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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

What full-duplex

2

What full-duplex ... and why?

3

Current wireless radios

· Time Division Duplexing

Node 1

TX/RX

Node 2

TX/RX

Node 1

TX/RX

Node 2

TX/RX

Timeslot 1

Timeslot 2

4

Current wireless radios

· Time Division Duplexing

Node 1

TX/RX

Node 2

TX/RX

Node 1

TX/RX

Node 2

TX/RX

Timeslot 1

Timeslot 2

· Frequency Division Duplexing

Node 1

RX Frequency 1

Node 2

TX

TX

RX

Frequency 2

5

Single channel full-duplex

Node 1

RX TX

Node 2

TX RX

6

Single channel full-duplex

Very strong self-interference: ~70dB for 802.11

Node 1 SelfInterference

RX TX RX

Node 2

TX

7

Single channel full-duplex

Very strong self-interference: ~70dB for 802.11

Node 1 SelfInterference

RX TX RX

Node 2

TX

Main idea: cancel self-interference

Combine RF and digital techniques for cancellation

8

The story so far...

Mobicom'10[1]: Antenna Cancellation + other techniques

TX1 d

RX d + /2

TX2

[1] Choi et al. "Achieving single channel, full duplex wireless communication", Mobicom 2010

9

The story so far...

Mobicom'10[1]: Antenna Cancellation + other techniques

TX1 d

RX d + /2

TX2

· · ·

Does not adapt to channel changes Interference pattern can affect intended receivers Frequency dependent, narrowband cancellation

10

[1] Choi et al. "Achieving single channel, full duplex wireless communication", Mobicom 2010

· · ·

New, better RF and digital cancellation techniques Adaptive algorithms for auto-tuning cancellation Real-time full-duplex MAC layer implementation

11

Talk Outline

· · · · ·

RF Cancellation using Signal Inversion Adaptive RF Cancellation System Performance Implications to Wireless Networks Looking Forward

12

Talk Outline

· · · · ·

RF Cancellation using Signal Inversion Adaptive RF Cancellation System Performance Implications to Wireless Networks Looking Forward

13

Cancellation using Phase Offset

SelfInterference

Cancellation Signal

Cancellation using Phase Offset

SelfInterference

Cancellation Signal SelfInterference

Cancellation Signal

Frequency dependent, narrowband

15

Cancellation using Signal Inversion

SelfInterference

Cancellation Signal

16

Cancellation using Signal Inversion

SelfInterference

Cancellation Signal SelfInterference

Cancellation Signal

Frequency and bandwidth independent

17

BALUN : Balanced to Unbalanced Conversion

Xt

+Xt/2

-Xt/2

BALUN

18

BALUN : Balanced to Unbalanced Conversion

TX

RX

Cancellation Signal

Xt +Xt/2 +Xt/2 -Xt/2 Xt TX RF Frontend -Xt/2

BALUN

RX RF Frontend

19

TX

Over the air attenuation and delay

RX

Xt +Xt/2 +Xt/2 -Xt/2 Xt TX RF Frontend RX RF Frontend -Xt/2

BALUN

20

Signal Inversion Cancellation

TX Over the air attenuation and delay Attenuator and Delay Line +Xt/2 -Xt/2 Xt TX RF Frontend RX RF Frontend -Xt/2 RX

Xt

+Xt/2

BALUN

21

Signal Inversion Cancellation: Wideband Evaluation

· · ·

Measure wideband cancellation Wired experiments 240MHz chirp at 2.4GHz to measure response

/2 Delay

TX

Xt

+Xt/2 -Xt/2 RX TX Xt RF Signal Splitter

+Xt/2

+Xt/2

RX

Signal Inversion Cancellation Setup

Phase Offset Cancellation Setup

22

Higher is better

Lower is better

23

Higher is better

Lower is better

~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation.

Significant improvement in wideband cancellation

24

Other advantages

TX Attenuator and Delay Line +Xt/2 Xt TX RF Frontend RX RF Frontend -Xt/2 RX

· ·

From 3 antennas per node to 2 antennas Parameters adjustable with changing conditions

25

TX Attenuator and Delay Line +Xt/2 Xt TX RF Frontend -Xt/2

RX

RX RF Frontend

Passive components better than active components · No gain required · Saturation can lead to non-linearity · Passive components are more frequency flat

26

Talk Outline

· · · · · ·

RF Cancellation using Signal Inversion: ~50dB for 20Mhz Adaptive RF Cancellation Adaptive Digital Cancellation System Performance Implications to Wireless Networks Looking Forward

27

Adaptive RF Cancellation

TX RX

RF Reference

Attenuation & Delay

Balun

Wireless Transmitter

RF Cancellation

Wireless Receiver TX Signal Path RX Signal Path

· ·

Need to match self-interference power and delay Can't use digital samples: Saturated ADC

28

Adaptive RF Cancellation

TX RX

RF Reference

Attenuation & Delay

RSSI

Balun

Wireless Transmitter

RF Cancellation

Wireless Receiver TX Signal Path RX Signal Path

· ·

Need to match self-interference power and delay Can't use digital samples: Saturated ADC

RSSI : Received Signal Strength Indicator

29

Adaptive RF Cancellation

TX RX

RF Reference

Attenuation & Delay

Control Feedback

RSSI

Balun

Wireless Transmitter

RF Cancellation

Wireless Receiver TX Signal Path RX Signal Path

· ·

Need to match self-interference power and delay Can't use digital samples: Saturated ADC

Use RSSI as an indicator of self-interference

30

TX

RX

RF Reference

Attenuation & Delay

Control Feedback

RSSI

Balun

Wireless Transmitter

RF Cancellation

Wireless Receiver TX Signal Path RX Signal Path

Objective: Minimize received power Control variables: Delay and Attenuation

31

Objective: Minimize received power Control variables: Delay and Attenuation

32

Objective: Minimize received power Control variables: Delay and Attenuation Simple gradient descent approach to optimize

33

Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler

Signal + Interference Interference Sample

/4 Delay Gain I

Gain Q

Cancellation Signal

Clean Signal

34

Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler

TX RX

Gain I Balun

/4 Delay

Gain Q

Control Feedback

RSSI

Wireless Transmitter

Wireless Receiver

TX Signal Path

RX Signal Path

35

Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler But ...

· ·

Uses /4 delay to generate quadrature component: Not precise for all frequencies Active components for gain: saturation leading to non-linearities

36

Off-the-shelf electronically tunable hardware approximation: QHx220 noise canceler

37

Typical convergence within 8-15 iterations (~1ms total)

38

39

Saddle Point

Recovery from local minimas and saddle points possible, needs more iterations

40

· ·

~65% converge without going through a local minima 98% converge in <20 iterations

41

Talk Outline

· · · · ·

RF Cancellation using Signal Inversion: ~50dB for 20Mhz Adaptive RF Cancellation: ~1ms convergence System Performance Implications to Wireless Networks Looking Forward

42

Digital Cancellation

· ·

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

43

TX Signal

Analog Conversion and Shaping

TX

RX

Filtering and Digital Conversion

Residual Self-interference

Digital Receiver

44

TX Signal

Analog Conversion and Shaping

TX

RX

Filtering and Digital Conversion

Residual Self-interference

Channel Model

Cancellation Signal

-

+

Digital Cancellation

Digital Receiver

45

Bringing It All Together

TX RX

Control Feedback

RF Reference

Attenuation & Delay

RSSI

Balun

Baseband RF DAC

RF Cancellation

RF Baseband ADC

Digital Interference Cancellation

Digital Interference Reference

FIR filter

Channel Estimate

Decoder RX Signal Path

46

Encoder TX Signal Path

Performance

· · ·

~73 dB cancellation WLAN full-duplex:Yes, with reasonable antenna separation Not enough for cellular full-duplex

47

Channel Coherence

~3dB reduction in cancellation in 1-2 seconds ~6dB reduction in <10 seconds

48

Talk Outline

· · · · ·

RF Cancellation using Signal Inversion: ~50dB for 20Mhz Adaptive RF Cancellation: ~1ms convergence System Performance: ~73dB cancellation Implications to Wireless Networks Looking Forward

49

Implications to Wireless Networks

· ·

Breaks a basic assumption in wireless Can solve some fundamental problems with wireless networks today[1,2]

· ·

Hidden terminals Network congestion and WLAN fairness

[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 Selfinterference Cancellation", in WiOpt 2011

50

Implementation

· ·

WARPv2 boards with 2 radios OFDM reference code from Rice University

· · ·

10MHz bandwidth OFDM signaling CSMA MAC on embedded processor

Modified for Full-duplex

51

Mitigating Hidden Terminals

Current networks have hidden terminals

N1 AP N2

· ·

CSMA/CA can't solve this Schemes like RTS/CTS introduce significant overhead

52

Mitigating Hidden Terminals

Current networks have hidden terminals

N1 AP N2

· ·

CSMA/CA can't solve this Schemes like RTS/CTS introduce significant overhead

Full Duplex solves hidden terminals

N1

AP

N2

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

53

Mitigating Hidden Terminals

Current networks have hidden terminals

N1 AP N2

· ·

CSMA/CA can't solve this Schemes like RTS/CTS introduce significant overhead

Full Duplex solves hidden terminals

N1

AP

N2

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

Reduces hidden terminal losses by up to 88%

54

Network Congestion and WLAN Fairness

·

Without full-duplex: 1/n bandwidth for each node in network, including AP Downlink Throughput = 1/n Uplink Throughput = (n-1)/n

55

Network Congestion and WLAN Fairness

·

Without full-duplex: 1/n bandwidth for each node in network, including AP Downlink Throughput = 1/n Uplink Throughput = (n-1)/n

·

With full-duplex: AP sends and receives at the same time Downlink Throughput = 1 Uplink Throughput = 1

56

Network Congestion and WLAN Fairness

1 AP with 4 stations without any hidden terminals

Throughput (Mbps) Fairness (JFI) Upstream Half-Duplex Full-Duplex 5.18 5.97 Downstream 2.36 4.99 0.845 0.977

Full-duplex distributes its performance gain to improve fairness

57

Talk Outline

· · · · · ·

RF Cancellation using Signal Inversion: ~50dB for 20Mhz Adaptive RF Cancellation: ~1ms convergence Adaptive Digital Cancellation: ~30dB cancellation System Performance: ~73dB cancellation Implications to Wireless Networks: Collisions, Fairness Looking Forward

58

·

Other cancellation techniques Digital estimation for analog cancellation[1]

TX RX

Baseband RF DAC

Baseband RF DAC

RF Baseband ADC

TX Signal

Cancellation Signal

RX Signal

[1] Duarte et al. "Full-Duplex Wireless Communications Using Off-The-Shelf Radios: Feasibility and First Results.", in Asilomar 2010.

59

· ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response

60

· ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers

High Power Amplifier

TX Signal

61

· ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers

High Power Amplifier

TX Signal

+ Estimate Distortion

62

· ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation

63

· · ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation Single antenna solution: circulators

TX Signal

RX Signal

Circulator

64

· · · ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation Single antenna solution: circulators Device precision: 1 ps resolution for delay line

65

· · · · ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation Single antenna solution: circulators Device precision: 1 ps resolution for delay line Going mobile: Higher cancellation, faster adaptation

66

· · · · · ·

Other cancellation techniques Digital estimation for analog cancellation[1] Non-linear channel response Reduce distortion: feedforward amplifiers Compensate: non-linear digital cancellation Single antenna solution: circulators Device precision: 1 ps resolution for delay line Going mobile: Higher cancellation, faster adaptation MIMO full-duplex

67

Full-duplex Networking

Access Point networks

68

Full-duplex Networking

Access Point networks

Cell Basestation

Relay

Cellular networks

69

Full-duplex Networking

Access Point networks

Cell Basestation

Relay

Cellular networks

Multi-hop Networks

70

Full-duplex Networking

Access Point networks

Cell Basestation

Relay

Cellular networks

Secure Networks[1,2]

Multi-hop Networks

[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.

71

Full-duplex Networking

Access Point networks

?

Cell Basestation

Relay

Cellular networks

Secure Networks[1,2]

Multi-hop Networks

[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.

72

Thank You Questions?

73

Backup

74

Talk Outline

· · · · · ·

RF Cancellation using Signal Inversion: ~50dB for 20Mhz Adaptive RF Cancellation: ~1ms convergence Adaptive Digital Cancellation System Performance Implications to Wireless Networks Looking Forward

75

Digital Cancellation

· ·

Create a precise "digital replica" of the selfinterference signal using TX digital samples Subtract self-interference replica from received digital signal

Requires ADC not saturated: RF cancellation

76

OFDM processing

Signal Band

77

OFDM processing

Sub-bands

78

OFDM processing

Channel Distortion

79

OFDM processing

Channel Distortion Equalization

80

OFDM processing

Channel Distortion Equalization

RX

RF Mixer

ADC Carrier Frequency Demapping Carrier Frequency Offset Correction Packet Detect FFT Engine

Equalization Channel Estimation

81

Step 1: Estimation

Self-interference Sounding

Training Preamble Sequence FIR Filter

IFFT RX

Self-interference Estimate

RF Mixer

ADC Carrier Frequency Demapping Carrier Frequency Offset Correction Packet Detect FFT Engine

Equalization Channel Estimation

Estimation includes effect of RF cancellation

82

Step 2: Cancellation

Cancellation Signal TX Signal

FIR Filter

IFFT RX

Self-interference Estimate

RF Mixer

ADC Carrier Frequency

+

Carrier Frequency Offset Correction Packet Detect

FFT Engine

Demapping

Equalization Channel Estimation

83

Step 2: Cancellation

Cancellation Signal TX Signal

FIR Filter

IFFT RX

Self-interference Estimate

RF Mixer

ADC Carrier Frequency

+

Carrier Frequency Offset Correction Packet Detect

FFT Engine

Demapping

Equalization Channel Estimation

30dB Cancellation

84

Talk Outline

· · · · · ·

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

85

Phase Offset Cancellation: Block Diagram

TX1

d

RX

d + /2

TX2

Attenuator Power Splitter RX RF Frontend TX RF Frontend

Digital Processor

86

Phase Offset Cancellation: Performance

TX1 TX2

-25 -30 Only TX1 Active

RSSI (dBm)

-35 -40 -45 -50 -55 -60 0 5 10 15 20 25

87

Position of Receive Antenna (cm)

Phase Offset Cancellation: Performance

TX1 TX2

-25 -30 Only TX1 Active Only TX2 Active

RSSI (dBm)

-35 -40 -45 -50 -55 -60 0 5 10 15 20 25

88

Position of Receive Antenna (cm)

Phase Offset Cancellation: Performance

TX1 TX2

-25 -30 Only TX1 Active

Both TX1 & TX2 Active

Only TX2 Active

RSSI (dBm)

-35 -40 -45 -50 -55 -60 0

Null Position

5 10 15 20 25

89

Position of Receive Antenna (cm)

Phase Offset Cancellation: Performance

TX1 TX2

-25 -30 Only TX1 Active

Both TX1 & TX2 Active

Only TX2 Active

RSSI (dBm)

-35 -40 -45 -50 -55 -60 0

Null Position

5

~25-30dB

10 15 20 25

90

Position of Receive Antenna (cm)

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

·

30 y axis (meters) 20 10 0

Different transmit powers for two TX helps

-58 dBm -52 dBm y axis (meters) 30 20 10 0 -52 dBm

-10 -20 -30 -30 -20 -10 0 10 20 30 x axis (meters)

-10 -20 -30 -30 -20 -10 0 10 20 30 x axis (meters)

Single Transmit Antenna

Two Transmit Antennas

91

Sensitivity of Phase Offset Cancellation

Cancellation (dB) Cancellation (dB)

Higher is better

Higher is better

dB

Error (mm)

Amplitude Mismatch between TX1 and TX2

Placement Error for RX

92

Sensitivity of Phase Offset Cancellation

Cancellation (dB) Cancellation (dB)

dB

Error (mm)

Amplitude Mismatch between TX1 and TX2

Placement Error for RX

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

93

Sensitivity of Phase Offset Cancellation

Cancellation (dB) Cancellation (dB)

dB

Error (mm)

Amplitude Mismatch between TX1 and TX2

Placement Error for RX

· ·

Rough prototype good for 802.15.4 More precision needed for higher power systems (802.11)

94

Bandwidth Constraint

A /2 offset is precise for one frequency

TX1 d

RX d + /2

TX2

fc

95

Bandwidth Constraint

A /2 offset is precise for one frequency not for the whole bandwidth

TX1 d

RX d + /2

TX2

fc -B

fc

fc+B

96

Bandwidth Constraint

A /2 offset is precise for one frequency not for the whole bandwidth

TX1 d1 TX1 d RX d + /2 RX d2 d2 + +B/2 TX2 RX d1 + -B/2 TX2 TX2

fc -B

fc

fc+B

TX1

97

Bandwidth Constraint

A /2 offset is precise for one frequency not for the whole bandwidth

TX1 d1 TX1 d RX d + /2 RX d2 d2 + +B/2 TX2 RX d1 + -B/2 TX2 TX2

fc -B

fc

fc+B

TX1

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

98

Bandwidth Constraint

5.1 G Hz Hz

2.4 G

300 M Hz

Edge frequency fc

99

Bandwidth Constraint

5.1 G Hz Hz

2.4 G

300 M Hz

· · ·

WiFi (2.4GHz, 20MHz): Max 47dB reduction Bandwidth => Cancellation Carrier Frequency => Cancellation

100

Mitigating Hidden Terminals

0.9 Packet Reception Ratio 0.8 0.7 0.6 0.5 Full Duplex Half Duplex

0

2000

4000

6000

8000

Data Load (Kbps)

101

Mitigating Hidden Terminals

0.9 Packet Reception Ratio 0.8 0.7 0.6 0.5 Full Duplex Half Duplex

0

2000

4000

6000

8000

Data Load (Kbps)

·

Full-duplex reduces hidden terminal related losses by 88% at 2 Mbps

102

Mitigating Hidden Terminals

0.9 Packet Reception Ratio 0.8 0.7 0.6 0.5 Full Duplex Half Duplex 1.000 0.925 0.850 0.775 0.700 Full Duplex

Fairness (JFI)

Half Duplex

0

2000

4000

6000

8000

0

2000

4000

6000

8000

Data Load (Kbps)

Data Load (Kbps)

· ·

Full-duplex reduces hidden terminal related losses by 88% at 2 Mbps At higher loads, half-duplex improves PRR at the expense of fairness

103

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