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Full Duplex Wireless
Philip LevisStanford University
@Princeton 10.12.2012
also Mayank Jain, Jung Il Choi, Sachin Katti, Kannan Srinivasan, Taemin Kim, Dinesh Bharadia, Prasun Sinha, and Siddarth Seth.
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2
Chuck Thacker InterviewCommunications of the ACM, July 2010
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“It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results. Thus, bidirectional systems must separate the uplink and downlink channels into orthogonal signaling dimensions, typically using time or frequency dimensions.” - Andrea Goldsmith, “Wireless Communications,” Cambridge Press, 2005.
• We’ve built working full duplex radios‣ Breaks a fundamental assumption in wireless‣ Current limitations: transmit power: ≤ WiFi
(200mW), bandwidth (≤ 20MHz)
• How full duplex could transform wireless
• Open technical barriers and challenges
Talk In a Nutshell
Why Half Duplex?
TX RX TX RX
• Self-interference is millions to billions (60-90dB) stronger than received signal
• If only we could “subtract” our transmission...
• 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
6
[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 saturates
• 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 = 40dB
• Digital cancellation: subtracting known interference digital samples from received digital samples.
ZigZag[1], Analog Network Coding[2] etc. ~15dB
Ineffective if ADC saturates
• Hardware cancellation: RF noise cancellation circuits with transmit signal as noise reference
Radunovic et al.[3] ~25dB
Existing Techniques
8
First Design: Antenna Cancellation
d d + λ/2
TX1 TX2RX
9
First Design: Antenna Cancellation
~30dB self-interference cancellation
Enables full-duplex when combined with digital (15dB) and hardware (25dB) cancellation.
d d + λ/2
TX1 TX2RX
10
Three techniques give ~70dB cancellation
• Antenna Cancellation (~30dB)
• Hardware Cancellation (~25dB)
• Digital Cancellation (~15dB)
11
Our Prototype
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AntennaCancellation
HardwareCancellation
Digital InterferenceCancellation
Bringing It Together
QHX220
ADC
HardwareCancellation
TX Signal
AntennaCancellation
13
RX
DigitalCancellation
∑TX Samples
+-
Clean RX samples
RF
Baseband
Bringing It Together
QHX220
ADC
HardwareCancellation
TX Signal
AntennaCancellation
14
RX
DigitalCancellation
∑TX Samples
+-
Clean RX samples
RF
Baseband
Bringing It Together
QHX220
ADC
HardwareCancellation
TX Signal
AntennaCancellation
15
RX
DigitalCancellation
∑TX Samples
+-
Clean RX samples
RF
Baseband
Bringing It Together
QHX220
ADC
HardwareCancellation
TX Signal
AntennaCancellation
16
RX
DigitalCancellation
∑TX Samples
+-
Clean RX samples
RF
Baseband
-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 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
18
Only TX1 Active
-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 TX1 ActiveOnly TX2 Active
Both TX1 & TX2 Active
Antenna Cancellation: Performance
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NullPosition
-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 TX1 ActiveOnly TX2 Active
Both TX1 & TX2 Active
Antenna Cancellation: Performance
20
~25-30dBNull
Position
Sensitivity of Antenna Cancellation
21
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
22
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
25
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⬆
• 802.15.4 based signaling on USRP nodes
• Two nodes at varying distances placed in an office building room and corridor
Experimental Setup
30
• Full-duplex should double aggregate throughput31
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
CDF
Throughput (Kbps)
Half-Duplex Full-Duplex Ideal Full-Duplex
1.84x
32
0
0.2
0.4
0.6
0.8
1.0
0 50 100 150 200 250 300
CDF
Throughput (Kbps)
Throughput
Half-Duplex Full-Duplex Ideal Full-Duplex
1.84x
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Performance loss at low SNR
34
First prototype gives 1.84x throughput gain with two radios compared to half-duplex with a single radio.
Limitation 1: Need 3 antennas
Limitation 2: Limited to 802.15.4
Limitation 3: Difficult farfield effects
Limitation 4: Doesn’t adapt to environment
Why Half Duplex?
TX RX TX RX
• Self-interference is millions to billions (60-90dB) stronger than received signal
• If only we could “subtract” our transmission...
Poor Man’s Subtraction
2.4 GHz
5.1 GHz
300 MHz
Cancellation using Phase OffsetSelf-Interference
Cancellation Signal
∑
Cancellation using Phase Offset
38
Self-Interference
Cancellation Signal
∑
Self-Interference
Cancellation Signal
∑
Frequency dependent, narrowband
Cancellation using Signal Inversion
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Self-Interference
Cancellation Signal
∑
Cancellation using Signal Inversion
40
Self-Interference
Cancellation Signal
∑
Self-Interference
Cancellation Signal
∑
Frequency and bandwidth independent
Time
41
Xt +Xt/2
-Xt/2
BALUN
Second Design: Balanced to Unbalanced Conversion
Traditional Design
R
TXRF Frontend
RXRF Frontend
T R+aT
aT
1. Invert the Signal
aT R
TXRF Frontend
RXRF Frontend
2T
+T -T R+aT
balun
2. Subtract Signal
R
TXRF Frontend
RXRF Frontend
ΣR+aT-T
R+aT
balun
aT
2T
-T+T
3. Match Signals
R
TXRF Frontend
RXRF Frontend
Σv
-vT R+aT-vT
R+aT
balun
attenuator anddelay line
aT
2T
-T+T
Can Receive If v = a!
R
TXRF Frontend
RXRF Frontend
Σv
-vT
R+aTattenuator anddelay line
balun
aT
2T
-T+T
R+aT-aT
+Xt/2
• Measure wideband cancellation
• Wired experiments
• 240MHz chirp at 2.4GHz to measure response
Time
Signal Inversion Cancellation: Wideband Evaluation
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TX
RX
Signal Inversion Cancellation Setup
∑ TX RX
Phase Offset Cancellation Setup
∑RF
Signal Splitter
Xt +Xt/2
-Xt/2
Xt
+Xt/2
λ/2 Delay
Time
48
Lower isbetter
Higher isbetter
Time
49
~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation.
Significant improvement in wideband cancellation
Lower isbetter
Higher isbetter
Time
50
• From 3 antennas per node to 2 antennas
• Parameters adjustable with changing conditions
Attenuator andDelay Line
TX RX
TXRF Frontend
Xt
+Xt/2 -Xt/2
∑
RXRF Frontend
Other advantages
• Need to match self-interference power and delay
• Can’t use digital samples: saturated ADC
Adaptive RF Cancellation
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TX RX
Attenuation & Delay
Wireless Receiver
Wireless Transmitter
RF Cancellation
TX Signal Path RX Signal Path
RF ReferenceΣ
Balun
• Need to match self-interference power and delay
• Can’t use digital samples: saturated ADC
Adaptive RF Cancellation
52
RSSI : Received Signal Strength Indicator
TX RX
Attenuation & Delay
Wireless Receiver
Wireless Transmitter
RF Cancellation
TX Signal Path RX Signal Path
RF ReferenceΣ
Balun
RSSI
• Need to match self-interference power and delay
• Can’t use digital samples: saturated ADC
Adaptive RF Cancellation
53
Use RSSI as an indicator of self-interference
TX RX
Attenuation & Delay
Wireless Receiver
Wireless Transmitter
RF Cancellation
TX Signal Path RX Signal Path
RF ReferenceΣ
Balun
RSSI
Control Feedback
54
Objective: Minimize received power
Control variables: Delay and Attenuation
TX RX
Attenuation & Delay
Wireless Receiver
Wireless Transmitter
RF Cancellation
TX Signal Path RX Signal Path
RF ReferenceΣ
Balun
RSSI
Control Feedback
55
Objective: Minimize received power
Control variables: Delay and Attenuation
56➔ Simple gradient descent approach to optimize
Objective: Minimize received power
Control variables: Delay and Attenuation
57
Typical convergence within 8-15 iterations (~1ms total)
58
Digital Interference Cancellation
TX RX
Attenuation & Delay
RF ➔ Baseband
ADC
Baseband ➔ RF
DAC
Encoder Decoder
Digital Interference Reference
RF Cancellation
TX Signal Path RX Signal Path
RF ReferenceΣ
FIR filter ∆
RSSI
Control Feedback
Channel Estimate
Balun
Bringing It All Together
59
Channel Coherence
~3dB reduction in cancellation in 1-2 seconds
~6dB reduction in <10 seconds
60
Performance
• WiFi full-duplex: with reasonable antenna separation
• Not enough for cellular full-duplex: need 20dB more
Implications
• WARPv2 boards with 2 radios
• OFDM reference code from Rice University
• 10MHz bandwidth OFDM signaling
• CSMA MAC on embedded processor
• Modified for full-duplex
Implementation
62
• CSMA/CA can’t solve this
• Schemes like RTS/CTS introduce significant overhead
APN1 N2
Current networks have hidden terminals
Mitigating Hidden Terminals
63
• 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
64
• 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
65Reduces hidden terminal losses by up to 88%
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
66
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
67
With full-duplex:
• AP sends and receives at the same timeDownlink Throughput = 1 Uplink Throughput = 1
Network Congestion and WLAN Fairness
68
1 AP with 4 stations without any hidden terminals
Throughput (Mbps)Throughput (Mbps)Fairness (JFI)
Upstream DownstreamFairness (JFI)
Half-Duplex 5.18 2.36 0.845
Full-Duplex 5.97 4.99 0.977
Full-duplex distributes its performance gain to improve fairness
Wide Area
(LTE)_
Apps Processor
BT
Media Processor
GPS
WLAN
UWB
DVB-H
FM/XM
All of these different radios interfere with one another
Today: limit which radios can be active at the same time
Coexistence Problem
Tomorrow: Full Duplex
Wide Area
(LTE)_
Apps Processor
BT
Media Processor
GPS
WLAN
UWB
DVB-H
FM/XM WLAN
LTE
Full duplexsignal cancellation
71
Access Point networks
Full-duplex Networking
72
Access Point networksCellular networks
Cell Basestation
Relay
Full-duplex Networking
73
Access Point networks
Multi-hop Networks
Cellular networks
Cell Basestation
Relay
Full-duplex Networking
74
Access Point networks
Multi-hop NetworksSecure Networks[1,2]
Cellular networks
Cell Basestation
Relay
Full-duplex Networking
[1] Gollakota et al. “They Can Hear Your Heartbeats: Non-Invasive Security for Implantable Medical Devices.”, in Sigcomm 2011.[2] Lee et al. “Secured Bilateral Rendezvous using Self-interference Cancellation in Wireless Networks”, in IFIP 2011.
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Access Point networks
Multi-hop NetworksSecure Networks[1,2]
Cellular networks
Cell Basestation
Relay
Full-duplex Networking
[1] Gollakota et al. “They Can Hear Your Heartbeats: Non-Invasive Security for Implantable Medical Devices.”, in Sigcomm 2011.[2] Lee et al. “Secured Bilateral Rendezvous using Self-interference Cancellation in Wireless Networks”, in IFIP 2011.
?
Possible Dealbreakers
v != a
R
TXRF Frontend
RXRF Frontend
Σv
-vT
R+aTattenuator anddelay line
balun
aT
2T
-T+T
R+aT-aT
Frequency Selectivity
Lower isbetter
Multipath
TXRF Frontend
RXRF Frontend
Σv
attenuator anddelay line
balun
-T+T
Second path
Greatest Potential?
• Where capacity is ultra-scarce (MIMO?)?
• Fixed channels/bands?
• Drop-in improvements (incremental)?
• Channel coherence < packet time?
• We can only guess.
• We’ve built working full duplex radios‣ Breaks a fundamental assumption in wireless‣ Current limitations: transmit power: ≤ WiFi
(200mW), bandwidth (≤ 20MHz)
• How full duplex could transform wireless
• Open technical barriers and challenges
Talk In a Nutshell
Backup
83
0 10 20 30-30 -20 -10
0
10
20
30
-30
-20
-10
x axis (meters)
y ax
is (
met
ers)
84
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
30
-30
-20
-10
x axis (meters)
y ax
is (
met
ers)
85
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