Achieving Single Channel Full-Duplex Wireless Communication
Jung Il Choi, Mayank Jain, Kannan Srinivasan,Philip Levis and Sachin Katti
1
Can a wireless node transmit AND receive at the same time on a single band?
2
Can a wireless node transmit AND receive at the same time on a single band?
3
Status quo: NO
Current wireless radios
• In-band half-duplex
• Full-duplex through other dimensions
• E.g. different frequencies
• Bandwidth is a precious resource
4
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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-60
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-50
-45
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-35
-30
-25
0 5 10 15 20 25
RSS
I (dB
m)
Position of Receive Antenna (cm)
TX1 TX2
Only TX1 Active
Antenna Cancellation: Performance
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-60
-55
-50
-45
-40
-35
-30
-25
0 5 10 15 20 25
RSS
I (dB
m)
Position of Receive Antenna (cm)
TX1 TX2
Only TX2 Active
Antenna Cancellation: Performance
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Only TX1 Active
-60
-55
-50
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-40
-35
-30
-25
0 5 10 15 20 25
RSS
I (dB
m)
Position of Receive Antenna (cm)
TX1 TX2
Only TX1 ActiveOnly TX2 Active
Both TX1 & TX2 Active
Antenna Cancellation: Performance
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NullPosition
-60
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0 5 10 15 20 25
RSS
I (dB
m)
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
0
10
20
30
-30
-20
-10
x axis (meters)
y ax
is (
met
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
0
10
20
30
-30
-20
-10
x axis (meters)
y ax
is (
met
ers)
0 10 20 30-30 -20 -10
0
10
20
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-20
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x axis (meters)
y ax
is (
met
ers)
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Equal Transmit Power Unequal Transmit Power
0 10 20 30-30 -20 -10
0
10
20
30
-30
-20
-10
x axis (meters)
y ax
is (
met
ers)
-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
0.4
0.6
0.8
1.0
0 50 100 150 200 250 300
CD
F
Throughput (Kbps)
Half-Duplex Full-Duplex Ideal Full-Duplex
1.84x
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0
0.2
0.4
0.6
0.8
1.0
0 50 100 150 200 250 300
CD
F
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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0
0.25
0.50
0.75
1.00
0 10 20 30 40
Pac
ket
Rec
eptio
n R
atio
SNR (dB)
Half-Duplex Full-Duplex
0
0.25
0.50
0.75
1.00
0 10 20 30 40
Pac
ket
Rec
eptio
n R
atio
SNR (dB)
Link Reception RatioHalf-Duplex Full-Duplex
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• Loss at High SNR: Due to spurious signal peaks in USRP
Loss atHigh SNR
0
0.25
0.50
0.75
1.00
0 10 20 30 40
Pac
ket
Rec
eptio
n R
atio
SNR (dB)
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
50
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
61
• 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
63
From 3 antennas to 2 antennas
! solves bandwidth problem
Demo on Wednesday
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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
65
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
66
PS: We’re looking for jobs starting mid-2011 :)
Kannan: Academic Mayank and Jung IL: Industrial Research