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Source
Full Duplex Radios*
ACM SIGCOMM 2013
Dinesh Bharadia Emily McMilin Sachin Katti
*All source information and graphics/charts
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Full Duplex Radios Daniel J. Steffey Source Full Duplex Radios* - - PowerPoint PPT Presentation
Full Duplex Radios Daniel J. Steffey Source Full Duplex Radios* - - PowerPoint PPT Presentation
Full Duplex Radios Daniel J. Steffey Source Full Duplex Radios* ACM SIGCOMM 2013 Dinesh Bharadia Emily McMilin Sachin Katti *All source information and graphics/charts 2 Problem It is generally not possible for radios to receive and
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Problem
“It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results.”
- Andrea Goldsmith, Wireless Communications 2005
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Motivation
Double Throughput* Or Half Spectrum Requirements
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How
❏ Capture / Estimate transmitted signal ❏ Subtract transmitted signal from received total signal ❏ Process residual received signal as received transmission
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Prior Work
❏ “Achieving single channel, full duplex wireless communication” ❏ Choi, Jain, Srinivasan, Levis, and Katti ❏ “Experiment-driven characterization of full-duplex wireless systems” ❏ Duarte, Dick, and Sabharwal ❏ “Practical, real-time, full duplex wireless” ❏ Jain, Choi, Kim, Bharadia, Seth, Srinivasan, Lewis, Katti, and Sinha ❏ Requires at least two antennas (transmit and receive) ❏ Spaced >20cm apart ❏ Provides at most 85 dB of cancellation ❏ <5m distance required for improvement over half duplex
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Current Research
❏ Completely cancel self-interference from received signal ❏ 110 dB of cancellation required ❏ Transmit at 100 mW = 20 dBm ❏ Noise floor is -90 dBm ❏ Single antenna for transmit and receive ❏ Ability to adapt to changing environment ❏ Analog cancellation ❏ Digital cancellation ❏ Frequency Independent ❏ Range of bandwidths (80 MHz WiFi, smaller LTE bands) ❏ No MIMO (future research)
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Transmission
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❏ What you want to send is not what you actually send
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Cancellation Requirements
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Design
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Analog Cancellation
❏ At least 60 dB of cancellation ❏ Provides multiple copies of transmitted signal ❏ Delayed by various fixed amounts using parallel circuits of varying lengths; 16 delay lines is sufficient ❏ Programmatically attenuated by different variable amounts using tunable attenuators ❏ Voodoo Magic ❏ Using Nyquist Theorem and sinc interpolation can construct the self interference signal at instant d and remove from received signal
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Analog Cancellation (cont.)
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Analog Cancellation (cont.)
❏ What attenuation settings to use? ❏ Solve: ❏ Use a combination of precomputed values and a modified gradient descent algorithm to solve ❏ Algorithm takes ~900-1000µs ❏ Need to run every ~100ms ❏ Tuned and solved by looking at a WiFi preamble
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Digital Cancellation - Linear Components
❏ 50 dB of cancellation for linear main signal component ❏ Complicated formula to “undo” distortions ❏ Feed in distorted digital signal ❏ Apply appropriate coefficients to the formula ❏ Output the undistorted signal ❏ What are the coefficients? ❏ Calculated on a per packet basis ❏ Determined by sampling the packet preamble
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Complicated Formula Clean Preamble Distorted Preamble Complicated Formula Clean Packet Remainder
- f Packet
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Digital Cancellation - Non-Linear Components
❏ 20 dB of cancellation required (out of 80 dB total) ❏ Estimate transmitted signal as Taylor Series ❏ Analysis shows only odd-terms contribute non-zero energy in frequency band of interest (can ignore even terms) ❏ Ignore m=1 here as that is covered in the Linear Component ❏ Each successive term is lower power ❏ Empirically learn 224 terms are needed ❏ How do we get the factors of the terms? ❏ Same as Linear Component ❏ Complicated formula to “undo” distortions ❏ Feed in distorted digital signal ❏ Apply appropriate coefficients to the formula ❏ Output the undistorted signal ❏ What are the coefficients? ❏ Calculated on a per packet basis ❏ Determined by sampling the packet preamble
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Project Proposal
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Implementation
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Evaluation
❏ Metrics ❏ Increase in Noise Floor ❏ SNR Loss ❏ Comparison ❏ Balun Design ❏ Rice Design ❏ Complicated formula to “undo” distortions ❏ Feed in distorted digital signal ❏ Apply appropriate coefficients to the formula ❏ Output the undistorted signal ❏ What are the coefficients? ❏ Calculated on a per packet basis ❏ Determined by sampling the packet preamble
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Cancellation and Increase in Noise Floor
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SNR Loss
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Cancellation in Stages
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SNR Loss vs Constellations and Bandwidths
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Analog Cancellation - How Many Lines?
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Effects of Digital Cancellation
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Analog Cancellation Retuning
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Throughput
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What Do I Think?
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❏ Very Interesting!!! ❏ MIMO ❏ When is it actually useful? ❏ Point-to-point systems with constant bi-directional transmission ❏ Cellular Systems ❏ General WiFi use
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Questions / Comments
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