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CSMA/CN: Carrier Sense Multiple Access with Collision Notification
Souvik Sen, Romit Roy Choudhury, Srihari Nelakuditi
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Collision in Wireless Networks
T1 R T2 time 2
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Collision in Wireless Networks
T1 R T2 time 2
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Collision in Wireless Networks
T1 R T2 time 2
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Collision in Wireless Networks
T1 R T2 Collision time 2
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Collision in Wireless Networks
T1 R T2 Collision ACK Timeout time 2
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Collision in Wireless Networks
T1 R T2 Collision ACK Timeout time Retransmit 2
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Collision in Wireless Networks
T1 R T2 time Not Efficient! Collision ACK Timeout Retransmit 3
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Collision in Wireless Networks
T1 R T2 Collision ACK Timeout time Not Efficient! Retransmit Better if T1 stops right after collision 3
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Collision in Wired Networks
R T2 Collision Ethernet 4
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Collision in Wired Networks
R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision 4
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Collision in Wired Networks
R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting 4
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Collision in Wired Networks
R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting If (sensed != transmitted), abort 4
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Collision in Wired Networks
R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting If (sensed != transmitted), abort Collision Detection (CSMA/CD) 4
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Why not do CSMA/CD in Wireless?
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Why not do CSMA/CD in Wireless?
Has been hard because.....
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Wireless Signal Propagation
Collision Signal power Distance 6
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Wireless Signal Propagation
T1 does not send and listen in parallel T1 R T2 Collision Signal power Distance 6
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Wireless Signal Propagation
T1 does not send and listen in parallel T1 R T2 Collision Signal not same at different locations Signal power Distance 6
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But what if we could do CSMA/CD in wireless?
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Is CSMA/CD Beneficial in Wireless?
T1 R1 8
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Is CSMA/CD Beneficial in Wireless?
T2 T1 R2 R1 8
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Is CSMA/CD Beneficial in Wireless?
T2 T1 R2 R1 8
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Is CSMA/CD Beneficial in Wireless?
Dont Transmit! T2 T1 R2 R1 8
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Is CSMA/CD Beneficial in Wireless?
Dont Transmit! T2 T1 T3 R2 R1 R3 8
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Is CSMA/CD Beneficial in Wireless?
Dont Transmit! T2 T1 T3 R2 R1 R3 Collision 8
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Is CSMA/CD Beneficial in Wireless?
Dont Transmit! Collision Detected T2 T1 T3 R2 R1 R3 Collision 8
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Is CSMA/CD in Wireless Beneficial?
Abort Tx! T2 T1 T3 R2 R1 R3 9
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Is CSMA/CD in Wireless Beneficial?
Channel free now Abort Tx! T2 T1 T3 R2 R1 R3 9
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Is CSMA/CD in Wireless Beneficial?
Lets Transmit! T2 T1 T3 R2 R1 R3 10
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Is CSMA/CD in Wireless Beneficial?
Lets Transmit! T2 T1 T3 R2 R1 R3 CSMA/CD frees the channel for other transmissions 10
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Can we imitate CSMA/CD on Wireless?
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Practical Requirements?
1. Transmitter cannot detect collision ✦ Receiver needs to detect it Rx Tx 12
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Practical Requirements?
1. Transmitter cannot detect collision ✦ Receiver needs to detect it Collision! Rx Tx 12
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Practical Requirements?
1. Transmitter cannot detect collision ✦ Receiver needs to detect it 2. Receiver needs to convey collision notification to the transmitter Collision! Rx Tx 13
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Practical Requirements?
1. Transmitter cannot detect collision ✦ Receiver needs to detect it 2. Receiver needs to convey collision notification to the transmitter Collision! Rx Tx 13
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Practical Requirements?
1. 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 Collision! Rx Tx 14
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Practical Requirements?
1. 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 Collision! Rx Tx 14
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Practical Requirements?
1. 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 Collision! Rx Tx 15
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Overview MAC MAC CrossLayer CrossLayer PHY PHY Tx Rx 16
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Overview Tx Rx 16 MAC S=S1 MAC PHY PHY Data Transmission (S1)
CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16
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Overview Tx Rx Detect Collision 16 MAC S=S1 MAC PHY PHY
CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16
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Overview Tx Rx Detect Collision 16 Look for Notification MAC S=S1 MAC
CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16
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Overview Tx Rx 17 If Collision, Notify Tx Look for Notification MAC
S=S1 MAC CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 17
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Overview Tx Rx 17 If Collision, Notify Tx Look for Notification MAC
S=S1 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 17
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Overview Tx Rx 18 If Collision, Notify Tx Look for Notification MAC
CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 18
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Overview Tx Rx 18 If Collision, Notify Tx Look for Notification MAC
S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 18
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Overview Tx Rx If Notification, Abort Tx If Collision, Notify Tx MAC
S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx
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Two Key Challenges Tx Rx 20 If Notification, Abort Tx
If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20
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Two Key Challenges Detect Collision in Real Time Tx Rx 20
If Notification, Abort Tx If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20
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Two Key Challenges Find Notification on Listening Antenna
Detect Collision in Real Time If Notification, Abort Tx If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20
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Find Notification on Listening Antenna
Detect Collision in Real Time We propose CSMA/CN
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Find Notification on Listening Antenna
Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation
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Signal Correlation Background Signal Known Signal
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Signal Correlation Correlation Background Signal Known Signal
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Signal Correlation Correlation Sample Number 27
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Signal Correlation Whenever there is a known signal,
Sample Number Whenever there is a known signal, there is a jump in correlation 27
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Find Notification on Listening Antenna
Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation
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Detect Collision in Real Time
Find Notification on Listening Antenna We propose CSMA/CN Our key idea: Signal Correlation
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Challenge 1: Collision Detection
Data R2 R R1 30
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Challenge 1: Collision Detection
Data Data R2 R R1 30
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Challenge 1: Collision Detection
Data Data R2 R R1 Correlate for Preamble + SoftPHY hints 30
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Challenge 1: Collision Detection
Data Data R2 R R1 Collision Correlate for Preamble + SoftPHY hints 30
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What if transmitter starts second?
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My signal starts after interferer
Data R2 R R1 32
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My signal starts after interferer
Data Data R2 R R1 32
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My signal starts after interferer
Data Data Sign(R1) Sign(R2) R2 R R1 32
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My signal starts after interferer
Data Data Sign(R1) Sign(R2) R2 R R1 Correlate (Sign(R1)) 32
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My signal starts after interferer
Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 32
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Signal Correlation and Abort
Sign(R1) Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 33
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Signal Correlation and Abort
Notification! Stop Tx Corr (Sign(R1)) T1 T2 Sign(R1) Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 33
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Signal Correlation and Abort
ACK Sign(R1) ACK Sign(R2) T2 T1 Data Data R2 R R1 34
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Find Notification on Listening Antenna
Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation
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Challenge 2: Detecting Notification
Collision notification is on same channel Hard to decode notification MAC PHY ✦ Self-signal too strong ✦ Our approach: use correlation ✦ Let Tx and Rx share a unique signature Tx correlates with shared signature Detects collision notification, aborts 36
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Challenge 2: Detecting Notification
Collision notification is on same channel Hard to decode notification MAC PHY ✦ Self-signal too strong ✦ Our approach: use correlation ✦ Let Tx and Rx share a unique signature Tx correlates with shared signature Detects collision notification, aborts Observe: No decoding, just correlate 36
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Notification Detection at Tx
MAC PHY Notification Detection at Tx Notification Signal << Self Signal 37
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Notification Detection at Tx
MAC PHY Notification Detection at Tx Notification Signal << Self Signal How weak can the notification signal be? 37
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How weak the notification signal be?
Signal power Self Signal Notification Signal 38
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}16 dB ✔ How weak the notification signal be? Signal power Self Signal
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}16 dB ✘ How weak the notification signal be? Signal power Self Signal
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How can we do better for weaker clients?
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How can we do better for weaker clients?
Transmitted signal is a known signal - Can be effectively canceled Opportunity: Transmit and listening antenna are on the same device
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Detecting Notification: Signal Cancelation
Wireless ✦ Pass the Tx signal over wire Wired ✦ Listen antenna has two copies of the Tx signal ✦ One over wire, other over wireless Both the copies have same filter, freq. offset effects Align the two signals using sampling offset info Subtract the wired signal from the wireless
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Correlation Performance with Cancelation
+ /- False of Fraction 0.35 0.3 0.25 0.2 0.15 0.1 0.05 Fraction of False +/- False positives False negatives False positivies False negatives (Self Signal) - (Notification Signature)(dB) (Self Signal) - (Notification Signature)(dB) Reliable detection for upto 34dB below
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Detecting Notification: Antenna Orientation
✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self 1ft. antenna separation Self 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 #2 SNR in dB 40 30 #3 20 #4 10 #1 #2 #3 #4 Configuration Number
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Detecting Notification: Antenna Orientation
✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self 1ft. antenna separation Self 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 18dB #2 SNR in dB 40 30 #3 20 #4 Best Configuration 10 #1 #2 #3 #4 Configuration Number 44
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Detecting Notification: Antenna Orientation
✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self 1ft. antenna separation Self 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 18dB #2 SNR in dB 40 30 #3 20 6dB #4 Best Configuration 10 #1 #2 #3 #4 Configuration Number 44
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How dissimilar should signatures be?
Each CSMA/CN receiver needs 2 signatures Different frequency offsets add diversity Results show 6 out of 20 bytes diff enough Hamming Distance (bytes) 2 4 6 8 10 False Positive 17% 9% 2.8% 2.2% 1.6%
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Performance Evaluation
10 node USRP testbed BPSK, QPSK modulation Signature size: 20 bytes Topologies with three links doing CSMA/CN Compare with like and PPR ✦ PPR retransmits only suspected bits of the packet
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Collision Detection at Rx
MAC Collision Detection at Rx PHY Collision detection accuracy of 92% Receiver detects collision within 20 bytes Total turnaround time for CN signature 18us ✦ Quicker turnaround Faster Tx abortion
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Testbed Throughput Gain
1 like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 25% 0.4 0.2 1 1.2 Throughput in Mbps 2.2 2.4 Tput in Mbps 48
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Testbed Throughput Gain
1 like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 25% 0.4 0.2 1 1.2 Throughput in Mbps 2.2 2.4 Tput in Mbps Upto 25% median throughput gain over 48
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Why CSMA/CN has gain over PPR?
1 like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 0.4 0.2 (Correct Bits/Transmitted Bits) Equivalent success ratio 49
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Why CSMA/CN has gain over PPR?
1 like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 0.4 0.2 (Correct Bits/Transmitted Bits) Equivalent success ratio PPR continues to transmit under collision CSMA/CN aborts colliding transmission 49
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Does CSMA/CN gain at high rates?
( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps Mbps 54Mbps 14 12 10 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) 50
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Does CSMA/CN gain at high rates?
( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps Mbps 54Mbps 14 12 10 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) CSMA/CN gains also at higher rates 50
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Does CSMA/CN gain at high rates?
( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps Mbps 54Mbps 14 12 10 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) Alleviates collision losses due to hidden terminals and same backoff
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Limitation and Future Work
✦ Improve Correlation ✦ Analog interference cancelers ✦ Interference due to notification ✦ Recover using FEC ✦ Coexistence with MIMO ✦ One listen antenna can be shared by MIMO antennas CSMA/CN allows aggressive network design Low CS threshold, backoff etc.
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Taking a Step Back Signals Decoded Bits Control Data 53
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Taking a Step Back Signals Decoded Bits Control Data Full Duplex 53
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Taking a Step Back 53 Correlation Control Full Duplex Signals
Decoded Bits Control Control Data Full Duplex 53
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Taking a Step Back CSMA/CN 53 Correlation Control Full Duplex Signals
Decoded Bits Control Control Data CSMA/CN Full Duplex 53
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Summary CSMA/CN imitates CSMA/CD in wireless
Uses correlation, enough for 1 bit feedback Traditional protocols: recovers collision CSMA/CN aborts collision to summarize imitates CD in wireless .. uses corr to convey one bit info about collision .. while traditional collisions recover much later .. imeediately aborts collisions .. 54
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Summary Prevention is Better than Cure
CSMA/CN imitates CSMA/CD in wireless Uses correlation, enough for 1 bit feedback Traditional protocols: recovers collision CSMA/CN aborts collision Prevention is Better than Cure 54
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Duke SyNRG Research Group
Questions, comments? Thank you Duke SyNRG Research Group
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Collision Detection at Rx
MAC Collision Detection at Rx PHY 1.6 Correctly decoded Preamble decoded but packet lost Preamble lost Detection Accuracy 1.4 Fraction of packets 1.2 1 0.8 0.6 0.4 0.2 2 SIR in dB (SNR S - SNR I) 14 56
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Collision Detection at Rx
MAC Collision Detection at Rx PHY 1.6 Correctly decoded Preamble decoded but packet lost Preamble lost Detection Accuracy 1.4 Fraction of packets 1.2 1 0.8 0.6 0.4 0.2 2 SIR in dB (SNR S - SNR I) 14 Collision Detection Accuracy of 92% 56
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Throughput Evaluation using Simulation
Throughput gain over PPR (%) 24 22 20 18 10% Hidden Terminal 16 6Mbps 18Mbps Mbps 54Mbps 14 12 10 Number of clients 57
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Throughput Evaluation using Simulation
Throughput gain over PPR (%) 24 22 20 18 10% Hidden Terminal 16 6Mbps 18Mbps Mbps 54Mbps 14 12 10 Number of clients More Collisions due to same backoff with more clients: More Throughput Gain 57
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Detecting Notification: Correlation Performance
1.2 Correlation when Signature absent Correlation when Signature present Normalized Correlation Value 1 0.8 0.6 0.4 0.2 (Self Signal) - (Notification Signature)(dB) Reliable detection for until below 16dB 58
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