The University of Iowa. Copyright© 2005 A. Kruger 1 Introduction to Wireless Sensor Networks Energy Considerations in WSNs I 3 February 2005

The University of Iowa. Copyright© 2005 A. Kruger 2 Organizational Monday 4:30-5:20Room 4511 SC Thursday12:30-1:20Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Monday5:20-6:20Room 1126 SC Thursday1:30-2:30Room 1126 SC OtherBy appointmentRoom 523C SHL Class Website Class Time Office Hours

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 4 Organizational Update Lecture 3 with two slides –Path loss Added supplementary material to website Lab demonstration next Thursday

The University of Iowa. Copyright© 2005 A. Kruger 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/R 2 but when the transmitter and receiver is close to the ground, the loss can be ~1/R 4 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?

The University of Iowa. Copyright© 2005 A. Kruger 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?

The University of Iowa. Copyright© 2005 A. Kruger 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/ R 2 or ~ 1/ R 4 ?. Is the communication LOS?

The University of Iowa. Copyright© 2005 A. Kruger 8 Energy/Power Considerations Terms –Cell, Battery –Energy (Joule) –Power (J/s or Watt) –Ampere-hour (AH) –Deep-cycle –MCU –Sleep Modes –ADC –BPS

The University of Iowa. Copyright© 2005 A. Kruger 9 Where Does The Power Go?

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 13 Radio Power Consumption

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 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

The University of Iowa. Copyright© 2005 A. Kruger 17 Capacity 1.25 Ah

The University of Iowa. Copyright© 2005 A. Kruger 18 May be possible to use curve to gauge battery state. Must be under load conditions.