Simple MAC for Pico Radio

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Simple MAC for Pico Radio

• Lizhi Charlie Zhong
• January 12, 2001

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Why Simple?

• Simpler means lower power
• Simpler means smaller size
• Simpler means lower cost
• Simpler means more robust
• Simpler means less design effort
• Keep it simple as long as design requirements can
  be met!

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How to Achieve Ultra-low Power

• Turn off main radio whenever it is not
  transmitting or receiving
• Uses a 1uw wakeup radio
• Wakes up only a particular node
• Tells the main radio which channel to listen to
• Trade bandwidth for power
• Exploit redundancy to save power

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Multi-channel CSMA

• Simple procedure
• Randomly pick a channel
• If it is idle, use it
• If it is busy, go to step 1) and repeat on the
  remaining channels until all channels have been
  checked
• If all channels are busy, set a random timer for
  each of them
• Use the channel whose timer expires first and
  clear all other timers
• Channels can be code, sub carrier, or space

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Advantages

• No power wasted on channel assignments
• No processing of channel allocation
• Quickly adapt to network changes (mobile)
• No dedicated control channel
• Less chance of secondary collision
• Better usage of bandwidth
• No RTS/CTS overhead
• Channel allocation may have conflict in code, and
  is not optimal yet

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Advantages

• Reduce collision, therefore retransmission
• CDMA radio supports 60 channel each node has
  less than 10 neighbors
• less than 1 user per channel on average
  collision possibility comparable to dedicated
  channel assignment
• Increase throughput
• Exposed/hidden terminals are more of
  single-channel problems
• No exposed terminal problem
• Hidden terminal secondary collision (no
  collision if parallel correlators available,
  which are needed for rake receiver)

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What to do with Redundancy?

• Temporal and spatial correlation are very high
• Difference between samples has smaller range, so
  it can be represented by fewer bits
• Joint decoding using previous samples
• Distributed source coding
• Source compression
• Trade redundancy for reliability

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Exploiting Redundancy (Charlie and Rahul)

• Obtain the highly correlated data
• Destination itself monitor
• Select based on geographic range and time
• Get the data from neighbors
• Slowly increase the network span
• Use simple data estimation to recover lost data
  from those correlated data
• Eliminate acknowledgment, end-to-end error
  detection triggers estimation

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Simple Data Estimation

• Last sample of the same sensor
• Moving average
• Yule-Walker predictor
• Best linear MMSE estimator using data from the
  same sensor and its neighbors

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Comparison

• Save at least 304 bits for every data packet
• ACK packet length 144 bits preamble 48 bits
  PLCP header ack packet fields( gt112 bits)
• compared to the saving of 1 or 2 bits using
  complicated signal processing seeking to remove
  redundancy directly
• And no constructive method available yet for
  distributed source coding

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Simulation Setup
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Light Intensity Vs. Time (W100 P10)
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Down-sampled Light Intensity Vs. Time (W250
P2)
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Simple Method for Connectivity

• Fixed transmission power
• Placement of static nodes to ensure connectivity
• Mobile nodes may offer short cut, but are not
  counted on for connectivity
• Single power level, always connected, no OH in
  adapting to power level changes

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Conclusion

• Simple means low power
• Why pay one dollar to save one cent?
• Why not simple?