Slides
A Low Power Asynchronous GPS Baseband Processor Benjamin Z. Tang, Stephen Longfield, Jr., Sunil A. Bhave, Rajit Manohar
Cornell University
05/07/2012 - 1/18
Benjamin Tang
Motivation Augmented reality
Micro robotics navigation
Location-based services
1980s
1990s
2000s
2010s
Decreasing power, but still too high 05/07/2012 - 2/18
Benjamin Tang
FUTURE
Need continuous operation, much lower power
How Does GPS Work? • GPS L1 civil signal Satellite τ3 Receiver
L1 carrier Pseudorandom noise code (PRN) • 1ms repeat period, 1.023MHz • Unique for each satellite
τ1
Navigation data τ2 L1 carrier
PRN code
Navigation data
05/07/2012 - 3/18
Benjamin Tang
GPS satellite transmitted signal
How Does Receiver Know… • Which satellite’s signal was received? Use CDMA
• Where the satellite is? Orbital information in navigation data
• When was the signal transmitted? Navigation data + PRN code phase L1 carrier GPS satellite transmitted signal
PRN code
Navigation data
05/07/2012 - 4/18
Benjamin Tang
GPS Receiver Our focus
GPS RF Frontend
Digital Samples
Medium power
GPS Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband Measurements & Decoded Processing Message
More power-hungry
~20-100mW
> sampling clock, unnecessary power 05/07/2012 - 7/18
Benjamin Tang
Asynchronous GPS Baseband Processor • 6 channels • Selected optimizations • QDI and bundled data
Bundled-data Shared tracking loops QDI Tracking
Digital DigitalSamples Samples
Controls Controls
Accumulators Accumulators Tracking Signal SignalReplica Replica Buffer
Asymmetric acquisition
05/07/2012 - 8/18
Data Data Decode Decode
Benjamin Tang
Measurements Measurements&&Decoded Decoded Message Message
Asymmetric Acquisition Full Acquisition (Other receivers)
Asymmetric Acquisition (Our receiver)
(+) Acquires: satellite ID, code phase offset and Doppler frequency
(-) Acquires: code phase offset, the rest from software
(-) FFT engine and memory or thousands of correlators
(+) Use pre-existing correlators
Full acquisition not needed often. Use asymmetric acquisition scheme. Reduced hardware, Reduced area, Reduced power Digital Samples
Controls
Accumulators Tracking Signal Replica
Data Decode
05/07/2012 - 9/18
Benjamin Tang
Measurements & Decoded Message
Accumulators • • • • •
Operate at input frequency 6 accumulators per channel 3-bit inputs iteratively added to 16-bit sum Only dump output once every 1ms Higher order bits do not switch often
DUMP
IN
3
16 DUMP
Bit=0 Bit=1 Reg
16
OUT
16 05/07/2012 - 10/18
Benjamin Tang
Bit=0
Accumulators • Standard 3-bit accumulator coupled with a 13-bit constant time counter • Concatenate results at DUMP IN
3
16 3 DUMP
Reg
16 3
OUT a
16 3
Carry out MSB
{b,a} b
Counter
OUT
16
13
• Naïve 16-bit accumulator: ~40μW Counter-based accumulator: ~10 μW 05/07/2012 - 11/18
Benjamin Tang
4X less power
Tracking Loops • Defer Tightlyupdates coupled feedback loops Slow Needtracking to provide loops, updates shared before between the next all channels, data sample saves power Fast tracking loops, power hungry
Tracking
Digital Digital Samples Samples
Controls Controls
Accumulators Accumulators Tracking Signal Replica Replica Signal Buffer Data Data Decode Decode
05/07/2012 - 12/18
Benjamin Tang
Measurements Measurements && Decoded Decoded Message Message
Tracking Loops • Frequency Locked Loop (FLL), Phase Locked Loop (PLL) and Delay Locked Loop (DLL) • Computations involve vector magnitude, arctangent, multiplication and division operations. Simplify: Fixed point arithmetic, bundled-data 1 Apply Taylor series small angle approximation: tan Apply modified version of Robertson approximation: A I 2 Q2 max I 14 Q , Q 14 I
Position error increases by ~1m on average
05/07/2012 - 13/18
Benjamin Tang
Receiver Performance Simulations • Transistor-level implementation of our system • Position accuracy simulation 60 seconds of signal from commercial GPS signal simulator No added atmospheric, ionospheric and multipath errors
3D-RMS error
Cornell University
05/07/2012 - 1/18
Benjamin Tang
Motivation Augmented reality
Micro robotics navigation
Location-based services
1980s
1990s
2000s
2010s
Decreasing power, but still too high 05/07/2012 - 2/18
Benjamin Tang
FUTURE
Need continuous operation, much lower power
How Does GPS Work? • GPS L1 civil signal Satellite τ3 Receiver
L1 carrier Pseudorandom noise code (PRN) • 1ms repeat period, 1.023MHz • Unique for each satellite
τ1
Navigation data τ2 L1 carrier
PRN code
Navigation data
05/07/2012 - 3/18
Benjamin Tang
GPS satellite transmitted signal
How Does Receiver Know… • Which satellite’s signal was received? Use CDMA
• Where the satellite is? Orbital information in navigation data
• When was the signal transmitted? Navigation data + PRN code phase L1 carrier GPS satellite transmitted signal
PRN code
Navigation data
05/07/2012 - 4/18
Benjamin Tang
GPS Receiver Our focus
GPS RF Frontend
Digital Samples
Medium power
GPS Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband GPSProcessing Baseband Measurements & Decoded Processing Message
More power-hungry
~20-100mW
> sampling clock, unnecessary power 05/07/2012 - 7/18
Benjamin Tang
Asynchronous GPS Baseband Processor • 6 channels • Selected optimizations • QDI and bundled data
Bundled-data Shared tracking loops QDI Tracking
Digital DigitalSamples Samples
Controls Controls
Accumulators Accumulators Tracking Signal SignalReplica Replica Buffer
Asymmetric acquisition
05/07/2012 - 8/18
Data Data Decode Decode
Benjamin Tang
Measurements Measurements&&Decoded Decoded Message Message
Asymmetric Acquisition Full Acquisition (Other receivers)
Asymmetric Acquisition (Our receiver)
(+) Acquires: satellite ID, code phase offset and Doppler frequency
(-) Acquires: code phase offset, the rest from software
(-) FFT engine and memory or thousands of correlators
(+) Use pre-existing correlators
Full acquisition not needed often. Use asymmetric acquisition scheme. Reduced hardware, Reduced area, Reduced power Digital Samples
Controls
Accumulators Tracking Signal Replica
Data Decode
05/07/2012 - 9/18
Benjamin Tang
Measurements & Decoded Message
Accumulators • • • • •
Operate at input frequency 6 accumulators per channel 3-bit inputs iteratively added to 16-bit sum Only dump output once every 1ms Higher order bits do not switch often
DUMP
IN
3
16 DUMP
Bit=0 Bit=1 Reg
16
OUT
16 05/07/2012 - 10/18
Benjamin Tang
Bit=0
Accumulators • Standard 3-bit accumulator coupled with a 13-bit constant time counter • Concatenate results at DUMP IN
3
16 3 DUMP
Reg
16 3
OUT a
16 3
Carry out MSB
{b,a} b
Counter
OUT
16
13
• Naïve 16-bit accumulator: ~40μW Counter-based accumulator: ~10 μW 05/07/2012 - 11/18
Benjamin Tang
4X less power
Tracking Loops • Defer Tightlyupdates coupled feedback loops Slow Needtracking to provide loops, updates shared before between the next all channels, data sample saves power Fast tracking loops, power hungry
Tracking
Digital Digital Samples Samples
Controls Controls
Accumulators Accumulators Tracking Signal Replica Replica Signal Buffer Data Data Decode Decode
05/07/2012 - 12/18
Benjamin Tang
Measurements Measurements && Decoded Decoded Message Message
Tracking Loops • Frequency Locked Loop (FLL), Phase Locked Loop (PLL) and Delay Locked Loop (DLL) • Computations involve vector magnitude, arctangent, multiplication and division operations. Simplify: Fixed point arithmetic, bundled-data 1 Apply Taylor series small angle approximation: tan Apply modified version of Robertson approximation: A I 2 Q2 max I 14 Q , Q 14 I
Position error increases by ~1m on average
05/07/2012 - 13/18
Benjamin Tang
Receiver Performance Simulations • Transistor-level implementation of our system • Position accuracy simulation 60 seconds of signal from commercial GPS signal simulator No added atmospheric, ionospheric and multipath errors
3D-RMS error
