Fundamentals of Interference in Wireless Network

8/7/2019 Fundamentals of Interference in Wireless Network

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Fundamentals of Interference

in Wireless Networks

1.0 Introduction

As wireless systems proliferate worldwide, the n um ber one enemy of wireless systems designers and serviceproviders is signal inter ference. Inter ference hampers coverage and capacity, and limits the effectivenessof both n ew and existing systems. It is an un avoidable fact that wireless comm un ications systems must coexistin extrem ely complicated signal environm ents. These environm ents are comp rised of multiple operating wire-less networks ranging from mobile commun ication services to specialized mobile rad io and paging/ broad castsystems. At the same time, wireless local area networks (WLANs) and digital video broadcasting are introducingnew techn ologies and signal sources that further th reaten to disrupt wireless comm un ications service.

Compounding the problem are regulatory and environmental restrictions which have effectively limited thenumber of suitable new base station tr ansceiver sites that can be p ut in place. Hence, m any wireless service

providers are n ow faced with co-location issues furthe r con tributing to the potential for signal inter ferenceas more antennae are p laced on individual cell towers.

This application note presents the subject of interference and its degrading effects on the performance ofwireless networks. It provides a br ief theor y of operation of commu nications receivers and antenn ae, as well asinstructions on how to locate an d identify an inter fering signal. It also reviews the operating pr inciples of theAnr itsu Spectru m Master MS2711B and some of its functional routines which make it an ideal inter ferencetroubleshooting tool.

With the information con tained in thisApplication Note and a complementarynote, Practical Tips on MeasuringInterference (referred to here as

Measurements Note, Anritsu Publicationnumber 11410-00303), you will under-stand the basic facts about what causesinter ference, how it sneaks into a typi-cal affected system receiver, and howyou can locate and identify typicalsources of those interfering signals. AGlossary of Terms useful for interfer-ence m easurements is included in theMeasurements Note.

APPLICATION NOTE

SPECTRUM MASTER MS2711B


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Application Note

2.0 What is Interference?

Its pretty easy to comp reh end th e th eory of signal inter ference and to see its effect on an installed system.As a mobile wireless customer, you m ay have experienced loss of service due to the effects of inter ference.You may not have thought an ything of it as you suffered th e incon venience of having to d ial back in to th ewireless ne twork.

As a service provider, your system mon itors might repor t a high rate of service impairmen ts in one of your

wireless network coverage areas. Based on locale, you may send an engineer or techn ician to th e suspect basestation only to find out that the equipmen t is fun ctioning pr operly. If something is still corrupting your service,suspecting signal inter ference as the culprit is your logical conclusion.

2.1 Basic RF Signal TheoryA properly-operating communication system depends on a radio frequency (RF) carrier signal whose frequencyhas been assigned by the U.S. Federal Commun ications Commission ( FCC). The FCC receives its auth orityfrom an adjunct organization of the United Nations, the Intern ational Telecommun ications Union (ITU) .

By modulating voice and data signals on to th e RF carrier signal, power ful communications links serve p eople,businesses and governments around th e world. Most mod ern systems have been migrating to d igital modula-tion formats which are very efficient in the use of the frequen cy spectrum and highly compatible with the digi-tal microcircuit revolution. Th ese systems conveniently han dle both voice and data.

At the R&D phase, every system design aims at transmitting and receiving data or voice with some specified bit-error-rate at optimum signal conditions, usually for a specified car rier-to-inter ference (C/ I) r atio. But th edesign must also allow for marginal signal conditions and allow for signal fading, reflections, atmosphericeffects, rain attenu ation, multi-path fades, and in ter fering signals.

2.2 Licensed vs. UnlicensedIn most worldwide applications of the electrom agnetic spectru m, channel frequencies are assigned under alegal license. This licensing process stringen tly controls the installation practices and the system per formancespecifications. Transmit powers have maximum limits, and the pointing of the system antennas is carefully regu-lated. These proced ures ensure th at most of the worlds licensed systems have a reasonable chan ce of oper atingindepen den tly and secure ly, without having to worry about oth er legal signals inter fering.

In recent years, unlicensed communications system designs have been allocated to certain narrow frequencyspectrum bands previously reserved for a variety of industrial, scientific and medical (ISM) applications. ISMbands are authorized under FCC Rules, and can be used for unlicensed communications applications. FCCPart 15.247 covers the 2.4 2.5 GHz band . For the ban ds at 5.7 5.8 GHz, operating un der FCC Part 15.401,higher powers are permitted using directional antennae which increase the chances of unlicensed communica-tion systems interfering with other systems.

Unlicensed systems do n ot require FCC approval for th eir pu rchase and installation. Instead, th e system equip-ment manufacturer must certify that the operating equipment meets Type approval, which insures that prop-er use is made of the frequency spectrum and the emitted power envelope.


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Application Note

3.0 What are the Causes and Sources of Interference?

Inter ference results from a variety of sources, usually other tr ansmitters in the area, bo th licensedand u nlicensed.

Whether licensed or unlicensed, sources of interference cause the same resultsimpaired system perform-ance. The only difference is that there are more p otential uncontrolled sources of inter feren ce in the un li-

censed bands. Figure 1 diagrams a variety of possible in ter ferers. The Affected System, a m icrowave linkfor an Ethernet data bridge in th e 2.4 2.5 GHz ISM band, is shown in the center.

When signals from other systems (shown as Figures 1 [ a], [ b], [ c], [ d] , [e ] and [f]) reduce the affectedsystems carrier/ interference ratio (C/ I) below its specification margins, the radios data processing fails.[a] is a high-power broadcast signal. [b] is a leaking microwave oven in the ISM band. [c] is a similar sys-tem whose transmitter signal overflies its own receiver. [d ] , [e] , and [f] have signals that en ter by reflec-tion, sidelobes or backlobes. Each of these inter ferer modes will be explained individually in Section 3.2.

UHF TVBroadcastStation 50,000W[a]

EntersBack lobes [f]

EntersSidelobes[e]

MicrowaveOven inCoffee Room [b]

InterfererSystem

Overfly[c]

Building

InterfererReflection[d]

Affected System

Figure 1. Virtually any real- li fe system (Affected System i n this picture) is subject to a variety of continuous or i ntermittent interfering signals.


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Application Note

3.1 Receiver Designs Contain VulnerabilitiesThe ideal receiver design and the modulation schem es it employs minimize its susceptibility to sources ofinter ference. Practically, however, all real-life receivers are susceptible to a cer tain d egree to th e in ter feringmechanisms described in Section 3.2 Sources of Interference below. Figure 2 helps explain how receivers fallaffected to inter fering signals.

S u p e r - h e t e r o d yn e R e c e iv e r s Virtually all communications receivers use the super-heterodyneprinciple shown in the block diagram in Figure 2. It is relatively inexpensive, highly sensitive to weak signals,

and uses well-un der stood circuit theor y from th e 1930s.

Figure 2( a) shows how system signalsfrom the receiving antenna are firstfiltered with an RF pre-selector ( bandpassfilter), then mixed with a local oscillator(LO) frequency to yield an intermediatefrequency (IF). The frequency chart ofFigure 2(b) shows that the LO and mixerfunction accepts two d ifferen t recep tionchann els spaced on either side of theLO frequency by the amoun t of the IFfrequ en cy. On e is the system design

channel (usually the one below the LOfrequency) and the oth er is called th eimage. The pre -selector filter rejectssignals in the unwanted image response.

M odula t ion Most modern data systems use some form of digital phase modulation for transporting thedata on the RF or m icrowave carr ier. The simplest, binary-ph ase-shift-keying (BPSK), switches the ph ase of thetransmitted carrier between 0 degree phase and 180 degrees to signify a bit zero or bit one. Other typical

digital formats include QPSK (quadra-, or 4-phase, -shift-keying), 16 QAM (quadrature-amplitude-modulation,with 16 ph ase states) , or even 256 QAM. Many other popu lar modulation formats may be em ployed, such asQPR (quadrature-partial response), OFDM (orthogonal frequency division multiplex), etc.

S p r e a d S p e c t r um For the ISM bands, manufacturers have relied on spread-spectrum modulation tech-niques to make the ir systems less susceptible to inter ference from other in-band ISM signals. The re are twobasic techniques for spreading a transmit spectrum, frequency hopped spread spectrum (FHSS) and direct-sequence-spread-spectrum (DSSS). Both achieve considerable insensitivity to in-channel noise and singlesource inter ferers, e.g., microwave ovens in the 2.4 GHz ISM band.

The wireless, cellular, and PCS communications sector has also introduced its own version of DSSS which iscalled code-division-multiple-access (CDMA). This alternate system modulation strategy offers the advantageof supporting multiple chann els in the same ban d allocations.

Ante nna s In general, antennas fall into two categories: 1) omnidirectional, used in an office wireless-LANwhere even coverage is needed, an d 2) directional, used in poin t-to-poin t app lications where you want alltran smitted power aimed at a single receiver station.

Antenna

Local

Oscillator

Pre-selector

RF Filter MixerIF

Amplifiers

Data

DemodProcessing

Data

(a)

(b)

FrequencyData Passband Image

RF Filter

Passband L.O.Frequency

IFFrequency

IFFrequency

Figure 2(a). A simpli fied block diagram of a super-heterodyne data receiver.Figure 2(b). A pre-selector bandpass filter rejects signals in the image passband and other bands.


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Application Note

The most basic antenna, called anisotropic emitter, is shown in Figure3(a). It transmits equal power in everydirection of a spherical space around it,much like a light bu lb at a distance.The term om ni-directional is similar toisotrop ic. It emits in all directions equal-ly, but usually is taken to mean in

azimuth (horizontal, 360 directions).In most comm un ications systems, theowner doesnt wish to pay for powerthat heads toward th e sky, so the patternis flattened as shown in Figure 3(b) .This not on ly forces the transmit signaldirection d own to ground level but alsoproduces a 5 6 dB antenna gainimprovemen t in the process.

For fixed point-to-poin t app lications, the directional an tenna is ideal because tran smit power is focused towardthe receive antenna and the directional performance helps eliminate interferers. Figure 3(c) shows two typicalsignal patterns of directional antennas plotted on a polar scale to indicate their directional performance.Plot (P) is an antenn a product which is specified to h ave a beam-width ( B/ W) of 7 in the frequen cy band of2.4-2.5 GHz. It provides an antenn a gain of approximately 26 dB and requires a 4-foot diameter parabolicreflector. Plot (F) is a produ ct which is specified to h ave a B/ W of 24, also in the 2.4-2.5 GHz band, with again of 16 dB, but which requires only a 1-foot sized flat panel anten na. It is pretty easy to see that the greaterthe d ish size, the sharper the antenn a beam focus.

In polar patterns, the direction of the main lobe is called the boresight direction. The radius of the plot isusually calibrated in dB from the center. Upon closer examination of the plots of Figure 3(c), it can be seenthat an in ter ferer system which is positioned at a considerable an gle off boresight of the directional beam stillcan enter the affected receiver, with only 10 or 15 dB of angle rejection. Usually the main lobe is specified asshown by the -3 dB power curve, meaning th at the power at those points is only down 50% from th e bor esightlevel. Sidelobes and backlobes vary from 15-30 dB below the main lobe and can allow interfering signals toenter from the side or back directions.

Figure 3(d ) shows a polar plot pattern of an actual wireless LAN system antenn a. These are commonlymounted on the ceilings of office configurations where un iform signal coverage is needed arou nd the officeto interconnect mobile laptop computers.

Signal polarization is an antenn a characteristic which can furn ish added signal rejection capab ility. Electro-magnetic (EM) signals can be horizontally or vertically polarized, and sometimes, circularly polarized. Two

similar systems operating, one vertically- and one horizontally-polarized, can exhibit considerable isolationfrom each other, typically 20 dB.

Emitter Ground Plane

(a)

Emitter Ground Plane

(c) (d)

(b)

Main

Lobe

-3 dB

Plot P

Plot F SideLobe

Back

Lobe

4 Ft. Parabolic

Flat Panel

30

60

90

120

150

180

210

240

270

300

330

0

Figure 3(a). An elevation view of an isotropi c emitter.Figure 3(b). Adjusting the emitting elements can flatten the sky-bound pattern of (a).Figure 3(c). Adding a parabolic reflector produces an antenna pattern with ahighly-directive main-lobe of desired power.Figure 3(d). A pol ar (azimuth) plot of a wireless-LAN antenna.


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Application Note

3.2 Sources of Interference

3.2.1 Out-of-Band EmittersThe following paragraph s describe some typical sources of inter fering signals and the ir effects. Even thoughthe affected system is designed to reject signals outside its assigned band, there are effects caused by out-of-band emitters which can impact the in-band per formance.

De se ns it iza t ion When a h igh-powered tr ansmitter, such as a UHF TV broadcast station , is nearby, theaffected receiver can be driven into RF overload even though its signal is well out-of-band. See Figure 1[a].This happ ens when the affected receivers pre-selection filter is no t adequate. The high power signal leakinginto the affected receiver will drive the operating poin t of the front-end amp lifier up th rough its dynam ic rangecharacter istic. This destroys the n orm ally-requ ired linear am plification process, introducing interm odu lationdistortion and serious data errors.

In term od ula t ion S igna ls f rom N on- l inea r Pow er Am plif ie rs Modern wireless systemsreceive, transmit, and process hu nd reds of chann els of voice or data at a common base station. Most of thosechann els are processed via common antenn as and broadband power amp lifiers at the final stage for costeffectiveness. The multiple chan nel signals are com bined in fron t of the final power stages and then amplifiedtogether. The linearity specifications of those final power stage amplifiers are very tight since a non-linear char-acteristic can cause cross-frequency signal products to be produced and emitted. Those cross-frequency signalsmight cause inter ference within th eir own systems operating band or cross over into o ther systems.

In termodula t ion f rom Non- l inear Externa l E lements This interference mechanism is calledth e rusty fence syndrome. If two high-powered transmitted signals, f1 and f2, impinge on some ran dom rustyelemen t such as a steel fence, a ru sty metal roof, or even corrod ed coaxial cable elements, an e lectric effectsometimes takes place. The corrosion junction acts like a rectifying d iode and mixes all the tran smit signalshitting it. Th is results in a whole list of n ew signals, called interm odu lation products, which are re-transmitted.These signals are mathematical combinations of the original transmitted signals, such as (f1 f2) ,(2f1 f2), (3f1+2f2), etc.

While th is effect is typically a random problem, th ere are certain tran smitter frequency assignmen ts whichcause the mixed produ cts to fall right on top of other assigned bands. When the f1 and f2 signals have exactfrequency spacing equal to th e affected receivers input, it tries to accept these re-transmitted intermodulationproducts as its own in-band data.

3.2.2 In-Band EmittersNon-licensed wireless Local Area Networks (LANs) using frequency-hopping (FHSS) or direct-sequence-spread-spectrum (DSSS) techniques spread the useful data mod ulation over a wider band. They operate in the ISMband which is also the home frequency of the typical microwave oven, as shown in Figure 1[b]. Microwaveovens operate at the 2.4 GHz resonance frequency of the water molecule. Although the spread spectrummodulation schemes defend against interference from the oven leakage, the location and power level of theleakage may overcome the interference resistance.

Ha rm onic a nd Pa ra s it ic O utputs If broadband output power amplifiers are driven far into satura-tion, signals begin to get compressed resulting in more than the intermodulation effects described in Section3.2.1. The signal clipping produces harmonics of the broadband transmit signal, and these spurious signalscoming out of the an tenn a might interfere in oth er receiver passbands.

Parasitic signals are caused when a power amplifier d eteriorates into a ran dom oscillation mode. Ham radiotransmitters deliver high power and can occasionally degrade to the point of emitting unintended spuriousoutputs.

3.2.3 Overlap of Antenna PatternsAntenn a patter n overlaps between commun ication systems frequently occur. There are a variety of inter ferencecauses attributed to antenna pattern conflicts.

ISM Sy s te m Ante nna A lignm e nt P rob le m s In non-licensed systems, users simply install theirnewly purchased equipmen t without any licensing needed. Th is commonly occurs in business parks wherecompanies install data links between office buildings. It is easy to foresee how one system signal becomes aninterferer in an affected receiver. The signal pattern might be as shown in Figure 1[c] where the intendedtransmit signal over flies its own receiver and comes right into th e aligned an tenna pattern of the affected receiver.


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Application Note

While modulation designs are supposed to offer some rejection of interference due to different frequency-hopping parameters or differen t DSSS code patterns, it is possible that the inter fering signal levels at theaffected receiver might still overwhelm the re jection to lerance of the modulation scheme .

It should be noted that even if the antenna pattern lobes of the affected system are relatively narrow (highgain), there is still considerable sensitivity to signals that are as much as 20 to 30 degrees off boresight.

Ba c k lobe s a nd S ide lobe s As shown in Figure 3(c) , ther e are sidelobe an d backlobe ch aracteristicsin every antenna. This means that interfering signals might cause problems if they enter one of the sidelobes

or the backlobe of the affected system as in Figure 1 [e] and [ f]. Typical sidelobe an d backlobe sensitivity isonly 15 30 dB down from the main lobe.

Re f le c t ions a nd Fad ing The affected system often operates in signal environm en ts which affect itssystem signals. Heavy rainfall attenuates microwave frequencies. Buildings, hills, and other natural obstructionsbend or cause multiple paths to form between transmitter and receiver. These multiple paths, or multipaths,lead to destructive signal cancellations and cause random fades in signal strength .

Other buildings, Figure 1 [ D], might reflect interference into the side of the affected an tenn as main lobe. Lowflying airplanes can cause a moving reflection which might degrade data r and omly.

Ce llu lar Ante nna O ver lap Cellular systems, with the ir th eore tical hexagonal base station cell patternspacing, take ad vantage of frequency band re-use by assigning the same frequen cies to cells that are spaced justone cell distance away. As such, any given cell antenna that happens to be misadjusted for tilt can easily overflythe adjacent cell and impinge on an affected receiver two cells over where the signal frequen cy assignments arethe same.

4.0 How to Determine if Interference is Corrupting your SystemA down-system complaint might be the result of equipment performance degradation leading to marginal data.But just as likely, an in ter fering signal could be causing poo r data or voice recep tion.

4.1 Recognizing the InterferenceSection 3.0 reviewed th e generic causes and sources of inter fering effects. The frequency of an inte rferingsignal is the most common parameter leading to the identification of the interfering source. Thus, aninterference problem can often be categorized by its frequency characteristics.

It should be noted that whe ther th e inter fering signal is in-band or out-of-band, the signal is almost certainlycoming through the antenna, down the cable, and into the affected receiver. Therefore, a spectrum analyzerconnected to the operating system antenna will serve as a substitute measuring receiver which will display andhelp iden tify unwanted signals. Remember that the systems band pre-selection filters are inside its receiver,so many out-of-band signals are naturally presen t at its antenn a input conn ector.

Inter ference gene rally on ly affects receiver p er formance. Although it is possible that a source of inter ferencecan be physically close to a transmitter, the characteristics of the transmitted signal will not be affected. Thus,the first step in recognizing if inter ference has corrup ted a receiver is to learn th e characteristics of the signalthat the affected system is intended to receive.

Reviewing the system's operations manual will indicate what modulated signal should be received. By analyzingthe frequency domain using a spectrum analyzer ( as opposed to the time domain u sing an oscilloscope) the

signal frequen cy, power, harmonic conten t, modulation qu ality, distortion and no ise or inter ference can easilybe measured. If inter ference is overlapp ing the in tended receiver signal, it will be relatively obvious on thespectrum analyzer d isplay.

A displayed inter ference fingerprint contains importan t iden tification character istics. A modulated signalwill have un ique characteristics dep ending on the type of modulation used. Section 5.0 reviews many typicalsignal character istics.


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Application Note

4.2 Selecting the Appropriate Test EquipmentThe most useful and accurate tool for qualitative an d quan titative an alysis of RF and microwave in ter ferencein the field is the broad band, hand held spectru m analyzer. The Anritsu Spectrum Master MS2711B is such atool that features power ful user-convenience p arameters (soft keys) , calibration-correction routines, and dataman ipulation an d storage capabilities.

Section 4.2.1 presents some key operating principles and d iscusses parameters relating to inter ferenceconsiderations. Following th at, Section 4.2.2 describes some more versatile measuring functions that areimportant and convenient for a typical inter ference measurement procedur e.

4.2.1 Spectrum Analyzer ParametersWhat do we need to know about a spectrum analyzer to make sure that we can measure the signal environmentadequately? Very basically, we need to know the frequency range, sensitivity, dynamic range, frequencyresolution an d accuracy.

F r e q u e n c y Ra n g e Frequen cy range should be th e easiest criteria, since you have a good idea of yoursystems frequ ency band an d h ence the spectru m span you want to observe. Just be sure to give yourself plentyof display width to work with, by setting the frequency span wide enough to include both your affected receiversignals and adjacent inter fering signals.

Se nsitivity Sensitivity, although fairly straightforward , can be somewhat con fusing. The key is to under-stand your system specifications and the level of sensitivity required to make your measurements of expectedreceiver inputs. For example, if your system s receiver signal stren gth specification is expected to be on theorder of60 dBm, then you will typically on ly need an additional 20 to 30 dB of measurement r ange. Th us,a spectrum analyzer that exh ibits a sensitivity of80 to 90 dBm should do the job nicely.

Fre que nc y Re s o lu t ion , Dy na m ic Ra nge , a nd Sw e e p T im e Frequency resolution, dynam -ic range, and sweep time are inter-related. Think of resolution as the shape of a scanning window whichsweeps across an unknown band of signals. The shape of that sweeping window is similar to that shown inFigure 4. Spectrum analyzers provide for selectable resolutions, and call it resolution bandwidth (RBW). RBWrepresents the3 dB width of the p assband of the an alyzers intermediate frequency (IF) amplifier chain. Resolution becomesimpor tant when you are tr ying to measure signals that occur close togethe r in frequency, and you need to beable to distinguish one from the other.

Example: If two perfectly pure signal frequencies are 10 kHz apart, the spectrum analyzer display would showtwo renditions of the Figure 4 shapes spaced at 10 kHz. Thus, a 10 kHz RBW setting would show that therewere two signals present. If the RBW was set to 30 kHz, the two signals 10 kHz apar t will ten d to display as one .In general, two equ al-amplitude signals can be adequately iden tified if the selected RBW is less than or equalto the separation of the two signals.

Selecting the smallest possible RBW will increasethe ability to resolve signals that are close together,but the trade-off is that it will take longer to sweepacross a given frequency band of interest. Also,dynamic range decreases as RBW is increased asmore noise is integrated as part of the measure-men t with the wider RBW. So, the optimum RBWsetting depends on the spacing of the signals thatare to be resolved, the dynamic range required ,and the maximum acceptable sweep time.

Selectivity In some inter ference applica-tions, there will be signals that have amplitudesthat are quite unequal. In this case, selectivitybecomes an impor tant criteria. It is very possiblefor the smaller of the two signals to becomeburied under the filter skirt of the larger signal.See Figure 4.

IF Center Frequency

10 kHz

-50

-60

Freq

Unequalsignal60 dB down

-40

-30

-20

-10

-0-3

dB

110kHz

Figure 4. A spectrum analyzers shape factor is defined as the ratio of its IF bandwidths at 60 and3 dB, in this case 11:1.


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Application Note

S h a p e F a c t o r A spectrum analyzers shape factor defines the ratio of the 60 dB bandwidth to the3 dB band width of the IF amplifiers. In th e Figure 4 example, the 10 kHz RBW filter has a typical shap e factorof 11:1, with a resultant 60 dB bandwidth of 110 kHz and a half-bandwidth value of approximately 60 kHz.If two signals are separated by 60 kHz, but one of them is 60 dB lower in amplitude, it will be almost buriedin the selectivity skirt of the main signal.

Ac c ura c y The measurem en t accuracy of any spectrum an alyzer results from the add ition o f many differ-en t accuracy components. Measurement accuracy is impor tant when comparing measured values on unkn own

signals to published specifications of a system un der test. Luckily, when making typical inter ference m easure-men ts, the user is looking for ratios, such as C/ I, which determines the op erating margin of the desired carrierover the inter fering signal in th e same operating ban dwidth. Th us, absolute accuracy is less critical thanrelative accuracy.

4.2.2 Spectrum Analyzer Functional Controls and RoutinesThe Anr itsu MS2711B has the traditional con trol an d inpu t keys of most advanced spectrum analyzers. But italso features more soph isticated one-button measurement routines which provide powerful computationalassistance when making interference measurements.

C e n t e r Fr e q u e n c y a n d S p a n All modern spectru m an alyzers have flexible control of their tu ningparam eters with cen ter frequency and d isplay sweep width (span ) being m ost popular. Span is simply the widthof the swept frequency band between the start frequen cy and the stop frequency. It can be en tered with theCenter Frequency and the Span soft keys. The GHz, MHz, kHz or Hz soft keys set the units value. Alternatively,the sweep width can be en tered by keying in the Start an d Stop frequencies.

Re s o lut ion B a ndw id th (RBW ) With a given RBW, a sweep speed that is too fast will pass by theunknown signal before the detection system has a chan ce to respond to the signal. But sweeping too slow willjust waste the operators time.

Thus, for each span width, and each sweep time, there will be an optimum resolution bandwidth (RBW) forbest accuracy. Fortunate ly, the operator can re ly on th e automatic setting feature of the MS2711B to provideoptimum combinations of span, sweep speed and bandwidth settings which enhance accuratemeasurement results.

S a v e - R e c a ll M e n u s The Save and Recall menu functions in the MS2711B allow you to store theinstruments many control settings for u p to 10 differen t cond itions. Use the Save Setup and Recall Setupmodes to achieve qu ick chan ges from one comm only-used set of measurement settings to ano ther. Setup

location 0 is the factory-preset settings state. The Save/ Recall function essen tially pre-sets all the instru mentcontrols to user-defined settings, which is quite u seful when the job is rou tine an d r epetitive. If don e be foredriving to th e field, th is mean s minimum keyboard setup times for p arameters like carrier level in frequency-allocated ch ann el assignmen ts.

M a r k e r P e a k S e a r c h a n d C e n t e r in g In a normal search routine, the operator will often set thedisplay span for a spectrum th at is wider th an th e affected r eceiver chann el assignmen t. This allows a pan oram-ic view of all signals on the air for that span. If a large am plitude possible-inter ferer signal shows up, the op era-tor can choose a convenient MS2711B key function called Marker to Peak. This arithmetically looks across thedisplay and picks out the highest signal. It then tun es the selected marker, say M1, to that h ighest peak andann otates the screen with the M1 frequ ency. Such information might give immediate clues to theinterfering culprit.

Another h andy key function, after captu ring the M1 marker frequency, is the Marker Freq to Center softkey.This immediately re-centers the display so that the M1 frequency is right at the center. The operator can thennarrow the sweep span to get some idea of the signal characteristics of the interferer, e.g., determine whetherit is random ly occurr ing, noisy, or perhaps exhibiting some o ther revealing character istic.


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Application Note

Dis p la y C a pture A very important feature of the MS2711B spectrum analyzer is the ability to capturespectrum displays that are encountered during a long day in the field. The Save Display function permitsthe operator to take many measurem ents, name th em for the measured situation, and bring the instrumen tback to the office where th ey can be downloaded or p rinted out. Figure 5 in Section 5.3 (page 13) is anexample of a field-captured d isplay, with all its annotated data.

Th e Recall Display key brings an index list to the screen for selecting and calling back saved waveforms.In the MS2711B, up to 180 screen displays may be stored for later use.

Preampl i f ie r and Input At tenuator For most field interference applications, signals will be rela-tively weak. The preamplifier feature offers a selectable, full-spectrum amplification o f 20 dB. The p ream plifierhas a very low noise figure , meaning that it does not add appreciable noise to the signal it amplifies. Thepreamplifier can han dle 20 mW (+13 dBm) without damage, and if it goes into saturation, an annotatedSAT ind icator shows on the display.

The inpu t attenuator is a standard feature o f all spectrum analyzers that p rotects the relatively delicate frontend mixer and preamplifier stage. The standard input of the analyzer can handle 200 mW (+23 dBm) withoutdamage, so good practice dictates that you first set the input atten uator to 40 or 50 dB if you expect un knownsignals with h igh amplitude. Th en adjust the attenu ator setting for less attenuation after the signals appearon screen.

M a x H o ld In a number of measurements, the randomness of an interfering signal will make the display

jump and difficult to visualize. It is helpful in such situations to use the MS2711Bs M a x H o ld function. Thisfeature digitally processes the display trace such that it always remembers and displays the highest signal levelat every point on the display.

D e m o d u la t o r For add itional power in iden tifying inter ferer signals, sometimes an aud io demodu lationof the waveform can assist. The MS2711B features an AM function plus two FM demodulator functions,narrowband and wideband. The audio outpu t can be heard on accessory earph ones or th rough the built-inspeaker on the front panel. The analyzer mu st be set to zero span in ord er to use the demodu lator feature,as explained in th e Application Note Practical Tips on Measuring Interference Anritsu Publicationnumber 11410-00303.

O c c u p ie d B a n d w id t h a n d C h a n n e l P o w e r The Spectrum Master MS2711B features twocomputational modes which are very powerful for measurements on wideband data channels such as ISM datalinks. Occupied Bandwidth (OBW) allows the ope rator to define the band edges of an occupied band, such as

the 20 dBc power p oints. After th e measuremen t, the M1 an d M2 markers show as annotated frequencies onthe display and define the band edge frequencies where the signal is 20 dB relative to the carrier.

C h a n n e l P o w e r Channel Power arithmetically computes the integrated power contained in a widebandspectru m after its defined bandwidth is set into th e an alyzer. This mode is particularly useful in measuringsystem p ower in an ISM spread spectrum com mun ications signal, and is described further in the Practical Tipson Measuring Interference Anritsu Publication number 11410-00303.

Ante nna Ac c e s s o r ie s Many field interference measurements will be made with an independentantenn a, i.e., not the comm unication systems operating an tenn a. The most versatile kind of indepen den tantenn a is the so-called whip design. A whip antenn a is a linear con ductor con nected to the MS2711B coaxialinput connector. Whip antennas are sized for 1/4 wavelength at th e specified cen ter frequency. Whip designsare omn idirectional, and insensitive to directional e ffects.

For additional diagnostic power on certain interference measurements, the operator will sometimes needto use a directional anten na. Microwave data systems are an example where th e system an tenna d irectionalper formance is highly critical. In those cases, a directional anten na attached to th e spectrum an alyzer candetermine the direction of the inter fering signal. Directional antenn a kits are comm ercially available forcommon application bands.


11/1611

Application Note

Fie ld Streng th C a l ibra t ions The MS2711B features the ability to an notate an d display spectru mpro files in term s of field strength quan tities, such as dBm/ meter. This can be useful when affected receivershave specifications in terms of field strength and an interferer signal can be measured with reasonable accuracywith calibrated antennas.

When using the MS2711B, its data processing function can make corrections for antenna pattern characteris-tics which highly improves the accuracy of the measured results. To use th is power ful and impor tant function,the specific antenna sensitivity factor data is uploaded from a PC using available Software Tools furnished

by Anritsu with the product. Multiple anten na data is easily stored and called up in the field using the menu-driven selection process.

5.0 Identifying an Interferer by the Characteristics of its SignalArmed with the understanding of causes and sources of interference and an appreciation of how receiverdesign can fail to reject the wide spectrum of possible inte rfering signals, the field tech nician n ow faces thetask of running down the interferer. The MS2711B will show the affected signal, and often will reveal theoverlapp ing inter ference as well. The Measurement Note offers some practical tips on signal and antennahookup for interference iden tification.

5.1 Broadcast StationsAnalog TV TV stations are power ful tran smitters, all completely legal and licensed, but often overpower-ing for co-located lower-power communications systems. UHF TV stations operating in the 450 850 MHz bandcan be licensed for 50,000 watts and transmit equally in all directions. Chann el spacing in the U.S. is 6 MHz.Video m odu lates the main carrier, while TV audio is contained in an FM-modulated sub-carrier spaced4.5 MHz away (6-7 MHz in Europe) . The d istinctive 4.5 MHz spacing of the sub-carr ier helps iden tify aTV signal as a possible interferer.

TV transmitters cause signal inter feren ce in two ways. In their tower vicinity, the TV carr ier power can dr iveaffected receivers into overload. But the FCC specifications also allow harmonics up to 30 dBc (50 Watts ona 50 kWatt station) . Such harmon ic signals can easily swamp commu nications chann els which are assigned tothose doubled or tripled harmon ic frequencies.

D igital TV The newest video transmission technology authorized by the FCC is digital video. By sophisti-

cated d igitization of the video signal, it is possible to pack four separate chann els of video in to th e same chan-nel bandwidth p reviously allocated to one analog TV chan nel. Harmonics of the carrier can likewise be a prob-lem with these digital power transmitters.

F M B r o a d c a s t FM broadcast stations also feature very high transmit powers in the 87.7 to 107.9 MHzband. FM broad casters can choose quite h igh power tr ansmitters, and if co-located on mountain tops withcommunication systems, they cause the same overload as TV stations. The broadcast FM spectrum profile isrelatively flat with sloping sides depend ing on the par ticular audio conten t being broadcast at the time. Musictends to have a wide flat top, while silence exh ibits a narrower p attern . Gener ally, the more comp lex the au dio,the wider the spectru m of the modulated signal. The d emodulator function of the MS2711B will assist iniden tifying an FM inter ferer. Choose the wideban d-dem odu lator function if you suspect FM Broadcast as theinter fering signal.

A M B r o a d c a s t AM broadcast channels are spaced every 10 kHz from 500 to 1,500 kHz. AM modulation

sidebands on the analyzer display show evident amplitude jumps with voice, and less activity with music. Usethe MS2711B AM demodulator function to determine if the signal is a broadcast AM station. Th e frequ encyband of AM broadcast is a long way from wireless radio, so the interfering mode is usually the out-of-bandoverload effects noted earlier in co-located en vironm en ts.


12/1612

Application Note

5.2 Traditional Communications SystemsF M M o b ile Before the emergence of the personal mobile wireless (cellular) phones in the 1990 s, publicsafety applications, such as police, fire and forest service, used narrowband FM technology. These applicationsstill exist in the 50, 150 and 450 MHz FM bands. Typical mobile FM transmitters emit 5 to 150 Watts whilethe ir permanent base stations often tr ansmit at 150 Watts with an omn i-directional footprin t. The spectrumprofile of nar rowband FM spans about 5 kHz. For he lp in iden tifying, use the MS2711B narrowband FMdemodulator function.

AM Airc ra f t Com m unic a t ions Using th e VHF frequencies in th e 118-136 MHz region , auth oritiesallocated 25 kHz-wide ch ann els for a h igher voice quality AM for aircraft commu nications. Being exceed inglymobile, aircraft inter ferers are also d ifficult to p in down since an y one aircraft is on ly in th e area for tens ofseconds. But again, th eir grou nd transmitters can be a con stant source of re latively high signal powers. Thespectru m p rofile again r eflects the voice nature of th is app lication . The MS2711B AM demodulator functioncould be useful here in iden tifying these types of signals as potential inter ferers.

Pa g ing Sy s te m s Simp le paging systems typically use a frequency-shift-keyed (FSK) modu lation formatwhich exhibits a spectrum profile with two separated peaks, each rep resenting one of the two frequ encieswhich shift according to the digital on e or zero being transmitted. As more com plex data, such as analphan umeric message, is transmitted, th e space be tween the two peaks fills in.

A m a t e u r R a d io ( H a m R a d io ) Scattered th roughou t the frequency spectrum are a n umber of

allocated frequency bands dedicated to Ham radio operators. While th eir transmitters largely use AMmodu lation, they are also au thorized to run experimental transmissions in other formats. Their emittedpowers can be quite high since they intend to transmit to others around the earth.

Ham s often use large, steerab le directional arr ays of HF an tennas to increase th eir directional power, so theirinterfering power can be quite high. Further, these transmitters are mostly found in residential areas wherewireless base stations are located. If the affected receiver is not well filtered at its input and is in the boresightdirection of the Ham transmitter, there is a possibility of interference. Ham transmitters can contain harmonicswhich extend into wireless bands. The MS2711B AM demodulator function will assist in identifying these typesof interfering signals.

5.3 Wireless CommunicationApplications

The wireless technology and cellular explosionhas filled th e allocated frequency spectru m withmillions of cell phones and base stations. On e h ard-ly has to mention that all those little transmittersessen tially saturate the ir assigned spectrum. Table 1serves as a starting poin t to pinpoint conflictingsignals. It lists some popular formats, but doesnot list all U.S. or global applications.

Adv a nc e d M obile Phone Se rv ic e( AM P S ) AMPS originally operated as an analogsystem in th e 800 MHz frequen cy band u sing30 kHz wide ch annels. A varian t of AMPS known

as N-AMPS uses 10 kHz wide channels and con se-quen tly almost trip led ch anne l capacity. AMPS isstill in common use throughout the Americas butis declining in the face of digital cellular standards.Wireless carriers that began service with AMPSsystems have generally turned to TDMA and CDMAdigital operation in the 800 MHz cellular band.

NameTitle/SpecReference

Frequency bandMHz

Analog Cellular

AMPS/NAMPS Adv MobilePhone Syst

MS 824849BS 869 - 894

Digital Cellular

TDMA IS-54/136 MS 824849BS 869894

CDMA IS-95 MS 824849BS 869894

GSM Global SystMobile Comm

MS 880915BS 925960

DCS 1800 Dig Comm Syst MS 17101785BS 1805 - 1880

Personal Communications Systems

PCS-TDMA Based on IS-136 MS 18501910

BS 1930

1990PCS-CDMA Based on IS-95 MS 18501910

BS 1930 - 1990

PCS-1900 Based on GSM MS 18501910BS 1930 - 1990

Digital Cordless Phones

DECT Dig enhanced 18801900

Note: The bands shown are the US transmitter frequency allocationsfor Base Station (BS) or Mobile Station (MS).

Table 1. Some Frequency Allocations for Typical Wireless Applications.


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Application Note

N orth Am er ican D ig ita l Ce llu lar (N ADC , now IS-13 6 ) This is one of th e or iginal ro llouts ofthe new cellular technology. It was designed to utilize the existing 30 kHz channel of the Advanced MobilePhone Service (AMPS) cellular techn ology. Its spectrum p rofile fills the 30 kHz channel with a relatively flattop spectrum characteristic.

Co de -D ivis ion-M ult ip le -Ac c e s s (CD M A) CDMA is a very inn ovative technology that exploits theidea of interleaving hun dreds of individual digitized voice signals into one fast digital data stream . That datastream , combined with a special encoding data stream, then modulates the RF carrier. The effect of this is to

spread the spectru m over the en tire 1.23 MHz frequency allocation for the service, and essen tially explains why798 users can fit into the one 1.23 MHz channel.

A typical CDMA spectrum profile looks likethe flat-topp ed characteristic of Figure 5. Theprofile is 1.23 MHz wide with r elatively sharpslopes at the band edges. If a CDMA signalis interfering with another signal, it can beiden tified by that 1.23 MHz wide pro file.

W id e b a n d - C D M A (W - C D M A ) This3GPP system is the th ird generation wide-bandversion of CDMA which utilizes chann els

approximately 5 MHz wide. It is a verysoph isticated modulation system, in tendedfor higher data rate connectivity. The 3GPP2is a parallel project and a collaborative effortof multiple country telecommunicationsorganizations; ARIB (Japan), TIA (NorthAmerica), CWTS (Ch ina) , TTA (Korea)and TTC (Japan).

Glob a l System for M ob ileC o m m u n ic a t io n s ( G S M ) GSM is anintern ational wireless standard which is

used heavily outside of th e US. GSM isassigned two frequen cy band s at 900 and1800 MHz. Each band supports 124 chann elsat 200 kHz spacing and each is broken into8 time slots operating in a TMDA (time-division-multiple-access) mode. Themodulation type is Gaussian MinimumShift Keyed ( GMSK). See Figure 6.

START= 1.931 000 000 GHz

STOP= 1.937 000 000 GHz

10 dB/ DIV

ATTEN*

0 dB

RECALL

RFIn 50

Pos Peak

MAX HOLD

AMP

SWP TIME

RECALL START 1.931GHz STOP 1.937GHz

REF LVL 40.00 dBm VBW 3 kHzRBW 10 kHz

Figure 5. Three adjacent CDMA channels of 1.23 M Hz each in the 1900 MHz PCS band.

START= 931.638 960 MHzSTOP= 931.838 960 MHz

10 dB/ DIVATTEN*0 dB

RECALLRFIn 50

Avg DetMAX HOLD

AMPSWP TIMERECALL START 931.638MHz STOP 931.838MHz

REF LVL60.00 dBm VBW 3 kHzRBW 10 kHz

Figure 6. TDMA technology of the GSM system shows 4 of 8 users in a 200 kHz channel.


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Application Note

Pe rs ona l Co m m unic a t ions Sy s te m s (PCS) The Personal Comm un ications System (PCS) is anam e given to wireless commun ications systems in the 1800 1900 MHz frequency band. PCS was supposed tobe a more comprehensive specification than the earlier cellular specification at 800 MHz. However, the on lytechnologies that were implemented were upbanded cellular standards. Thus, the change was simply oneof expanding the available spectru m by using the same signal formats at the h igher 1800 MHz band . Nowconsumers rare ly know whether the ir cellular ph one is operating in the cellular or PCS band.

PCS1900 - Upbanded GSM cellular

TIA/ EIA-136 Upbanded TDMA digital cellular (ANSI-136)

TIA/ EIA-95 or IS-2000 Upban ded CDMA digital cellular ( ANSI-95, cdmaOne or cdma2000)

All PCS systems are digital. The PCS frequency allocation in the US is three 30 MHz allocations andtwo 10 MHz allocations in th e 1850 - 1990 MHz frequen cy band.

5.4 Unlicensed ISM Data Systems

Table 2. Some Frequency Allocations for Typical Unlicensed ISM Applications.

Table 2 shows some popular ISM-allocated ban ds. In addition to a myriad of unlicensed applications like microwaveovens and atomic particle accelerators, they now support thousands of unlicensed data communications systems.Customers often prefer these systems because of their inexpensive nature and the ability to install them without a

tedious licensing process. They are popular for point-to-point and point-to-multipoint data link applications such asEthernet bridging for intracompany data bridges. Recent Bluetooth technology promises to further fill the spectrumwith close-range personal data applications.

Wire less LANs Wireless LAN technology in the ISM band was conceived for short-range connectivitysystems. Its uses include laptop computer s and data man agemen t within bu ildings. Both WLAN techn ologies,frequencyhopping (FHSS) and direct-sequence (DSSS), depend on spread spectrum technology for datamodulation. Th ese schem es trade wider bandwidth for transmission reliability. To a nar row band system,spread spectrum signals just look like rand om noise.

The typical FHSS system utilizes a 1 MHz power spectrum which is frequency-hopped three times per secondacross a 75 MHz channe l. A typical DSSS system u tilizes a constan t 25 MHz wide spectru m, from a 1 Watt(+30 dBm) transmitter, which tr anslates to +16 dBm per MHz.

When ISM interference from another similar system brings down a affected receiver, it is highly likely thatthe interferer is completely legal. It is then up to the affected to d etermine h ow to arrange other elementslike antenn as or perhap s modulation alternatives to solve the p roblem. There is no ap peal to officialregulators since these are un licensed bands.

NameTitle/SpecReference

FrequencyBand MHz

Wireless Data

Bluetooth ISM band 24002497

Wireless LAN IEEE 802.11b 24002484

ISM Applications

ISM 902928

FCC Part 15.247 57255850

FCC Part 15.407

UNII-1 51505250

UNII-2 52505350

UNII-3 57255825


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Application Note

IS M M ic r o w a v e D a t a L in k s ISM-band systems provide fast installation for applications such asEthernet-bridges which connect backbone data systems with new wireless base stations without the need fordigging u ndergroun d cables. ISM data links at 5725 5850 MHz have considerable advantage over UNII(Unlicensed National Information Infrastructure) systems because they are allowed higher output powersand very high gain antenn as. These often give them a 48 dB inter ference advantage.

Here are the FCC bands:

Part 15.247 Technical requirements for intentional radiators

57255850 MHz Emission B/W20 dB points

Part 15.407 Technical requirements for UNII

51505250 MHz UNII-1 Emission B/W26 dB points

52505350 MHz UNII-2 same

57255825 MHz UNII-3 same

(The word "Part" refers to the FCC regulations indexing format.)

6.0 ConclusionOne of the industry trade associations web sites shows statistics which estimate that worldwide ownership ofwireless devices exceeded 800 million in 2002.[1] Does anyone wonder that competing wireless systems will

continue to inter fere with each oth er in regions of high density installations?

Under stand ing the characteristics of your affected receivers modulated signal and the effect that noise orinter ference has on that signal is the first step in detecting inter ference within your commu nications system.Selecting the appropr iate test equipment, such as the Anritsu Spectrum Master MS2711B, and employingproper measurement techniques can en hance the likelihood of locating an d identifying sources ofinterference within your system.

A companion application note from Anritsu Company, Practical Tips on Measuring Interference, pn: 11410-00303

offers actual measurement examples and practical routines used by field technicians who search for interference as a

lifes work. It also explains the advantages of Anritsus Spectrum Master MS2711B that is designed to simplify your

search for interfering signals.

For those with deeper interest or questions about antennas, there are plenty of Internet resources for further study. The

footnote lists the National Spectrum Management Association (NSMA) website www.nsma.org. A typical antennamanufacturer website www.gabrielnet.com is also listed.

_______Footnote:[1] Cellular Networking Perspectives Ltd. See their very useful, impartial and informative web-site: www.cnp-wireless.com.

It contains a super-comprehensive acronym list of wireless terminology.


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Fundamentals of Interference in Wireless Network