Cambium College
Interference and Mitigation Techniques
Seth Poche
Senior Technical Consultant-Global Services
Summary
• What is Interference?
• Signal to Noise Ratio
• Types of Interference
• Detecting Interference
• Basic Interference Mitigation Techniques
• Advanced Interference Mitigation Techniques
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Setting the Stage
• Session 02: “Channel Must Be Clear” –Dave Hensley (Radio Wave Propagation and Antennas/Reflectors)
transmit
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formatter
modulator
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What is interference?
• Definition: …anything which modifies, or
disrupts a signal as it travels along a channel
between a source and a receiver. (Wikipedia)
• A form of Noise
• Examples:
–
–
–
–
Wi-Fi Disconnecting
AM Radio cutting out
Wireless mouse freezing
Static on wireless baby monitor
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https://support.metageek.com/hc/en-us/articles/200628894-WiFi-and-non-WiFi-Interference-Examples
Perspective
• Interference largely only affects Receivers
NOT Transmitters
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Types of Interference
• Co-Channel Interference
– Example: FM Radio station from neighboring town
• Adjacent Channel Interference
– Example: FM Radio station from neighboring
channel
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Signal to Noise Ratio
• Measured in dB (relative measurement)
• Abbreviated SNR
• Also called CNR, CINR, CIR, C/I, S/I,…
dBm
-20
-30
-40
Signal Level
-50
-60
-70
-80
30dB
Noise Level
-90
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Detecting Interference
• e400 WiFi Detector
• Spectrum Analysis
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Basic Interference Mitigation Techniques
• Avoid the Interference
• Make the Signal more robust
• Attempt to Filter the Interference
• Antenna Selection
• Shielding
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Avoiding Interference
• Change channels
• Change bands
– Wide-band radios
– Dual-band radios
• Smaller channels
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Make the Signal More Robust
• Talk louder => Increase Tx Power
• Talk slower => Lower Modulation Rate
• Repeat => Add error correction into signal
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Attempt to Filter the Interference
Static Filter
5.925 GHz
Interference from
Neighboring
Channels
Impacting the
Radio
RADIO
Clean
Channel
Dynamic Filter
Selected Channel
Selected Channel
4.900 GHz
Static Filter
Spectrum
Dynamic Filter
Traditional Fixed Filter
Dynamic Filtering
Makes radio susceptible to
neighboring channel interference
Wraps around selected channel,
filtering out interference from
neighboring channels
https://en.wikipedia.org/wiki/Superheterodyne_receiver
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Antenna Selection
Parabolic Microphone (JGlover)
• Benefits of higher gain antennas
– Better gain
– More directional (selective)
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Shielding
• Physically separate the interference
from the receiver.
• Attenuate the interfering signal
• Example:
– Wireless Router on top of Microwave Oven
– Water Towers
– Antenna Shielding
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Advanced Interference Mitigation Techniques
• Frequency Hop
• GPS Synchronization
• Beamforming
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Frequency Hopping
• Hedge your bets
• Examples:
– Bluetooth
– Wireless Controllers (Xbox, Playstation,…)
• Disadvantages
– Lower data rates
– Typically limited range*
2400
2483
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GPS Synchronization
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Beamforming
• Achieved by combining elements in a
phased array in such a way that signals at
particular angles experience constructive
interference while others experience
destructive interference. (Wikipedia)
• Possible for both Transmit and Receive
Delay
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Beamforming Concept
INTERFERENCE
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Sources
• By JGlover - Attribute to J. Glover, Atlanta, Georgia in any use (including
derivative) outside of WikiMedia Foundation projects. - Own work, CC BY-SA
3.0, https://commons.wikimedia.org/w/index.php?curid=2896587
• Interference (communication). (2016, December 29). In Wikipedia, The Free
Encyclopedia. Retrieved 20:05, April 12, 2017, from
https://en.wikipedia.org/w/index.php?title=Interference_(communication)&oldid
=757275439
• Spectrum Dashboard. Retrieved April 12, 2017, from
http://reboot.fcc.gov/spectrumdashboard/searchSpectrum.seam
• Beamforming. (2017, April 5). In Wikipedia, The Free Encyclopedia. Retrieved
17:42, April 13, 2017, from
https://en.wikipedia.org/w/index.php?title=Beamforming&oldid=773901460
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