1. 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

2. Outline Smart Dust Technology Power Passive & Active Communications
Networking
Summary

3. “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

4. Smart Dust Mote

5. Concept of Operations

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

7. 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

8. First-Generation Dust Mote
CCR Control Circuitry
Type 5 Hearing Aid Battery
(smallest commercially
available battery)

9. Optical Communication with Passive Dust Mote Transmitters
Asymmetric Link assummed: high power laser xmit from BS, with larger scale imaging array

10. Optical Communication with Passive 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

11. Optical Communication with Passive 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

12. Optical Communication with Passive 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

13. Active Dust Mote Transmitter
Two-axis beam steering assembly
Active dust mote transmitter
Beams have divergence << 1º
Steerable over a full hemisphere

14. Optical Communication with Active 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

15. Optical Communication with Active 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)

16. Optical Communication with Active 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

17. Optical Communication with Active 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

18. 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

19. 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

20. 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