1. Practical, Real-time, Full Duplex Wireless
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
Mobicom 2011
Yuchen Wu

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
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. The paper’s Goals New, better RF and digital cancellation techniques
Adaptive algorithms for auto-tuning cancellation
And… Real-time full-duplex MAC layer implementation

9. RF Cancellation using Signal Inversion
Our Work
RF Cancellation using Signal Inversion

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
Fixed bandwidth
Varying bandwidth
~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation.

15. Adaptive RF Cancellation
Our Work
Adaptive RF Cancellation
empirically

16. 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

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

18. Our Work
Digital Cancellation

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

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

21. Our Work
Implications to
Wireless Networks

22. 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

23. Full duplex MAC
The full duplex MAC protects primary and secondary transmissions from hidden terminal losses. A busytone
is used to protect periods of single-ended data transfer.

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

25. WLAN Fairness Without full-duplex: With full-duplex:
Downlink Throughput = 1/n Uplink Throughput = (n-1)/n
full duplex improves the downstream throughput 111%, while the upstream throughput increases only by 15%.
With full-duplex:
Downlink Throughput = 1 Uplink Throughput = 1

26. Our Work
Discussion&
conclussion

27. Dicussion RF Engineering Asymmetric Traffic Handling in MAC
In-Packet Channel Estimation
Half Duplex Compatibility

28. Dicussion
Full duplex radio based on a novel cancellation technique called balun cancellation.
Work on wideband, high power signals .
Automatically tune their cancellation circuits
Evaluates a full duplex MAC layer