Achieving Single Channel Full-Duplex Wireless Communication

Jung Il Choi, Mayank Jain, Kannan Srinivasan,Philip Levis and Sachin Katti

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Can a wireless node transmit AND receive at the same time on a single band?

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Can a wireless node transmit AND receive at the same time on a single band?

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Status quo: NO

Current wireless radios

• In-band half-duplex

• Full-duplex through other dimensions

• E.g. different frequencies

• Bandwidth is a precious resource

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Why not full-duplex on the same band?

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Why not full-duplex on the same band?

• Very strong self-interference

• ~70dB stronger for 802.15.4

• Analog to Digital converter (ADC) saturates

TX RX

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TX RX

• Digital cancellation: Subtracting known interference digital samples from received digital samples.

ZigZag[1], Analog Network Coding[2] etc.

• Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference

Radunovic et al.[3]

Existing Techniques

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[1] Gollakota et al. “ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, ACM SIGCOMM 2008[2] Katti et al. “Embracing Wireless Interference: Analog Network Coding”, ACM SIGCOMM 2007[3] Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", WiMesh (SECON Workshop),, 2010

• Digital cancellation: Subtracting known interference digital samples from received digital samples.

ZigZag[1], Analog Network Coding[2] etc.

Ineffective if ADC is saturated

• Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference

Radunovic et al.[3]

Existing Techniques

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[1] Gollakota et al. “ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, ACM SIGCOMM 2008[2] Katti et al. “Embracing Wireless Interference: Analog Network Coding”, ACM SIGCOMM 2007[3] Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", WiMesh (SECON Workshop),, 2010

These are not enough 25dB +15dB < 70dB

• Digital cancellation: Subtracting known interference digital samples from received digital samples.

ZigZag[1], Analog Network Coding[2] etc. ~15dB

Ineffective if ADC is saturated

• Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference

Radunovic et al.[3] ~25dB

Existing Techniques

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Our innovation: Antenna Cancellation

d d + λ/2

TX1 TX2RX

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Our innovation: Antenna Cancellation

~30dB self-interference cancellation

Enables full-duplex when combined with Digital (15dB) and Hardware (25dB) cancellation.

d d + λ/2

TX1 TX2RX

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Can a wireless node transmit AND receive at the same time on a single band?

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Can a wireless node transmit AND receive at the same time on a single band?

YES, IT CAN!

Full-duplex prototype achieves 92% of the throughput of an “ideal” full-duplex system

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• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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Three techniques give ~70dB cancellation

• Antenna Cancellation (~30dB)

• Hardware Cancellation (~25dB)

• Digital Cancellation (~15dB)

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Attenuator

Antenna Cancellation: Block Diagram

d d + λ/2TX1 TX2RX

RXRF Frontend

Digital Processor

TXRF Frontend

Power Splitter

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Hardware and Digital Cancellation

* Radunovic et al. , "Rethinking Indoor Wireless: Lower Power, Low Frequency, Full-duplex", MSR Tech Report, 2009

Digital Cancellation

• Subtract known transmit samples from received digital samples

Hardware Cancellation

• Use existing interference cancellation circuits (QHx220)*

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Bringing It Together

QHX220

ADC

HardwareCancellation

TX Signal

AntennaCancellation

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RX

DigitalCancellation

∑TX Samples

+-

Clean RX samples

RF

Baseband

Bringing It Together

QHX220

ADC

HardwareCancellation

TX Signal

AntennaCancellation

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RX

DigitalCancellation

∑TX Samples

+-

Clean RX samples

RF

Baseband

Bringing It Together

QHX220

ADC

HardwareCancellation

TX Signal

AntennaCancellation

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RX

DigitalCancellation

∑TX Samples

+-

Clean RX samples

RF

Baseband

Bringing It Together

QHX220

ADC

HardwareCancellation

TX Signal

AntennaCancellation

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RX

DigitalCancellation

∑TX Samples

+-

Clean RX samples

RF

Baseband

Our Prototype

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AntennaCancellation

HardwareCancellation

Digital InterferenceCancellation

• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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RSS

I (dB

m)

Position of Receive Antenna (cm)

TX1 TX2

Only TX1 Active

Antenna Cancellation: Performance

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RSS

I (dB

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Position of Receive Antenna (cm)

TX1 TX2

Only TX2 Active

Antenna Cancellation: Performance

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Only TX1 Active

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Position of Receive Antenna (cm)

TX1 TX2

Only TX1 ActiveOnly TX2 Active

Both TX1 & TX2 Active

Antenna Cancellation: Performance

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NullPosition

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Position of Receive Antenna (cm)

TX1 TX2

Only TX1 ActiveOnly TX2 Active

Both TX1 & TX2 Active

Antenna Cancellation: Performance

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~25-30dBNull

Position

Sensitivity of Antenna Cancellation

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Amplitude Mismatch between TX1 and TX2

Placement Errorfor RX

dB

Red

uctio

n Li

mit

(dB)

Red

uctio

n Li

mit

(dB)

Error (mm)

Sensitivity of Antenna Cancellation

30

dB

Red

uctio

n Li

mit

(dB)

Red

uctio

n Li

mit

(dB)

Error (mm)

30dB cancellation < 5% (~0.5dB) amplitude mismatch < 1mm distance mismatch

Amplitude Mismatch between TX1 and TX2

Placement Errorfor RX

Sensitivity of Antenna Cancellation

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dB

Red

uctio

n Li

mit

(dB)

Red

uctio

n Li

mit

(dB)

Error (mm)

• Rough prototype good for 802.15.4

• More precision needed for higher power systems (802.11)

Amplitude Mismatch between TX1 and TX2

Placement Errorfor RX

Bandwidth Constraint

A λ/2 offset is precise for one frequency

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fc

d d + λ/2

TX1 TX2RX

Bandwidth Constraint

A λ/2 offset is precise for one frequencynot for the whole bandwidth

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fc fc+Bfc -B

d d + λ/2

TX1 TX2RX

Bandwidth Constraint

A λ/2 offset is precise for one frequencynot for the whole bandwidth

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fc fc+Bfc -B

d d + λ/2

TX1 TX2RX

d2 d2 + λ+B/2

TX1 TX2RX

d1 d1 + λ-B/2

TX1 TX2RX

Bandwidth Constraint

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fc fc+Bfc -B

d d + λ/2

TX1 TX2RX

d2 d2 + λ+B/2

TX1 TX2RX

d1 d1 + λ-B/2

TX1 TX2RX

WiFi (2.4G, 20MHz) => ~0.26mm precision error

A λ/2 offset is precise for one frequencynot for the whole bandwidth

Bandwidth Constraint

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2.4 GHz

5.1 GHz

300 MHz

Bandwidth Constraint

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2.4 GHz

5.1 GHz

300 MHz

• WiFi (2.4GHz, 20MHz): Max 47dB reduction

• Bandwidth⬆ => Cancellation⬇• Carrier Frequency⬆ => Cancellation⬆

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What about attenuation at intended receivers?Destructive interference can affect this signal too!

0 10 20 30-30 -20 -10

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y ax

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ers)

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Equal Transmit Power

Deep Nulls at 20-30m

Unequal Transmit Power

What about attenuation at intended receivers?Destructive interference can affect this signal too!

• Different transmit powers for two TX helps

0 10 20 30-30 -20 -10

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Equal Transmit Power Unequal Transmit Power

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-52 dBm

-58 dBm-52 dBm -100 dBm

What about attenuation at intended receivers?Destructive interference can affect this signal too!

• Different transmit powers for two TX helps

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What about attenuation at intended receivers?Destructive interference can affect this signal too!

• Different transmit powers for two TX helps

• Diversity gains in indoor environments

• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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• 802.15.4 based signaling on USRP nodes

• Two nodes at varying distances placed in an office building room and corridor

Experimental Setup

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• Full-duplex should double aggregate throughput44

Half-Duplex :- Nodes interleave transmissions

Node 1 ➜ 2

Node 2 ➜ 1

Node 1 ➜ 2

Node 2 ➜ 1

Full-Duplex :- Nodes transmit concurrently

Median throughput 92% of ideal full-duplex

Throughput

0

0.2

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1.0

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Throughput (Kbps)

Half-Duplex Full-Duplex Ideal Full-Duplex

1.84x

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0

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Throughput (Kbps)

Throughput

Half-Duplex Full-Duplex Ideal Full-Duplex

1.84x

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Performance loss at low SNR

Little loss in link reliability: 88% of half-duplex on average

Link Reception Ratio

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Half-Duplex Full-Duplex

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Link Reception RatioHalf-Duplex Full-Duplex

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• Loss at High SNR: Due to spurious signal peaks in USRP

Loss atHigh SNR

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Half-Duplex Full-Duplex

Link Reception Ratio

• Loss at High SNR: Due to spurious signal peaks in USRP

• Loss at low SNR: Due to imprecisions in prototype49

Loss atLow SNR

• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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The prototype gives 1.84x throughput gain with two radios compared to half-duplex with a single radio

So what? PHY gains similar to 2x2 MIMO

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The prototype gives 1.84x throughput gain with two radios compared to half-duplex with a single radio

So what? PHY gains similar to 2x2 MIMO

True benefit lies beyond the physical layer

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• Breaks a basic assumption in wireless

• Can solve some fundamental problems with wireless networks today

• Hidden terminals

• Primary detection in whitespaces

• Network congestion and WLAN fairness

• Excessive latency in multihop wireless

Implications to Wireless Networks

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• CSMA/CA can’t solve this

• Schemes like RTS/CTS introduce significant overhead

APN1 N2

Current networks have hidden terminals

Mitigating Hidden Terminals

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• CSMA/CA can’t solve this

• Schemes like RTS/CTS introduce significant overhead

APN1 N2

Since both sides transmit at the same time, no hidden terminals exist

Current networks have hidden terminals

Full Duplex solves hidden terminals APN1 N2

Mitigating Hidden Terminals

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Primary Detection in Whitespaces

Secondary transmitters should sense for primary transmissions before channel use

Time

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Primary TX(Wireless Mics)

Secondary TX(Whitespace AP)

Primary sensing

Primary TX(Wireless Mics)

Secondary TX(Whitespace AP)

Traditional nodes may still interfere during transmissions

Interference

Primary Detection in Whitespaces

Secondary transmitters should sense for primary transmissions before channel use

Time

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Primary sensing

Primary TX(Wireless Mics)

Secondary TX(Whitespace AP)

Full-duplex nodes can sense and send at the same time

Primary sensing

Primary TX(Wireless Mics)

Secondary TX(Whitespace AP)

Network Congestion and WLAN Fairness

Without full-duplex:

• 1/n bandwidth for each node in network, including APDownlink Throughput = 1/n Uplink Throughput = (n-1)/n

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Network Congestion and WLAN Fairness

Without full-duplex:

• 1/n bandwidth for each node in network, including APDownlink Throughput = 1/n Uplink Throughput = (n-1)/n

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With full-duplex:

• AP sends and receives at the same timeDownlink Throughput = 1 Uplink Throughput = 1

Long delivery and round-trip times in multi-hop networks

Solution: Wormhole routing

N1 N2 N3 N4

N1

N2

N3

N4

N1

N2

N3

N4

Time Time

Half-duplex

Time

Full-duplex

Reducing Round-Trip Times

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• Design of Full-Duplex Wireless

• 3 Techniques: Antenna, Hardware and Digital Cancellation

• Analyzing Antenna Cancellation

• Performance Results

• Implications to Wireless Networks

• Limitations of Design, Future Work

Talk Outline

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• Bandwidth Constraint

Working on a frequency independent signal inversion technique

• Time-varying wireless channel

Auto-tuning of the hardware cancellation circuit

• Multi-path

Estimate and incorporate in digital cancellation: Some existing work does this

• Single stream

Extension to MIMO-like systems

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Summary

• Prototype for achieving in-band full-duplex wireless

• Constraints of current prototype can be overcome with some neat ideas and careful engineering

• Rethinking of wireless networks

• We’ve discussed some applications like mitigating hidden terminals and WLAN fairness

• Many more possibilities

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