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Wireless Sensor Networks Radio Realities

Wireless Sensor Networks Radio Realities. Professor Jack Stankovic University of Virginia 2006. Motivation. Significant Evidence of radio irregularity in physical environments Theoretical Practical (empirical evidence)

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Wireless Sensor Networks Radio Realities

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  1. Wireless Sensor NetworksRadio Realities Professor Jack Stankovic University of Virginia 2006

  2. Motivation • Significant Evidenceof radio irregularity in physical environments • Theoretical • Practical (empirical evidence) • Too many current solutions are via simulation with circular radio range assumed • Need for simulation tools to model irregularity • Need for better protocols to address irregularity • Many current protocols won’t work in practice

  3. B, C, and D are the same distance from A. Note that this pattern changes over time. Example Irregular Range of A A and B are asymmetric

  4. Outline • Aradio energy model that considers irregularity and that can be used in simulators • Study the impact of radio irregularity on • MAC layer • Routing layer • Other protocols (such as localization, topology control) • Result: Common and non-negligible • Solutions to deal with radio irregularity • Implicit • Explicit

  5. Antenna Types • Half-wave dipole (most efficient transmission) • Quarter wave vertical Quarter Wave Vertical Half-wave dipole Perfect Isotropic Antenna Radiation pattern Radiation pattern

  6. Line of Sight Impairments • Attenuation • Strength of the signal falls with distance • Attenuation is greater at higher frequencies • Strength of signal must be detectable by circuitry AND above noise • Free Space Loss • Ratio of radiated power to the power received by the antenna (antenna of certain area size)

  7. Line of Sight Impairments • Noise • Thermal • Crosstalk • Impulse (e.g., lightning) • Atmosphere absorption • Vapor and oxygen contribute to attenuation

  8. Line of Sight Impairments • Multipath • Reflection – bounce off objects are arrive at destination late, together with original signal • Diffraction – occurs at edge and looks like a new source (can have signal received even when no line of sight) • Scattering – if size of obstacle is on order of size of wavelength

  9. Summary - Causes of Radio Irregularity • Devices • Antenna type (directional, omni-directional) • Sending power (non-linear) • Antenna gains • Receiver sensitivity (circuits) • Propagation Media • Media type (air, water) • Background noise • Temperature, humidity • Obstacles • Rain But how significant in WSN devices

  10. Signal Strength over Time in Four Directions Real Measurements - Radio Signal • Non-isotropic Path Loss:The radio signal from a transmitter has different path loss in different directions. (RSSI – Received Signal Strength Indicator)

  11. Signal Strength Values in Different Directions Non-isotropic Path Loss • Reasons: • Reflection, diffraction and scattering in environment • Hardware calibration (non-isotropic antenna gain)

  12. Signal Strength Values in Different Directions Radio Signal Property • Continuous variation:The signal path loss varies continuously with incremental changes of the propagation direction from a transmitter.

  13. (a) One mote with different battery status (b) Different motes with the same battery status Radio Signal Property • Heterogeneity:Different nodes have different signal sending power • Reasons • Different hardware calibration and circuits

  14. Degree of Irregularity RIM – Radio Irregularity Model • Degree of Irregularity(DOI): • Definition: the maximum received signal strength percentage variation per unit degree change in the direction of radio propagation. • Account for non-isotropic path loss Max range Min range Actual Range For this node

  15. RIM - VSP • Variance of Sending Power(VSP): • Definition: the maximum percentage variance of the signal sending power among different devices. • Account for heterogeneous sending power

  16. RIM – Propagation Formula Signal receiving power = signal sending power - path loss + fading Signal receiving power = signal sending power – DOI adjusted path loss + fading DOI adjusted path loss = path loss* KD Signal receiving power = VSP adjusted signal sending power – DOI adjusted path loss + fading VSP adjusted signal sending power =

  17. (a) Carrier Sense Technique (b) Handshake Technique Impact – MAC layer • Impact on: • Carrier Sensetechnique • Handshake technique • Used in CSMA, MACA, MACAW, 802.11 DCF

  18. Route Discovery Using Multi-Round Technique Impact on Path-Reversal Technique Impact - Routing • Impact on: • Path-Reversal technique • Multi-Round technique • Used in AODV, DSR, LAR

  19. Impact on Neighbor Discovery Technique Impact - Routing • Impact on: • Neighbor-Discovery technique • Used in GF, GPSR, SPEED

  20. Simulation Test

  21. Quantify the Impact Increase DOI Increase VSP

  22. Increase DOI Increase VSP Quantify the Impact

  23. Summary of the Impact • Radio irregularity has a greater impact on the routing layer than on the MAC layer. • Routing protocols, such as AODV and DSR, that use multi-round discovery technique, can deal with radio irregularity, but with a high overhead. • Routing protocols, such as geographic forwarding, which are based on neighbor discovery technique, are severely affected by radio irregularity.

  24. GF always choose to node that is closest to the destination. Geographic Forwarding s d

  25. Solution: Symmetric Geographic Forwarding • Beacon to discover neighbors • Exchange neighbor tables to detect asymmetry • Delete asymmetric links from valid neighbor table 1 4 3 X 3 4 1 3 1 x 2 1 4

  26. Increase DOI Increase VSP Symmetric Geographic Forwarding (SGF)

  27. Percentage of Reporting Nodes Bounded Distance Forwarding • Bounded Distance Forwarding restricts the distance over which a node can forward a message in a single hop. • Implemented in a surveillance/tracking system with 70 MICA2 motes

  28. Bounded Distance Forwarding • 8 ft – not enough nodes that close so some/many paths not possible • 16 ft – best tradeoff • 24 ft and greater – too many asymmetric links A Weaker signal 8 16

  29. Other Radio Realities? • Interference Range • Normally, interference range is greater than communication range • Some protocols assume if more than 2 hops away then zero interference • Not true: sum of energy from many distant communication nodes may cause interference (must deal with SNR and not hop count)

  30. Range 1 1 Range2 Interferes C A OK B Radio Interference

  31. Other Radio Realities • Logically, if two nodes are both transmitting and within 1 hop, then both messages are lost • Not necessarily true – one packet may have enough signal strength to still be received correctly even if another node is transmitting at the same time (e.g., the second node may have a weak signal)

  32. Spread Spectrum • Spread spectrum is a transmission technique in which a pseudo-noise (PN) code, independent of the information data, is employed as a modulation waveform to “spread” the signal energy over a bandwidth much greater than the signal information bandwidth. • At the receiver the signal is “despread” using a synchronized replica of the pseudo-noise code.

  33. Two Types • Frequency Hopping Spread Spectrum • Easier to explain • Direct Sequenced Spread Spectrum • Used in MicaZ

  34. Basic Idea Sender Receiver 0100100100 Know the PN code and reverse the encoding 00 at freq A 01 at freq B 10 at freq C 00 at freq D 01 at freq E Might have 16 freq channels to choose from

  35. Advantages • Jam resistant • If you jam on a freq you only knock out a few bits (can be corrected) • Eavesdroppers on a freq can only hear a few bits • More resistant to noise and multi-path distortion • Multiple users can transmit simultaneously with no (little) interference

  36. Example • Use Spread Spectrum with a code • User A has code that provides freq 3,7,2,8 • User B has code that provides disjoint set of freq, e.g., 5, 6, 14, 1, 4

  37. Example: Radio Chip CC 2420 • DSSS • 250kbps effective data rate • Q-QPSK with half sine pulse shaping modulation • Low current consumption (RX: 19.7 mA, TX: 17.4 mA) • Programmable output power • 16 available frequency channels (IEEE 802.15.4 standard) • Fc = 2450 + 5 (k-11) MHz, k = 11, 12, …, 26 • Hardware MAC encryption

  38. More on Spread Spectrum • Tutorials on WEB • Wireless Communications and Networks, W. Stallings, Prentice Hall, 2nd edition.

  39. Summary • Radio irregularities are commonplace • Many current protocols are susceptible to poor performance because they ignore this problem (MAC, routing, localization, topology control) • They just don’t work in practice • SGF, Bounded Distance, …solutions do exist for radio irregularities • Radio interference realities are just being considered now • Spread spectrum will likely become common

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