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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
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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
Prepared By Ms.Sreenu.G, Department Of Computer Science, RASET
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.
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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
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Powersplitter
Frequency A
BPFA
combiner
Freque
ncyB
BPFB
RFout
If
transmitter BPF
Channelcombiner
IFin
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Prepared By Ms.Sreenu.G, Department Of Computer Science, RASET
Headendbridge
Transmitter
transmitterrepeater
controller
receiver
Ifswitch
Receiver
repeaterIfin
Ifout
Working channel
Spare channel
RF
RFRF
DIVERSITY
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MICROWAVE TERMINAL STATION
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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
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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.
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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)
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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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