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					                                                                         Motivation
                                                      • Goal: sensor network
                                                        hardware that can run for 2-5
                  WiseNET                               years off a single 1.5V AA
                                                        alkaline battery.
   (Or, how to make a sensor net node that            • This means average power
                                                        less than 10-100 µW.
   lasts as long as your battery’s shelf life)
                                                      • Typical commercial radio
                                                        transceivers require > 10mW




                                                               Communication Power
                    Outline
                                                                  Consumption
• WiseMAC Protocol                                    • How to communicate when nodes sleep
  – Optimized for low duty cycle operation              most of the time?
• Optimized Radio Transceiver                         • Design MAC to reduce wasted power due
                                                        to:
  – Low power
                                                        –   Idle listening
• System On Chip design                                 –   Overemitting
                                                        –   Overhearing
                                                        –   Collisions




          Preamble Sampling                                   Fixed-Length Preamble
• Carrier Sense, Multiple Access                      • All nodes sample medium with same
• Preamble Sampling                                     period, independent offsets.
                                                        – If busy, listen until receive packet or not busy.
  – I.e. don’t listen all the time, just sample and
    wait for a preamble                               • Transmitter sends preamble longer than
                                                        sampling period before each packet.
  – Sampling = measure received signal strength
                                                      • + Low power for low traffic
• (This isn’t the new part…)
                                                      • - Power overhead for long preamble
                                                        – xmit, plus rcv for all nodes that hear preamble




                                                                                                              1
WiseMAC: Minimized Preamble                             WiseMAC Preamble Minimization
• Nodes also send time to next sample in
  acknowledge packets.
• Nodes maintain table of offsets for common
  destination nodes.
• Duration of wake-up preamble adjusted to
  cover maximum drift between clocks.




WiseNET Wakeup Preamble Size

•   Tp = min(4θTc, Tw)
•   Tp = duration of the preamble
•   θ = frequency tolerance of time-based quartz
•   Tc = interval between communications
•   Tw = sampling period
•   Covers potential drift between clocks




            Power Comparison                                         Radio Design
• CSMA limited at low traffic                           • Transceiver is biggest power drain in the
    – Receiver never turned off                           sensor node.
• T-MAC                                                 • WiseMAC is designed for low duty cycle
    – Choose duty cycle                                   operation. Optimize the transceiver, too.
    – Drops packets at low duty cycles, so get either
      low power in low traffic or high throughput
• WiseNET is ultralow-power for low traffic,
  efficient for high traffic.




                                                                                                      2
        Radio Design Choices                              Dual-Band, Multi-Channel
• Chose high constant transmit power (max             • 433MHz ISS, 868 MHz SRD
  allowed in Europe for 434MHz ISM)                   • Avoid interference from other sources.
  – Nodes usually listen a lot, transmit rarely       • Why not 2.4GHz? (popular, globally
• Dual-band, multi-channel to reject strong             available) Power.
  interference                                          – 50% of receiver power due to circuits operating
• Reduce energy consumption and wakeup                    at the carrier frequency, and power is
  time in receive mode.                                   proportional to the frequency.




              Receiver Power                                WiseNET Optimizations
• Receiver power much larger for RF blocks            • Flexible wake-up sequence
  – Current directly related to frequency of            – Low frequency reference clock
    operation or required bandwidth.
                                                        – Baseband path of channel filters, limiters, &
  – These are also the blocks that wake up quickly.       RSSI.
• Turn-on time varies inversely with                    – Frequency synthesizer
  frequency.
                                                        – Intermediate frequency amplifiers
• So save power by waking up baseband
                                                        – RF frequency low-noise amplifier & mixers
  components first, then RF circuits.




      WiseNET Optimizations                                 Fast Rx/Tx Turnaround
• Use RSSI to determine whether to power up           • You’re burning power when switching from
  rest of the broadband receive chain                   Rx to Tx or vice versa, so do it fast.
  – I.e. if there’s no signal, don’t process it       • WiseNET shares the core (intermediate
• Minimize wakeup time for baseband and                 frequency) circuitry for the receivers and
  intermediate frequencies.                             transceivers, so there’s very little
  – Deep submicron process, trade speed for             turnaround time.
    power.
  – Circuit tricks to wake up the baseband fast.




                                                                                                            3
           WiseNET—SOC                               WiseNET Node Architecture
• Custom System-On-Chip (SOC)
• Most sensor node functionality on a single
  chip to reduce power consumption.
  – Sensing, processing, storing, communicating




              Bottom Line                                         To Sum Up…
• WiseNET transceiver with WiseMAC                • SystemOnChip design
  protocol consumes 25uW when forwarding          • WiseMAC protocol
  56-byte packets every 100 seconds.                 – Minimal preamble size reduces xmit and receive
                                                       overhead.
• For comparison, Motes…                             – Preamble filtering reduces overhearing.
  – 24mW in receive mode                          • Cool radio transceiver
  – SMAC or TMAC, 10% duty cycle => 2.4mW            – Low power.
                                                     – Efficient wakeup.
                                                     – Quick turnaround between Rx and Tx.




                                                                                                        4

				
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posted:11/9/2010
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