Download Wireless Sensor Networks

Introduction to Wireless
Sensor Networks
Energy Considerations in WSNs I
3 February 2005
The University of Iowa.
Copyright© 2005
1
A. Kruger
Organizational
Class Website
www.engineering.uiowa.edu/~ece195/2005/
Class Time
Monday
4:30-5:20
Room 4511 SC
Thursday
12:30-1:20
Room 3220 SC
Please note that the room numbers are different for
Mondays and Thursdays.
Office Hours
Monday
5:20-6:20
Room 1126 SC
Thursday
1:30-2:30
Room 1126 SC
Other
By appointment
Room 523C SHL
The University of Iowa.
Copyright© 2005
2
A. Kruger
Organizational
• Class Topics (3 students/per topic)
• Students read and help prepare a lecture on one of:
– ZigBee Protocol
• Brief overview
• Example application
– What is RFID?
• Brief overview
• Explanation of terminology
– Sample Energy Budget
• Worked example of energy considerations in WSNs
– Directed Diffusion
• Explanation of the algorithm
– Overview of TinyDB
– Sensor Characteristics
• Soil moisture, temperature, humidity, wind speed, vibration
– Analog/Digital Conversion
• Number of bits, linearity, sampling rate, power, etc.
– Smart Dust
• Will post a signup sheet
The University of Iowa.
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Organizational
• Update Lecture 3 with two slides
– Path loss
• Added supplementary material to
website
• Lab demonstration next Thursday
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Review Questions
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What is “LOS”?
What is WSN?
True or false – Visual LOS implies RF LOS
A 1-km 2.4 GHz link has two antennas that are 2 m
above the ground. Do we have LOS?
True of false: In free space RF power loss ~1/R2 but
when the transmitter and receiver is close to the
ground, the loss can be ~1/R4
What is RSSI? How is it used in WSNs?
What are the ISM bands
True or false: ISM bands are unregulated
Explain with a simple sketch and paragraph how
multipath propagation can diminish or enhance radio
propagation
What is BER?
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Review Questions
• A manufacturer claims its radio can
make reliable reception if the received
power is -105 dBm. How many mW is
this? Are you impressed?
• Explain what TDMA is.
• Explain what FDMA is.
• Explain (to grandma) what Spread
Spectrum communication is.
• What is S/N and SNR?
• What are the common units of SNR?
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Review Questions
• True or false – everything else being
equal, RF path loss are higher at 2.5
GHz than at 900 MHz
• Estimate the path loss in dB at 900 MHz
in an indoor environment. There are
two floors are six walls between
transmitter and receiver.
• The antennas of a 2.4 GHz RF link are 1
m above the ground, and are 100 m
apart. Is the path loss ~ 1/R2 or ~ 1/R4?.
Is the communication LOS?
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Energy/Power Considerations
• Terms
– Cell, Battery
– Energy (Joule)
– Power (J/s or Watt)
– Ampere-hour (AH)
– Deep-cycle
– MCU
– Sleep Modes
– ADC
– BPS
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Where Does The Power Go?
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Microcontroller Unit (MCU)
• Intel’s StrongARM, Atmel AVR (PIC?)
• Low power modes
– Active, Idle, nap, shutdown, sleep modes
– For some MCUs, in deep sleep modes, the
power consumption can almost be
negligible
– Takes longer to wake from a deep sleep
than just a nap
– Wakeup time also takes power
– Wakeup impact processing
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Radio
• Radio typically contain embedded controller
that provides many functions
– Uses RSSI to adjust transmit power
– Error detection and correction in hardware
• Several modes
– Receive only, transmit + receive, idle, etc.
• Transmit in general requires most power
• Careful consider radio spec and modes
• Mode change can consume a lot of power
– May be better to shutdown completely rather than
go into idle mode
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Bandwidth vs. Power Consumption
• Higher bandwidth (BPS) generally
requires
– Better S/N
• => more signal power
– More sophisticated modulation
encoding/decoding algorithms
• => more powerful CPU, more power
• Receive power normally much lower
than transmit power
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Radio Power Consumption
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Conventional vs WSN Power
Management
• Conventional
– Well developed techniques
– Objective is to minimize power consumption of
individual device: sleep modes, low voltage, low
clock speed, etc.
• WSNs
– Objective is to maximize lifetime of individual
node, but more importantly
– The network as a whole
• For example
– It may be better for a node to deplete its power
source for the greater good of the network
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Sensors
Passive & low power (~mW and smaller)
– Soil moisture, temperature, light, humidity
• Active & high power
– Anemometers, disdrometers, cameras
• Many sensors are inherently analog, but some
sensors have digital interfaces (provided by
embedded controllers)
• Conditioning/wakeup times need to be considered
• Analog-Digital Converters (ADC)
– Can be a major power consumer
– More bits and high conversion rate requires more power
– Don’t over specify
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Battery
• Uses chemical reaction to provide
electrical energy
– Temperature depended
• Batteries are often the most bulky part
of a mote
• Capacity measured in Ampere-hours or
Ah. Note that the capacity does
consider voltage…
– The capacity is the nominal number of
hours it can supply a given current
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Capacity 1.25 Ah
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May be possible to use
curve to gauge battery
state. Must be under
load conditions.
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