Microwave Net

8/10/2019 Microwave Net

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icrowave Network esignPresented by

Eng / Yahia Ahmed


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AGENDA

Introduction to Microwave Network DesignTypes of Microwave Transmissions.Microwave Transmission Frequency Bands.

Performance and availability objectivesDesign aspects and main concepts

Design Parameters Antenna Theory

Microwave Link Budget Assumptions in designDesign Steps


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Introduction to Microwave Network Design

The main objective for system planning is to ensure thatthe radio relay system will meet the given performance andavailability requirements.

Quality and availability of communications line-of-sight(LOS) radio are closely related to propagation conditions.


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Transmission Types

Two methods of classifications

1. Point to point and point to multi point.

2. Line of sight and non line of sight.


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Transmission Types 1): Point to point and point to multi point

Point to point:

1 transmitter and 1 receiver.Directive antennas used.

Low radiated power.

Point to multi point:

1 Base station connected to many stations.Omni directional antenna used for base station.High radiated power.


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Line of sight

Line of Sight LOS):

Simple design.Full or partial clearance of fresnel zone.Suitable for long links.


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Frequency Bands

The following bands are available:

Long Haul:2 , 6 , 7 and 8 GHz

Short Haul:11 , 13 , 15 , 18 , 23 , 25 , 28 and 32 GHz

Micro Links:38 GHz.


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Frequency Plans

Frequency

Lower band Upper band

Channel bandwidth

Channel spacing

According to ITU recommendation for each band.The recommendation specifies the channel bandwidth, spacing and total number ofavailable channels.


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Performance and availability objectives


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Link Availability

A microwave link is available if communication is establishedin the two directions with an acceptable bit error rate (BER).

If the BER of the communication in at least one directionexceeds the BER specified, the link is considered unavailable.

%Availability=100-%Unavailability.


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Error performance parameters are derived from thefollowing events

Errored second ES):It is a one second period in which one or more bits are inerror or during which loss of signal or alarm indication isdetected..

Severely errored second SES):It is a one second period which has a bit error ratio 10 -3 .


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Parameters are:

Errored second ratio ESR):

the ratio of ES to total seconds in available time during afixed measurement interval.

Severely errored second ratio SESR):

the ratio of SES to total seconds in available time during afixed measurement interval.


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Availability performance parameters and objectives

Period of unavailable timebegins at the onset of 10 consecutive SES events. These10 s are part of unavailable time.

Period of available timebegins at the onset of 10 consecutive non SES events.These 10 s are part of available time.

A path is available if, and only if, both directions areavailable.


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Availability performance parameters and objectives

Quality (SES) and Availability objectives are chosenaccording to different ITU recommendations.

Different ITU recommendations depend on the capacitiesand hop lengths.


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Design Aspects and Main Concepts


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Design Parameters

1. Propagation related issuesFree space lossSurface reflectionThe Line of Sight Concept

Atmospheric multipathRain Scattering propertyPolarizationGaseous attenuation

2. Equipment related aspectsModulationRadio protection switching

Antennas


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Design Param eters Propagation related issues

Free space loss:The free space loss (FSL) value is given in the equationbelow:

FSL= 92.44 + 20 log f) + 20 log d)

Where:

FSL: free space loss (dB)f: frequency of radio (GHz)d: distance between transmitter and receiver (km)


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Surface reflectionThe influence of the reflected signal from the surface of the

Earth on the performance of the Microwave link is importantwhen it is sufficiently strong to interfere significantly with thedirect signal, either constructively or destructively.

The strength of the reflected signal at the receiving antennaterminals will depend upon:

the directivity of the antennas,

the height of the terminals above the Earths surface, the nature of the surfaceand the length of the path.


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The Line of Sight Concept

An optical line of sight exists if an imaginarystraight line can be drawn connecting the antennason either side of the link.

A clear line of sight exists when no physicalobjects obstruct viewing one antenna from thelocation of the other antenna. A radio wave clear line of sight exists if a defined

area around the optical line of sight (Fresnel Zone)is clear of obstacles.


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Fresnel Ellipsoid

The free space loss formula can only be applied if the direct line-of-sight(LOS) between transmitter and receiver is not obstructedThis is the case, if a specific region around the LOS is cleared from anyobstaclesThe region is called Fresnel ellipsoid

Transmitter

Receiver

LOS


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Fresnel Ellipsoid

21

21

d d

d d r

The Fresnel ellipsoid is the set of

all points around the LOS wherethe total length of the connectinglines to the transmitter and thereceiver is longer than the LOSlength by exactly half awavelengthIt can be shown that this region iscarrying the main power flow fromtransmitter to receiver

Transmitter Receiver

LOS

LOS + /2

Fresnel zone


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Atmospheric multipath

Under normal propagation conditions a radio wave follows a single pathfrom the transmitter to the receiver.

Anomalous propagation conditions however make two or more pathspossible. This phenomenon is known as multipath.

In the presence of multipath several rays arrive at the receiving antennaat slightly different angles in the vertical plane. The resulting signal isthen the sum of various components whose mutual interferenceproduces more or less deep fades, according to the relative amplitudesand phases of the components.


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Rain Scattering property

Rain precipitation scattering of microwaves is very important atfrequencies above about 10 GHz.

At these frequencies the rain droplet sizes become comparable to thewave length of the radio waves and cause scattering of microwaveenergy.

The main effect of scattering is a heavy attenuation in the path.

Due to the asymmetrical approximately oblate spheroidal shape of therain drops which has a vertical rotation axis, it cause larger attenuationfor horizontally polarized waves than that for vertically polarized ones.


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PolarizationThe plane of polarization is not affected by normal passage

of the wave through the atmosphere except in case of rainor during multipath formation.

The wave is received by the receiver antenna as either Hor V polarized.

Polarization is a very convenient and simple method

available by which it is possible to increase the isolationbetween two signals and hence to increase the spectrumusage.


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Gaseous attenuation

Gases in the atmosphere such as water vapour andoxygen create additional attenuation over and above that

produced during propagation in free space.

13 GHz 18 GHz 23 GHz 38 GHz

0.03 dB/km 0.08 dB/km 0.19 dB/km 0.12 dB/km


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Design Param eters Equipment related aspects

Modulation Lower modulation schemes use larger Bandwidth but provide higher systemgain (higher TX power and lower Rx threshold), while higher modulationschemes use smaller bandwidth but provide lower system gain.

As a result, lower modulation schemes are used for long links to provideoptimum performance.

Higher modulation schemes are used in congested city areas to providemaximum use of the frequency bands.

For example:4 QAM 16 QAMHigher BW m) Lower BW m/2)High system gain [TX power Rx threshold] spectrum efficiency

F d gi P t ff ti g th Li k


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Fade margins Param eters affecting the LinkQuality

Thermal Fade Margin:Is the difference between the free space received signal and the receiverthreshold level.

Interference Fade Margin:Is the additional attenuation to the free space received signal required toproduce an outage due to interference (independent of thermal noise).

Flat Fade Margin:Is the combination of the thermal and the interference fade margins.

Dispersive Fade Margin:This is an equipment parameter which depends on the equipment design andis defined as the average depth of multi-path fade which causes an outageindependent of thermal noise and interference.

Effective Fade Margin:Is the combination of the flat and dispersive fade margin components.


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Antenna Theory


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Antenna Theory

50 is the impedance of the cable377 is the impedance of the air

Antennas adapt the different impedances

They convert guided waves, into free-space waves(Hertzian waves) and/or vice versa

Z =377=50


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Antenna Characteristics

Isotropic Antenna A hypothetical, lossless antenna having equal radiation intensityin all directions. Used as a zero dB gain reference in directivitycalculation (gain).Gain

Antenna gain is a measure of directivity. It is defined as the ratioof the radiation intensity in a given direction to the radiationintensity that would be obtained if the power accepted by theantenna was radiated equally in all directions (isotropically).

Antenna gain is expressed in dBi.

Radiation PatternThe radiation pattern is a graphical representation in either polaror rectangular coordinates of the spatial energy distribution of anantenna.


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Antenna Characteristics Cont.)

Antenna BeamwidthThe directiveness of a directional antenna. Defined as the anglebetween two half-power (-3 dB) points on either side of the mainlobe of radiation.

EIRP Effective Isotropic Radiated Power)The antenna transmitted power. Equal to the transmitted outputpower minus cable loss plus the transmitting antenna gain.

)()()( dBGdBC dBm P EIRP t t out


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Antenna Pattern and HPBW

0 dB

-3 dB

-10 dB

0 dB

-3 dB

-10 dB

verticalhorizontal

sidelobe

null direction

main beam

H P B W


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Microwave antennas, feeders and accessories

Microwave point to point systems use highly directional antennasGain

with G = gain over isotropic, in dBiA = area of antenna aperture

e = antenna efficiency

Used antenna types parabolic antenna high performance antenna horn lens antenna horn antenna

GA e

104

2lg


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Antenna Data

PolarizationSpecification due to certain wave polarization (linear/elliptic,cross-polarization)

Half power beam width (HPBW)

Related to polarization of electrical fieldVertical and Horizontal HPBW

Antenna patternYields the spatial radiation characteristics of the antenna

Front-to-back ratioImportant for interference considerations


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Panel Antenna with Dipole Array

Many dipoles are arranged in a grid layoutNearly arbitrary antenna patterns may be

designedFeeding of the dipoles with weighted and phase-shifted signalsCoupling of all dipole elements


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Dipole Arrangement

t Dipolearrangement

Typical flat panelantenna

Dipole

element

Weightedand

phaseshiftedsignals


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X 65 T6 900MHz 2.5m

Rural road coverage with mechanical uptilt Antenna

RFS Panel Dual Polarized Antenna 872-960 MHz

APX906516-T6 SeriesElectrical specification

Gain in dBi: 17.1Polarization: +/-45HBW: 65VBW: 6.5Electrical downtilt: 6

Mechanical specificationDimensions HxWxD in mm: 2475 x 306x 120Weight in kg: 16.6

HorizontalPattern


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Parabolic antenna

Parabolic dish, illuminated by a feed horn at itsfocus

Available sizes: 1 (0.3 m) up to 16 (4.8 m)Sizes over 4 seldom used due to installationrestrictions

Single plane polarized feed vertical (V) orhorizontal (H)

Also: dual polarized feeder (DP), with separateV and H connections (lower gain)Front-to-back ratios of 45 dB not high enoughfor back-to-back configuration on the same

frequency Antenna patterns are absolutely necessary forinterference calculations


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High performance antenna

Similar to common parabolic antenna, exceptfor attached cylindrical shieldImprovement of front-to-back ratio and wideangle radiation discrimination

Available in same sizes as parabolic, single ordual polarizedSubstantially bigger, heavier, and moreexpensive than parabolic antennas

Allow back-to-back transmission at the samefrequency in both directions (refer tointerference calculation)


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Horn antennas

Horn lens antenna n For very high frequencies > 25 GHzn Replacement for small parabolic

antennas (1 )n Same electrical data, but easier to

install due to size and weight

Horn reflector antenna n Large parabola, energy from the feed

horn is reflected at right angle (90)n Gain like 10 parabolic antenna ( 60

dBi), but higher front-to-back ratios >70 dB

nBig and heavy, requires a complex installation procedurenOnly used on high capacity microwave backbones e.g. MSC-MSC interconnections)


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Specific Microwave Antenna Parameters (1)

Cross polarization discrimination (XPD)highest level of cross polarisation radiation relative to themain beam; should be > 30 dB for parabolic antennas

Inter-port isolation

isolation between the two ports of dual polarised antennas;typical value: better than 35 dB

Return loss (VSWR)Quality value for the adaption of antenna impedance to theimpedance of the connection cable

Return loss is the ratio of the reflected power to the powerfed at the antenna input (typical> 20 dB)


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Radiation pattern envelope (RPE)Tolerance specification for antenna pattern (specification ofantenna pattern itself not suitable due to manufacturingproblems)

Usually available from manufacturer in vertical and horizontalpolarisation (worst values of several measurements)

Weight

Wind load

Specific Microwave Antenna Parameters (2)


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Microwave Link Budget


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Radio Link Design

Unlike terrestrial cellular networks, in a mobile-satellite network, transmissions areconstrained by available power.Efficient coding and modulation techniques need to be employed in order toachieve a system margin above the minimum needed to guarantee a particularQuality of Service (QoS).

Li k B dg t A l i


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Link Budget Analysis

Link budgets are performed in order to analyze the critical factors inthe transmission chain and to optimize the performancecharacteristics.The strength of the received signal power is a function of thetransmitted power, the distance between transmitter and receiver, thetransmission frequency, and the gain characteristics of the transmitterand receiver antennas.If we have an isotropic antenna as transmitter and receiver then theloss in dB is given by:

Where:

d is the path length in kilometers and f is the frequency in MHz.

f d Loss log20log204.32

The apert re antenna


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The aperture antenna

The antenna forms the interface between the guided wave(for example in a coaxial cable) and the electromagneticwave propagating in free space. Antennas act in a similar manner irrespective of whether

they are functioning as transmitters or receivers, and it ispossible for an antenna to transmit and receivesimultaneously.Parabolic dishes used for microwave communications or

satellite Earth stations are good examples of apertureantennas.

The aperture antenna (Cont )


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The aperture antenna (Cont.)

The gain of a circular parabolic dish type of aperture antenna is givenby the approximation :

where D is the diameter of the dish in meters and f is the frequency ofoperation in GHz.

The beamwidth is usually measured in degrees. A usefulapproximation is:

where D is the diameter of the dish in metres and f is the frequency ofoperation in GHz

f DdBi gain log20log2018)(

rees Df Beamwidth deg22

Power Density


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Power Density

Assuming that the transmitting antenna is perfect, the power enteringthe antenna from its feed, Pt, is measured in watts. Once it has leftthe antenna, it creates a power density, Pd, in space that is measuredin watts per square meter:

This equation reveals a very valuable generalization in radio wave

propagation: the inverse square law. It can be seen that the powerdensity produced by an antenna reduces with the square of thedistance.

24 r P P t

d

Power at the Receiver


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Power at the Receiver

The power entering the aperture (the received power, Pr ) depends onthe size of the aperture, Ae square meters (the suffix e taken to standfor effective when referring to a receiving antenna), and the powerdensity of the radio wave:

This gives us the power received at distance r meters by an antennawith effective aperture Ae square meters when an isotropic antennatransmits power Pt watts.

ed r A P P

24 r A P

P et r

Power at the Receiver (Cont )


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Power at the Receiver (Cont.)

The power density is given more generally by:

where Gt is the gain of the transmitting antenna in any direction.

we can modify the equation for the received power:

where the effective aperture of the receiving antenna is now called Aerto make it clear that it is the receiving antenna that is being referred to.

24 r

G P P t t

d

24 r

AG P P er t t

d

The effective aperture


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The effective aperture

The effective aperture of an isotropic antenna, Aei,depends on the wavelength, , and is given by:

Practical antennas have a smaller aperture than thatcalculated from the diameter of the dish.

Where is the aperture efficiency, which lies between 0 and 1 and D is thediameter of the parabolic dish.

4

2

e i A

4

2 D Ae

General rules


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General rules

If you double the frequency, the gain of an antennawill quadruple. If you double the frequency, the beamwidth of an

antenna will halve.

If you double the antenna diameter (keeping thefrequency the same), the gain of the antenna willquadruple. If you double the antenna diameter (keeping the

frequency the same),the beamwidth of an antenna willhalve.

Point to point transmission


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Point-to-point transmission

With knowledge about the gain of antennas and the free-space lossbetween two points it is possible to predict the received signalpower for a particular situation and, thence, to design a link to adeliver a particular power to the receiver.The power required by any radio receiver depends on a number of

things:the quality of the receiver. the noise and interference being received. the required bit error ratio.the modulation scheme used and.the bit rate being transmitted.

Point to point transmission (cont )


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Point-to-point transmission (cont.)

For Digital communication we almost use the term Eb / No topresent the signal to noise ration.The actual value of N0 depends on the quality of the particularreceiver ,N0 can be written as generally:

Where: K is Boltzman constant, T is the temperature in Kelvin.

)1038.1( 23

0 k kT N

EXAMPLE1


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EXAMPLE1

A point-to-point system operates over a distance of 20 kilometers at afrequency of 26 GHz. The antennas are each of diameter 90 cm.Estimate the beamwidths of the antennas deployed and the powerreceived if the transmit power is 20 dBm.

SOLUTION ?

EXAMPLE2


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EXAMPLE2

A geostationary satellite is 39 000 km from an Earth station.It is transmitting a digital signal at a bit rate of 36 Mbit/s using a 40-dBm transmitter at a frequency of 11.2GHz. The transmitting antennahas a diameter of 80 cm. Determine the required size of a receiving

antenna if the required Eb/N0 ratio is 12 dB and the noise temperatureof the receiving system is 160 kelvin.

SOLUTION ?

EXAMPLE3


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EXAMPLE3

a particular transmitter delivers a power of 20 dBm into the feeder of thetransmitting antenna. We are operating at a frequency of 30 GHz over adistance of 12 km. The transmitting and receiving antennas are of 0.9meters diameter. Estimate the gain of the two antennas

SOLUTION ?


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Assumptions in design



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More available information Less assumptions

Network design cannot begin without the following:Sites coordinatesEstimation of transmitted capacity for equipment specificationFrequency band used

Assumptions:Configuration 1+0 / 1+1 Terrain DatabaseAntenna heights ensuring LOS and following assumed clearance criteriaQuality and availability objectivesFrequency channels Co -polar / dual polar / XPIC Network topologyTrafficProtection

Design Steps


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Design Steps

Dimensioning:1. Sites coordinates and network topology entering2. Path profiles generation and achieving LOS criteria Antenna Heights 3. Climatic parameters setting4. Equipment specification Antenna and radio models

5. Performance evaluation------ End of dimensioning ------

6. Frequency plan7. Interference analysis

------ Design complete ------

Synchronization between Tendering and Designer


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Information needed for Network Design:

Sites CoordinatesConfiguration

Path profile and surveyRadio and capacityProtectionNetwork TopologySpectrum and frequency bandQuality and availability objectivesFrequency channels Co -polar / dual polar / XPIC

Traffic

Necessary

CoordinatesPath profile and surveyRadio, configuration, and

capacityProtectionTopologySpectrum and frequency band



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More Information available Fewer assumptions & fasterdesign

Network Design needs time

Output of Network Design: Design + report

Better output needs time


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Thank you

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