Mobile Networking for Smart Dust

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Mobile Networking for Smart Dust

• J. M. Kahn, R. H. Katz, K. S. J. Pister
• Department of Electrical Engineering
• and Computer Sciences
• University of California, Berkeley
• Berkeley, CA 94720-1776

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Outline

• Smart Dust Technology
• Power
• Passive Active Communications
• Networking
• Summary

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Smart Dust Mote

• Device being developed by Kahn and Pister as part
  of DARPA MTO MEMS program
• System support being developed under DARPA
  Information Technology Expeditions program
• Autonomous node incorporating sensing, computing,
  communications power source in 1 mm3 volume
  (current prototype 8 cm3)
• Dispersed through (outdoor) environment
• Exploit wireless communication to relay sensor
  info to BS over distances of 10s1000s m

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Smart Dust Mote
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Concept of Operations
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Power Management

• Sources
• Solar cells
• Thermopiles
• Storage
• Batteries 1 J/mm3 (Advantage higher power
  density)
• Capacitors 1 mJ/mm3
• Usage
• Digital control 10-15 J/typ. 8-bit instruction
• Analog circuitry nJ/sample
• Communication nJ/bit
• Several hours of useful life achievable

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Corner-Cube Retroreflector

• Fabricate CCR using MEMS technology
• Light striking within 30 of body diagonal
  undergoes 3 bounces returns to source in a
  narrow beam (ltlt 1)
• Deflect one mirror electrostatically, modulating
  return beam up to 10 kbps (simple on-off
  keying)
• Major benefit transmit passively with no
  radiated energy, no beam aiming

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First-Generation Dust Mote
CCR Control Circuitry
Type 5 Hearing Aid Battery (smallest commercially
available battery)
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Optical Communication withPassive Dust Mote
Transmitters
Asymmetric Link assummed high power laser xmit
from BS, with larger scale imaging array
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Optical Communication withPassive Dust Mote
Transmitters
Transmission appears as blinking light at BS

• Requires each dust mote to have LoS to BS
• Uplink transmissions are multiplexed using
  space-division multiplexing
• Separation depends on the resolution of imaging
  array at BS

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Optical Communication withPassive Dust Mote
Transmitters

• Power Efficient Probe Protocol
• Dust motes are asleep BS broadcasts a wakeup
  signal, then a query Dust mote wakes up,
  receives query
• BS broadcasts periodic interrogating signal
  synchronized to its imaging sensor
• Dust motes transmit simultaneously to BS,
  synchronized to the interrogating signal
• Reliability
• Dust mote positions and orientations are random
• Not all in field-of-view of BS
• To insure adequate coverage, use excess of dust
  motes
• Centralized control scheme BS is single point of
  failure

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Optical Communication withPassive Dust Mote
Transmitters

• Passive Communications Pros
• Dust motes need not radiate power, nor steer beam
• Exploits asymmetry powerful BS, low-power dust
  motes
• Utilizes space-division multiplexing
• Only baseband electronics required
• Passive Communications Cons
• Requires line-of-sight path to BS
• Short range (up to about 1 km)
• Bit rate limited to about 10 kbps
• Affected by rain, fog, atmospheric turbulence

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Active Dust Mote Transmitter
Two-axis beam steering assembly
Active dust mote transmitter

• Beams have divergence ltlt 1º
• Steerable over a full hemisphere

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Optical Communication withActive Dust Mote
Transmitters
Wall

• BS uses CCD or CMOS camera (operate at up to 1
  Mbps)
• Using multi-hop routing, not all dust motes need
  LoS to BS

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Optical Communication withActive Dust Mote
Transmitters

• Minimizing Transmitted Energy/Bit
• Advantageous to transmit in short bursts at high
  bit rate
• More efficient to use narrow beam at high scan
  rate than wide beam at lower scan rate
• Topology Discovery
• Protocols for dust motes to discover location of
  neighbor dust motes, to actively aim their
  directional transmitters towards nearby nodes
• Stereo imaging at multiple BSs can yield 3D
  information (centralized routing algorithms)

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Optical Communication withActive Dust Mote
Transmitters

• Links Not Bi-Directional
• Directional transmitters but non-directional
  receivers waste power communicating with nodes
  unable to receive transmission
• Costs power to steer and actively ping nearby
  neighbors
• Establish bi-directional links nodes that
  acknowledge receipt of ping transmissions
• Hidden terminals not eliminated collisions at
  dust motes during mote-to-mote communications are
  possible

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Optical Communication withActive Dust Mote
Transmitters

• Active Communications Pros
• Longer range than passive links (10 km)
• Higher bit rates than passive links (1 Mbps)
• With multi-hop, avoids need for LoS to BS
• Utilizes space-division multiplexing
• Only baseband electronics required
• Active Communications Cons
• Requires protocol to steer directional
  transmitters
• Requires higher power than passive transmitter
• Affected by rain, fog, atmospheric turbulence

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Packet Radio vs. Smart Dust

• Omnidirectional
• Simpler bi-directional link establishment
• No LoS blockage
• Power limited
• Rapid topology changes
• Scarce radio spectrum
• Available spectrum limits overhead messages

• Directional xmit non-directional receive
• Harder bi-directional link establishment
• LoS blockage
• Severely power limited
• Slower topology changes
• Optical imaging for spatial division high b/w
• Available pwr limits active xmit for blocked nodes

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Multi-Hop Routing Issues

• Collecting Disseminating Route Information
• BS Visible Dust Motes
• Stereo imaging for 3D location within BS
  field-of-view
• Topology information disseminated via BS
  broadcast
• Dust motes within sight of BS are landmark nodes
• Blocked Dust Motes
• Discover blockage via absence of BS probe
• Go active to determine links to neighbors
• Budget intensity/frequency to conserve power
• Exchange topology info with bi-directional
  neighbors
• Build routing table to landmark dust motes

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Summary

• Smart dust motes incorporate sensing,
  computation, communications power in 1 mm3
• Free-space optical communication offers
  advantages in size, power network thruput
• Passive dust mote optical transmitters
• Use corner-cube retroreflector (CCR)
• Extremely low power
• Require LoS to BS
• Active dust mote optical transmitters
• Use laser and beam-steering mirror
• Enable higher bit rates, longer ranges, multi-hop
  routing