cross-layer approach to wireless collisions dina katabi
TRANSCRIPT
![Page 1: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/1.jpg)
Cross-Layer Approach to Wireless Collisions
Dina Katabi
![Page 2: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/2.jpg)
802.11 devices are increasingly prevalent
distributed and bursty access
Home
office
City mesh
Two devices access medium together Collision
![Page 3: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/3.jpg)
Job of the MAC:
Avoid Collisions!
And when they happen?
Be in denial!
![Page 4: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/4.jpg)
In This Talk
• Collisions are not harmful– We can decode colliding packets as
efficiently as if they were sent separately
• Collisions are beneficial– We can exploit strategic collisions to
increase throughput Analog Network Coding
![Page 5: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/5.jpg)
The Hidden Terminals Problem
Collision!
Alice Bob
![Page 6: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/6.jpg)
The Hidden Terminals Problem
More Collisions!Retransmissions
Can’t get any useful traffic!
Alice Bob
![Page 7: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/7.jpg)
Can we take two collisions and produce the two packets?
Pa
Pb
Pa
Pb
Yes, we can!
Yes, we can!
![Page 8: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/8.jpg)
ZigZagExploits 802.11’s behavior• Retransmissions
Same packets collide again• Senders use random jitters
Collisions start with interference-free bits
∆1 ∆2Pa
Pb
Pa
Pb
Interference-free Bits
![Page 9: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/9.jpg)
How Does ZigZag Work?
∆1 ∆2
Find a chunk that is interference-free in one collisions and has interference in the other
11
∆1 ≠∆2
Decode and subtract from the other collision
11
![Page 10: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/10.jpg)
∆2
11
22
11
∆1
How Does ZigZag Work?
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 11: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/11.jpg)
∆211
2222∆1
How Does ZigZag Work?
33
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 12: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/12.jpg)
∆211
22 44∆1
How Does ZigZag Work?
33 33
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 13: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/13.jpg)
∆211
22 4444∆1
How Does ZigZag Work?
33 55
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 14: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/14.jpg)
∆211
66∆1
How Does ZigZag Work?
33 55 55
22 44
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 15: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/15.jpg)
∆211
6666∆1
How Does ZigZag Work?
22 44
33 55 77
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
![Page 16: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/16.jpg)
∆2
11
66 88∆1
How Does ZigZag Work?
22 44
33 55 7777
Find a chunk that is interference-free in one collisions and has interference in the other
∆1 ≠∆2
Decode and subtract from the other collision
• Delivered 2 packets in 2 timeslotsAs if packets didn’t collide
• Low-complexity linear decoder• No need for synchronization
![Page 17: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/17.jpg)
How does the AP know it is a collision and where the second packet starts?
Time
AP received a collision signal
∆
![Page 18: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/18.jpg)
Detecting Collisions and the Value of ∆
Time
AP received signal
Packets start with known preamble
AP correlates known preamble with signal
Correlation
Time
Correlate
∆
![Page 19: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/19.jpg)
How Does the AP Subtract the Signal?
• Channel’s attenuation or phase may change between collisions
• Can’t simply subtract a chunk across collisions
Alice’ s signal in first collision
Alice’ s signal in second collision
![Page 20: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/20.jpg)
Subtracting a Chunk
![Page 21: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/21.jpg)
What if AP Makes a Mistake?
![Page 22: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/22.jpg)
∆1 ∆211 11
2222
Bad News: Errors can propagate
33
Can we deal with these errors?
What if AP Makes a Mistake?
![Page 23: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/23.jpg)
∆1 ∆2
What if AP Makes a Mistake?
Good News: Temporal DiversityA bit is unlikely to be affected by noise in both
collisions
Get two independent decodings
![Page 24: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/24.jpg)
Errors propagate differently in the two decodings
For each bit, AP picks the decoding that has a higher PHY confidence [JB07, WKSK07]
Which decoded value should the AP pick?
∆1 ∆2
11 11
2222
33
AP Decodes Backwards as well as Forwards
![Page 25: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/25.jpg)
ZigZag Generalizes
![Page 26: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/26.jpg)
ZigZag Generalizes
∆1
∆211
22
11
22
• Flipped order
![Page 27: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/27.jpg)
• Flipped order• Different packet sizes
ZigZag Generalizes
∆1 ∆2
11
22
11
22
![Page 28: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/28.jpg)
ZigZag Generalizes
1
2
3
1
2
3
1
2
3
• Flipped order• Different packet sizes• Multiple colliding packets
11
22
11
22 22
11
333333
![Page 29: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/29.jpg)
ZigZag Generalizes• Flipped order• Different packet sizes• Multiple colliding packets • Capture effect
Pa1
Pb
Pa2
Pb
3 packets in 2 timeslots better than no collisions
![Page 30: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/30.jpg)
Performance
![Page 31: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/31.jpg)
Implementation
• USRP Hardware• GNURadio software• Carrier Freq: 2.4-2.48GHz• BPSK modulation
![Page 32: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/32.jpg)
USRPsTestbed
• 10% HT, 10% partial HT, 80% perfectly sense each other
• Each run randomly picks an AP and two clients
• Co-located 802.11a nodes that measure HTs. The USRPs use the same collision patterns as 802.11
802.11a
![Page 33: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/33.jpg)
Throughput Comparison
Throughput
CD
F o
f co
ncur
rent
flo
w p
airs
![Page 34: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/34.jpg)
Throughput Comparison
802.11
Throughput
CD
F o
f co
ncur
rent
flo
w p
airs
Hidden Terminals
Partial Hidden Terminals
Perfectly Sense
![Page 35: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/35.jpg)
Throughput Comparison
ZigZag
Throughput
CD
F o
f co
ncur
rent
flo
w p
airs
802.11
Hidden Terminals get high throughput
![Page 36: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/36.jpg)
Throughput Comparison
ZigZag
Throughput
CD
F o
f co
ncur
rent
flo
w p
airs
802.11
ZigZag Exploits Capture Effect
ZigZag improved average Throughput by 25%
![Page 37: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/37.jpg)
Throughput Comparison
ZigZag
Throughput
CD
F o
f co
ncur
rent
flo
w p
airs
802.11
Improved hidden terminals loss rate from 72% to 0.7%
Hidden Terminals
![Page 38: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/38.jpg)
Is ZigZag as efficient as if the colliding packets were sent in separate slots?
• For every SNR,
Check that ZigZag can match the BER of collision-free receptions
![Page 39: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/39.jpg)
Is ZigZag as efficient as if packets were collision-free Receptions?
SNR in dB
Bit E
rror
Rat
e (B
ER)
![Page 40: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/40.jpg)
Collision-Free Receptions
Is ZigZag as efficient as if packets were collision-free Receptions?
SNR in dB
Bit E
rror
Rat
e (B
ER)
![Page 41: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/41.jpg)
Collision-Free Receptions
Is ZigZag as efficient as if packets were collision-free Receptions?
ZigZag-Decoded Collisions
SNR in dB
Bit E
rror
Rat
e (B
ER)
ZigZag is as efficient as if the colliding packets were sent separately
![Page 42: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/42.jpg)
But can collisions be beneficial?
ZigZag makes collisions harmless
![Page 43: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/43.jpg)
Current Wireless
Alice Bob
![Page 44: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/44.jpg)
Current Wireless
Current approach requires 4 time slots
Can we do better?
Alice Bob
![Page 45: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/45.jpg)
Naive Application of ZigZag
Collision!Alice Bob
![Page 46: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/46.jpg)
Decode Collisions
Naïvely applying ZigZag requires 4 time slots
Decoding collisions at AP doesn’t reduce timeslots
But the AP doesn’t need to decode!
Alice Bob
Naive Application of ZigZag
![Page 47: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/47.jpg)
What if the AP doesn’t decode?
Collision!
1) Alice and Bob transmit simultaneously
Alice Bob
![Page 48: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/48.jpg)
What if the AP doesn’t decode?
Collision!
1) Alice and Bob transmit simultaneously2) AP amplifies and broadcasts the collision
Alice Bob
![Page 49: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/49.jpg)
1) Alice and Bob transmit simultaneously2) AP amplifies and broadcasts the collision 3) Alice subtracts her packet from the collision
1fhj
What if the AP doesn’t decode?
1fhj11
Bob’s pkt11
Alice’s pkt
2 time slots instead of 4
Throughput Gain of 2x
Alice Bob
![Page 50: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/50.jpg)
Extends Network Coding to Signals
Traditional network coding: nodes forward linear combinations of received packets
New approach: channel naturally creates linear combinations of signals
Analog Network Coding (ANC)!
![Page 51: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/51.jpg)
Theoretical Limits
Theorem [Katti et al. ISIT’07]:Informal version: “For a symmetric two-way relay channel the
rate achieved by analog network coding at high SNR is double the rate achieved by pure forwarding”
![Page 52: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/52.jpg)
Implemented in software radio
Evaluated in a similar testbed
Metric
Gain = Throughput in ANC /Current Throughput
Performance
![Page 53: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/53.jpg)
Throughput Gain for the Alice-Bob Scenario
Throughput gain
CD
F
Median Throughput Gain is 1.7x
![Page 54: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/54.jpg)
Related Work
![Page 55: Cross-Layer Approach to Wireless Collisions Dina Katabi](https://reader034.vdocuments.us/reader034/viewer/2022042718/5697bfc81a28abf838ca8892/html5/thumbnails/55.jpg)
Conclusion
Softcast: cross-layer wireless video without CSI