Post on 07-Jun-2018
Practical, Real-time,Full-Duplex Wireless
Mayank Jain, Jung Il Choi, Taemin Kim,Dinesh Bharadia, Kannan Srinivasan, Siddharth Seth,
Philip Levis, Sachin Katti, Prasun Sinha
September 22, 2011
1
Single channel full-duplex
2
TX
RX
RX
TXNode 1 Node 2
Mobicom’10[1]:
Antenna Cancellation + other techniques
3
The story so far...
d d + λ/2
TX1 TX2RX
[1] Choi et al. “Achieving single channel, full duplex wireless communication”, Mobicom 2010
Mobicom’10[1]:
Antenna Cancellation + other techniques
4
The story so far...
d d + λ/2
TX1 TX2RX
• Frequency dependent, narrowband
• Requires manual tuning
• Two transmit antennas cause complex far-field behavior
• New full-duplex radio design: signal inversion cancellation
• Wideband, frequency independent
• Adaptive
• One transmit antenna design
• Real-time full-duplex MAC layer implementation
• Show MAC layer gains with full-duplex
5
Contributions
• RF Design using Signal Inversion
• Adaptive RF Cancellation
• System Performance
• Implications to Wireless Networks
• Open Questions
Talk Outline
6
• RF Design using Signal Inversion
• Adaptive RF Cancellation
• System Performance
• Implications to Wireless Networks
• Open Questions
Talk Outline
7
The challenge of full-duplex
➔ Very strong self-interference: ~70dB for 802.11
8
TX
RX
RX
TXNode 1 Node 2
Self-Interference
9
TX
RX
RX
TXNode 1 Node 2
Self-Interference
Combine RF and digital techniques for cancellation
The challenge of full-duplex
➔ Very strong self-interference: ~70dB for 802.11
Cancellation using Phase OffsetSelf-Interference
Cancellation Signal
∑
Cancellation using Phase Offset
11
Self-Interference
Cancellation Signal
∑
Self-Interference
Cancellation Signal
∑
Frequency dependent, narrowband
Cancellation using Signal Inversion
12
Self-Interference
Cancellation Signal
∑
Cancellation using Signal Inversion
13
Self-Interference
Cancellation Signal
∑
Self-Interference
Cancellation Signal
∑
Frequency and bandwidth independent
Time
14
Xt +Xt/2
-Xt/2
BALUN
BALUN : Balanced to Unbalanced Conversion
Time
15
TX RX
TXRF Frontend
Xt
+Xt/2 -Xt/2
∑
RXRF Frontend
Xt +Xt/2
-Xt/2
BALUN
BALUN : Balanced to Unbalanced Conversion
Cancellation Signal
Time
16
TX RX
TXRF Frontend
Xt
+Xt/2 -Xt/2
∑
RXRF Frontend
Xt +Xt/2
-Xt/2
BALUN
Over the air attenuation and delay
Time
17
TX RX
TXRF Frontend
Attenuator andDelay Line
Xt
+Xt/2 -Xt/2
∑
RXRF Frontend
Xt +Xt/2
-Xt/2
BALUN
Signal Inversion Cancellation
Over the air attenuation and delay
+Xt/2
• Measure wideband cancellation
• Wired experiments
• 240MHz chirp at 2.4GHz to measure response
Time
Signal Inversion Cancellation: Wideband Evaluation
18
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
19
Lower isbetter
Higher isbetter
Time
20
~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation.
Significant improvement in wideband cancellation
Lower isbetter
Higher isbetter
Time
21
• 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
• RF Design using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation
• System Performance
• Implications to Wireless Networks
• Open Questions
Talk Outline
22
• Need to match self-interference power and delay
• Can’t use digital samples: Saturated ADC
Adaptive RF Cancellation
23
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
24
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
25
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
26
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
27
Objective: Minimize received power
Control variables: Delay and Attenuation
28➔ Simple gradient descent approach to optimize
Objective: Minimize received power
Control variables: Delay and Attenuation
29
Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler
Gain Q
Gain I
λ/4 Delay
Σ Σ
InterferenceSample
Signal + Interference
CancellationSignal Clean
Signal
30
Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler
Gain Q
Gain I
λ/4 Delay
Σ Σ
TX RX
Wireless Receiver
Wireless Transmitter
TX Signal Path RX Signal Path
RSSI
Control FeedbackBalun
31
Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler
32
Typical convergence within 8-15 iterations (~1ms total)
• RF Design using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation: ~1ms convergence
• System Performance
• Implications to Wireless Networks
• Open Questions
Talk Outline
33
Digital Cancellation
34
• Measure residual self-interference after RF cancellation
• Subtract self-interference from received digital signal
35
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
36
Channel Coherence
~3dB reduction in cancellation in 1-2 seconds
~6dB reduction in <10 seconds
37
Performance
• WiFi full-duplex: with reasonable antenna separation
• Not enough for cellular full-duplex: need 20dB more
• RF Design using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation: ~1ms convergence
• System Performance: ~73dB cancellation
• Implications to Wireless Networks
• Open Questions
Talk Outline
38
• Breaks a basic assumption in wireless
• Can solve some fundamental problems with wireless networks today[1,2]
• Hidden terminals
• Network congestion and WLAN fairness
Implications to Wireless Networks
39
[1] Choi et al. “Achieving single channel, full duplex wireless communication”, in Mobicom 2010[2] Singh et al. “Efficient and Fair MAC for Wireless Networks with Self- interference Cancellation”, in WiOpt 2011
• 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
40
• CSMA/CA can’t solve this
• Schemes like RTS/CTS introduce significant overhead
APN1 N2
Current networks have hidden terminals
Mitigating Hidden Terminals
41
• 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
42
• 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
43Reduces 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
44
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
45
With full-duplex:
• AP sends and receives at the same timeDownlink Throughput = 1 Uplink Throughput = 1
Network Congestion and WLAN Fairness
46
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
• RF Design using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation: ~1ms convergence
• System Performance: ~73dB cancellation
• Implications to Wireless Networks: Collisions, Fairness
• Open Questions
Talk Outline
47
48
Improving Full-duplex
• Non-linear channel response
• Non-linear channel responseReduce distortion: feedforward amplifiers
49
Improving Full-duplex
50
• Non-linear channel responseReduce distortion: feedforward amplifiersCompensate: non-linear digital cancellation
Improving Full-duplex
51
TX Signal RX Signal
Circulator
• Non-linear channel responseReduce distortion: feedforward amplifiersCompensate: non-linear digital cancellation
• Single antenna solution: circulators
Improving Full-duplex
52
• Non-linear channel responseReduce distortion: feedforward amplifiersCompensate: non-linear digital cancellation
• Single antenna solution: circulators
• MIMO full-duplex
Improving Full-duplex
53
Access Point networks
Full-duplex Networking
54
Access Point networksCellular networks
Cell Basestation
Relay
Full-duplex Networking
55
Access Point networks
Multi-hop Networks
Cellular networks
Cell Basestation
Relay
Full-duplex Networking
56
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.
57
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.
?
Summary
• Design for real-time full-duplex wireless
• Makes full-duplex WiFi possible
• Still some way to go for full-duplex cellular
• Made practical using adaptive techniques
• Rethinking of wireless networks
• WiFi: hidden terminals and fairness
• Many more possibilities
58
59
Thank You
Questions?
Backup
60
• Other cancellation techniques
Digital estimation for RF cancellation[1]
61
TX RX
RF ➔ Baseband
ADC
Baseband ➔ RF
DAC
TX Signal RX Signal
Σ
Baseband ➔ RF
DAC
Cancellation Signal
[1] Duarte et al. “Full-Duplex Wireless Communications Using Off-The-Shelf Radios: Feasibility and First Results.”, in Asilomar 2010.
• RF Cancellation using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation: ~1ms convergence
• Adaptive Digital Cancellation
• System Performance
• Implications to Wireless Networks
• Looking Forward
Talk Outline
62
Digital Cancellation
63
• Create a precise “digital replica” of the self-interference signal using TX digital samples
• Subtract self-interference replica from received digital signal
Requires ADC not saturated: RF cancellation
64
OFDM processing
SignalBand
65
OFDM processing
Sub-bands
66
OFDM processing
ChannelDistortion
67
OFDM processing
ChannelDistortion
Equalization
OFDM processing
68
ADC
RX RF Mixer
CarrierFrequency
Carrier FrequencyOffset Correction
Packet DetectFFT Engine
Channel Estimation
EqualizationDemapping
ChannelDistortion
Equalization
Step 1: Estimation
69
Self-interference Sounding
Preamble TrainingSequence Self-interference
Estimate
FIR Filter
ADC
RX RF Mixer
CarrierFrequency
Carrier FrequencyOffset Correction
Packet DetectFFT Engine
Channel Estimation
EqualizationDemapping
IFFT
Estimation includes effect of RF cancellation
Step 2: Cancellation
70
ADC
RX RF Mixer
CarrierFrequency
Carrier FrequencyOffset Correction
Packet DetectFFT Engine
Channel Estimation
EqualizationDemapping
TX Signal
+-
Self-interference Estimate
FIR Filter
IFFT
CancellationSignal
Step 2: Cancellation
71
ADC
RX RF Mixer
CarrierFrequency
Carrier FrequencyOffset Correction
Packet DetectFFT Engine
Channel Estimation
EqualizationDemapping
TX Signal
+-
Self-interference Estimate
FIR Filter
IFFT
CancellationSignal
30dB Cancellation
• RF Cancellation using Signal Inversion: ~50dB for 20Mhz
• Adaptive RF Cancellation: ~1ms convergence
• Adaptive Digital Cancellation: ~30dB cancellation
• System Performance
• Implications to Wireless Networks
• Looking Forward
Talk Outline
72
Attenuator
Phase Offset Cancellation: Block Diagram
d d + �/2TX1 TX2RX
RXRF Frontend
Digital Processor
TXRF Frontend
Power Splitter
73
-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
Phase Offset Cancellation: Performance
74
-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
75
Only TX1 Active
Phase Offset Cancellation: Performance
-60
-55
-50
-45
-40
-35
-30
-25
0 5 10 15 20 25
RSS
I (dB
m)
Position of Receive Antenna (cm)
NullPosition
TX1 TX2
76
Only TX1 ActiveOnly TX2 Active
Both TX1 & TX2 Active
Phase Offset Cancellation: Performance
-60
-55
-50
-45
-40
-35
-30
-25
0 5 10 15 20 25
RSS
I (dB
m)
Position of Receive Antenna (cm)
NullPosition
TX1 TX2
77
~25-30dB
Only TX1 ActiveOnly TX2 Active
Both TX1 & TX2 Active
Phase Offset Cancellation: Performance
78
What about attenuation at intended receivers?Destructive interference can affect this signal too!
• Different transmit powers for two TX helps
Single Transmit Antenna Two Transmit Antennas
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
0 10 20 30-30 -20 -10
0
10
20
30
-30
-20
-10
x axis (meters)
y ax
is (
met
ers)
-52 dBm
Sensitivity of Phase Offset Cancellation
79
Amplitude Mismatch between TX1 and TX2
Placement Errorfor RX
dB
Can
cella
tion
(dB)
Can
cella
tion
(dB)
Error (mm)
Higher is better
Higher is better
Amplitude Mismatch between TX1 and TX2
Placement Errorfor RX
dB
Can
cella
tion
(dB)
Can
cella
tion
(dB)
Error (mm)
80
30dB cancellation < 5% (~0.5dB) amplitude mismatch < 1mm distance mismatch
Sensitivity of Phase Offset Cancellation
81
• 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
dB
Can
cella
tion
(dB)
Can
cella
tion
(dB)
Error (mm)
Sensitivity of Phase Offset Cancellation
Bandwidth Constraint
82
fc
d d + λ/2
TX1 TX2RX
A λ/2 offset is precise for one frequency
Bandwidth Constraint
A λ/2 offset is precise for one frequencynot for the whole bandwidth
83
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
84
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
A λ/2 offset is precise for one frequencynot for the whole bandwidth
85
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
Bandwidth Constraint
86
2.4 GHz
5.1 GHz
300 MHz
fc
Edge frequency
Bandwidth Constraint
87
• WiFi (2.4GHz, 20MHz): Max 47dB reduction
• Bandwidth⬆ => Cancellation⬇• Carrier Frequency⬆ => Cancellation⬆
2.4 GHz
5.1 GHz
300 MHz
Mitigating Hidden Terminals
88
0.5
0.6
0.7
0.8
0.9
0 2000 4000 6000 8000Data Load (Kbps)
Pack
et R
ecep
tion
Ratio
Full Duplex
Half Duplex
0.5
0.6
0.7
0.8
0.9
0 2000 4000 6000 8000
Mitigating Hidden Terminals
89
Data Load (Kbps)
Pack
et R
ecep
tion
Ratio
Full Duplex
Half Duplex
• Full-duplex reduces hidden terminal related losses by 88% at 2 Mbps
Mitigating Hidden Terminals
90
0.700
0.775
0.850
0.925
1.000
0 2000 4000 6000 8000Data Load (Kbps)
Fairn
ess
(JFI
)
Full Duplex
Half Duplex
• Full-duplex reduces hidden terminal related losses by 88% at 2 Mbps
• At higher loads, half-duplex improves PRR at the expense of fairness
0.5
0.6
0.7
0.8
0.9
0 2000 4000 6000 8000Data Load (Kbps)
Pack
et R
ecep
tion
Ratio
Full Duplex
Half Duplex
Time
91
Passive components better than active components
• No gain required
• Saturation can lead to non-linearity
• Passive components are more frequency flat
TX RX
TXRF Frontend
Attenuator andDelay Line
Xt
+Xt/2 -Xt/2
∑
RXRF Frontend
92
• ~65% converge without going through a local minima
• 98% converge in <20 iterations
93
AnalogConversionand Shaping
TX Signal TXFiltering and
DigitalConversion
RX
∑+-Channel
Model
DigitalReceiver
CancellationSignal
Digital Cancellation
Residual Self-interference
• Other cancellation techniques
Digital estimation for RF cancellation[1]
• Non-linear channel response
Reduce distortion: feedforward amplifiers
94
TX Signal
EstimateDistortion
∑+ -
High PowerAmplifier