Course Material for Module 2 Mcs

8/3/2019 Course Material for Module 2 Mcs

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T 703 Modern Communication Systems

MODULE 2MICROWAVE COMMUNICATION

What Are Microwaves

Microwave frequencies range from 300 MHz to 30 GHz, corresponding to wavelengthsof 1 meter to 1 cm. These frequencies are useful for terrestrial and satellitecommunication systems, both fixed and mobile. In the case of point-to-point radio links,antennas are placed on a tower or other tall structure at sufficient height to provide adirect, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites.In the case of mobile radio systems, a single tower provides point-to-multipoint coverage,which may include both LOS and non-LOS paths. LOS microwave is used for bothshort- and long-haul telecommunications to complement wired media such as opticaltransmission systems. Applications include local loop, cellular back haul, remote andrugged areas, utility companies, and private carriers. Early applications of LOSmicrowave were based on analog modulation techniques, but todays microwave systemsused digital modulation for increased capacity and performance.

Standards

In the United States, radio channel assignments are controlled by the FederalCommunications Commission (FCC) for commercial carriers and by the NationalTelecommunications and Information Administration (NTIA) for government systems.

The FCC's regulations for use of spectrum establish eligibility rules, permissible userules, and technical specifications. FCC regulatory specifications are intended to protectagainst interference and to promote spectral efficiency. Equipment type acceptanceregulations include transmitter power limits, frequency stability, out-of-channel emissionlimits, and antenna directivity.

The International Telecommunications Union Radio Committee (ITU-R) issuesrecommendations on radio channel assignments for use by national frequency allocationagencies. Although the ITU-R itself has no regulatory power, it is important to realizethat ITU-R recommendations are usually adopted on a worldwide basis.

Historical Milestones

1950s Analog Microwave Radio

Prepared By Ms.Sreenu.G, Department Of Computer Science, RASET


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Used FDM/FM in 4, 6, and 11 GHz bands for long-haul

Introduced into telephone networks by Bell System

1970s Digital Microwave Radio

Replaced analog microwaves

Became bandwidth efficient with introduction of advanced modulation techniques(QAM and TCM)

Adaptive equalization and diversity became necessary for high data rates

1990s and 2000s

Digital microwave used for cellular back-haul

Change in MMDS and ITFS spectrum to allow wireless cable and point-to-multipoint broadcasting

IEEE 802.16 standard or WiMax introduces new application for microwave radio

Wireless local and metro area networks capitalize on benefits of microwave radio

Principles and Operation

Microwave Link Structure. The basic components required for operating a radio linkare the transmitter, towers, antennas, and receiver. Transmitter functions typically includemultiplexing, encoding, modulation, up-conversion from base band or intermediatefrequency (IF) to radio frequency (RF), power amplification, and filtering for spectrumcontrol. Receiver functions include RF filtering, down-conversion from RF to IF,amplification at IF, equalization, demodulation, decoding, and demultiplexing. Toachieve point-to-point radio links, antennas are placed on a tower or other tall structure atsufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the

transmitter and receiver sites.

Microwave System Design. The design of microwave radio systems involvesengineering of the path to evaluate the effects of propagation on performance,development of a frequency allocation plan, and proper selection of radio and linkcomponents. This design process must ensure that outage requirements are met on a perlink and system basis. The frequency allocation plan is based on four elements: the local



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frequency regulatory authority requirements, selected radio transmitter and receivercharacteristics, antenna characteristics, and potential intrasystem and intersystem RFinterference.

Microwave Propagation Characteristics. Various phenomena associated with

propagation, such as multipath fading and interference, affect microwave radioperformance. The modes of propagation between two radio antennas may include adirect, line-of-sight (LOS) path but also a ground or surface wave that parallels the earth'ssurface, a sky wave from signal components reflected off the troposphere or ionosphere,a ground reflected path, and a path diffracted from an obstacle in the terrain. Thepresence and utility of these modes depend on the link geometry, both distance andterrain between the two antennas, and the operating frequency. For frequencies in themicrowave (~2 30 GHz) band, the LOS propagation mode is the predominant modeavailable for use; the other modes may cause interference with the stronger LOS path.Line-of-sight links are limited in distance by the curvature of the earth, obstacles alongthe path, and free-space loss. Average distances for conservatively designed LOS links

are 25 to 30 mi, although distances up to 100 mi have been used. For frequencies below 2GHz, the typical mode of propagation includes non-line-of-sight (NLOS) paths, whererefraction, diffraction, and reflection may extend communications coverage beyond LOSdistances. The performance of both LOS and NLOS paths is affected by severalphenomena, including free-space loss, terrain, atmosphere, and precipitation.

Strengths and Weaknesses

Strengths

Adapts to difficult terrain Loss versus distance (D) = Log D (not linear) Flexible channelization Relatively short installation time Can be transportable Cost usually less than cable No back-hoe fading

Weaknesses

Paths could be blocked by buildings Spectral congestion Interception possible Possible regulatory delays Sites could be difficult to maintain Towers need periodic maintenance Atmospheric fading



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Business Implications and Applications

The tremendous growth in wireless services is made possible today through the use of

microwaves for backhaul in wireless and mobile networks and for point-to-multipoint networks. Towers can be used for both mobile, e.g. cellular, and point-to-point applications, enhancing the potential for microwave as wireless systems grow.Increases in spectrum allocations and advances in spectrum efficiency throughtechnology have created business opportunities in the field of microwave radio.Telecommunications carriers, utility companies, and private carriers all use microwave tocomplement wired and optical networks.

Advantages of microwave communication

The many advantages of microwave fiber-optic communication links over conventional

coaxial or waveguide links are well known. They include reduced size, weight and cost,

low and constant attenuation over the entire modulation frequency range, imperviousness

to electromagnetic interference, extremely wide bandwidth, low dispersion, and high

information transfer capacity. These advantages mean that they are currently viable

contenders for a number of applications of commercial importance including:

Personal Communications Networks,

Where micro cells in a wide area are connected by optical fibers and use radioextension links to the home allowing increased bandwidth (and the possibility of

mobile broadband systems) and high spectrum reuse efficiency without the

significant expense of installing fiber to the home;

Millimeter-wave radio LANs,

With the radio nodes (possibly picocells) are fed by optical fiber with similar

advantages to the above;

Antenna Remoting

At satellite earth stations where optical fiber transmission allows the

concentration of the remote equipment at a central location and the quick re-routing of traffic to different antennas via centralized switching;

Broadband video distribution networks,

Using sub carrier multiplexing and exploiting the advantages of the broad

bandwidth and low loss of optical fiber;

Signal distribution for phased array antennas,



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where the use of monolithic microwave integrated circuits (MMIC) has reduced

the cost of active elements so that the main cost and weight is in the signal

distribution.

The advantages have long been recognized, however, except in some high-bandwidthpoint-to-point military links, their practical exploitation has been limited due to the poor

microwave performance of practical microwave optical sources. Many of these

limitations have now been overcome, with, for example, the use of the optical phase-

locked loop, and so we are therefore likely to see increased use of radio-on-fiber systems,

particularly at higher frequencies in the future. This area of research is becoming, if

anything, more important than ever. Many companies looking towards developing

27GHz broadband cellular network technologies in the near term and there is an

increased interest in the possibilities of using the 60 GHz and 70GHz bands, with their

excellent frequency reuse, for broadband Pico-cellular operations.

Recently, research in this area has blossomed with studies looking at many issues

including basic optical-microwave interaction (e.g., radio frequency signal generation),

photonics devices operating at microwave frequencies, photonic control of microwave

devices, high frequency transmission links, and the use of photonics to implement various

functions in microwave systems. Novel applications where that have been investigated

include spectrum analysis, frequency conversion, high performance oscillators, and

analogue-to-digital conversion. Continued progress in photonic components andtechnology sustains great interest in this field and expanding acceptance of photonics for

microwave systems. With these new applications there are increasing requirements for

high performance devices for microwave and mm-wave systems.

In our work we are seeking to develop new applications of fiber Bragg gratings to

microwave and millimeter-wave signal processing. We have successfully established that

complex and flexible high performance photonic signal processing elements can be

fabricated using fiber Bragg gratings. We have also identified problems that need further

study. We are investigating several new paths of research that may overcome some ofthese problems and enable us to achieve more practical and higher performance devices.

Analog vs. Digital Microwave



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Analog Microwave

AM Properties : ----- Suppress; run Maintain for digitalamp in compression

Signal-to-Noise : --- Declining SNR above threshold

Burst Errors : --- Non-catastrophic transients

Crosstalk I : --- Intermodulation problem

Effects of decreasing bandwidth: --- Adjacent channel interference

Performance : --- Noise, loss of audio, just above loss of colorhreshold

Performance --- Radio squelchesat or belowThreshold

Digital Microwave

AM Properties : --- Maintain for digitalmodulation; runamp in linear mode

Signal-to-Noise : --- Stable SNR above threshold

.Burst Errors. Loss of Sync;

: --- loss of new frames

Crosstalk : --- No intermodulation problem



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Effects of decreasing bandwidth : --- Increased modemComplexity, morenoise & interferenceProblems

Performance : --- Perfectjust above Threshold

.

Performance : --- Catastrophic; signalat or below off digital cliffThreshold

Frequency modulated microwave radio system

FM microwave systems used with the appropriate multiplexing equipment are capable ofsimultaneously carrying from a few narrowband voice circuits up to thousands of voiceand data circuits. Microwave radios can also be configured to carry high speed data,facsimile, broadcast quality audio and commercial television signal. A simplified block

diagram of FM microwave radio is shown in figure. The base band is the compositesignal that modulates the FM carrier and may comprise one or more of the following:

1. Frequency division multiplexed voice band channels

2. Time division multiplexed voice band channels

3. Broadcast quality composite video or picture phone

4. Wideband data

Simplified block diagram of a microwave radio: a) transmitter b) receiver

A.



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Microwave transmitter

B.

Microwave Receiver


Preemphasisn/w n/w Modulato

r

IFamplifiers andband passfilters

Mixer/convertor

RFpoweramplifier andbandpassfilter

Basebandi/p

Transmitantenna

Transmissionline

Microwavegenerator

basebandsectionmodulator and if section Mixer&upconvertor section

Deemphasisn/w

Modulator

IFamplifiers andband passfilters

Mixer/convertor

RFpoweramplifier andbandpassfilter

Basebando/p

Receiveantenna

Transmissionline

Microwavegenerator

BasebandsectionModulator and if section

Mixer&down

convertor section

RF section

RF section


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FM microwave Radio repeaters

With systems that are longer than 40 miles or when geographical obstructions, such asmountain, block the transmission path, repeaters are needed.

A microwave repeater is a receiver and transmitter placed back to back or in tandem withthe system. The location of intermediate repeater sites is greatly influenced by the natureof the terrain between and surrounding the sites. In relatively flat terrain, increasing pathlength will dictate increasing the antenna tower heights. The exact distance is determined

primarily by line of sight path clearance and received signal strength.


MicrowaveTransmitter

Receiver Transmitter

Microwave Repeater

MicrowaveReceiver


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Basically there are three type of repeaters IF, Base band and RF repeaters. The receivedRF carrier is down converted to an IF frequency , then amplified , reshaped , and upconverted to RF frequency and then retransmitted. Since the signal never demodulatedbelow IF, base band intelligence is un modified by repeater.

There could be two types of base band repeaters. In the first type the received RF carrieris down converted to an IF frequency , amplified , filtered, and then further modulated to



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base band. The BB signal is typically FDM voice band channels which is furtherdemodulated to a master group, super group, group or even channel level. This allowsthe BB signal to be reconfigured to meet the routing needs of over all communicationnetwork. Once BB signal is reconfigured , it frequency modulate an IF carrier and thenretransmitted.

DIVERSITY

Microwave systems use line of sight transmission.,therefore a direct signal mpath mustexist between the transmit and receive antennas.If that signal path undergoes a severedegradation a service interruption will occur.

Diversity suggests that more than one transmission path or method of transmissionavailable between a transmitter and a receiver.The purpose of diversity is increase thereliability of the system.

Frequency diversity and space diversity are the two types of diversities.

Frequency diversity is simply modulating two different RF carrier frequencies with thesame IF intelligence,than transmitting both RF signals to a given destination.At thedestination both carriers are demodulated,and the one that yields better quality IF signalis selected.

With space diversity the o/p of a transmitter is fed to two or more antennas that arephysically separated by an appreciable number of wavelengths.Similarly at the receivingend ,there may be more than one antenna providing the i/p signal to the receiver.

PROTECTION SWITCHING ARRANGEMENTS

Two types of protection switching arrangements are there.Hot Standby

Diversity

With hotstandby ,each working radio channel has a dedicated backup or sparechannel.With diversity protection a single backup channel is made available to as manyas 11 working channels.

Frequency diversity transmitter



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Space diversity transmitter

Hotstand By


Powersplitter

Frequency A

BPFA

combiner

Freque

ncyB

BPFB

RFout

If

transmitter BPF

Channelcombiner

IFin


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Headendbridge

Transmitter

transmitterrepeater

controller

receiver

Ifswitch

Receiver

repeaterIfin

Ifout

Working channel

Spare channel

RF

RFRF

DIVERSITY


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MICROWAVE TERMINAL STATION


Transmitter

transmitter

receiver

Receiver

Ifin

Ifout

transceiver

Ifswitch

transmitter

transceiver

Ifswitch

Qualitycontroller

Ifswitch

Ifinch22

receiver

Qualitydetector

Channel1

Spare channel

Channel 2

Auxiliarychannel

Qualitydetector

Qualitydetector


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Terminal station consists of four major sections: the base band, wireline entrance link(WLEL), FM-IF, and RF sectionsWLEL: Often in large communication networks the building that houses the radio stationis quite large. Consequently it is desirable that similar equipment be physically placed ata common location. Dissimilar equipment may be separated by a considerable distance. AWLEL serves as the interface between the multiplex terminal equipment and the FM-IF


Protectionswitch

IFamp

Compressionamp

transmod

Poweramp

Iso;atorCombine n/w

Microwavegenerator

RFout

Terminal station transmitter

Protectionswitch

If ampand AGC

Receivemod

BPFChannelcombiningn/w

Microwavegenerator

RFin

IF

Terminal station Receiver

IF

Upconverter RF

IF


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equipment. A WLEL generally consists of an amplifier and an equalizer and levelshaping devices commonly called pre-and deemphasize networks.

IF section:The FM terminal equipment generates a frequency modulated IF carrier. This is

accomplished by mixing the output of two deviated oscillators that differ in frequency bythe desired IF carrier. These oscillators are deviated in phase opposition, which reducesthe magnitude of phase deviation required of a single deviator by a factor of 2.

RF Section

The IF and compression amplifiers help keep the IF signal power constant and atapproximately the required i/p level to the transmit modulator. A transmod is a balancedmodulator that, when used in conjunction with a microwave generator power amplifier

and band pass filter up-converts the if carrier to an RF carrier and amplifies the RF to thedesired o/p power.

The RF receiver is essentially the same as the transmitter except that it works in theopposite direction. One difference is the presence of an IF amplifier in the receiver. Thishas an automatic gain control circuit. There are no RF amplifiers in the receiver.Typicalya highly sensitive low noise balanced demodulator is used for the receive demodulator.

High/ low microwave system



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A high/low microwave repeater stationneeds two microwave carrier upplies for the downand up converting process. Rather than use two microwave generators a single generatorwith a shift oscillator ,a shift modulator and a bandpass filter can generate the tworequired signals.One o/p from the generator is fed directly to the transmod.and other ismixed with oscillator signal to produce a second microwave carrier frequency.

LINE OF SIGHT PATH CHATACTERISTICS

The free space path is the line of sight path directly between the transmit and receiveantennas .The ground reflected wave is the portion of the transmit signal that is reflectedoff Earths surface and captured by the receive antenna. The surface wave consists ofelectric and magnetic fields associated with the currents induced in Earths surface. Allpaths exist in any microwave radio system, but some are negligible in certain frequencyranges.The sky wave is the portion of the transmit signal that is returned back to earth ssurface by the ionized layers of earths atmosphere.


A B C D

F1 F2 F1 F2

High/low microwave system

Combining

n/w

BPF

RECEIVEMOD

Shiftmod

oscillator

ISOLATOR

BPFTRANSMOD

IFAMP

Microwavegenerator

Channeln/w


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FREE SPACE PATH LOSS

Free space path loss assumes ideal atmospheric conditions, so no electro magnetic energyis actually lost or dissipated-it merely spreads out as it propagates away from the source,resulting in lower relative power densities.(Spreading loss)

Spreading loss occurs simply because of the inverse square low .The mathematicalexpression for free space path loss is

Lp=(4D/ 2)

And because =c/f above equation can be written as

Lp= (4fD/c)2

Where Lp=free space path lossD=distancef=frequency =wavelengthC=velocity of light in free space.

Converting to dB yields

Lp (dB)=10 log(4fD/c)2

Or Lp (dB) =20 log (4fD/c)

Separating the constants from the variables gives

Lp=20 log (4/c20logf+20 log D

For frequencies in MHz and distances in kilometers,

Lp= [4 (106)(103)/3x108]+20 log f(MHz)+20 log D(km)

Or

Lp=32.4+20 log f(MHZ)+ 20 log D(km)

When the frequency is given in GHZ and the distance in km,

Lp=92.4+20 log f(GHZ)+ 20 log D(km)

When the frequency is given in GHZ and the distance in miles,



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Lp=926.6+20 log f(GHZ)+ 20 log D(miles)

PATH CLEARANCE AND ANTENNA HEIGHTS

The amount of clearance is generally described in terms of Fresnel zones. All points fromwhich a wave could be reflected with an additional path length of one half wavelengthsform an ellipse that defines the first Fresnel zone. Similarly the boundary of the nthFresnel zone consists of all points in which the propagation delay is n/2 wavelengths. Forany distance d1, from antenna A, the distance Hn from the line of sight path to theboundary of the nth Fresnel Zone is approximated by a parabola described as

Hn= (n (d1 (d-d1)/d

Where h=distance between direct path and parabola surrounding it

=wavelength

d=direct path length

d1=reflected path length

FADING

Fading is a general term applied to the reduction in signal strength at the input to areceiver.It applies to propagation variables in the physical radio path that affect changesin the path loss between transmit and receive antennas. The changes in the characteristicsof a radio path are associated with both atmospheric conditions and the geometry of the[path itself. Fading can occur under conditions of heavy ground fog .The result is asubstantial increase in path loss over a wide frequency band. The magnitude and rapidityof occurrence of slow, flat fading of this type can generally be reduced only by usinggreater antenna heights.

Median duration of fast fading



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Figure shows median duration of radio fades on 4-GHZ signal for various depths with anaverage repeater spacing of 30 miles. As shown in figure median duration of a 20-dBfade is about 30 seconds, and a median duration of a 40 dB fade is about 3 seconds. Atany given depth of fade is, the duration of 1% of the fades may be as much as 10 times oras little as one-tenth of the median duration.

System Gain

Gs=Pt-Cmin

Gs=System gain(dB)

Pt=transmitted o/p power

Cmin=minimum receiver i/p power for a given quality objective.

Pt-Cmin>losses-gains

Gains: Transmit antenna gain (At)

Receive antenna gain (Ar)

Losses: free space path loss ( Lp)

Feeder loss (Lf)

Total coupling or branching loss (Lb)


0 10 20 30 40 50

2

5

10

20

50

4GHZsignal


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Fade margin ( Fm)

Fade Margin (Fm) =30 log D+10 log (6ABf)-10 log(1-R)-70

D-distance (Kilometers)

F-frequency (gigahertz)

R-reliability expressed as a decimal

A=roughness factor

=4 over water or a very smooth terrain

=1 over an average terrain

=.25 over a very rough, mountainous terrain

B=factor to convert a worst month probability to an annual probability.

=1 to to convert an annual availability to a worst month basis

=.5 for hot humid areas

=.25 for average inland areas=.125 for very dry or mountainous areas

NOISE FACTOR

F=I/P SIGNAL TO NOISE RATIO

O/P SIGNAL TO NOISE RATIONOISE FIGURE

NF=10 log F

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Course Material for Module 2 Mcs