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Introduction to Wireless Sensor Networks

Introduction to Wireless Sensor Networks. Energy Considerations in WSNs I 3 February 2005. Organizational. Class Website. www.engineering.uiowa.edu/~ece195/2005/. Class Time. Office Hours. Organizational. Class Topics (3 students/per topic)

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Introduction to Wireless Sensor Networks

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  1. Introduction to Wireless Sensor Networks Energy Considerations in WSNs I 3 February 2005

  2. Organizational Class Website www.engineering.uiowa.edu/~ece195/2005/ Class Time Office Hours

  3. 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

  4. Organizational • Update Lecture 3 with two slides • Path loss • Added supplementary material to website • Lab demonstration next Thursday

  5. Review Questions • 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?

  6. 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?

  7. 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?

  8. Energy/Power Considerations • Terms • Cell, Battery • Energy (Joule) • Power (J/s or Watt) • Ampere-hour (AH) • Deep-cycle • MCU • Sleep Modes • ADC • BPS

  9. Where Does The Power Go?

  10. 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

  11. 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

  12. 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

  13. Radio Power Consumption

  14. 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

  15. 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

  16. 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

  17. Capacity 1.25 Ah

  18. May be possible to use curve to gauge battery state. Must be under load conditions.

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