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 2011.11.16

What Duplex is? Simplex Half-duplex Full-duplex

What Duplex is? Time Division Duplexing Frequency Division Duplexing

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

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

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

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

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

RF Cancellation using Signal Inversion Our Work RF Cancellation using Signal Inversion

Previous work: Cancellation using Phase Offset

Cancellation using Signal Inversion

How to inverse BALUN : Balanced to Unbalanced Conversion

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

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

Adaptive RF Cancellation Our Work Adaptive RF Cancellation empirically

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

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

Our Work Digital Cancellation

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

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

Our Work Implications to Wireless Networks

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

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.

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

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

Our Work Discussion& conclussion

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

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