Current 802'11x Wireless Routing uplink

1
Current 802.11x Wireless Routing (uplink)

• Dina in a lossy environment with multiple routers

2
Current 802.11x Wireless Routing (uplink)

• A single packet on the uplink

3
Current 802.11x Wireless Routing (uplink)

• An erroneous packet received

4
Current 802.11x Wireless Routing (uplink)

• Wireless spatial diversity is free
• Both routers overhear Dinas transmitted packet

5
Current 802.11x Wireless Routing (uplink)

• How can these two copies of an erroneous packet
  be combined to create a correct packet?

6
Previous Work Multi Radio Diversity (MRD)



Continue recombining packets on a block basis
until a packet with a correct CRC checksum occurs
7
Much Lower Retransmission Rate
Instead of using hard decision higher layer
recombining
Exploit wireless spatial diversity and soft
information
8
The SOFT approach



Continue recombining packets on a block basis
until a packet with a correct CRC checksum occurs
9
Analog Network Coding
Router
10
Analog Network Coding
Router
Interference

• Dina and Jon transmit simultaneously

11
Analog Network Coding
Router

• Dina and Jon transmit simultaneously
• Router amplifies and broadcasts interfered signal

12
Analog Network Coding
Router

• Dina and Jon transmit simultaneously
• Router amplifies and broadcasts interfered signal
• Dina subtracts known signal from interfered
  signal

13
Analog Network Coding
Router

• Dina and Robert transmit simultaneously
• Router amplifies and broadcasts interfered signal
• Dina subtracts known signal from interfered
  signal

• Analog Network Coding requires 2 time slots
• Higher throughput

14
It Is More Than Going From 3 To 2!

• Philosophical shift in dealing with interference
• Strategically exploit interference instead of
  avoiding it
• Promises new ways of dealing with hidden terminals

15
Hidden Terminal Scenario
Src
R1
R2
Dst
16
Hidden Terminal Scenario
Src
R1
R2
Dst
17
Hidden Terminal Scenario
Src
R1
R2
Dst
P2

• Src and R2 transmit simultaneously

18
Hidden Terminal Scenario
Src
R1
R2
Dst
P1
P2

• Src and R2 transmit simultaneously
• R1 subtracts P1, which he relayed earlier to
  recover P2 that he wants

19
Hidden Terminal Scenario
Src
R1
R2
Dst
P1
P2

• R2 and Src are hidden terminals
• Today Simultaneous transmission ? Collision
• ANC Simultaneous transmission ? Success!

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Hidden Terminal Scenario
Src
R1
R2
Dst

• Other Benefits of ANC
• First step toward addressing hidden terminals
• ANC extends network coding to new scenarios

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How do we make it work?
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Practical Challenges

• Interfered signal is not exactly the sum
• Channel distorts signals
• Two signals are never synchronized
• It is not sD(t) sJ(t) but f1(sD(t))
  f2(sJ(t-T))

• Prior work assumes full synchronization and
  ignores channel distortion

Not Practical!
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Key Idea Exploit Asynchrony!
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Key Idea Exploit Asynchrony!

• Dina uses interference-free parts to estimate
  channel and timing
• Dina compensates for her interfering signal
• Jon runs the same algorithm backwards!

Exploit asynchrony to make it practical
25
Cross layer realization of our idea
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Protocol

• Router senses idle medium and broadcasts a
  trigger to Dina and Jon
• Dina and Jon jitter their start times randomly
  and transmit
• Router amplifies and forwards interfered signal
• Dina and Jon receive and decode

How do they decode?
27
Primer on Modulation

• Nodes transmit vectors on channel
• Focus on MSK (Minimum Shift Keying) modulation

D2 lags D1 by 90 degrees ? Bit 0
D2 leads D1 by 90 degrees ? Bit 1
D2
D1
D2
D1
28
Primer on Channel Effects

• Attenuation

D2
D2
Channel
D1
D1
D2 and D1 are attenuated by the same amount
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Primer on Channel Effects

• Attenuation
• Rotation

D2
D2
Channel
D1
D1
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Primer on Channel Effects

• Attenuation
• Rotation

D2
D1
Channel
D2
D1
To decode, receiver computes angle between
received vectors Angle (D2, D1) 90 degrees?
Bit 1 was transmitted
Angle between vectors is preserved
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So, How Does Dina Decode?
No Interference
No Interference
Dinas Signal
Jons Signal
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So, How Does Dina Decode?
No Interference
No Interference
Interference

• Small uninterfered part at the start
• Decodes uninterfered part via standard MSK
  demodulation
• Once interference starts, Dina changes decoding
  algorithm

33

• What did Dina send?

D2
D1
34

• What did Dina send?
• What did Jon send?

J1
D1
D2
J2
35

• What is Interference ? Vector addition

X1
D1
J1
D2
J2
X2
36

• What does Dina know?

D2
X1
D1
J1
D2
D1
J2
X2
37

• What does Dina know?

D2
X1
ß
a
D1
X2
No Interference
Amplitude of Jons Vectors ? ß
Amplitude of her Vectors ? a
38

• What does Dina know?

D2
X1
D1
D1
J1
J1
D1
X2

• Dina finds solutions for X1 and X2

39

• What does Dina know?

D2
X1
D1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Two solutions for each interfered vector!
40

• What does Dina know?

D2
X1
D1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Four possible angles!
41

• What does Dina know?

D2
D1
X1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Four possible angles!
42

• What does Dina know?

D2
D1
X1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Four possible angles!
43

• What does Dina know?

D2
X1
D1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Four possible angles!
44

• What does Dina know?

D2
X1
D1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Four possible angles!
45

• What does Dina know?

D2
X1
D1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Pick the correct angle ? 90 degrees
46

• What does Dina know?

D2
D1
X1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Pick the correct angle ? 90 degrees
47

• What does Dina know?

D2
D1
X1
D1
J1
J1
J2
D2
D1
J2
X2
D2

• Dina finds solutions for X1 and X2

Dictates solution for Jons vectors!
48

• What does Dina know?

D2
D1
X1
J1
D2
D1
J2
X2

• Dina finds angle between J1 and J2 and decodes

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Decoding Algorithm Decoding interference
Interference

• Decode rest of the interfered part using this
  algorithm
• Decode final uninterfered part from Jon via
  standard MSK demodulation

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Performance
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ANC Implementation

• Software GNURadio codebase
• Hardware USRP frontend
• 2.4-2.48 GHz frequency range
• SNR of 20-30 dB
• Canonical topologies in mesh networks

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Dina and Jon
Router

• ANC throughput gain over current 4/2 2
• ANC throughput gain over COPE 3/2 1.5

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Throughput gain for Dina-Jon scenario
CDF
Gain over Routing
Throughput gain
Median Gain over Routing 70
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Throughput gain for Dina-Jon scenario
CDF
Gain over Routing
Gain over COPE
Throughput gain
Median Gain over Routing 70
Median Gain over Routing 70 Median Gain over
COPE 30
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X topology
Router
Interference
Capture!
Capture!
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X topology
Router
Interference
Capture!
Capture!
57
X topology
Router

• ANC throughput gain over current 4/2 2
• ANC throughput gain over COPE 3/2 1.5

ANC decodes interference using overheard signals
58
Throughput gain X topology
CDF
Gain over Routing
Throughput gain
Median Gain over Routing 65
59
Throughput gain X topology
CDF
Gain over Routing
Gain over COPE
Throughput gain
Median Gain over Routing 65
Median Gain over Routing 65 Median Gain over
COPE 28
60
Chain topology
Src
R1
R2
Dst

• ANC throughput gain over current 3/2 1.5

61
Throughput gain Chain topology
CDF
Throughput gain
Median Gain over Routing 37
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Conclusion

• Shifts in the design of wireless networks to
  recognize wireless for what it is
• Embrace Broadcast
• Embrace Interference

• Implementation that yields large throughput gains