Towards Collision Detection in Wireless Networks

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Towards Collision Detection in Wireless
Networks
Souvik Sen, Naveen Santhapuri, Romit Roy
Choudhury, Srihari Nelakuditi
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Collision in Wireless Networks
T1
R
T2
3
Collision in Wireless Networks
T1
R
T2
t0
t1
time
Not Efficient!
Retransmit
T1 should have stopped right after collision
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Collision in Wired Networks

• Transmitter aborts transmission on collision
• Transmitter senses the signal while transmitting
• If (sensed ! transmitted), abort

Collision Detection (CSMA/CD)
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Can we do CSMA/CD in Wireless?
Seems hard because.....
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Wireless Signal Propagation
T1 cannot send and listen in parallel
T1
R
T2
Signal power
Distance
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But what if we could do CSMA/CD in wireless?
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Is CSMA/CD Beneficial in Wireless?
T2
T3
T1
R2
R1
R3
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Is CSMA/CD Beneficial in Wireless?
Collision Detected
T2
T3
T1
R2
R1
R3
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Is CSMA/CD in Wireless Beneficial?
Collision Detected
T2
T3
T1
R2
R1
R3
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Is CSMA/CD in Wireless Beneficial?
Collision Detected
T2
T3
T1
R2
R1
R3
CSMA/CD frees the channel for other transmissions
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Can we imitate CSMA/CD on Wireless?
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Practical Requirements?

• Transmitter cannot detect collision
• Receiver needs to detect it

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Practical Requirements?

• Transmitter cannot detect collision
• Receiver needs to detect it
• Receiver needs to convey
• collision notification to the transmitter

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Practical Requirements?

• Transmitter cannot detect collision
• Receiver needs to detect it
• Receiver needs to convey
• collision notification to the transmitter
• Transmitter needs an additional antenna
• To receive notification

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Practical Requirements?

• Transmitter cannot detect collision
• Receiver needs to detect it
• Receiver needs to convey
• collision notification to the transmitter
• Transmitter needs an additional antenna
• To receive notification

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Overview
SS1
CrossLayer
CrossLayer
Tx
Rx
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Overview
SS1S2
SS1
CrossLayer
CrossLayer
Tx
Rx
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Two Key Challenges
2. Detect Collision in real time
1. Find Notification on Listening Antenna
SS1S2
CrossLayer
CrossLayer
Tx
Rx
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2. Detect Collision in real time
1. Find Notification on Listening Antenna
We propose CSMA/CN
Our key idea Correlation
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Challenge 1 Detecting Notification

• Hard to decode notification on same channel
• Self-signal too strong
• Let Tx and Rx share a unique signature
• Tx correlates with shared signature
• Detects collision notification, aborts

Observe No decoding, just correlate
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Challenge 1 Detecting Notification
Self Signal
Notification Signature
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Challenge 1 Detecting Notification
Correlation
Self Signal
Notification Signature
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Challenge 1 Detecting Notification
Correlation
Self Signal
Notification Signature
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Challenge 1 Detecting Notification
Correlation
Self Signal
Notification Signature
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Challenge 1 Detecting Notification
Correlation
Self Signal
Notification Signature
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Challenge 1 Detecting Notification
Correlation
Sample Number
Whenever there is a notification, there is a
jump in correlation
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Challenge 2 Interference Detection
T2
T1
R2
R
R1
Correlate for Preamble SoftPHY
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What if transmitter starts second?
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SOI starts after interference
T2
T1
R2
R
R1
Correlate (Sign(R1))
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Signal Correlation and Abort
T2
Corr (Sign(R1))
T1
R2
R
R1
Correlate (Sign(R1))
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Performance Evaluation

• 7 node USRP testbed
• Zigbee CC2420 PHY
• Max data rate 250Kbps
• Signature size 5 bytes
• Compare with 802.11-like and PPR
• PPR detects interfered portion of received packet
  
• Transmitter sends only the interfered portion

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Notification Detection at Tx
Notification Signal ltlt Self Signal
How weak can the notification signal be?
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How weak the notification signal be?
Signal power
Self Signal
Notification Signal
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How weak the notification signal be?
Signal power
?
Self Signal
Notification Signal
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Interference Detection at Rx

• Interference detection accuracy of 93
• Receiver should detect interference quickly
• Quicker detection Faster Tx abortion

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Interference Detection Speed
Bytes after interferer started
CSMA/CN predicts collision within 7 bytes
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Testbed Experimentation

• One link doing CSMA/CN
• CSMA/CN link has an exposed and hidden terminal
• Whenever CSMA/CN link fails due to interference
• CSMA/CN link stops
• Exposed terminal transmits reducing channel
  wastage

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Testbed Throughput
PPR continues to transmit under collision, worse
than CSMA/CN
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Traced Based Evaluation
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Throughput in Kbps
Upto 50 gain in per link throughput
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Summary

• CSMA/CN imitates CSMA/CD in wireless
• Rx uses correlation to detect interference
• Tx uses correlation to detect notification
• Others can utilize freed-up channel

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Limitation and Future Work

• Improve Correlation
• Suppress known self signal
• Multiple Interferers
• Interference detection logic yet to be tested
• Interference due to notification
• Collision notification can interfere with other
  links
• Recover using FEC

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Duke SyNRG Research Group http//synrg.ee.duke.edu