Z IG Z AG D ECODING : C OMBATING H IDDEN T ERMINALS IN W IRELESS N ETWORKS Shyamnath Gollakota and Dina Katabi MIT CSAIL SIGCOMM 2008 Presented by Paul Wang

H IDDEN T ERMINAL P ROBLEM Leads to low utilization of bandwidth and unfairness in channel access RTS/CTS induced too much overhead – disabled by default Collided packets may still be decodable! Alice Bob AP X

2 C HARACTERISTICS OF E XPLOITED 1. An sender retransmits a packet until it is acked or timed out, and hence when two senders collide they tend to collide again on the same packets senders jitter every transmission by a short random interval, and hence collisions start with a random stretch of interference free bits

B ASIC IDEA OF Z IG Z AG D ECODING Chunk 1 from user A from 1 st copy of collided packet can be decoded successfully Subtract from 2 nd copy to decoded the Chunk 1 of user B Subtract from 1 st copy of collided packet to decode Chunk 2 from user A Subtract from 2 nd copy of collided packet to decode Chunk 2 from user B

W AIT ! W HAT ABOUT S HANNON C APACITY ? Requires retransmissions if collision occurs No overhead if no collision R1 R2 TDMA

O THER ALTERNATIVES CDMA – Incompatible with WLAN – Low efficiency in high SNR Successive interference cancellation (SIC) – Chunk == packet – Decode the strong signal first, subtract from the sum and then decode the weak signal – No need for retransmissions – Both transmitters need to transmit at a lower rate

P ATTERNS THAT Z IG Z AG A PPLICABLE Both backward and forward decoding can be used Sudoku?

P RELIMINARY ON COMMUNICATION BPSK: 0 -> > 1

T ECHNICAL B ARRIERS How do I know packets collide Matching collision happened? (P1, P2) and (P1’, P2’) Frequency offset between transmitter and receiver Sampling offset Inter-symbol interference What if errors occur in chunks Acknowledgement? } subtraction is non-trivial

C OLLISION D ETECTION Preamble Pseudo random number, independent of shifted versions of itself as well as the data Correlation with moving window thresholding

M ATCHING COLLISION Given (P1, P2(  )) and (P1’, P2’(  ’)), how to determine that P1 = P1’ and P2 = P2’ Determine offset first – you know this from the collision detection scheme in the prior slide Align P2(Δ) and P2’(Δ’) accordingly Calculate correlation of P2(  ) and P2’(  ’) If high correlation, then packet matched!

D ECODE MATCHING COLLISION Decode iteratively Re-encoding Computing channel parameters Channel gain estimated from Frequency offset and sampling error 1) coarse estimation from previously successful reception 2) iterative estimation Inter-symbol interference: take the inverse of linear filter (for removal of ISI)

D ECODE MATCHING COLLISION ( CONT ’ D ) Re-encoding Account for sampling offset error ( μ ) – based off of Nyquist criteria

W HAT ABOUT ERRORS ? Will errors in decoding have a cascading effect? Error propagation dies out exponentially Error correction capability of modulation Forward and backward decoding

A CKNOWLEDGEMENT Use as much synchronous acknowledgement as possible for backward compatibility

E VALUATION 14-node GNURadio testbed – USRP with RFX2400 radio (2.4 GHz) – BPSK – Bit rate 500kbs – 32-bit preamble – 1500-byte payload, 32-bit CRC Deficiency in GNURadio – Cannot coordinate transmission and reception very closely – CSMA, ACK TransmitterReceiver Software

M ICRO - BENCHMARK

A LICE & B OB Bob’s location is fixed, Alice moves closer to the base-station

I MPACT OF SNR ON BER Alice & Bob at fixed and equal location Vary transmission power level

T ESTBED R ESULTS Pick two senders randomly 10% hidden terminals, 10% partial, 80% perfect

T HREE HIDDEN TERMINALS

C ONCLUSION ZigZag improves fairness & throughput Further research Combination of analog network coding

S ECRET TO S UCCESS Work on something novel! Write effectively and clearly… the paper must be understandable by the readers Don’t assume your reader knows as much as you about the specific topic Made numbers sound good… but BER/Packet loss numbers don’t mean there isn’t any error… bit errors are just very low!