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Spectrum Monitoring and Interference Analysis
James West
Senior Software Engineer Summitek Instruments
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Summitek Instruments• Formed in July 1996• Became a part of Smiths Group November 2001• Products
– Passive IM Measurement Solutions• Bench top
– Engineering design– Manufacturing QA
• Portable– Site certification– Ongoing maintenance
– OASIS Spectrum Monitoring– Spartan Automated Test & Data Management
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Presentation Overview
– The Interference Problem
– Sources of Interference
– Spectrum Monitoring Process
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
The Interference Problem• Increasing numbers of transmitters create an
environment where interference is more prevalent.• Compliant signals in the licensed and unlicensed
spectrum are potential components of an interfering signal.
• Spectrum analyzers can only show the spectrum; the user must determine the source of the interference.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Solving the Interference Problem• Characterize the local environment;
Find neighboring transmitters.• Locate the source of the interference and
identify the problem.• Perform an Intermodulation (IM)
analysis based on transmitters in the area.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
CDMA
Analog Calls
Attenuator/Noise Floor Change
Typical 1.25 MHz wide CDMA
Carrier
Control Channels
Instantaneous Noise
Call Start
Call End
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Analog FM
Approximately 20 kHz wide FM
paging transmitters.
A powerful FM transmitter that has what
appears to be a wider than normal bandwidth
due to its signal strength.
Note the deviation of the FM transmitters. Approximately 6-10 kHz of deviation of
the center frequency.
FM paging transmitter that is not always on-air.
Two paging transmitters
occupying 40 kHz of bandwidth.
Five Paging transmitters occupying
approximately 500 kHz of bandwidth.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Indications of Interference
Switch Data• Idle Channel
Squelch Opening• Noise floor data• Dropped calls• Access/Setup
failures• Reverse Power
Control
Customer Complaints• Noise
• Dropped Calls
• Poor Voice Quality
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Non-IM Interference – Fundamental Transmitters
A fundamental transmitter is a signal that is not mixed with another signal.
Some examples:– Off frequency transmitters– Improperly tuned transmitters– Defective transmitters– Abandoned transmitters that drift – Illegal transmitters
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Co-Channel Interference occurs when a distant base station signal is too strong.
• Creates hand-off problems• Minimize by proper system
deployment and design.• Detect by using an instrument
that can decode the base station ID.
Non-IM Interference– Co-Channel Interference
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Non-IM Interference – Equipment Issues
• Spectral re-growth (excessive sideband lobes)• Broadband noise• Power fluctuations • Sub-carrier signals
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Non-IM Interference –Adjacent Channel
Adjacent Channel Interference is caused by “bleed through” from transmitters operating at adjacent frequencies (channels).
F1 F2
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Intermodulation Interference
F1 F2
XX
F1+F2-X F1+F2+X
2X
IM2 : F1 + F2
Power
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Intermodulation Interference• Not a clearly defined bandwidth, may have
“fuzzy” RF envelope• Bandwidth may shift as the fundamental
transmitters deviate• May be intermittent as one or more of the
contributing transmitters turns on and off• May cover up entire protected band if IM product
is of higher order or protected band is narrow
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Passive IntermodulationAny passive component may serve as the mechanism
of IM interference.
• Semi-Conducting Junctions
• Poor alignment of parts
• Inadequately torqued screws and fasteners
• Bad solder joints
• Plating related problems
• Any Ferrite Material
• Degradation Due to Environment:
• Contaminated conductorsand interfaces (Dirt, Dust, Moisture)
• Wind induced vibrations
• Temperature cycles
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Evaluation
• Monitor the Protected Band and determine if the interference is present.
• Examine 3-10 times the bandwidth of the protected band.
• Identify Adjacent Channel Interference• Examine the site for changes.• Inspect the antenna system.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Evaluation
• Characterize the Interference: – Duration– Bandwidth– Strength– Sub-carriers– Time of occurrence
• Characterization of the signal will help begin the process of eliminating interference suspects in the area.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Monitor
• Make a broadband peak hold measurement of the local RF spectrum.
• Use a broadband antenna.• Use a fine resolution bandwidth (RBW). 30kHz
or smaller is recommended.• Conduct the survey when the interference occurs!
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Identification
• Examine each peak detected by the monitoring.– Compare each peak to the characteristics of the
interfering signal.– Verify that each peak is compliant with the
license and/or applicable standard.
High priority in the process will be given to:– Peaks that are similar to the interference.– Peaks that are unknown or non-compliant.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – IM Suspect Calculation
• Calculate the IM frequency ranges. • Calculation of low order IM products is
recommended. (up to IM3).
Note that high power broadcast transmitters may cause IM products of the 13th order or higher to be present.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Categorization
Demodulation• It is more likely that you can demodulate a
fundamental transmitter.• IM interference mixing may prevent
demodulation.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Categorization
Bandwidth• The bandwidth of a fundamental transmitter
will match the bandwidth of the interference• IM interference mixing will change the
bandwidth to reflect the results of the calculation.
• IM interference cannot be created by component with a bandwidth larger than the bandwidth of the interference.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Categorization
Duration• Interference that is continuous implies that
all components are continuous. • Interference that is intermittent implies that
at least one component is intermittent.Strength• The power level of an IM interference
cannot be higher than any component signal.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Categorization
Start
End
Intermittent Interference
• Use Spectrogram to monitor sweep history.
• On/Off correlation may provide easy way to identify interference source.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Direction Finding
• Yagi Type Antenna• Take peak strength and
direction readings from various locations.
• Isolate the location of the interference.
Location could be internal or external to the system with the problem.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Troubleshooting Process – Signal Mapping
Log sweep data using spectrum analyzer, GPS receiver.
Overlays logged signal strength on the map.
Export the data to your mapping software package.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Illustrative Examples – Case 1 – Bad Transmitter
BA
AA BB
• ‘AA’ was experiencing poor signal quality.
• Examination of protected band found Adjacent Channel Interference.
• Direction Finding found ‘B’.
• FCC license data indicated that ‘B’ was using wrong frequency.
• ‘B’ was repaired.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Illustrative Examples – Case 2 – Metal Shed
• ‘A’ had intermittent interference.
• IM calculation showed ‘A’ and‘B’ could create an IM signal.
• Broadband scan found the signal.
• Direction Finding located IM signal emanating from shed.
• ‘A’ moved antenna.
AB
IM
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Illustrative Examples – Case 3 – LNA Overload• A Base Station had
dropped calls.• Broadband scan and IM
calculation showed IM suspects were present.
• DF found only IM signal from Base Station.
• Interference was internally generated.
• A pad was used to isolate.• LNA was overloaded.
AB
IMBTS
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Illustrative Examples – Case 4Frequency Drifting Transmitter
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Illustrative Examples – Case 5Broadband Noise – Bad Equipment
Offending Transmitter Idle Offending Transmitter Active
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
200 kHz wide, off frequency ISM transmitter – shifted
down 100 MHz in frequency.
TDMA Calls covered up by
the interference.
Interference has a discrete RF envelope.
Illustrative Examples – Case 6Off Frequency ISM
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
3 Order IM Product
Illustrative Examples – Case 7IM3 Signal Capture
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Summary• Interference can be located using a process.• Searching for interference requires an
understanding of wireless standards and the operating environment.
• Characteristics of the interference signal offer the best way to isolate the signal.
• Use Direction Finding to locate the signal.• Elimination of interference requires knowledge of
wireless systems and how IM is created.
NSMA ConferenceMay, 2007
Spectrum Monitoring and Interference Analysis
Thank you for coming to my presentation!
Jim West
Contact Information– Email - jwest@summitekinstruments.com– Phone - 303-768-8080– www.summitekinstruments.com