1. Achieving Single Channel, Full Duplex Wireless Communication
Jung Il Choi, Mayank Jain, Kannan Srinivasan, Philip Levis, Sachin Katti
Stanford University
Mobicom 10’

2. Wireless Channel – Half Duplex?
“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,” Cambridge Press, 2005.
Reason: The signal from a local transmitting antenna is hundreds of thousands of times stronger than transmissions from other nodes.

3. Challenge for Full Duplex
Decoding method
Subtract transmit signal from receive antenna signal
Problem: Very strong self-interference ~70dB

4. We need 70 dB of self signal cancellation to enable full duplex communication

5. Existing cancellation techniques

6. Digital cancellation Rx Input Analog to Digital Converter (ADC)
Digital Signal Processing
Analog to Digital Converter (ADC)
Analog Rx Frontend
Rx signal
Rx signal
Digital Signal Processing
Digital to Analog Converter (DAC)
Analog Tx front end
Tx Output
Tx signal
Tx signal

7. Digital cancellation
Digital Signal Processing
Rx signal – Tx signal (subtract)
Rx Input
Analog to Digital Converter (ADC)
Analog Rx Frontend
Rx signal
Rx signal
Digital Signal Processing
Digital to Analog Converter (DAC)
Analog Tx front end
Tx Output
Tx signal
Tx signal
Achieves 15dB of cancellation. We need 70dB, but we take what we get

8. Why is Digital cancellation not perfect
Rx Signal
Discrete values for digitizing
disc values for digitizng .. any values would be mapped to closest of these samples and saved in memory ..
During digital conversion, rx signal is mapped to discrete values

9. Why is Digital cancellation not perfect
Rx Signal
Self Signal

10. Self Signal
Rx Signal
Tx Signal is much stronger than Rx signal, thus in presence of strong self interference, the resolution for Rx signal decreases on ADC digitized samples.

11. Analog cancellation
Rx Input
Digital Signal Processing
Rx signal – Tx signal (subtract)
Analog to Digital Converter
Analog Cancellation circuits
Analog Rx Frontend
Rx signal
Rx signal
Analog Tx front end
Tx Output
Digital Signal Processing
Digital to Analog Converter
Tx signal
Tx signal
Achieves 25dB of cancellation. Together with 15dB of digital cancellation, we have 40dB of total cancellation
Still need 30dB more cancellation to achieve 70dB of cancellation

12. Antenna Cancellation Two Transmit antennas, one Receive antenna ~30dB
Where to place the receive antenna?
Distance from the two transmit antennas differs by an odd multiple of half the wavelength of the center frequency of transmission -- Signal add destructively

13. Antenna Cancellation - Performance

14. Antenna Cancellation - Performance

15. Antenna Cancellation - Performance

16. Antenna Cancellation - Performance

17. Antenna Cancellation - Bandwidth
Bandwidth constraint
λ/2 offset is precise for one frequency
BUT not for the whole bandwidth
WiFi (2.4G, 20MHz) => ~0.26mm precision error

18. Antenna Cancellation - Bandwidth
Higher Bandwidth => Less Cancellation
Higher Carrier Frequency => More Cancellation⇧

19. System Diagram Three steps Achieves ~70dB cancellation
Antenna Cancellation ~30dB
Hardware Cancellation ~25dB
Digital Cancellation ~15dB
Achieves ~70dB cancellation

20. Evaluation – Aggregate Throughput
Median throughput 92% of ideal full-duplex

21. Evaluation – Link Reliability
Little loss in link reliability: 88% of half-duplex on average

22. Full Duplex - Applications
True benefit lies beyond the physical layer
Breaks a basic assumption in wireless
Can solve some fundamental problems with wireless networks
Hidden terminals
Reducing Congestion with MAC Scheduling
Wormhole Routing in Multihop Networks
Cognitive Radios

23. Mitigating Hidden Terminals
Full Duplex solves hidden terminals
N2 hears the transmission from the access point and delays its transmission, thereby avoiding a collision.

24. Reducing Congestion with MAC Scheduling
Aggregate Network throughput: 1/n -> 1*link_cap (2n+1 nodes, star topology)

25. Wormhole Routing in Multihop Networks
Problem: Long delivery and round-trip times in multihop networks
Solution: Forward a packet while receiving it

26. Limitations High Transmit Power Bandwidth Constraint
Time-varying wireless channel
Auto-tuning of the hardware cancellation circuit
Mayank Jain, Jung Il Choi, Taemin Kim, Dinesh Bharadia, Kannan Srinivasan, Siddharth Seth, Philip Levis, Sachin Katti and Prasun Sinha. “Practical, Real-time, Full Duplex Wireless.” In Proceedings of the 17th Annual International Conference on Mobile Computing and Networking (Mobicom 2011).

27. Questions