ZigZag Decoding: Combating Hidden Terminals in Wireless Networks

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ZigZag Decoding Combating Hidden Terminals in
WirelessNetworks

• Shyamnath Gollakota and Dina Katabi
• MIT CSAIL
• SIGCOMM 2009

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Hidden Terminal Problem
Alice
Bob
AP
X

• Leads to low utilization of bandwidth and
  unfairness in channel access
• RTS/CTS induced too much overhead
• Collided packets may still be decodable!

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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

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Wait! What about Shannon Capacity?
R1

• Requires retransmissions if collision occurs
• No overhead if no collision

TDMA
R2
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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

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Patterns that ZigZag Applicable

• Both backward and forward decoding can be used

Sudoku? ?
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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
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Preliminary on communication

• BPSK 0 -gt -1 1 -gt 1
• http//en.wikipedia.org/wiki/QPSK

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Collision Detection

• Preamble
• Pseudo random number
• Correlation with moving window
• thresholding

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Matching collision

• Given (P1 P2(?)) and (P1, P2(?)), how to
  determine that P1 P and P2 P2
• Determine offset first
• Correlation of P2(?) and P2(?)

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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)

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Decode matching collision (contd)

• Re-encoding
• Account for sampling error

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What about errors?

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

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Acknowledgement

• Use as much synchronous acknowledgement as
  possible for backward compatibility

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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

Software
Transmitter
Receiver
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Micro-benchmark
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Alice Bob

• Bobs location is fixed, Alice moves closer to
  the base-station

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Impact of SNR on BER

• Alice Bob at fixed and equal location
• Vary transmission power level

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Testbed Results

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

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Three hidden terminals
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Conclusion

• ZigZag improves fairness throughput
• Further research
• Combination of analog network coding