Full Duplex Radios. Daniel J. Steffey

Transcription

1 Full Duplex Radios Daniel J. Steffey

2 Source Full Duplex Radios* ACM SIGCOMM 2013 Dinesh Bharadia Emily McMilin Sachin Katti *All source information and graphics/charts 2

3 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

4 Motivation Double Throughput* Or Half Spectrum Requirements 4

5 How Capture / Estimate transmitted signal Subtract transmitted signal from received total signal Process residual received signal as received transmission 5

6 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 6

7 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) 7

8 Transmission What you want to send is not what you actually send 8

9 Cancellation Requirements 9

10 Design 10

11 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 11

12 Analog Cancellation (cont.) 12

13 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 ~ µs Need to run every ~100ms Tuned and solved by looking at a WiFi preamble 13

14 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 Distorted Preamble Complicated Formula Clean Preamble Remainder of Packet Complicated Formula Clean Packet 14

15 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 15

16 Project Proposal 16

17 Implementation 17

18 Evaluation Metrics Increase in Noise Floor SNR Loss Comparison Balun Design Rice Design 18

19 Cancellation and Increase in Noise Floor 19

20 SNR Loss 20

21 Cancellation in Stages 21

22 SNR Loss vs Constellations and Bandwidths 22

23 Analog Cancellation - How Many Lines? 23

24 Effects of Digital Cancellation 24

25 Analog Cancellation Retuning 25

26 Throughput 26

27 What Do I Think? Very Interesting!!! MIMO When is it actually useful? Point-to-point systems with constant bi-directional transmission Cellular Systems General WiFi use 27

28 Questions / Comments 28