Slide 1 Feb 2009 Company Confidential Finding Interferers Using Handheld Spectrum Analyzers Steve Thomas Senior Product Manager
Slide 1 Feb 2009 Company Confidential
Finding Interferers Using Handheld Spectrum Analyzers
Steve Thomas
Senior Product Manager
Slide 2 January 2011
Who Needs to Find Interference?
Anyone who transmits signals over the air
Cell phone companies
First responders (police, fire, etc.)
Wi-Fi providers
Commercial Land mobile (taxi, delivery companies, truckers)
Broadcast stations doing remotes or using microwave links
Users of wireless microphones, especially when multiple microphones are in use simultaneously.
Slide 4 January 2011
Why Hunt Interference?
Capacity
Removing interference can increase capacity i.e. do more with less
Data Rate
Removing interference can increase data rates keeping the customers happy
Return On Investment
Removing interference can make your return on investment happen earlier
Poor Signal Quality on analog systems
Slide 5 January 2011
Crowded Frequency Bands
Over 3,703 licensed radio transmitters in Santa Clara County alone
According to the FCC Licensing Data Base
Does not include Cell phones, Wi-Fi, Government users, FRS radios, other
unlicensed transmitters such as baby monitors & microwave ovens
Individual police, fire, mobile and handheld radios
It’s growing daily
Slide 6 January 2011
Interference will happen
New carriers
Changes in band plans
Equipment failures
Self-interference
Deliberate Interference
Slide 7 January 2011
Carrier to Interference Ratio (C/I)
For voice systems
Less receive sensitivity Dead areas
» Land Mobile
Dropped calls
For data systems
Low data rate Adaptive modulations
respond to C/I » HSDPA » EVDO » WiMAX » WCDMA for data » CDMA for data
Low capacity
[1]
[1] HSDPA for Improved Data Transfer, Qualcomm, October 2004
Cellular data-centric transmission methods set data rate by signal quality
Slide 8 Feb 2009 Company Confidential
What is Interference?
To fix interference, we must first understand interference.
Slide 9 January 2011
It’s a Receiver Issue
Signals passed by the receive filter (pre-selector) affect the receiver’s front end causing
A reduction in sensitivity
Apparent lower C/I
It’s called Receiver De-Sense, or De-Sensitization
Interfering signals do not need to be on your receive channel!
They only need to make it through the Rx filter
Sometimes huge signals that are at wildly different frequencies can cause fundamental overload
Slide 10 January 2011
Co-Channel Interference
Your own system creates overlap
Land Mobile or Cellular Antenna tilt Valleys Higher than expected antennas Better propagation over water Errors in frequency settings
» How many transmitters in a typical metro area GSM system?
Multi-path for CDMA type systems
Other radio services on your frequency
With larger than expected coverage
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Slide 11 January 2011
In-Band Interference
Carriers from other services
Distortion products from other signals, or combinations of signals
Impulse Noise
Harmonics
Intermodulation
Near-far problem
Intentional Interference
Signals do not need to be on your channel
Only need to make it through the pre-selector/pre-filter
Slide 12 January 2011
Impulse Noise
Impulse Noise comes from
BTS Lighting Suppression
Electrical Motors Elevators (and elevator controllers) Floor Buffers FAX machines
Bakery ovens
Welders
Tower lighting
Electric fences
Power Lines Arcing and sparking
Light Dimmers
Slide 13 January 2011
What is Impulse Noise?
A sudden step in power
Arcing, sparking, signal switching
Looks like a rise in the spectrum noise floor over a wide frequency range
If you are in a narrow span, it will look like the entire noise floor takes a vertical jump of 10 - 20 dB
Use Max-Hold and a wide span to see the full envelope
Lighting Arrestor Spark Gaps are a common culprit
Breakdown voltage lowers over time
Eventually, peak power will cause arcing This generates harmonics and broadband junk
Slide 14 January 2011
Harmonic Interference
Harmonics of powerful broadcast transmitters may interfere with other radio services
What’s 60 dB down from 1,000,000 Watts ERP? 1 Watt
What’s a receiver looking for? ~ 0.000,001 Watt
Broken transmitters can produce strong harmonics
If a transmitter loses one output transistor the output can become distorted.
Slide 15 January 2011
Intermodulation Distortion
Intermodulation Distortion requires
Two or more strong signals At least +7 dBm or higher
A non-linear device Transistor Diode Corrosion
» especially with dissimilar metals
IM caused by corrosion is called
The “Rusty Bolt” effect Environmental Diodes are created
Slide 16 January 2011
Transmitter Back-feed Intermodulation
IM can be generated by:
Transmitter to transmitter interference Back-feed from
closely located transmitters » Transmitter 1’s signal in
Transmitter 2’s output stage » The output transistors
provide the non-linear device » Antennas work in
both directions!
TX2 TX1
Slide 17 January 2011
Multiple Signals in One Amplifier
IM can be generated by:
A Multi Carrier Power Amplifier (MCPA) MCPAs must be very linear IM and Spectral Regrowth can both be caused by a less-than-
ideal amplifier Two or more carriers, 1 amp 1 antenna.
Slide 18 January 2011
Multiple Signals in One Antenna: Passive Intermodulation (PIM)
PIM can be generated by:
A environmental diode in the antenna system when Two transmitters share one antenna
» GSM and WCDMA, for example
- Or there is- One wideband carrier such as WCDMA
Corroded connectors or antennas are likely causes
Problems may be created by micro-arcing if power levels are high enough. Adding additional signals in a system that is working OK may
reach a threshold where micro-arcing occurs » levels >1 kW.
PIM often is created outside the antenna system by environmental diodes
Slide 19 January 2011
External Corrosion and PIM
Environmental diodes
Rust on tower
Rusty fence around cell site
Corroded metal roof Can come and go with the wind!
Dissimilar metals
Slide 20 January 2011
Intentional Interference
Some interference is intentional
Sad, but true Employers keeping
employees off the phone Drivers wanting other drivers
to stay off their phones Some military applications Some churches & theaters have installed
jammers Mobile GPS jammers
» Plug into cigarette lighter » Designed to disable GPS trackers
The Pocket Jammer
TheSignalJammer.com
Slide 22 January 2011
Signs of Interference
First indicators
Noisy analog links
Low throughput in specific sectors
High dropped call rate
Second indicator
Receive Noise Floor is high on a specific sector
This is enough to warrant an interference hunt
?
Slide 23 January 2011
Spotting Interference in the Field
Check for interference at the base station
Measure noise floor from a receive antenna Same receive pattern as the base station Receive Channel Power measurement should match the switch
Noise Floor number
Get a visual ID on the interfering signal Characterize signal so you will know it later
» Bandwidth » General appearance » Modulation type » Audio, if you can demodulate it
Slide 24 January 2011
Characterizing Interference
Which signals belong?
Which signals do not belong?
Use the receive pre-filter to eliminate signals that the receiver cannot see.
If not possible, ignore signals that you know the filter will eliminate
Slide 25 January 2011
Characterizing Interference
Spot the strong signals on the up-link
Dual trace spectrum for the consistent signals Max-Hold and Normal
Spectrogram for the intermittent ones
Auto-Mask for “save on event” waveform capture
Strong signals that are not from a system’s transmitters may be the problem
Slide 26 January 2011
Identifying Signals
Quickest way to locate is to identify the type of signal
AM/FM demod for traditional signals Can give station ID Pagers and land mobile systems have a Morse Code ID
Next step FCC Licensing Data Base
» Gives GPS coordinates of tower and contact phone number
Slide 27 January 2011
Signal Monitoring for Odd Signals
Use the spectrogram
To look for signals that change over time
For long term monitoring Days of monitoring time
» Useful for later analysis on computer
User can set time resolution
To further characterize the signal You will need to
recognize the signal during the hunt
This signal is unstable in frequency
From a cellular repeater with insufficient input to output isolation
Slide 28 January 2011
Extended Signal Monitoring Envelope Creation
To look for intermittent signals near or on a legitimate signal
Save-on-Event can save signals when either mask is violated Allows for quick analysis
of captured data
Upper and lower masks created automatically
» Easy to do » Can be sloped or square » User sets number of points (up to 41)
Capture unusual events within the channel or band
Slide 29 January 2011
Save-on-Event Results on the PC
Create a spectrogram of auto-saved results
Only view mask violations Eliminate unnecessary data Don’t worry about the
good signal!
Colors can be customized to highlight desired data Can create a pass/fail
spectrogram
3D Spectrogram View from any angle to better
understand the signal
Min/Max/Peak markers
Zoom capability
Slide 31 January 2011
Spotting the Signal at Ground Level
Signals that are strong at tower level can be weak at ground level
Hills, buildings and ground clutter may between you and the source
Ways to find that elusive ground level signal:
For cellular systems check other BTS sectors for interfering signal strength This gives you a general idea of direction
Drive around and look for the signal at ground level An Omni-directional rooftop antenna is useful for this task A Max-Hold and normal dual trace setup will also help
Find a building top or open area for direction finding
Slide 32 January 2011
Direction Finding in Urban Areas
You may receive only multi-path signals
Use “urban canyons” to help you
At an intersection measure signal strength down all the roads
Go in the direction of the strongest signal.
Eventually you will reach a place where the direction of arrival changes dramatically
That means you are close
Use traditional DFing to find the source
Slide 33 January 2011
Locating the Source
Directional Antenna Techniques Find direction to signal
Record direction on a map or in the instrument
Take multiple measurements Multiple measurements help resolve multi-path issues
Move closer to the source and repeat the process
Shield antenna to get help nail down direction to emitter Step behind a structure between you and the suspected arrival
direction If amplitude goes down, that means your suspicions are right
Spectrum Analyzer Tools Dual trace mode
» Normal & max-hold traces Use Signal Strength meter
» Tone and visual display Interference Mapping
Slide 35 January 2011
Locating the Source
Signal Strength Meter
Use when taking directional measurements Tone makes swinging the directional antenna quick Also allows users to sight along the antenna while swinging
When very close and multi-path is an issue Use an Omni-directional antenna Hunt for the strongest signal This is similar to the child’s game of “Hot and Cold”
» It works fast if close
You can record measurements with GPS coordinates » For display on a map later » For evidence
Slide 36 January 2011
Locating the Source
Once you are close
Look around for potential sources Older shared antennas Other radio transmitters Rust near transmitters Homes or business Cell phone jammers
Use your directional antenna or Omni to locate the emitter
Slide 37 January 2011
Summary
Why Hunt Interference?
Capacity, Data Rate, ROI
Eliminate dead areas
How does Interference Happen?
Receiver De-Sense
Impulse Noise
Harmonics
Intermodulation
Near-Far problem
Intentional interferer
Rx Band
Freq.
Pow
er
Rx C
han
nel
Inte
rfere
nce
Slide 38 January 2011
Questions
Please submit questions using the Questions function on your gotowebinar control panel.
You can also send questions to [email protected] to be answered after the Webinar
A PDF version of these slides will be made available through RCR wireless.
The archived Webinar will be available a few days after the Webinar.