GHz Spectrum Acquisition in Realtime - MIT CSAIL · 2014-05-18 · Spectrum Crisis • The FCC:...
Transcript of GHz Spectrum Acquisition in Realtime - MIT CSAIL · 2014-05-18 · Spectrum Crisis • The FCC:...
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GHz Spectrum Acquisition in Realtime
Haitham Hassanieh
Lixin Shi, Omid Abari, Ezz Hamed and Dina Katabi
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Spectrum Crisis• The FCC: spectrum crunch started in 2013• But at any time, most of the spectrum is unused
Seattle January 7, 2013(Microsoft Spectrum observatory)
Frequency (GHz)1.5 2 2.5 3 65.554.543.51
Occup
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Spectrum Crisis• The FCC predicts a spectrum crunch starting 2013• But at any time, most of the spectrum is unused
Seattle January 7, 2013(Microsoft Spectrum observatory)
Frequency (GHz)1.5 2 2.5 3 65.554.543.51
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Dynamic Spectrum AccessSense to find unused bands; Use them!How do you capture GHz of spectrum?
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Realtime GHz Spectrum Sensing is Difficult
• Today, sequential scanning of tens of MHz Can easily miss radar signals
• Key Challenge: high‐speed ADCs
Tens of MHz ADCLow‐power
High resolutionCheap
A Few GHz ADC10x more powerPoor resolution
Expensive
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Idea: Leverage Sparsity
Sparse recovery show that one can acquire sparse signals using sub‐Nyquist sampling
Compressive Sensing however is difficult• Random sampling Can’t use low‐speed ADCs
• Compute million‐point FFT High power
Seattle January 7, 2013(Microsoft Spectrum observatory)
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Idea: Leverage Sparsity
Sparse FFTNo random sampling Use a few low‐speed ADCsSub‐linear algorithm Computes large FFT cheaply
Seattle January 7, 2013(Microsoft Spectrum observatory)
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Divide spectrum into a few buckets
Estimate the large coefficients in each non‐empty bucket
How Does Sparse FFT Work?
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1‐ Bucketize
2‐ Estimate
Can ignore empty bucket
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How Does Sparse FFT Work?Bucketize Estimate
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Bucketize Estimate
How Does Sparse FFT Work?
Sub‐sampling time Aliasing the frequencies
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Subsample1 2 3 4 5 7 8 9 106
FFT
FFT
Time Frequency
1 2 3 4 5Alias
1 2 3 4 5
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How Does Sparse FFT Work?
Empty Bucket Isolated Freq. Estimate
Collision Cannot Estimate
Bucketize Estimate
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How Does Sparse FFT Work?Bucketize Estimate
Bucketize multiple time using co‐prime sub‐sampling Same frequencies don’t collide in two bucketizations
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How Does Sparse FFT Work?Bucketize Estimate
Identify isolated freq. in one bucketization and subtract them from the other; and iterate …
Output Result:
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How Does Sparse FFT Work?Bucketize Estimate
Repeat bucketization after shifting the signal in time by a time shift
Freq‐Domain
Phase Rotation :
Time‐Domain
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BigBand: GHz Receiver for Sparse Signals
• Sub‐sample the data Can use low‐speed ADCs• Very fast algorithm Lower‐power consumption
• Used sparse FFT to build a GHz receiver from three tens of MHz ADCs
• Both senses and decodes the spectrum
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Realtime GHz Spectrum SensingCambridge, MA January 15 2013
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Realtime GHz Spectrum SensingCambridge, MA January 15 2013
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3 ADCs with a combined digital Bandwidth of 150 MHz can acquire a GHz
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Decoding Senders Randomly Hopping in a GHz
Number Transmitters
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Decoding Senders Randomly Hopping in a GHz
Number Transmitters
18
SFFT enables realtime GHz sensing and decoding for low‐power portable devices
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Differential BigBand
• Even if the spectrum is 100% occupied, changes in occupancy are sparse Apply sFFT to Changes/Diffs
• Can’t subtract signals; operate over power
• Realtime GHz sensing; but no decoding
But, what if the spectrum is not sparse?!
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Conclusion• BigBand provides GHz‐wide realtime spectrum sensing and decoding using sFFT
• Differential‐BigBand provides GHz sensing using sFFT
• Imagine multi‐GHz of unlicensed open spectrum operating with carrier sense (a la WiFi)