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Enhancing Radio Channel Quality for Industrial Sensor Networks

Explore the anatomy of a mesh network system, the challenges of radio reliability in factories, and the benefits of channel diversity and fading factors in industrial environments. Learn about IEEE 802.15.4 standards, diversity gains, and power levels for optimized performance.

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Enhancing Radio Channel Quality for Industrial Sensor Networks

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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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