTV
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Final
Accumulation
Tightly-Coupled Opportunistic Navigation
Estimator
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Benefits of Proposed TCON
1. Frequency stability transfer
Correct its local clock variations to allow extended integration
2. Search space contraint
Aiding of precise time and frequency information
Decreases time & frequency uncertainty
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Ambient Signal Analysis
Analyzed ambient CDMA signals
Extremely stable and synchronous
Signals remain stable beyond 9 seconds
Time synchronized to 4 µs (2 of 3)
Frequency accurate to 0.3 Hz
Mean-squared Coherence vs Time
9.5 s 20 s
100+ s
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Field Demonstration
Cellular CDMA signals recorded in
Austin, TX
TCON used to estimate the local clock
variations using only CDMA signals
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Conclusions
Synchronization is required for femtocell operation
Location is required to meet FCC regulations
Existing approaches may not meet requirements
Propose TCON to exploit other available signals
Increases the GPS-sensitivity of the femtocell GPS meets the syncrhonization requirements
Allows femtocells to be located further indoors
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Conclusions
Requirements to acquire a 5 dB-Hz GPS signal: Coherent integration time of 7 seconds
Time certainty to within 6 µs of GPS time
Frequency certainty to within 0.5 Hz
TCON achieves the needed gain in sensitivity
Femtocells synchronized indoors using GPS
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Questions?