Practical, Real-time, Full Duplex Wireless

1. Mayank Jainy1, Jung Il Choiy1, Tae Min Kim1, Dinesh Bharadia1, Siddharth Seth1, Kannan Srinivasan2, Philip Levis1, Sachin Katti1, Prasun Sinha3 1:Stanford University 2:The University of Texas at Austin 3:The Ohio State University Practical, Real-time, Full Duplex Wireless Mobicom 2011 Yuchen Wu 2011.11.16

2. What Duplex is? • Simplex • Half-duplex • Full-duplex

3. What Duplex is? • Time Division Duplexing • Frequency Division Duplexing

4. Single Channel Full-duplex • Very strong self-interference: ~70dB for 802.11 • Main idea: cancel self-interference • Combine RF and digital techniques for cancellation

5. The story so far... • Mobicom’10[1]: • Antenna Cancellation + other techniques • [1] Choi et al. “Achieving single channel, full duplex wireless communication” Best Demo Award

6. The story so far... with disadvantages • Double throughput : two transmit antennas, and one receive antenna • But MIMO system can triple throughput with three antennas

7. With more disadvantages • Bandwidth constraint • A theoretical limit which prevents supporting wideband signals such as WiFi • Requires manual tuning • Null regions in the far field • Because of destructive interference

8. Our Goals • New, better RF and digital cancellation techniques • Adaptive algorithms for auto-tuning cancellation • And… Real-time full-duplex MAC layer implementation

9. Our Work • RF Cancellation using Signal Inversion • Adaptive RF Cancellation • Adaptive Digital Cancellation • System Performance • Implications to Wireless Networks • Looking Forward

10. Previous work: Cancellation using Phase Offset

11. Cancellation using Signal Inversion

12. How to inverse • BALUN : Balanced to Unbalanced Conversion

13. Signal Inversion Cancellation: Wideband Evaluation • Measure wideband cancellation • Wired experiments • 240MHz chirp at 2.4GHz to measure response

14. Wideband Evaluation • ~50dB cancellation at 20MHz bandwidth with balunvs ~38dB with phase offset cancellation. • Fixed bandwidth • Varying bandwidth

15. Our Work • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation • Adaptive Digital Cancellation • System Performance • Implications to Wireless Networks • Looking Forward

16. Adaptive RF Cancellation • Need to match self-interference power and delay • Objective: Minimize received power • Control variables: Delay and Attenuation

17. Hardware approximation: QHx220 noise canceller • Find best Gi and Gq: Algorithm works in steps • Compute the slope of the residual RSSI curve • Move to the new settings with lower RSSI

18. QHx220 : Find the Best Cancellation • Typical convergence within 8-15 iterations (~1ms total)

19. Our Work • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation • System Performance • Implications to Wireless Networks • Looking Forward

20. Digital Cancellation • Measure residual self-interference after RF cancellation • Subtract self-interference from received digital signal

21. Bringing It All Together • FIR filter : finite impulse response filter • FPGA implementation

22. Performance • ~73 dB cancellation • In practice: 40dB of attenuation (20 cm) from antenna separation • In sum: 113dB; 20dBm -> -93dBm : close to the noise floor

23. Our Work • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation : ~73dB • System Performance : ~113dB cancellation • Implications to Wireless Networks • Looking Forward

24. Implications to Wireless Networks • Breaks a basic assumption in wireless • Can solve some fundamental problems with wireless networks today • Hidden terminals • Network congestion and WLAN fairness

25. Implementation • USRP: too slow • Wi-Fi cards: hard to program • WARPv2 boards with 2 radios • FPGA • OFDM reference code from Rice University • 10MHz bandwidth OFDM signaling • CSMA MAC on embedded processor • Modified for Full-duplex

26. Mitigating Hidden Terminals • Reduces hidden terminal losses by up to 88%

27. WLAN Fairness • Without full-duplex: • Downlink Throughput = 1/n Uplink Throughput = (n-1)/n • With full-duplex: • Downlink Throughput = 1Uplink Throughput = 1

28. WLAN Fairness • 1 AP with 4 stations without any hidden terminals • 8 links, each 3Mbps

29. Our Work • RF Cancellation using Signal Inversion: ~50dB for 20Mhz • Adaptive RF Cancellation: ~1ms convergence • Adaptive Digital Cancellation : ~73dB • System Performance : ~113dB cancellation • Implications to Wireless Networks • Looking Forward

30. Other cancellation techniques • Digital estimation for analog cancellation[1] • Duarte et al. “Full-Duplex Wireless Communications Using Off-The-Shelf Radios: Feasibility and First Results.”, in Asilomar 2010.

31. Single antenna solution: circulators • Circulator: Radio frequency power entering any port is transmitted to the next port in rotation

32. And …. more • Going mobile: Higher cancellation, faster adaptation • MIMO full-duplex • Wu: power saving? New network model?

33. Full-duplex Networking • [1] Gollakota et al. “They Can Hear Your Heartbeats: Non-Invasive Security for Implantable Medical Devices.”, in Sigcomm 2011. • [2] Lee et al. “Secured Bilateral Rendezvous using Self-interference Cancellation in Wireless Networks”, in IFIP 2011.