1. Decoding Collisions
Shyamnath Gollakota
Dina Katabi

2. The Hidden Terminals Problem
Collision!
Alice
Bob
we know HT .. alice and bob cannot hear each other .. makes worse if obstacle in between .. however they can interfere with each other at a common receiver ..

3. The Hidden Terminals Problem
Retransmissions
More Collisions!
Alice
Bob
Can’t get any useful connections
The problem does not stop here. Alice and Bob retransmit their packets after increasing their contention window. But this does not help. Their transmissions still overlap causing more collisions.
Alice and Bob will continue increasing their contention window and retransmitting, until after many trials they get a packet through or they time out.

4. Can we take two collisions and produce the two packets?Pb Pb
Yes, we can!
suppose first tx collided ..re-tx also collided .. can we take these two and recover the original packet ..

5. ZigZag Pa Pa Pb Pb Exploits 802.11’s behavior Retransmissions
Same packets collide again
Senders use random jitters
 Collisions start with interference-free bits Pa Pa
this work proposes a coll decoding scheme called zigzag ..
…………
random jitters .. start time of interference is different in each collision
How do we leverage these interference free bits to decode the collisions? ∆1 Pb ∆2 Pb
Interference-free Bits

6. Decode and subtract from the other collision
How Does ZigZag Work? 1 1 ∆1 ∆2
∆1 ≠∆2
as a result .. because of rsndom jitters .. interference free duration in each packet is going to be different ..
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

7. Decode and subtract from the other collision
How Does ZigZag Work? 1 1 ∆1 ∆2 2
∆1 ≠∆2
the core idea is to exploit the difference in interference free bits in eliminating collisions ..
walk you through an example .. interference
this part is free .. you can decode .. in the seocnd part at this location .. you have the sum of two signals .. subtract .. 1 since you know it completely ..
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

8. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 ∆1 2 ∆2 2
∆1 ≠∆2
when you subtract 1 .. you can decode 2 here from second packet .. now in first collision two is buried under int .. if you subtract you can
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

9. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 3 ∆1 ∆2 2 4
∆1 ≠∆2
decode 3 .. now 3 is under interference here ..
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

10. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 5 ∆1 4 ∆2 2 4
∆1 ≠∆2
you can continue this chain of operations
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

11. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 5 5 ∆1 ∆2 2 4 6
∆1 ≠∆2
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

12. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 5 7 ∆1 6 ∆2 2 4 6
∆1 ≠∆2
and decode the whole packet ..
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

13. Decode and subtract from the other collision
How Does ZigZag Work? 1 3 5 7 7 ∆1 ∆2 2 4 6 8
Delivered 2 packets in 2 timeslots
As efficient as if the packets did not collide
∆1 ≠∆2
Thus, even in the face of collisions, we are able to correctly receive 2 packets in 2 timeslots.
Find a chunk that is interference-free in one collisions and has interference in the other
Decode and subtract from the other collision

14. ZigZag A receiver design that decodes collisions
As efficient as if the colliding packets were sent in separate time slots
Experimental results shows that it reduces hidden terminal losses from 72% to 0.7%

15. AP received a collision signal
Time ∆
How does the AP know it is a collision and where the second packet starts?
Now lets go through an detailed example of how ZigZag decodes.

16. Detecting Collisions and the Value of ∆
Time
AP received signal
Correlate
Packets start with known preamble
AP correlates known preamble with signal
Preamble Correlation
Detect collision and the value of ∆
Works despite interference because correlation with an independent signal is zero ∆
Correlation
Time

17. 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
Now that the AP knows where the second packet starts, it is going to take the interference-free chunk from at the start of the first collision and subtract it from the second.
In practice, it is not that simple…………..

18. Subtracting a Chunk Decode chunk into bits
Removes effects of channel during first collision
Re-modulate bits to get channel-free signal
Apply effect of channel during second collision
Use correlation to estimate channel despite interference
Now, can subtract!
The solution to this problem however is simple.
……..
Now the AP can subtract the first chunk. The AP iterates this process to decode both the whole packet.

19. What if AP Makes a Mistake?
But, as is the usual case, channel adds noise which could lead to decoding errors.

20. What if AP Makes a Mistake?
Bad News: Errors can propagate 1 3 1 ∆1 ∆2 2 2
For example, if the channel was noisy it can cause a decoding error in chunk 1.
Since we subtract chunks across collisions, errors in the first collision can propagate to the second collision
For example, Now when we subtract, the error in chunk 1 can lead to a decoding mistake in chunk 2, which itself could propagate further.
Can we deal with these errors?

21. What if AP Makes a Mistake?
Good News: Temporal Diversity
A bit is unlikely to be affected by noise in both collisions ∆1 ∆2
Get two independent decodings
The good new is that we have temporal diversity. Each bit appears twice on the channel.
It is unlikely that the same bit is in error in both collisions.
We can leverage this property by decoding each bit twice using two independent decodings.
But how do we get these two independent decodings?
I have already described to u … one way of decoding.

22. AP Decodes Backwards as well as Forwards∆1 ∆2 2 2 3 1 1
Errors propagate differently in the two decodings
Which decoded value should the AP pick?
But in addition to this forward decoding method we can also decode backwards.
For each bit, AP picks the decoding that has a higher PHY confidence [JB07, WKSK07]

23. ZigZag Generalizes

24. ZigZag Generalizes
Flipped order ∆2 1 1 2 2 ∆1

25. ZigZag Generalizes
Flipped order
Different packet sizes 1 1 2 2 ∆1 ∆2

26. ZigZag Generalizes Flipped order Different packet sizes
Multiple colliding packets 1 1 2 3 1 1
Intuitively, this is similar to solving three linear equations with three unknowns. The collisions are the equations and the packets are the unknowns. 2 2 2 3 3 3

27. ZigZag Generalizes Flipped order Different packet sizes
Multiple colliding packets
Capture effect
Subtract Alice and combine Bob’s packet across collisions to correct errors
Pa1
Finally, ZigZag makes a better use of capture effects.
Capture effect can happen when the AP can decode Alice’s packets despite interference from Bob. But in ZigZag, the AP also subtract Alice’s packets from the collision signals to get Bob’s packets. Since Bob’s signal is so weak it is unlikely that one collision is sufficient to decode it. But ZigZag can combine the results of decoding Bob’s packet across both collisions to improve performance.
Pa2 ∆1 ∆2 Pb Pb
3 packets in 2 time slots  better than no collisions

28. Performance

29. Implementation USRP Hardware GNURadio software
Carrier Freq: GHz
BPSK modulation

30. Testbed
USRPs
10% HT, 10% partial HT, 80% perfectly sense each other
Each run randomly picks an AP and two clients
Co-located a nodes to find out about HTs and created the same collision patterns by the USRPs
802.11a
Blah blah blah…
Since Software radios do all operations in software in user mode, they cannot perform carrier sense accurately. So, to capture carrier sense, each of our nodes have both and USRP radios. We first run the experiment using the nodes which apply carrier sense. Then we repeat the same experiment the USRP nodes, while forcing them to have the same number of collisions as the corresponding nodes.

31. Throughput Comparison
CDF of concurrent flow pairs
Throughput

32. Throughput Comparison
Perfectly Sense
Partial Hidden Terminals
Hidden Terminals
CDF of concurrent flow pairs
802.11
We see three regions. Some nodes are complete hidden terminals and hardly get any throughput. Others are partial hidden terminals and get sub-optimal throughput, and the rest are get good throughput.
Throughput

33. Throughput Comparison
CDF of concurrent flow pairs
Hidden Terminals get high throughput
ZigZag
802.11 …
what used to be hidden terminals, they get excellent throughput.
Throughput

34. Throughput Comparison
ZigZag Exploits Capture Effect
CDF of concurrent flow pairs
ZigZag
802.11
Also its interesting to note that some of the scenarios that used to get high throughput now get even higher throughput. This correspond to capture effect because as we said earlier ZigZag can exploit capture effect to deliver more than one packet per time slot. ……
Throughput
ZigZag improved average Throughput by 25%

35. Throughput Comparison
Hidden Terminals
CDF of concurrent flow pairs
ZigZag
802.11
Zooming in on the hidden terminals, ZigZag improves …….
This means that you can transform scenarios which were completely unusable to completely fine .
Improved hidden terminals loss rate from 72% to 0.7%
Throughput

36. Check that ZigZag can match the BER of collision-free receptions
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
…. But, is ZigZag really as efficient as if the colliding packets were sent in separate time slots?
To answer this question, it is not sufficient to look at packet loss rate because a packet may be lost because of one bit error or many bit errors.
Thus, to answer this question we check …….

37. Is ZigZag as efficient as if packets were collision-free Receptions?
Bit Error Rate (BER)
SNR in dB

38. Is ZigZag as efficient as if packets were collision-free Receptions?
Bit Error Rate (BER)
SNR in dB

39. Is ZigZag as efficient as if packets were collision-free Receptions?
Bit Error Rate (BER)
ZigZag-Decoded Collisions
As you can see at every SNR, ZigZag has slightly lower BER than collision free reception………
……How come packets that are decoded from collisions have a lower BER than those sent in separate time slots. This is because of temporal diversity. With ZigZag, every bit is received twice, once in every collision, improving its chance of being correctly decoded. By decoding both forward and backward, we can collect this diversity gain and use it to combat bit errors.
ZigZag is as efficient as if the colliding packets were sent separately
SNR in dB

40. Three Colliding Senders
Collision!
Alice
Bob
Chris
Nodes picked randomly from testbed

41. Three Colliding Senders
Alice
CDF of runs
Bob
Chris
The real point here is that ZigZag can decode collisions involving more than two senders.
Per-Sender Throughput
ZigZag extends beyond two colliding senders

42. Related Work RTS-CTS Interference Cancellation Rmax Rmax
Excessive Overhead; Administrators turn it off
Interference Cancellation
Unsuitable for because of bit rate adaptation
Rmax
Bob’s Info Rate
Interference cancelation operates on one collision
 Undecodable
Finally, related work falls in two areas.
First, RTS-CTS was originally proposed to deal with Hidden terminals. However, much prior work shows that RTS-CTS produces excessive overhead for the common case of no collisions. Thus, in practice, typically both card manufacturers …..
Second, interference cancellation was proposed for decoding concurrent transmissions in cellular networks. But interference cancellation is unsuitable for networks that use bit rate adaptation like
Consider again the hidden terminal example of Alice and Bob. Let’s plot Alice’s information rate on the x-axis and Bob’s information rate on the y axis.
Information theory allows us to plot the achievable rate region, where Rmax is the maximum rate supported by Alice’s and Bob’s channels. Any point inside the grey area is decodable, and any point outside it, is higher than the capacity and hence is not decodable.
Because of bit rate adaptation, Alice and Bob send as close as possible to Rmax, the maximum rate supported by their channels. Thus when they collide, <animate> the collision is outside the rate region and is impossible to decode.
Interference cancelation operates on this single collision , but if Bob and Alice are using an efficient bit rate selection, it is not possible to decode a single collision.
Alice’s Info Rate
Rmax

43. Related Work RTS-CTS Interference Cancellation Rmax Rmax
Excessive Overhead; Administrators turn it off
Interference Cancellation
Unsuitable for because of bit rate adaptation
Rmax
Bob’s Info Rate
ZigZag operates on two collisions
 Can decode
ZigZag operates on two collisions of the same packets
The information rate in two collisions is half the rate in one collision, and hence even if when using efficient bit rate selection ZigZag can still decode.
Alice’s Info Rate
Rmax

44. Conclusion ZigZag is a receiver design that resolves collisions
It is as efficient as if the colliding packets were sent in separate time slots
It reduces hidden terminal losses from 72% to 0.7%
It enables aggressive MAC  More concurrency ….
Also ZigZag opens up new avenues for future research…….
For example, since one can resolve collisions, one may try to change the carrier sense threshold to allow more concurrent transmissions and improve spatial reuse.
Taking it a step forward, one may let go of carrier sense and have senders transmit whenever they have traffic and resolve collisions as and when they happen.