Identifying and Eliminating Interference in Mobile Networks Webinar v 5.1

Identifying and eliminating interference in mobile networks

Peter Busch Sales Manager, Rohde & Schwarz Mobile Network Testing Version 5.1

Agenda ı

Sources of interference

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Impact of interference on network performance

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Identifying, locating and eliminating external interference

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Overview of test tools and methodologies

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Additional considerations Q&A / Discussion

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Identifying andeliminating interference

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Sources of Interference

Sources of interference ı

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The types and sources of interference are very frequencydependent. Lower frequencies tend to propagate and penetrate better than higher frequencies. For many cellular networks, common interferers tend to be: Harmonics / intermod products Spurious emissions from electronic devices Wrong-region devices 

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Intentional or malicious interference Note however there are many geography-specific interference types, e.g. from cable or broadcast television.

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Harmonics / intermod products ı

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In many cases, an interfering signal is actually a harmonic of a lower-frequency signal (e.g. a signal at 400 MHz affecting 800, 1200, etc. MHz) This is particularly true of narrowband interferers : it is always a good idea to “do the math” and check if a narrowband interferer is a harmonic. Inte rmodulation products are created when two or more signals mix in a non-linear device or junction. External intermod (the “rusty bolt effect”) is not uncommon.

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Spurious emissions from electronic devices ı

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Almost all electronic devices emit radio frequency energy at various frequencies. Interference is caused when high level spurious emissions (or their harmonics) fall into cellular bands. Spurs can be narrowband or wideband, possibly frequency-unstable. Common sources of spurs: 

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Amplifiers of any kind Lighting and displays (screens) Consumer electronics Industrial equipment

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Wrong-region devices ı ı

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Spectrum allocations vary from country to country Interference can be caused by devices using frequencies allocated in a different country / region For example, cruise and cargo ships may have systems that disrupt cellular services upon entering different regions.

Common wrong-region devices: Wireless phone systems DECT Baby monitors Microphones 

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Deliberate interference / jammers ı

Deliberate interference may be narrow-band (e.g. talking on a public safety frequency) or broad-band (jamming).

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Pirate or unlicensed (“free band”) operations can also cause issues to licensed users.

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Sources may be mobile.

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Although most businesses and individuals are very cooperative in resolving interference, deliberate interferers will usually deny or conceal their activities.

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Physical characteristics of jammers ı

Jammers can come in many shapes and sizes. Sometimes they are disguised or hidden.

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Typically has one antenna per jamming band/technology.

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Battery-powered jammers are small but have a limited jamming radius (10-25 meters).

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More powerful jammers usually require AC power and tend to run hot (look for devices with fans and/or large heat sinks).

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Spectral characteristics of jammers ı

Jammers are typically easy to identify and locate: strong, broad, always-on signal.

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Some increase the noise floor well outside of their nominal operating range.

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Jammers may generate very strong harmonics outside of their nominal operating range.

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Common commercial cellular jammer targets are cellular, GPS, WiFi, and car remotes.

TAKEAWAY: Interference in mobile networks is pervasive and increasing due to more technologies, the sensitivity of LTE, denser spectrum and proliferation of electronic devices 10.05.2017

Identifying and eliminating interference

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Impact of interference on network performance

Impact of interference on network performance ı

Network performance can be categorized on two levels: RF level : SNR, Ec/Io, RSSI, RSRP, etc. Subscriber level : Throughput, call drops, voice/video quality, etc. Interference is an RF level issue, but affects the subscriber quality of experience. Important therefore to test at both the RF level and the subscriber level. Generally speaking, you need a combination of tools to test at both levels. 

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Impact of interference on network capacity It is not easy to quantify the impact of interference towards the capacity of a network, because it’s depending on so many parameter. Here just a general graph with MCS areas:

Example MCS vs. SINR 800

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% / t u500 p h g u ro400 h t e v ti 300 a l e R

Translates in to: For every 10 dB decr ease in SIN R, you loos e 50% of your capacity 64QAM

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256QAM (MCS 20...27)

The graph is a simplified real-world example for EPA5, 2x2MIMO and HARQ. The MCS-SINR relation depends on the specific Base Station vendors' algorithms, performance and scheduler implementation, as well as on the channel fading profile etc.

(MCS 11...19) 16QAM (MCS 5...10)

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QPSK (MCS 0...4)

0 -15

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MCS: Modulation and Coding scheme SINR: Signal to Interference and Noise Ratio

SINR / dB

TAKEAWAY: Interference dramatically impacts network performance) - so interference hunting becomes essential 10.05.2017


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Identifying, locating and eliminating external interference

Steps in interference hunting ı

Detecting interference For network operators, usually things like high RSSI, call drops, poor throughput, etc. Determinin g genera l l ocation of in terfere r Attempt to narrow down location to ~100 meters Often involves driving/walking the entire affected area. Determinin g specific l ocation of int erfe rer Done on foot Handheld antennas Need to locate specific device generating RF 

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Detecting Interference ı

For cellular network operators, interference issues are almost always found in the uplink

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First indications of interference normally come from the base station statistics themselves

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Usually one sector is affected more than the others. Multiple sectors from multiple base stations may be affected by a single source of interference.

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Identifying Interference ı

Analyzing an interfering signal can provide important clues about its source. The most common analysis methods include : Spectral analysis Waterfall analysis Pattern analysis Content analysis / Demodulation Many common interference sources can be easily 

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identified by their spectral characteristics. Important to know what your spectrum and common interferers normally “look like”.


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Identifying Interference ı

Analyzing an interfering signal can provide important clues about its source. The most common analysis methods include: Spectral analysis Waterfall analysis Persistence Histogram Pattern analysis Content analysis 

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Many common interference sources can be easily identified by their spectral characteristics.

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Important to know what your spectrum should “normally look like”

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Spectral analysis ı

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Many interferers have a distinctive spectral “shape” that helps to identify them. For example: leaking cable television signals can be identified by the regularlyspaced, identical-width channels. Locating the interferer is greatly simplified, if you know (roughly) what you are looking for. Many providers build their own “library” of well-known interferers and their spectral characteristics.

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Waterfall analysis ı

A waterfall (or spectrogram) display shows frequency, time, and level information (as color)

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Extremely useful in analyzing signal behavior over time or for locating short duration signals.

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Can reveal the presence of interferers underneath desired signals.

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Waterfall intensity can also be helpful in locating an oscillating / drifting signal.

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Recommendation : always use spectrum and waterfall together during interference hunting.

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Persistence Histogram ı

What exactly is polychrome spectrum display? Persistence Histogram (statistical function that uses several measured levels as basis) Processes level-frequency pairs over time Color indicates relative occupancy over time (How “often” that signal is measured) or signal duration 

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Maxhold

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What can polychrome spectrum display be used for? Resolve pulsed signals that share same frequency 

Wanted Signal

GSM Signal (less occurrence) GSM Signal ClearWrite (more occurrence) Wanted Signal

WiFi

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Pulsed Interference

bands and are superimposed in spectral display Detection of short duration wideband interference signals (not possible via MAXHOLD due to averaging)

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Bluetooth

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Pattern analysis ı

Important questions to ask in analyzing interference patterns: When does the interference occur? Is the interference constant or intermittent? Does the interference coincide with any other (physical or spectral) events? Does the interferer affect multiple locations in a given sequence? In many cases, base station statistics can provide useful 

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information. Trying to find interference when it is not “on” is not productive.


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Content analysis ı

Simplest type of content analysis is audio demodulation.

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Always try to demod narrowband signals

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What are we listening for? Station identification or call signs, even “unofficial” ones. Language and content Combined FM audio usually indicates intermodulation products.   

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Recording audio signals for later analysis / documentation is useful.

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In some cases digitally modulated signals can also be identified through analog demod, but tools also exist for demodulating digitally modulated signals. TAKEAWAY: Analyzing an interfering signal can provide important clues about its source. Use spectral analysis, waterfall analysis, pattern analysis and content analysis 10.05.2017


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Identifying the general location ı

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The first indications of interference normally come from the base station statistics themselves. Usually one sector is affected more than the others. Multiple sectors from multiple base stations may be affected by a single source of interference. Note that base station statistics can only provide location with a sector-level resolution at best – where is the interferer within that sector?

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Driving around ı

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In most cases, the interferer CANNOT be seen on the ground at the base station. One typically has to drive (or walk) the sector(s) / affected area to try to narrow down the location of interfering signal. In the driving phase we use things such as: RF “heat maps” Manual and automatic bearings Network performance statistics. Typically we want to get within about 100 meters of the interferer. 

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Understanding Propagation ı

A solid practical knowledge of propagation is critical in interference hunting. Knowing how signals travel makes it easier to figure out where they could be coming from

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The lower the frequency, the better the propagation and penetration.

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Know the attenuation characteristics of common materials

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Multipath propagation can make interference hunting difficult in urban areas.

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Walking around ı

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The next step is to locate the exact source of the interference. Handheld antennas are used to sweep or scan. Level information in the form of spectrum amplitude, waterfall intensity, or tone output can be used to determine position. Ultimately, the only way to determine if a device is the source of interference is to power it off and examine base station statistics (subscriber-level vs. RF-level)

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Overview of test tools and methodologies

Table of content ı

Spectrum Analyser FSH/FPH 

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TDD LTE trigger (FSH/FPH)

Interference Receiver PR100

Mobile Locator DDF007 Scanner TSMA/E with Romes 10.05.2017

Identifyingandeliminatinginterference

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Spectrum Analyzer FSH/FPH

Directional Antenna R&S®HE400 ı

Automatic module and polarization recognition

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Low Noise Amplifier (powered by R&S® FSH/FPH or PR100)

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Easy and comfortable handling (trigger button, arm rest etc.)

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Automatic field strength display based on detected antenna module and stored k-factor tables in R&S® FSH/FPH or PR100

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Exact positioning system with GPS and GLONASS support

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Highly accurate bearing information in cellular bands due to delta mode function (R&S®HE400CEL)

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R&S®HE400 Handheld Directional Antenna New antenna for interference hunting ı ı

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Frequency range VSWR (except with HF module) Polarization Operatingtemperaturerange Max. dimensions Weight

8.3 kHz - 30 MHz

R&S®HE400HF

8.3 kHz to 8 GHz (covered by 5 antenna modules) < 3.0 (typical < 2.0) adjustableverticalorhorizontal -10 C to +55 C (Class C device) approx. 620 x 290 x 90 mm approx. 1 kg °

20 - 200 MHz

450 – 8000 MHz

R&S®HE400VHF 10.05.2017

°

700 – 2500MHz

R&S®HE400LP


30 – 6000 MHz

R&S®HE400CEL 32

R&S®HE400UWB

Interference Analyzer (FSH/FPH) ı

The Interference Analyzer mode provides the ability to visualize and measure spectrum

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Similar to spectrum analyzer mode

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Standard measurements include channel power, OBW and ACLR.

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Interference-specific measurements include Carrier to Noise and Carrier to Interference

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Spectrogram display, record, and playback are also included with the Interference Analyzer option

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Carrier-to-Noise ı

The Carrier-to-Noise (C/N) measurement is a tool to determine if a signal has sufficient power compared to the surrounding spectrum

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The FSH determines the distance between the level of the carrier and the lowest signal level that has been measured (usually the noise floor).

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Carrier to Interference ı

The Carrier-to-Interference (C/I) measurement is a tool to determine if a signal is affected by interference from neighboring channels.

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The FSH determines the distance between the level of the carrier and the second strongest level.

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Tone Function ı

The Tone function outputs an audible tone whose level varies according to the received signal strength level.

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The current receive level (dashed line) and squelch level (solid line) are displayed in the power bar.

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Tone can be used both for obtaining bearings as well as for sweeping an area to determine the precise location of an interferer.

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Spectrogram ı

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Spectrograms display amplitude vs.frequency over time. Amplitude is indicated by the color of the spectrogram display. Spectrogram reference, range, and other parameters can be specified Extremely useful in interference hunting– allows the observation of a signal’s behavior over time.

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Saving Positions and Azimuth ı

The azimuth (or bearing) is the direction in which the antenna is currently pointed

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This information can be using the menu or the trigger on the HL300 antenna

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The power bar and power result can help determine the direction of maximum receive power (i.e. bearings towards the transmitter) 10.05.2017


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R&S®HE400CEL Cellular Antenna Module Features and benefits ı ı ı

Frequency range 700 MHz to 2500 MHz Two broadband dipoles switched in-phase (normal mode) or out-of-phase (delta mode) Change of radiation pattern – boresight null in delta mode -> Very precise bearing determination

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Computing Triangulation ı

The Triangulate function computes the triangulation point for up to five selected bearings.

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The lat / long and error radius of the triangulation point are also displayed.

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The computed triangulation point can be saved to the SD card.

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R&S®Spectrum Rider FPH Long Time Recording ı

Long time recording

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R&S®Spectrum Rider FPH Long Time Recording

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R&S®Spectrum Rider FPH Signal Strength Mapping (FPH-K16) & Map Analysis (Instrument View) ı

Save on events

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TDD LTE Interference Hunting

TDD with interferer present (non triggered) ı

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This is an example of a TDD LTE signal in “normal” (free run) operating mode. Downlink frames dominate the spectrum display (no uplink traffic is present in this example). Without triggering, it is extremely difficult to see even a strong interferer within the TDD band. Having properly configured our Gate Settings, we now enable Gated Trigger 10.05.2017


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Determining time slot allocations ı

Visual inspection of the zero span screen is used to determine the location of one or more (depending on TDD configuration) uplink timeslots

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Gate Delay ı

Gate delay and gate length are shown as two vertical red lines in the zero span display – gate delay on the left and gate length on the right.

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Gate delay will vary each time you switch to the FSH internal clock.

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Make sure that the gate delay line is (slightly) inside the uplink timeslot Gate delay

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Gate Length ı

Gate length is the distance (time) from gate delay.

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Typically will be on the order of 1 ms per uplink timeslot.

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Make sure that the gate length line is also (slightly) inside the uplink timeslot Gate length

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TDD with interferer present (triggered) ı

With Gated Trigger enabled, the downlink frames are suppressed and we can clearly see a strong (~ -55 dBm) periodic interferer in both the spectrum and waterfall displays.

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All standard FSH functions can now be used with the Gated Trigger enabled. 10.05.2017


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Nice TDD LTE Video from a customer in the US

https://www.youtube.com/watch?v=wvL1cX4F9Uc

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Interference Receiver PR100

Preselection ı

Subdivides the input frequency range into subranges via switches and filters

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Spectrum analyzers : Do not have preselectors Measured signal is known and relatively stable Sum load of all signals is on the input of the first mixer 

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Monitoring receivers : 

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Preselection is a must Frequency range split in sub-ranges Reduce the signal sum load on the input of the first mixer Allows monitoring of widely different signals

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Speed vs. accuracy ı

Biggest difference is speed.

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Spectrum analyzers are (relatively) slow, but highly accurate over a wide frequency range.

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Monitoring receivers are less accurate, but are very fast (real-time) and gap-free. POI within the demodulation bandwidth is 100%.

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Short duration (low POI) signals 

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Digital data Frequency hoppers Radar pulses Noise sources Clandestine transmitters 10.05.2017

Reassembly of a frequency-agile signal


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Monitoring short duration signals e im t

e im t

frequency

frequency

Spectrum analyzer (swept / heterodyne principle)

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Monitoring receiver (FFT based)

Spectrum clearance and interference hunting

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R&S®PR100 family Polychrome Display Option: R&S®PR100-PC & R&S®DDF007-PC ı

What exactly is polychrome spectrum display? Persistence Histogram (statistical function that uses several measured levels as basis) Processes level-frequency pairs over time Color indicates relative occupancy over time (How “often” that signal is measured) or signal duration 

Maxhold

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Wanted Signal

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GSM Signal (less occurrence) ClearWrite GSM Signal (more occurrence)

What can polychrome spectrum display be used for? Resolve pulsed signals that share same frequency bands and are superimposed in spectral display Detection of short duration wideband interference signals (not possible via MAXHOLD due to averaging)

WiFi Pulsed Interference

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Bluetooth

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Wanted Signal

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Mobile Locator DDF007

What is Mobile Locator? ı

Mobil e Locator processes DDF007 bearings in real-time using a sophisticated mathematical algorithm.

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Probability clouds and estimated transmitter location are computed and displayed on attached control PC.

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It is referred to a mobile locator since the bearings are only processed while the system is in motion. 10.05.2017


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PR100 DF Option ı

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The PR100 DF option allows the PR100 to be used as a DF receiver when connected to a suitable antenna. Transition from vehicle to foot now simply a quick change of antenna cable. Bearing lines are automatically calculated and displayed on the PR100 interface.

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System Configuration ADDx0 7 ı

Three Hardware Components Control PC DDF007 ADDx07 

RF & Control cable

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DDF00 7

Control PC

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Ease of deployment ı ı

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Low profile, weather-proof Magnetic mounting plate allows mounting/unmounting in minutes Mount is stable to at least 80 mph, all weather conditions.

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Scanner TSMA/E with Romes

TSMA connected to PC

LAN

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R&S®ROMES4 (RF Power Scan) Visualization of spectrum for RF analysis

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R&S®ROMES4 (RF Power Scan)

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R&S®ROMES4 (automated identification via post processing)

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R&S®ROMES4 (automated identification via post processing)

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Summary

Summary ı

There are many different sources of interference in cellular networks; they are pervasive, increasing and particularly effect LTE.

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Interference can be measured at RF-level or the subscriber-level.

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Interference dramatically impacts network performance therefore interference hunting is becoming essential

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Identifying and locating interference can be partially automated using scanning receivers and direction finders

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Analyzing an interfering signal using spectral analysis, waterfall analysis, pattern analysis and content analysis can provide important clues about its source.

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New developments in mobile networks require the use of new solutions for resolving interference. 10.05.2017


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Q&A / Discussion

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Identifying and Eliminating Interference in Mobile Networks Webinar v 5.1

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