Decoding Collisions Shyamnath Gollakota Dina Katabi

The Hidden Terminals Problem Collision! Alice Bob

The Hidden Terminals Problem Alice Bob More Collisions! Retransmissions Can’t get any useful connections

Can we take two collisions and produce the two packets? PaPa PbPb PaPa PbPb Yes, we can!

ZigZag Exploits ’s behavior Retransmissions  Same packets collide again Senders use random jitters  Collisions start with interference-free bits ∆1 ∆2 PaPa PbPb PaPa PbPb Interference-free Bits

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

∆ ∆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

∆ ∆1 How Does ZigZag Work? 3 3 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

∆ ∆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

∆ ∆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

∆ ∆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

∆ ∆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

∆ ∆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 Delivered 2 packets in 2 timeslots As efficient as if the packets did not collide Delivered 2 packets in 2 timeslots As efficient as if the packets did not collide

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%

How does the AP know it is a collision and where the second packet starts? Time AP received a collision signal ∆

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

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

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!

What if AP Makes a Mistake?

∆1 ∆ Bad News: Errors can propagate 3 3 Can we deal with these errors? What if AP Makes a Mistake?

∆1 ∆2 What if AP Makes a Mistake? Good News: Temporal Diversity A bit is unlikely to be affected by noise in both collisions Get two independent decodings

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 ∆ AP Decodes Backwards as well as Forwards

ZigZag Generalizes

∆1 ∆ Flipped order

Different packet sizes ZigZag Generalizes ∆1 ∆

ZigZag Generalizes Flipped order Different packet sizes Multiple colliding packets

ZigZag Generalizes Flipped order Different packet sizes Multiple colliding packets Capture effect – Subtract Alice and combine Bob’s packet across collisions to correct errors ∆1 ∆2 P a1 PbPb P a2 PbPb 3 packets in 2 time slots  better than no collisions

Performance

Implementation USRP Hardware GNURadio software Carrier Freq: GHz BPSK modulation

USRPs Testbed 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 a

Throughput Comparison Throughput CDF of concurrent flow pairs

Throughput Comparison Throughput CDF of concurrent flow pairs Hidden Terminals Partial Hidden Terminals Perfectly Sense

Throughput Comparison ZigZag Throughput CDF of concurrent flow pairs Hidden Terminals get high throughput

Throughput Comparison ZigZag Throughput CDF of concurrent flow pairs ZigZag Exploits Capture Effect ZigZag improved average Throughput by 25%

Throughput Comparison ZigZag Throughput CDF of concurrent flow pairs Improved hidden terminals loss rate from 72% to 0.7% Hidden Terminals

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

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

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

Collision-Free Receptions Is ZigZag as efficient as if packets were collision-free Receptions? ZigZag-Decoded Collisions SNR in dB Bit Error Rate (BER) ZigZag is as efficient as if the colliding packets were sent separately

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

Three Colliding Senders ZigZag extends beyond two colliding senders CDF of runs Per-Sender Throughput Alice Bob Chris

Related Work RTS-CTS – Excessive Overhead; Administrators turn it off Interference Cancellation – Unsuitable for because of bit rate adaptation Interference cancelation operates on one collision  Undecodable Alice’s Info Rate Bob’s Info Rate Rmax

Related Work RTS-CTS – Excessive Overhead; Administrators turn it off Interference Cancellation – Unsuitable for because of bit rate adaptation ZigZag operates on two collisions  Can decode Alice’s Info Rate Bob’s Info Rate Rmax

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