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Photo: San Onofre Nuclear Generating Station

Radio Channel Quality in Industrial Sensor Networks. Photo: San Onofre Nuclear Generating Station. Daniel Sexton, Jay Werb SICon 05 February 9 th 2005. “Generic” Mesh Network Features. Bi-directional  Acknowledgements and commands Multi-hop  Extend solutions and resiliency

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Photo: San Onofre Nuclear Generating Station

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  1. Radio Channel Quality inIndustrial Sensor Networks Photo: San Onofre Nuclear Generating Station Daniel Sexton, Jay WerbSICon 05February 9th 2005

  2. “Generic” Mesh Network Features • Bi-directional  Acknowledgements and commands • Multi-hop  Extend solutions and resiliency • Self-healing  Reliable and flexible • Standards  IEEE 802.15.4 • Secure  Symmetric link to link encryption

  3. Anatomy of an installed system Bridge nodes interface the sensor network to the IP world Mesh nodes form a reliable backbone for routing sensor data Sensors connect to end nodes to access the network

  4. Value Proposition Δ = 25.61°C Motor Replaced

  5. IEEE 802.15.4Standard • Desirable features • 250 kbps → Flexibility, low duty cycle • 2.4 GHz → International • Lightweight MAC → low complexity • Simple ASIC → many sources → low cost • Network Security • But will it work in a harsh factory environment?

  6. Does the promise match the reality? • Research focuses on radio reliability in factories • Channel fading & multipath • Channel coherence • Radio performance • In simulation • On the wire • Statistical: lost packets in factories • Will channel diversity help?

  7. Channel Fading • Multipath effects • Varies by position • Varies by frequency • Varies over time • Overcome with diversity • Path diversity • Costs more routing nodes • Frequency diversity • Free with certain protocols

  8. Fading Factors Path Loss: L(dB) = 40 +10*N*LogD L= path loss at 2.4GHz D = distance in meters N=Exponential Path Loss Factor N=2 for free space Observed values of N: 1.3<N<3.7 Factors: Building Construction, Channel Obstructions This equation represents a singlestatic channel L(dB) = 40 +10*Ne *LogD Ne = Value based on installation type Large Scale:L(dB) = 40 +10*Ne *LogD + Ll Ll=RV based on loss from Obstructions (Walls, Doors, Ceilings) Ll usually a log normal distribution – from installations of same type Small Scale: L(dB) = 40 +10*Ne*LogD + Ls + Ll Ls = RV based on measured Channel Characteristics Both in time and space (Rayleigh, Rician, Nakagami, etc). The better we characterize Ne the less variance in Ll

  9. 6 1 5 2 4 3 IEEE 802.5.4 Channels Frequency Diversity In situ experiment

  10. Diversity Gains NLOS Channel Rayleigh Faded Methods to Obtain Diversity Gain (Small Scale) • Frequency Diversity (Greater than Coherence Bandwidth) • Good Margin for fading and interference • Path Diversity (Greater than 1 wave length) • Good Margin for fading, some for interference • Time Diversity (Greater than Coherence Time) • Good Margin for interference, some for fading We use all Three Need to quantify Large Scale Diversity Gain

  11. Power Level • How much power is enough? • 1 mW (0 dBm) is typical power level for 802.15.4 • Not enough for target 100 meter range • Especially when no line of sight available • Interference from Bluetooth and WiFi • Need similar power level to be heard • Supporting tests and simulations to be published • Regulatory limits • 36 dBm in US; 20 dBm in Europe (FH); 10 dBm some countries • Transmitter energy consumption • Transmitter percentage ~35% at 15 dBm; inflection point • Tested at 15 dBm • Seems about right; more field experience needed

  12. Covering the Bases • Assumptions • Spatial diversity supports a few paths • Frequency diversity supports many channels • Path diversity directs signal away from interferers • Interference is channel limited

  13. Early ResultsSummary • Industrial protocols should support diversity • Diversity gains >>10 dB • Frequency diversity is “free” • Some path diversity also provides redundancy • Increase 2.4 GHz radio power to about 15 dBm • Bluetooth/WiFi coexistence • Hostile radio environment, line of sight often unavailable • Range consistent with scale of industrial applications • Likely range somewhat less than 100 meters • Multihop architecture is necessary • More testing needed in a wider variety of sites

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