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Photo: San Onofre Nuclear Generating Station

Radio Channel Quality inIndustrial Sensor Networks

Daniel Sexton, Jay WerbSICon 05February 9th 2005

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“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

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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

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Value Proposition

Motor Replaced

Δ = 25.61°C

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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?

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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?

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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

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Fading FactorsPath Loss: L(dB) = 40 +10*N*LogDL= path loss at 2.4GHzD = distance in metersN=Exponential Path Loss Factor N=2 for free spaceObserved values of N: 1.3<N<3.7Factors: Building Construction, Channel Obstructions

This equation represents a single static channelL(dB) = 40 +10*Ne *LogDNe = Value based on installation type

Small Scale: L(dB) = 40 +10*Ne*LogD + Ls + Ll

Ls = RV based on measured Channel CharacteristicsBoth in time and space (Rayleigh, Rician, Nakagami, etc).

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

The better we characterize Ne the less variance in Ll

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Frequency Diversity In situ experiment

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IEEE 802.5.4 Channels

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Diversity Gains

• 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

Methods to Obtain Diversity Gain (Small Scale)

We use all Three Need to quantify Large Scale Diversity Gain

NLOS ChannelRayleigh Faded

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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

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Covering the BasesStatic Multipath

Time VariantMultipath

StaticInterference(e.g. Microwave)

Time VariantInterference(Other Networks)

Spatial Diversity(Mesh Routing)

Frequency Diversity (Channel Hopping)

Temporal Diversity(Retries)

Transmit Power(15 dBm)

Overall Risk Coverage

• Assumptions– Spatial diversity supports a few paths– Frequency diversity supports many channels– Path diversity directs signal away from interferers– Interference is channel limited

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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