ZIGZAG DECODING: COMBATING HIDDEN TERMINALS IN WIRELESSNETWORKSShyamnath Gollakota and Dina KatabiMIT CSAILSIGCOMM 2008Presented by Paul Wang

HIDDEN TERMINAL PROBLEM

• 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 BobAPX

2 CHARACTERISTICS OF 802.11 EXPLOITED

1. An 802.11 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

2. 802.11 senders jitter every transmission by a short random interval, and hence collisions start with a random stretch of interference free bits

BASIC IDEA OF ZIGZAG DECODING

Chunk 1 from user A from 1st copy of collided packet can be decoded successfully Subtract from 2nd copy to decoded the Chunk 1 of user B

Subtract from 1st copy of collided packet to decode Chunk 2 from user A

Subtract from 2nd copy of collided packet to decode Chunk 2 from user B

WAIT! WHAT ABOUT SHANNON CAPACITY?

Requires retransmissions if collision occurs No overhead if no collision

R1

R2

TDMA

OTHER 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

PATTERNS THAT ZIGZAG APPLICABLE

Both backward and forward decoding can be used

Sudoku?

PRELIMINARY ON COMMUNICATION

BPSK: 0 -> -1 1 -> 1http://en.wikipedia.org/wiki/QPSK

driftfrequency todue ][][][

collision of presencein ][][][][

channel)invariant time(a ][][][

2 nwenHxny

nwnynyny

nwnHxny

fTj

BA

TECHNICAL BARRIERS 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

COLLISION DETECTION

Preamble Pseudo random number,

independent of shifted versions of itself as well as the data

Correlation with moving window

thresholding

L

kB ksH

1

2|][|)('

1

1

1

2

1

2 2

1

2 2 2

1

2

1

*

*

*

* 0B

B

B B

L

k

L

A Bk

L

Bk

Lj k f T

B Bk

Lj k f T

Bk

Lj k f T j k f T

Bk

L

Bk

s k y k

s k y k y k w k

s k y k

s k H s k e f

H s k e

H s k e e

H s k

MATCHING 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!

DECODE 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)

L

kB ksH

1

2|][|)('

L

LlISIl lixhix ][][

DECODE MATCHING COLLISION (CONT’D)

Re-encoding Account for sampling offset error (μ) – based off

of Nyquist criteria

2[ ] [ ]

[ ] [ ]sin ( ( ))

Aj f TA A A

A A A Ai

y n H x n e

y n y i c n i

WHAT ABOUT ERRORS?

Will errors in decoding have a cascading effect?Error propagation dies out exponentially

Error correction capability of modulationForward and backward decoding

ACKNOWLEDGEMENT

Use as much synchronous acknowledgement as possible for backward compatibility

EVALUATION• 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

Transmitter Receiver

Software

MICRO-BENCHMARK

ALICE & BOB

Bob’s location is fixed, Alice moves closer to the base-station

IMPACT OF SNR ON BER

Alice & Bob at fixed and equal location

Vary transmission power level

TESTBED RESULTS

Pick two senders randomly 10% hidden terminals, 10% partial, 80% perfect

THREE HIDDEN TERMINALS

SECRET TO SUCCESS

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!