Cross-Layer Approach to Wireless Collisions Dina Katabi

devices are increasingly prevalent distributed and bursty access Home office City mesh Two devices access medium together  Collision

Job of the MAC: Avoid Collisions! And when they happen? Be in denial!

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

The Hidden Terminals Problem Collision! Alice Bob

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

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 if packets didn’t collide Low-complexity linear decoder No need for synchronization Delivered 2 packets in 2 timeslots  As if packets didn’t collide Low-complexity linear decoder No need for synchronization

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 P a1 PbPb P a2 PbPb 3 packets in 2 timeslots  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 that measure HTs. The USRPs use the same collision patterns as 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

But can collisions be beneficial? ZigZag makes collisions harmless

Current Wireless Alice Bob

Current Wireless Current approach requires 4 time slots Can we do better? Alice Bob

Naive Application of ZigZag Collision! Alice Bob

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

What if the AP doesn’t decode? Collision! 1)Alice and Bob transmit simultaneously Alice Bob

What if the AP doesn’t decode? Collision! 1)Alice and Bob transmit simultaneously 2)AP amplifies and broadcasts the collision Alice Bob

1)Alice and Bob transmit simultaneously 2)AP amplifies and broadcasts the collision 3)Alice subtracts her packet from the collision 1fhj What if the AP doesn’t decode? 1fhj 11 Bob’s pkt 11 Alice’s pkt 2 time slots instead of 4  Throughput Gain of 2x 2 time slots instead of 4  Throughput Gain of 2x Alice Bob

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)!

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”

Implemented in software radio Evaluated in a similar testbed Metric Gain = Throughput in ANC /Current Throughput Performance

Throughput Gain for the Alice-Bob Scenario Throughput gain CDF Median Throughput Gain is 1.7x

Related Work Network Coding and Physical-layer Network Coding – Our ANC decoder linear modulation-independent and works without synchronization Joint Decoding & 2-way relay – Requires senders synchronization  hard in practice Interference Cancellation – Requires exponential difference in power or code-rate

Conclusion Cross-layer design changes how we perceive collisions Collisions need not be harmful – ZigZag decodes collisions as efficiently as if the colliding packets were sent in separate time slots Collisions can be beneficial – Analog network coding induces strategic collisions to increase throughput Softcast: cross-layer wireless video without CSI