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

Outline Smart Dust Technology Power Passive & Active Communications Networking Summary

“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

Smart Dust Mote

Concept of Operations

Power Management Sources Storage Usage 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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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