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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