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) –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

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

Outline A radio 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

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

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)

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

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

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

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

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

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

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

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 Degree of Irregularity Max range Min range Actual Range For this node

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

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* K D Signal receiving power = VSP adjusted signal sending power – DOI adjusted path loss + fading VSP adjusted signal sending power =

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

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

Impact - Routing Impact on: –Neighbor-Discovery technique –Used in GF, GPSR, SPEED Impact on Neighbor Discovery Technique

Simulation Test ComponentsSetting SimulatorGloMoSim Terrain(150m,150m) Node Number100 Node PlacementUniform Payload Size32 Bytes Application6 randomly chosen periodic multi-hop CBR streams Routing ProtocolAODV, DSR, GF MAC ProtocolCSMA, (DCF) Radio ModelRIM Radio Bandwidth200Kb/s Runs140 Confidence IntervalsThe 95% confidence intervals are within 0~25% of the mean

Quantify the Impact Increase DOIIncrease VSP

Quantify the Impact Increase DOIIncrease VSP

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.

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

Solution: Symmetric Geographic Forwarding Beacon to discover neighbors Exchange neighbor tables to detect asymmetry Delete asymmetric links from valid neighbor table X x

Symmetric Geographic Forwarding (SGF) Increase DOIIncrease VSP

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 Percentage of Reporting Nodes

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 Weaker signal A 816

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)

Radio Interference B AC Range 1 1 Range 2 OK Interferes

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)

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.

Two Types Frequency Hopping Spread Spectrum –Easier to explain Direct Sequenced Spread Spectrum –Used in MicaZ

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

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

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

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 standard) –Fc = (k-11) MHz, k = 11, 12, …, 26 Hardware MAC encryption

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

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