1 Mobile Networking for Smart Dust J. M. Kahn, R. H. Katz, K. S. J. Pister Department of Electrical...
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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 10s—1000s m
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Smart Dust Mote
1-2 mm
Thick-Film Battery
Solar Cell
Power Capacitor
Analog I/O, DSP, Control
Active Transmitter with Laser
Diode and Beam SteeringPassive Transmitter with
Corner-Cube Retroreflector
Sensors
Receiver with Photodetector
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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 J/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 (<< 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
Light Collection Area
Body Diagonal
Direction
Reflected
of Incidence
RadiantIntensity
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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
DownlinkLaser
U plink
CCD Corner-Cube
Upl ink
Data In
Data
Im ageSensor
Retroreflector
D ata In
Photo-
DownlinkData Out
detector
Base-S tation Transceiver
Dust Mote
S ignal Selectionand P rocessing
UplinkData . . .
OutNOut1
Array
Unm odulated Interrogation
M odulated Reflected
Lens
Lens
M odulated Downlink Data or
B eam for Uplink
B eam for Uplink
Asymmetric Link assummed: high power laser xmit from BS, with larger scale imaging array
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Optical Communication withPassive Dust Mote Transmitters
– 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
Transmitter Radiant Intensity
Receiver Light Collection Area
Base
TransceiverStation
DustMote
DustMote
Transmission appears as blinking light 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
LaserCollimating
Beam
Mirror(s)
Lens
Steering
Diode
Two-axis beam steering assembly
Active dust mote transmitter
– Beams have divergence << 1º– Steerable over a full hemisphere
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Optical Communication withActive Dust Mote Transmitters
– BS uses CCD or CMOS camera (operate at up to 1 Mbps)– Using multi-hop routing, not all dust motes need LoS to BS
Transmitter Radiant IntensityReceiver Light Collection Area
Base
TransceiverStation
DustMote
DustMote
DustMote Wall
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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 blockagePower limitedRapid topology changesScarce 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