Photo: San Onofre Nuclear Generating Station

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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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(No Transcript)
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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
? 25.61C
Motor Replaced
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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 Factors
Path Loss L(dB) 40 10NLogD L path loss at
2.4GHz D distance in meters NExponential Path
Loss Factor N2 for free space Observed values of
N 1.3ltNlt3.7 Factors Building Construction,
Channel Obstructions
This equation represents a single static channel
L(dB) 40 10Ne LogD Ne Value based on
installation type
Large ScaleL(dB) 40 10Ne LogD Ll LlRV
based on loss from Obstructions (Walls, Doors,
Ceilings) Ll usually a log normal distribution
from installations of same type
Small Scale L(dB) 40 10NeLogD Ls Ll Ls
RV based on measured Channel Characteristics Bot
h in time and space (Rayleigh, Rician, Nakagami,
etc).
The better we characterize Ne the less variance
in Ll
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Frequency Diversity In situ experiment
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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
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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 Bases
Static 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 gtgt10 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