Spectrum Monitoring and Interference Analysis James West Senior Software Engineer Summitek Instruments

Spectrum Monitoring and Interference Analysis

James West

Senior Software Engineer Summitek Instruments

NSMA ConferenceMay, 2007


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



Presentation Overview

– The Interference Problem

– Sources of Interference

– Spectrum Monitoring Process



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.



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.



CDMA

Analog Calls

Attenuator/Noise Floor Change

Typical 1.25 MHz wide CDMA

Carrier

Control Channels

Instantaneous Noise

Call Start

Call End



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.



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



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



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



Non-IM Interference – Equipment Issues

• Spectral re-growth (excessive sideband lobes)• Broadband noise• Power fluctuations • Sub-carrier signals



Non-IM Interference –Adjacent Channel

Adjacent Channel Interference is caused by “bleed through” from transmitters operating at adjacent frequencies (channels).

F1 F2



Intermodulation Interference

F1 F2

XX

F1+F2-X F1+F2+X

2X

IM2 : F1 + F2

Power



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



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



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.



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.



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!



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.



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.



Troubleshooting Process – Categorization

Demodulation• It is more likely that you can demodulate a

fundamental transmitter.• IM interference mixing may prevent

demodulation.




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.




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.




Start

End

Intermittent Interference

• Use Spectrogram to monitor sweep history.

• On/Off correlation may provide easy way to identify interference source.



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.



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.



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.



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



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



Illustrative Examples – Case 4Frequency Drifting Transmitter



Illustrative Examples – Case 5Broadband Noise – Bad Equipment

Offending Transmitter Idle Offending Transmitter Active



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



3 Order IM Product

Illustrative Examples – Case 7IM3 Signal Capture



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.



Thank you for coming to my presentation!

Jim West

Contact Information– Email - [email protected]– Phone - 303-768-8080– www.summitekinstruments.com

Documents

Spectrum Monitoring and Interference Analysis James West Senior Software Engineer Summitek Instruments