Dynamic Sensor Networks Communications Board

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Dynamic Sensor NetworksCommunications Board

• Dr. Jack Wills
• Feb 3, 2000
• USC Information Sciences Institute

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Communications Board - Overview

• Low Power Communications Capability
• TDMA Burst Communications
• Power savings from low duty cycle
• GPS Time and Position
• Open loop time synchronization
• Position data help routing
• Micropower Alarm Channel always on

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Communications BoardFunctionality

• Medium Data rate UHF data comm Tx/Rx
• Low data rate VLF alarm channel Tx/Rx
• Location determination
• Time of Day
• Store and forward packets, based on header

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Dynamic Sensor NetworksNode Architecture
Processor Subsystem
Sensor Array Subsystem
Optional for Command Node
Long Range Radio
Temp
Network Connection
Vibration
Other
Serial Port
Serial Port
TOD
Communications Subsystem
GPS
UHF Radio
LF Radio
Comm Antenna
GPS Antenna
Loop Antennae
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Communications BoardGPS/Radio

• TDMA Burst Communications synchronized by GPS
  Time of Day
• Tx and Rx are scheduled on for power savings
• Commercial 915 MHz ISM Band Chipset
• RFMD 9901/9902 - FSK - 100 kbps
• Open loop bit sync - no sync preamble
• Commercial GPS
• Motorola Oncore UT

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Communications BoardGPS Performance

• Type - Motorola Oncore UT
• Channels - 5
• Power
• Active lt0.9 Watts at 5 volts
• Sleep 5?A typ. At 2.5 volts
• Position Accuracy
• 100 meter 2D RMS (selective availability on)
• Time Accuracy
• lt50 nS (1 sigma) with position hold on

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Communications Board - Form Factor

• Single PCB
• GPS on COTS daughter card
• Multipin connector to match Rockwell Node Design
• External Ferrite Rod Antennas for low frequency
  communications
• 4.00 inch long 1.00 inch diameter

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Communications BoardWakeup Problem

• A single sensor node (while in a low data rate,
  high latency communications mode) may identify an
  event which warrants high rate, low latency
  communications
• Solution
• Low power receiver continuously on
• Out of band signaling

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Communications BoardAlarm Channel

• Very Low Bit Rate Required
• Micropower receiver
• Extra Low Frequency
• 160-190 kHz
• Simple Waveform
• Square Wave AM (Pulse Modulation)
• Tuned RF receiver - no local oscillator

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Communications BoardELF Radio

• Large Wavelength
• 1750 meters
• Near Field Propagation
• Antenna Efficiency Issues
• Tuned Ferrite Rods
• Q50
• Bandwidth 3kHz at 175 kHz
• Coupled Inductors
• Orientation Sensitive

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Communications BoardELF Receiver

• Active Devices
• AD620 Instr. Amp (2 each) 2.7 mW each
• AD8310 Log Amp (2 each) 22.5 mW each
• AD630 Chopper Amplifier 50 mW
• BiFET Dual Op amp (Integrator and
  comparator) 3 mW
• Power consumption 100 mW

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Communications Board Transmission Sequence (1 of
2)

• Initialize GPS. Acquire position and time. Start
  TOD counter
• Power down GPS
• Accept data buffer and transmission time form
  processor subsystem. Calculate checksum and add
  to packet.
• Before transmission wake GPS. Acquire
  satellites. Select 0-D mode. Monitor time string
  from GPS serial port

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Communications Board Transmission Sequence (2 of
2)

• At 1 second before transmission, enable
  transmission at next 1 PPS pulse
• At edge of pulse, turn on transmitter, Wait 5
  milliseconds for PLL to lock, then send data
  buffer at fixed bit timing
• Power down GPS.

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Communications Board Timing Accuracy

• Maximum timing error - 1/10 of bit period
• 1 microsecond
• Allocation
• GPS time of day /- 45 nanosecond
• Clock Resolution (10MHz)/-0.1 microsecond
• Prop Delay(100 meters) /- .16 microsecond
• Oscillator Drift /- .69 microsecond (allocated)

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Communications Board Oscillator Accuracy

• Clock Oscillator - 1
• /- 100 ppm
• Max packet length 345
• TCXO (off the shelf) - 5
• /- 2.5 ppm
• max packet length 13800
• Custom TCXO - 100
• /- 0.5 ppm
• max packet length 69000

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Communications Board Command Set - Processor to
Comm Board

• Sleep/Wake
• Receive at time XXX
• Fetch receive data (from comm bd to processor)
• Transmit at time YYY
• Load Tx data (from processor to comm bd)
• Initialize/Reset
• Get time
• Get position

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Communications Board Milestones

• Test GPS module 28
  Feb
• Brassboard alarm channel 21 Feb
• Board Design
• Circuit design 13 March
• PCB Layout 27 March
• Board Fab 3 April
• Board Assemble/Test 24 April
• Integrate with Processor 29 June