Interference Hunting Webinar

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.


Technology Fundamentals


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


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


Interference will happen

  New carriers

  Changes in band plans

  Equipment failures

  Self-interference

  Deliberate Interference


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


What is Interference?

To fix interference, we must first understand interference.


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


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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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


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


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


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.


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


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


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.


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


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


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


Where is Interference Found? Monitoring Interference


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

?


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


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


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


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


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


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


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


How to Find Interference Hunting Interference


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


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


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


In-Instrument Interference Mapping


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


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


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

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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.

Documents

Interference Hunting Webinar