Cap 12 MW Digital Modulation

7/28/2019 Cap 12 MW Digital Modulation

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CapCaptulo 12tulo 12

Ing. Marcial Lpez [email protected]

2007

ModulacinDigital

(Digital Modulation)


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UNI - Sistemas de MW 2

Principles of Digital Modulation:

Outline of Lectures

l Introduction to digital modulationl Relevant Modulation Schemes (QPSK,GMSK, M-Ary Schemes)l Coherent and Differential Reception

l The impact of the mobile channel on digitalmodulation

noise and interference random FM (narrowband fading)

intersymbol interference (wideband fading)


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Digital Modulation Basics

l The bit rate defines the rate at which information

is passed.l The baud (orsignalling) rate defines the numberof symbols per second.Each symbol represents n bits, and has Msignal

states, where M = 2n.This is called M-ary signalling.l The maximum rate of information transfer througha baseband channel isgiven by:

Capacity fb = 2 W log2M bits per second where W = bandwidth of modulating basebandsignal


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l Pulse shaping can be employed to remove spectralspreading.

l ASK demonstrates poor performance, as it is heavilyaffected by noise and interference.

Amplitude Shift Keying (ASK)


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Frequency Shift Keying (FSK)

l Bandwidth occupancy of FSK is dependant on the spacing ofthe two symbols. A frequency spacing of 0.5 times the symbol

period is typically used. l FSK can be expanded to a M-aryscheme, employing multiple frequencies as different states.


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Phase Shift Keying (PSK)

l Binary Phase Shift Keying (BPSK) demonstrates better performancethan ASK and FSK.l PSK can be expanded to a M-ary scheme, employing multiple

phases and amplitudes as different states.l Filtering can be employed to avoid spectral spreading.


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Nyquist & Root-Raised Cosine Filters

l The Nyquist bandwidth isthe minimum bandwidththan can be used torepresent a signal.l It is important to limit the

spectral occupancy of asignal, to improvebandwidth efficiency andremove adjacent channelinterference.

l Root raised cosine filtersallow an approximation tothis minimum bandwidth.

Nyquist bandwidth onthe QPSK spectrum


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Modulation - QPSK

lQuadrature Phase Shift Keying is effectively two independent BPSKsystems (I and Q), and therefore exhibits the same performance but twice thebandwidth efficiency.

lQuadrature Phase Shift Keying can be filtered using raised cosine filters toachieve excellent out of band suppression.lLarge envelope variations occur during phase transitions, thus requiringlinearamplification.


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Types of QPSK

lConventional QPSK has transitions through zero (ie. 180o phasetransition). Highly linear amplifier required.lIn Offset QPSK, the transitions on the I and Q channels are staggered.Phase transitions are therefore limited to 90o.

lIn p/4-QPSK the set of constellation points are toggled each symbol, sotransitions through zero cannot occur. This scheme produces the lowestenvelope variations.lAll QPSK schemes require linear power amplifiers.


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GMSK - Gaussian Minimum Shift Keying

l GMSK is a form of continuous-phase FSK, in which the

phase is changed between symbols to provide a constantenvelope.Consequently, it is a popular alternative to QPSK.l The RF bandwidth is controlled by the Gaussian low-passfilter bandwidth.

l The degree of filtering is expressed by multiplying the filter3dB bandwidth by the bit period of the transmission, ie. by BT.l As BT is lowered the amount ofintersymbol-interferenceintroduced increases and this results in either a fixed powerpenalty or an irreducible error floor.l GMSK allows efficient class C non-linear amplifiers to beused, however even with a low BT value its bandwidthefficiency is less than filtered QPSK.


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Minimum Shift Keying (MSK)

l In MSK phase ramps up through 90 degrees for a binary one, and down90 degrees for a binary zero.

l For GMSK transmission, a Gaussian pre-modulation baseband filter isused to suppress the high frequency components in the data. The degreeof out-of-band suppression is controlled by the BT product.


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GMSK Signals

l In MSK , the BT is infinityand this allows the square bittransients to directly modulatethe VCO.l In GMSK, low values of BT

create significant intersymbolinterference (ISI). In thediagram, the portion of thesymbol energy a acts as ISIfor adjacent symbols.

l If BT is less than 0.3, someform of combating the ISI isrequired.


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Espectro GMSK

lGMSK has a main lobe 1.5 times that of QPSK.

lGMSK generally achieves a bandwidth efficiency lessthan 0.7 bits per second per Hz (QPSK can be as high as1.6 bits per second per Hz).


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Multi-level (M-ary) Phase and Amplitude Modulation

lAmplitude and phase shift keying can be combined to transmit severalbits per symbol (in this case M=4). These modulation schemes are often

refered to as linear, as they require linear amplification.l16QAM has the largest distance between points, but requires very linearamplification. 16PSK has less stringent linearity requirements, but hasless spacing between constellation points, and is therefore more affectedby noise. lM-ary schemes are more bandwidth efficient, but moresusceptible to noise.


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Shannon-Hartley Capacity Theorem

For error free communication, it is possible to define the

capacitywhich can be supported in an additive white gaussiannoise (AWGN)channel.fb/W = log2(1 + Eb fb /hW)

where fb = Capacity (bits per second)W = bandwidth of the modulating baseband signal (Hz)Eb = energy per bith = noise power density (watts/Hz)thus Ebfb = total signal power

. hW = total noise powerfb/W = bandwidth efficiency (bits per second per Hz)


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Comparison of Modulation Schemes

This graph shows thatbandwidth efficiency istraded off against powerefficiency.l MFSK is power efficient,but not bandwidth efficient.l MPSK and QAM arebandwidth efficient but notpower efficient.l Mobile radio systems arebandwidth limited,

therefore PSK is moresuited.


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Comparison of Modulation types


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Coherent Reception

An estimate of the channel phase and attenuation is

recovered. It is then possible to reproduce thetransmitted signal, and demodulate. It is necessary tohave an accurate version of the carrier, otherwise errorsare introduced.Carrier recovery methods include:

l Pilot Tone (such as Transparent Tone in Band)Less power in information bearing signal High peak-to-mean power ratiol Pilot Symbol Assisted Modulation Less power in information bearing signal

l Carrier Recovery (such as Costas loop) The carrier is recovered from the information signal


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Differential Reception

In the transmitter, each symbol is modulatedrelative to the previous symbol, for example indifferential BPSK: 0 = no change 1 = +180ol In the receiver, the current symbol is demodulatedusing the previous symbol as a reference. Theprevious symbol acts as an estimate of the

channel.l Differential reception is theoretical 3dB poorerthan coherent. This is because the differentialsystem has two sources of error: a corrupted

symbol, and a corrupted reference (the previoussymbol). l Non-coherent reception is often easier toimplement.


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Modulation Summary

Phase Shift Keying is often used, as it provides a highlybandwidth efficient modulation scheme.QPSK, modulation is very robust, but requires some formof linear amplification. OQPSK and p/4-QPSK can beimplemented, and reduce the envelope variations of thesignal.High level M-ary schemes (such as 64-QAM) are very

bandwidthefficient, but more susceptible to noise andrequire linear amplification.Constant envelope schemes (such as GMSK) can beemployed since an efficient, non-linear amplifier can beused.

Coherent reception provides better performance thandifferential, but requires a more complex receiver.


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ENG SYSTEMSUSING COFDM

TECHNOLOGY


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COFDM BASICSMulti-Carrier modulation scheme

200 to 8000 carriers.Most Popular - DVB-T (European)

Standard - 2048 carriers Tandberg / Nucomm / Link-MRC


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COFDM BASICS

Each Carrier individually modulated usingQPSK, 16QAM or 64QAM QPSK - Most RobustRecommended for best results

Amplifiers only 2dB down

16QAMAmplifiers 3dB down 64QAM - Highest Data Rates

Less resilient to multipathAmplifiers 4 to 5dB down


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Benefits of COFDM overtraditional FM

Performs very well in a multipathenvironment Superior reception from urban sites notaccessible using FM

Bounce RF signal off buildings Superior reception from movingHelicopters orENG Vans. Superior reception from camera mountedminitransmitters using OMNI antennas


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Benefits of COFDM over traditional FM.. Continued

COFDM modulation occupies only 6 8 MHz of RFchannel bandwidth

Accommodate pending FCC regulatory changes(phase 1 and 2)

Capable of transporting multiple multiplexed MPEG-2compressed video signals in a single channel

Capable of dual carrier operation in a single channel -2 x 6MHz = 12MHz


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Disadvantages

Still relatively expensive COFDM Mod/MPEG-2 Encoder - $30K COFDM Demod/MPEG-2 Decoder - $6K

RF Output Power Amplifiers must bebacked off by 2 to 5dB, depending on modulation type

chosen.


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COFDM SYSTEM DESIGN CONSIDERATIONS

ENG Vans Cost - All vehicles or single vehicle Transmitters must be replaced - if not digital ready

Low phase noise oscillatorsHeterodyne - Dual Conversion upconverter

Amplifiers must be biased for linear operation Add COFDM modulator and MPEG-2 encoder (single box) Add second antenna - Omni (optional)


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COFDM SYSTEM DESIGNCONSIDERATIONS

Receive Sites Existing 2 GHz antenna systems are OK 7Ghz antenna systems - must upgrade LNA/BlockDown-Converter

Central Receiver must be capable of passing digital -

(Nucomm CR4s can be upgraded)Low phase noise oscillators Add COFDM demodulator and MPEG-2decoder (single box)


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1.0) Introduction:In a gradual process starting late this year, volunteer TV stations will starttransmitting digital television signals direct to homes; the processculminates in the year 2006 when all present analog AM frequency

allocations will be revoked (although under certain circumstances stationsmay be allowed an extension to their license). For those stations thatpresently use microwave equipment for distribution and contribution ofvideo and audio signals, the conversion process will also includeupgrading their analog microwave links to digital transmission.This paper has been written to provide an objective summary of the stateof digital microwave link technology. Section 2 provides a brief overviewof the main components of a digital microwave link; section 3concentrates on the task of engineering a digital link to provide reliabletransmission. Section 4 lists 7 steps to choosing the right equipment foryour link. Finally the appendices cover in some detail the topics of digitalmodulation schemes, error correction and adaptive equalization.This paper is intended as a guide to new and emerging technology. RFTechnology strongly recommends that, when engineering any microwavelink, the greatest care be used to comply with all federal regulations. Wewould be more than happy to discuss how this papers contents relate toyour specific circumstances.


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The Digital Future:It is true to say that the future of almost all communications mediums is inthe transmission of digits. In general, digital transmission offers the enduser better quality and the provider more capacity for a given transmissionresource. But perhaps the greatest long term advantage of digitaltransmission is that any type of information can be encoded as 1s and0s; the network neither knows or cares what the 1s or 0s represent, just

that they be transmitted fast and error free. Still pictures, movingpictures, program audio, telephone conversations, data files, computerprograms, email, web pages, faxes and many other things can all betransmitted and received.For the transmission of video signals in particular, the digital future holdstwo major advantages over the analog present. Most importantly, a digitallink allows the transmission of high definition video in the same or less

bandwidth than the corresponding analog video link. Secondly, a digitallink has the capability to transmit multiple channels of standard or highdefinition video in the same bandwidth as an analog link.


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The Transition to Digital:Many factors have pushed the terrestrial microwave market toward adigital future; these include major advances in the availability of digitalvideo compression - primarily compliant to the MPEG II standard; andthe pressure, especially in the largest cities, to transmit more video signalsin an ever decreasing bandwidth. However, the overwhelming factor isthe need to comply with government legislation to transmit at least19.39MBit/s worth of digital TV to the consumer, over the air.Eventually the vast majority of microwave links will be converted from

analog to digital transmission; however for many stations the first stepwill be to install a digital Studio-Transmitter-Link to feed a digital signalto their ATSC transmitter. There are a number of forms that this digitalSTL could take. Many stations will co-locate their new ATSC transmitterwith their existing NTSC transmitter - enabling them to upgrade theirexisting analog STL to feed both transmitters! Out of choice or necessity,other stations will locate their ATSC transmitter at a different site from

their existing NTSC transmitter and will only transmit an ATSC signal ontheir digital microwave link. Finally, some stations will rely on multiplehops of microwave to feed their transmitters.


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The Components of a DigitalVideo Microwave Link:

Multiple Channel Digital

Microwave Link


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Figure 1 shows the components whichmake up a digital microwave link.There are five components, not all of whichmay be required: MPEG IIvideo compression encoder; multiplexer;modulator; microwave

transmitter; waveguide / feeder andantenna. The equipment used toreceive, demodulate, demultiplex anddecode the video is almost exactlythe mirror of the transmission endequipment. Hence it has not beenincluded in the following passages.


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Modulator:The modulator takes as its input a baseband digital bit stream and uses itto modulate either the frequency, the phase or a combination of the phase

and the amplitude of a carrier. Typically, the carrier is at an IF(intermediate frequency) of 70 or 140MHz. Two increasingly visibledigital modulation techniques are 8-VSB (HDTV broadcast scheme) and64QAM (digital cable scheme).There are four types of modulation scheme that are most commonly usedfor digital microwave links; these are FSK, QPSK, 8PSK and QAM. Theschemes, together with the advantages and disadvantages associated with

each, are described in more detail in appendix A of this paper.The table on the next page lists the bandwidth efficiency -m for each ofthe common modulation schemes. This is a very important figure of merit;it is a measure of how much data can be transmitted in a given bandwidth.In general, the higher the bandwidth efficiency of the scheme, the moresusceptible it is to noise, interference and multipath (and therefore to lossof signal). The table also shows, for data streams incorporating varyinglevels of error correction coding, how much data can be transmitted in a10 or 15MHz channel at 2GHz and in a 25MHz channel at 7 or 13GHz.


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There is a strong case for choosing amodulator which has a variable datarate interface. This will allow you totransmit only the data that isrequired, and to avoid the need to stuffthe data stream with emptyframes in order to achieve a fixed data rate

standard.The modulator and demodulator might alsohave several countermeasuresto combat adverse channel conditions.These include error correctioncoding and adaptive equalizers, which aredescribed in appendices B & C

respectively.


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Microwave Transmitter:The heterodyne microwave transmitter will accept the output of amodulator (usually at an IF of 70 or 140MHz); upconvert the signal to thefinal RF output frequency and then amplify and filter the digital signal.

This type of equipment has seen wide exposure in the broadcast industrywhen used as an IF repeater in analog multi-hop systems. The keyattribute that a heterodyne digital microwave transmitter has, that ananalog IF repeater will not have, is a linear Power Amplifier.Specifically, the transmission of PSK and QAM digital signal requiresreasonably linear amplification; amplitude variations in the signal need tobe passed through the RF signal chain without distortion. Analog FM and

Digital FSK signals contain no amplitude modulation component and canbe passed through saturated amplifiers.Digital PSK signals contain transient amplitude variations, which ifsurpressed in a saturated amplifier, will cause in-channel and adjacentchannel distortion. This distortion will degrade the performance of thelink and interfere with other users of the spectrum. Typically an amplifierbacked off from its saturation point by 3dB (i.e. to half power) will pass

amplitude components well enough to be suitable for PSK signals. Asalways, the greatest care must be taken to ensure that the modulator /transmitter combination does not produce out of channel energy.


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Digital QAM signals rely on amplitude andphase variations to transmitdata; if a QAM signal was passed througha saturated amplifier much ofthe data would be lost and theperformance of the link would beunacceptable. Typically an amplifier

backed off 6dB (i.e. to quarterpower) will pass amplitude componentswell enough to be suitable forQAM signals. As always, the greatest caremust be taken to ensure thatthe modulator / transmitter combinationdoes not produce out of channel

energy.


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Waveguide/Feeder and

Antennas:The hardware used to support analog microwave signals is just as capableof supporting digital microwave signals.As the signal is passed between radio and antenna feed, a fraction of thesignal energy will be reflected. The reflected signal interferes with themain, wanted signal and causes degradation in the performance of thelink. These reflections have a far greater affect on digital links than they

do on analog links. Reflections will occur at every transition in the signalpath and they will also occur in the transmission line if there aresignificant changes in the impedance of the line along its length.The key to avoiding reflections is to ensure that the antenna andtransmission line equipment is aligned and tuned. Typically, the returnloss into the transmission line at the radio flange should be at least 26dB.If the reflections continue to significantly degrade the performance of the

link, it may be time to consider swapping existing antennas for new, lowreflection (VSWR) models and/or existing line for lower loss types.


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Components of a Digital VideoMulti-hop system:

The equipment configuration required atthe repeater site of a digitalmulti-hop link is different from that used forthe analog equivalent. In theanalog system, IF repetition is used toavoid the need to demodulate andremodulate the FM signal.

As figure 2 shows, the best configurationfor a digital system is to receiveand demodulate the signal to a basebandbit stream before remodulatingand transmitting. This process allows errorcorrection and adaptiveequalization (if used) to be carried out after

each hop; it also preventsphase and amplitude inaccuracies fromaccumulating over several hops.


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Digital Microwave Repeater Configuration


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Engineering A Digital LinkThe keys to engineering an analog link are to ensure adecent flat or thermal fade margin (i.e. a good received

signal strength) and to ensure that antenna heights aresufficient to maintain line-of-sight between receive andtransmit antennas.These two factors are just as important in engineering adigital link, but there is also a third phenomenon that has

to be taken into consideration and accounted for -multipath, i.e. reflected versions of the transmitted signalinterfering with the main, line-of-sight signal.Whilst multipath does affect analog microwave links(typically causing the temporary loss of the color

subcarrier), it can be catastrophic in its effect on digitalmicrowave links, causing loss of the entire signal.


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Flat Fading:A flat fade is just another way of describing a fade (orreduction in input signal level) where all frequencies in the

channel of interest are equally affected. Flat fades areusually caused by temperature/pressure variationsin the atmosphere. These variations cause the signal tobend away from the receive antenna, and only a fraction ofthe signal power to be received.

Additionally, if precipitation occurs anywhere along thesignal path significant attenuation can result. The effect ofrain attenuation is negligible at 2 and 7GHz, but can causeproblems at 13GHz and is the prevalent cause of flat fadingat 18 and 23GHz.


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The flat fade margin is the difference between the receivedpower level when the link is operating under idealconditions and the threshold power level, below which theperformance of the link is unacceptable. Fixed microwavelinks should be configured to provide a flat fade margin ofapproximately 40dB (although this figure varies dependingon the local climate). This ideal fade margin is the same foranalog and digital microwave links.

The key to ensuring that a good flat fade margin ismaintained when an analog microwave link is upgraded fordigital transmission is to remember two things. Firstly,when transmitting with most digital modulation schemes,the transmitter has to be linearized and will produce less

power.Secondly, the receiver threshold for acceptable picturequality from a digital link may well be above that of ananalog link.


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Digital transmission using FSK doesnt require any back-offof the transmitter, and depending on bandwidth efficiency ofthe FSK modem, the threshold for an FSK system is very

similar to that of an analog FM system. These two thingscombined mean that on converting an analog FM link to adigital FSK link, there should be no significant difference inflat fade margin.Digital transmission using QPSK typically requires a 3dB

back-off of the transmitter; depending on the amount of errorcorrection used, the threshold for a QPSK system is typicallyequal to an analog FM system.So, on converting an analog FM link to a digital QPSK link,there will be around a 3dB reduction in fade margin. As an

illustration, for a 7GHz system, an analog FM link with two6ft. antennas would have the same flat fade margin as aQPSK digital link with one 8ft. antenna and one 6ft. antenna.


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Digital transmission using 16QAM typically requires a 6dBback-off of the transmitter; depending on the amount oferror correction used, the threshold for a 16QAM systemis typically 6dB worse than an analog FM system. So, onconverting an analog FM link to a digital 16QAM link,there will be around a 12dB reduction in fade margin.Using the same illustration at 7GHz, an analog FM linkwith 6ft. antennas has the same flat fade margin as a16QAM digital link with 12ft. antennas!It is important not to assume that because your analoglink had a good fade margin on a certain path, a digitallink will perform just as well. It is extremely worthwhilespending some time calculating the flat fade margin

of your new digital link.


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If necessary, there are several ways in which theflat fade margin of a link can be improved, including usinglarger antennas, a higher power microwave transmitter,

lower loss feed line and splitting a longer path into twoshorter hops.

Although the threshold levels for digital transmission maybe worse than the equivalent analog links, theperformance of the digital link just above threshold is far

superior. Figure 3, an idealized version of the relationshipbetween the video signal-to-noise ratio and the RFreceived signal level under flat fade conditions is shownon the following page for analog and digital modulationschemes.


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Threshold performance

Digital Vs. Analog


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Frequency SelectiveFading or Multipath:

Digital PSK and QAM links are far more susceptible tofrequency selective fading or multipath than analog FMor FSK links. Frequency selective fades, as their namesuggests, do not affect each frequency in thechannel equally. These fades occur when there are

multiple paths along which the microwave signal cantravel between transmitter and receiver.


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Causes of multipath in fixed microwave links include (butare not limited to) the following phenomena. Firstly, signals

reflected from the terrain between the transmit and receiveantennas - very likely if the hop is over a body of water ormarshy ground. Secondly, signals reflected intemperature/pressure layers in the atmosphere - likely tooccur if the local weather conditions often include rapid

temperature and humidity changes, (i.e. at dawn and duskin the Southern and South Eastern United States).An explanation of how these reflected signals causemultipath is contained in Appendix C. Several steps, listedon the following page, can be taken to minimize the effects

of multipath.


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The most effective way to improve the ability of a link toendurefrequency selective fades, is to ensure that the link has agood flat fademargin! Employing space diversity or adaptive equalizationwont helpthe performance of a link if the flat fade margin is so narrowthat even a

small amount of frequency selective fading will cause lossof signal.If there is any flexibility in the choice of the geographicallocation of thehop, avoid transmitting over large bodies of water and

marshy areas,which more readily reflect signals than regular or hillyterrain.


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One of the most successful methods of combating multipath is toemploy vertical space diversity at the receive end of the link. This

can be achieved by placing a second antenna 10 to 20ft. belowthe main receive antenna, and adding an extra receiver anddemodulator. Furthermore trials have proven that the performanceof a diversity system can be optimized if the main and diversityantennas are of different size.Selection between the two demodulated bit streams is typically

carried out on the basis of the bit error rate of each. Employingdiversity on a link with a good flat fade margin will yield asignificant improvement in system performance.


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Together with space diversity, adaptive equalization has proved to beone of the most successful methods of combating multipath. Adaptiveequalization is included in both the ATSC standards for direct-to-hometransmission and in digital cable systems, as well as being a key part

of TDMA cell phone systems. The theory behind adaptive equalizationis explained in some detail in appendix C. Well designed andimplemented equalizers will typically compensate for many of themultipath effects that are encountered in a point-to-point microwavesystem.The longer the path, the greater the statistical probability is that there

will be a reflection point which will cause multipath. Always choose theshortest path available; if necessary consider splitting a longer pathinto two shorter hops.


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7 Steps to Engineering a DigitalLink:1) Decide how many video channels, and of what kind need to betransmitted.

2) Assign 8MBit/s for every 4:2:0 encoded NTSC channel,20MBit/s for every 4:2:2 encoded NTSC channel and19.39MBit/s for every ATSC high definition channel. Add all ofthe data rates together to find the total bit rate to be transmitted.3) Note the channel bandwidth that is available for the link.4) Consult Table 1; starting from the top of the table, pick the first

modulation scheme / error correction combination whichaccommodates your desired data rate. This will be the optimalchoice and will utilize as much of the available bandwidth aspossible.


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5) By referring to section 3.1, calculate the transmitterback-off and the reduced threshold (compared to ananalog link) of the modulation scheme chosen in step 4.

6) Choose antenna sizes and feeder/waveguide type toprovide sufficient flat fade margin (at least 40dB). ContactRF Technology if help is required in carrying out thesecalculations.7) Decide whether your link will be prone to multipatheffects (see section 3.2 for a list of the major causes of

multipath). If there is a chance that your path will beaffected, seriously consider some of the remedies alsolisted in section 3.2.


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An example using the 7 Steps:

1) A broadcaster has decided to transmit an ATSC signal, and isfortunate enough to be able to co-locate his ATSC antenna on thesame tower as his NTSC antenna. However, he only has a singlemicrowave frequency allocation to his tower site. He needs totransmit his existing NTSC channel as well as his ATSC channelon the microwave link. The link is a single hop and fordistribution so only 4:2:0 encoding is required.

2) The channel of 4:2:0 NTSC requires 8MBit/s of data to betransmitted, the channel of ATSC requires 19.4MBit/s of data.The broadcasters total requirement is 27.4MBit/s, i.e. 28MBit/s.

3) The frequency allocation the broadcaster has is in the 7GHz band,

hence the bandwidth available is 25MHz.


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4) Consulting Table 1, the 28MBit/s data stream will beaccommodated by QPSK with 5/6 viterbi coding (forward errorcorrection).

5) The back off in the transmitter required for QPSK signals is 3dB.The flat fade threshold level of a digital QPSK link is very similarto that of an analog FM link i.e. -86dBm.


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6) The saturated output power of the transmitter is 2W, the backedoff output power is therefore 1W. The length of the hop is 20miles, and the antennas have to be placed at the 500ft. level on thetowers at either end of the link. The link will use EW63waveguide. The broadcaster also needs to take into account 2dBof miscellaneous loss at either end of the link and a 3dB fieldmargin.Using the above information, the broadcaster calculates that heneeds 8ft. antennas at either end of the link.

7) The terrain of the path and local weather conditions dont indicatethat there will be excessive multipath. If the broadcaster decidesto take extra precaution by installing space diversity at the receiveend of the link, an ideal solution would be to install a 6ft. antenna15ft. below the main antenna, run a second length of waveguide

down the tower and install a second receiver, demodulator and adiversity switch.


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Cap 12 MW Digital Modulation