Copy of FNE - Microwave

8/8/2019 Copy of FNE - Microwave

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2001 Flextronics Network Services

Page 2 Proprietary & confidential, not for use or disclosure outside Flextronics Network Services, without prior written permission.

CONTENTS

1 What is Microwave?....................................................................................................6

1.1 Overview .............................................................................................................62 Field (Site and Path) Survey........................................................................................7

2.1 Definition.............................................................................................................72.2 Equipment used for Field Survey ........................................................................7

2.3 Information Gathered During Field Survey.........................................................73 Line of Sight (LOS).....................................................................................................9

3.1 Definition.............................................................................................................9

3.2 Optical LOS.........................................................................................................93.3 Radio LOS ...........................................................................................................9

4 Path Profiling and Clearance Criteria........................................................................11

4.1 Definition...........................................................................................................114.2 Overview ...........................................................................................................12

5 Antenna......................................................................................................................14

5.1 Definition...........................................................................................................145.2 Parabolic Antenna..............................................................................................145.3 Characteristics and terms...................................................................................15

6 Modes of operation....................................................................................................17

6.1 Non Protected (NP) ...........................................................................................176.2 Monitored Hot StandBy (MHSB)......................................................................17

6.3 Diversity Systems ..............................................................................................17

6.3.1 Definition...................................................................................................176.3.2 Overview ...................................................................................................17

6.3.3 Space Diversity..........................................................................................17

6.3.4 Frequency Diversity ..................................................................................18

7 Repeaters ...................................................................................................................197.1 Active Repeater .................................................................................................19

7.2 Passive Repeater ................................................................................................19

8 Microwave Radio Propagation ..................................................................................228.1 Characteristics Of Microwave Transmission ....................................................22

8.2 Refraction ..........................................................................................................22

8.3 Atmospheric Duct..............................................................................................238.4 Multipath ...........................................................................................................23

8.4.1 Overview ...................................................................................................23

8.4.2 How to overcome multipath? ....................................................................24

8.5 Diffraction .........................................................................................................25

8.6 Reflection...........................................................................................................268.7 Fading................................................................................................................28

9 Microwave Path Losses And Gains...........................................................................319.1 Definition...........................................................................................................31

9.2 Free Space Loss.................................................................................................31

9.3 Link Budget .......................................................................................................339.4 System Gain.......................................................................................................34

9.4.1 Plane Reflector Insertion Loss...................................................................34


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Fixed Network EngineeringMicrowave Systems

Proprietary & confidential, not for use or disclosure outside Flextronics Network Services, without prior written permission. Page 3

9.4.2 Back-to-Back Antenna Insertion Loss.......................................................359.5 Fade Margin.......................................................................................................37

9.6 Reliability or Availability..................................................................................38

9.6.1 Unavailability Standards............................................................................389.6.2 Causes Unavailability................................................................................38

9.6.3 Unavailability calculation for loop protected network ..............................399.7 Report ................................................................................................................399.8 Design Optimization..........................................................................................40

10 Network Topologies ..............................................................................................41

10.1 Star Topology ....................................................................................................41

10.2 Ring Topology...................................................................................................4110.3 Implications to topology....................................................................................41

10.4 Implications of rain on loop topology ...............................................................41

11 Frequency Planning ...............................................................................................4311.1 Definition...........................................................................................................43

11.2 Causes of Interference .......................................................................................43

11.2.1 Internal Causes ..........................................................................................4311.2.2 External Causes .........................................................................................43

11.3 Effects of Interference .......................................................................................43

11.3.1 Co-channel Interference ............................................................................4311.3.2 Adjacent-channel interference...................................................................43

11.4 Frequency Channel Planning.............................................................................43

11.4.1 Basic ITU Arrangements...........................................................................43

11.4.2 High/Low Arrangements ...........................................................................4411.4.3 Alternate Polarization................................................................................44

11.5 Frequency Re-Use .............................................................................................44

11.5.1 Two-Frequency (One-pair) Plan................................................................4411.5.2 Four-Frequency (Two pairs) Plan..............................................................45

11.5.3 Sex-Frequency (Three pairs) Plan .............................................................45

11.6 Antenna Considerations.....................................................................................4511.7 Overcoming frequency interference ..................................................................45

12 Practices Using Software Planning Tool ...............................................................47


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Figures

Figure 1 Block Diagram Of A Microwave Radio System...............................................6

Figure 2 Tower ................................................................................................................8Figure 3 Path Profile......................................................................................................11

Figure 4 Different Ways Of Representing Path Profiling .............................................12Figure 5 Different Types Of Parabolic Antennas ..........................................................14

Figure 6 Antenna Beam width And Lobes ....................................................................15Figure 7 Active Repeaters .............................................................................................19

Figure 8 Passive Repeater: Flat Reflector .....................................................................21

Figure 9 Atmospheric Ducting ......................................................................................23Figure 10 Multipath As a Result Of Reflection From The Ground Surface And

NOT From Ducting ...................................................................................................24

Figure 11 Terrain Shielding To Cancel Multipath ........................................................27Figure 12 Antenna Polarization.....................................................................................29

Figure 13 Received Power From An Isotropic Antenna ...............................................31

Figure 14 Antenna Radiation Pattern ............................................................................32Figure 15 Different Types Of Waveguides ...................................................................36Figure 16 Network Topologies......................................................................................41

Figure 17 Two-Frequency Plan .....................................................................................45

Figure 18 Four-Frequency Plan With Alternated Polarization......................................45Figure 19 Four-Frequency Plan With Alternated Frequency ........................................45


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Tables

Table 1 Equipment used for Field Survey...................................................................7

Table 2 Information Gathered During Field Survey....................................................8Table 3 Radius Of First Fresnel Zone For different Frequencies And Distances .....10

Table 4 Relation Between Antenna Gain and Both Frequency And Antenna Size...16Table 5 Values Taken for Clearance % and k-factor According To Frequency And Distance 26

Table 6 Frequency And Fading .................................................................................29


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1 What is Microwave?

1.1 Overview

Microwave in the context of this document refers to point-to-point fixed links that operate in duplex mode.Duplex operation means that each radio frequency (RF) channel consists of a pair of frequencies for thetransmit and receive directions, respectively. The baseband signal, which contains the user information,occupies a limited bandwidth depending on the modulation scheme used. This signal is modulated onto an RFcarrier and is transmitted over the air as an electromagnetic waveform. The microwave radio links cover thefrequency spectrum from 300 MHz to approximately 60 GHz. Figure 1 shows the Microwave Radio System.

Figure 1 Block Diagram Of A Microwave Radio System

PCMMUX

Multiplexinputs

and addsservices

Modulateusing FSK

or QAMUp-Convert

to RF

Down-Convert

Demodulateto baseband

DemultiplexTo tributariesand services

Path

Path


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2 Field (Site and Path) Survey

2.1 Definition

The field survey is a detailed investigation of the sites and the path where the system will be deployed. In otherwords its the assembling of pertinent geographical and environmental data required to design a radiocommunication system.

2.2 Equipment used for Field Survey

Equipment Use / Description

Topographicmap(s)

1:50,000 or better. Alternatively, computer based Path Profile analysis software. Forurban links, three-dimensional map photographs, usually on a 1:10,000 scale, can beused to identify possible repeater sites and identify possible obstructions. Alternatively,computer based Path Profile analysis software with digital maps can be used. Thesedigital maps are based on pixel sizes of 200 m by 200 m resolution, however in manycases a resolution of 50 m by 50 m or better is available.

Route Map Used to investigate various route options.

Digital Camera Used to take pictures for sites, tower, potential obstructions, road, airport, etc.

BinocularsUsed to determine if optical LOS (Line Of Sight) exists. Whether one site (proposedantenna location) is visible from the other site. Used when we have clear weather andthe distance between the sites is reasonable.

Mirror Used to determine if optical LOS exists when we have bright sun.Strobe Light Used to determine if optical LOS exists when the weather conditions are foggy or hazy.High-powerFlashlight

Used to determine if optical LOS exists during the night.

Compass

Hand-held GPS(Global PositioningSystem)

Used to measure the co-ordinates and distance between sites (it has certain accuracy).Also it is used in determining the distance to potential obstructions and whether or notthey are within the first Fresnel zone. In most cases site coordination need to beaccurate to within 10 m to 20 m.

Altimeter Used to measure the elevation of the sites AMSL (Above Mean Sea Level). Some typesof GPSs provide this information.

Spectrum Analyzer Used to evaluate the general RF (Radio Frequency) environment during site survey.Measuring Tape

Safety hat

Ladder

Table 1 Equipment used for Field Survey

2.3 Information Gathered During Field Survey

Data Contents

Site Data

Name, Call Sign (If Available), Address, General Directions, Latitude, Longitude, UTMCoordinates (Calculated), Elevation (AMSL), Description, Power (If Available), County,Nearest Town, Topographical Map Name, FCC / FAA Data (If Available), General Security(Gates).

Road Data Type, Condition, Access, Type of Vehicle Required, Description, General Grade / Slope.Airport Data Nearest Airport, Airport Name, Airport Location, Airport Distance & Bearing.

Tower Data (IfExisting)

Current Structure, Type, Width at Base, Height, Overall Height, Joint Length, Cross MemberHeight, Steel Type (Angle / Round), Painting Description, Lighting, Number Of Guy Points,


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Grounding, Safety Equipment, Cable Ladders, Cable Bridges, Cable Mount Type, ExistingCables, Microwave Dishes (Size & Height), 2-Way & Cellular Antennas (Size & Height),Other Antennas (Height), Lighting, Height of Lighting, Strobes / Beacons, Near FieldObstructions. See Figure 2.

Figure 2 Tower

Wave GuideData (If Existing)

Type, Condition, Mounting, Description, Entry Type (If Available), Entry Capacity.

ExteriorBuilding Data(If Existing)

Type, Condition, Exterior Size, Distance from Tower, Outer Material, General Description.

InteriorBuilding Data(Optional - IfExisting)

Current Equipment Location, Batteries, Chargers, Power, Type of Racks, Floor Plans,Electrical Outlets, Grounding Points, Movable Objects, Non-Movable Objects, Cable Bridges& Ladders, Punch-block locations, Dehydrators, Dehydrator Capacity, Safety Stations, InsideDimensions, Security, General Descriptions.

Table 2 Information Gathered During Field Survey


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3 Line of Sight (LOS)

3.1 Definition

Propagation in which the direct ray from the transmitter to the receiver is unobstructed, i.e., the transmissionpath is notestablished by or dependent upon reflection or diffraction. The need for LOS propagation is mostcritical at VHF (Very High Frequency) and higher frequencies (e.g. Microwave).

3.2 Optical LOS

Confirmed using Binoculars, Mirror, high-power Flashlight, Strobe light and/or topographical or digital maps(Pathloss uses digital maps to confirm the optical LOS).

3.3 Radio LOS

Confirm if the Fresnel Zones exist as co-axial ellipsoids connecting the two antennas. The maximum diameterof each ellipsoid is located at the center point of the ellipsoids axis, and increases as the distance between theantennae increases. Typically, for optimally designed microwave paths, at least 60% of the first Fresnel Zonemust be free of obstructions. Any further clearance will yield negligible improvement in the received signal leveland increases multipath. Radio LOS is done using microwave link planning software tools through path profiling.

Fresnel Zone

The Fresnel zones are a series of concentric ellipsoids surrounding the radio path. TheFirstFresnel zone is the surface containing every point for which the sum of thedistances from that point to the two ends of the path is exactly 1/2 wavelength longerthan the direct end-to-end path.

Fresnel Zones are areas in which, if the radio wave is reflected back toward the other end

of the microwave path, this radio wave would arrive in some relative phase as the original(non-reflected). If the radio wave is in-phase with the original waves than the radio waveshave an adding effect (We dont want this, The receiver has a maximum input power). Onthe other hand, if the radio wave arrives in an out-of-phase relation to the original, theradio waves have a canceling effect. This is commonly known as multi-path fading. Inother words, all even (2, 4, 6, 8, etc.) reflected Fresnel zones cancel while all odd (1, 3, 5,7, etc.) reflected Fresnel zones add from a reflected point in a path.

The first Fresnel radius F1 can be expressed as:

( ) ( )21211 ddddF +=

Where:

d1 distance of the point from transmitter at site 1 (m)d2 distance of the point from receiver at site 2 (m)

wavelength (m)

The subsequent nth Fresnel zone can be determined as

nFF 11 = , n is integer

At the midpoint the radius will be at its maximum (d 1=d2).


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Table 3 shows the radius of first Fresnel zone for different frequencies and distances.

Distance (km)

2.5 5 10 20Frequency (GHz) Radius of first Fresnel zone (m)

2 9.7 13.7 19.4 27.47 5.2 7.3 10.4 14.6

18 3.2 4.6 6.5 9.1

38 2.2 3.1 4.4 6.3

Table 3 Radius Of First Fresnel Zone For Different Frequencies And Distances


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4 Path Profiling and Clearance Criteria

4.1 Definition

Path profile: A graphic representation of the physical features of a propagation path in the vertical plane

containing both endpoints of the path, showing the surface of the Earth and including trees, buildings, and otherfeatures that may obstruct the radio signal (see Figure 3). Profiles are drawn either with an effective Earthradius simulated by a parabolic arc (in which case the ray paths are drawn as straight lines) or with a "flatEarth"-- in which case the ray paths are drawn as parabolic arcs. Figure 4 shows the two different ways used inrepresenting the path profile.

Figure 3 Path Profile

Path clearance: In microwave line-of-sight communications, the perpendicular distance from the radio-beamaxis to obstructions such as trees, buildings, or terrain. The required path clearance is usually expressed, for aparticular k-factor, as some fraction of the first Fresnel zone radius.

Effective Earth Radius

The radius of a hypothetical Earth for which the distance to the radio horizon, assuming rectilinearpropagation, is the same as that for the actual Earth with an assumed uniform vertical gradient ofatmospheric refractive index. For the standard atmosphere, the effective Earth radius is 4/3 thatof the actual Earth radius.


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Figure 4 Different Ways Of Representing Path Profiling

k-factor

k-factor is the curvature correction factor to compensate for atmospheric diffraction. k-factor is theratio of the effective Earth radius to the actual Earth radius. The k-factor is approximately 4/3typically 4/3 for most radio propagation applications.

4.2 Overview

In microwave link planning; the transmitter and receiver must have a LOS path between them. This means thatthere must be no obstructions between the antennas of the transmitter and receiver. In addition to this the LOSpath between the receiver and transmitter must be a minimum height above objects in the path.

In most radio applications, atmospheric refraction is considered. Therefore, sufficient antenna heights should beused in order to ensure microwave path performance. Given the proposed antenna heights, the microwaveplanning software determines whether a path meets a set of clearance criteria above the terrain and any aboveground obstructions. Clearance requirements are usually stated as a combination of a percentage of the firstFresnel zone radius and a k-factor. For example, a common requirement is that 60% of the first Fresnel zone

radius should be clear of all obstructions at a k factor of 4/3.

The refractivity of any medium (like the atmosphere) depends on its density, so changing atmosphericconditions can change the effect on radio waves. The problem comes from the relative earth curvature as itapplies to the radio wave and atmospheric anomalies such as "ducting" and "layering". During certain parts ofthe day and night the atmosphere causes the radio wave to "bend" more or less due to atmospheric conditions.This is noticed at even a greater measure when comparing Autumn, Winter, Spring and Summer months. Forthis reason, microwave path planners often consider other effective earth curvature values. Therefore, it is often


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advisable to check the path at both 4/3 and another value, which is calculated by some microwave design

softwares (e.g. PathLoss).

Clearance is IMPORTANTbut NEVER over estimate it; remember the multipath, which always increases as theheight of the antenna above ground increases. Therefore; make your design so that the MINIMUMheights arechosen to meet the clearance criteria.

Also, for a range of possible antenna heights at one end of a path, a trade-off analysis can be provided thatshows the corresponding antenna heights that are required at the other end. This is related to installationlimitations or to minimize the multipath.


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5 Antenna

5.1 Definition

Antenna: Any structure or device used to collect or radiate electromagnetic waves.

5.2 Parabolic Antenna

An antenna consisting of a parabolic reflector and a radiating or receiving element at or near its focus (Figure5). A parabolic antenna is used in microwave systems to concentrate radiated energy into a narrow beam fortransmission through the air. This results in the most efficient transmission of radiated power with a minimum ofinterference. An effective gain of 25 to 48 dB over an omni-directional antenna is possible depending upon thesize of the antenna and the microwave frequency used.

Figure 5 Different Types Of Parabolic Antennas


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Radomes

A radome is a protective covering used to prevent snow, ice, water, or debris from accumulatingon a microwave antenna. Heated radomes are available for use in areas where severe ice andsnow conditions exist. The use of a radome results in lower antenna gain.

5.3 Characteristics and terms

Radiation Pattern: The variation of the field intensity of an antenna as an angular function with respect to theaxis. A radiation pattern is usually represented graphically for the far-field conditions in either horizontal orvertical plane.

Beam width: In the radio regime, of an antenna pattern, the angle between the half-power (3-dB) points of themain lobe, when referenced to the peak effective radiated power of the main lobe. Beam width is usuallyexpressed in degrees. It is usually expressed for the horizontal plane, but may also be expressed for the verticalplane. The beam width is of special importance when we deal with antenna alignment, multipath and frequencyplanning and interference. Figure 6 shows the antenna beam width and lobes.

Figure 6 Antenna Beam width And Lobes

Lobe: An identifiable segment of an antenna radiation pattern. A lobe is characterized by a localized maximumbounded by identifiable nulls.

Main Lobe: Of an antenna radiation pattern, the lobe containing the maximum power (exhibiting the greatestfield strength). The horizontal radiation pattern, i.e. , that which is plotted as a function of azimuth about theantenna, is usually specified. The width of the main lobe is usually specified as the angle encompassedbetween the points where the power has fallen 3 dB below the maximum value. The vertical radiation pattern,i.e. , that which is plotted as a function of elevation from a specified azimuth, is also of interest (for antennaalignment and multipath considerations) and may be similarly specified.

Side Lobe: In a directional antenna radiation pattern, a lobe in any direction other than that of the main lobe.The Side Lobe is of special importance when we deal with multipath and frequency planning and interference.

Antenna Gain: The ratio of the power required at the input of a loss-free reference antenna to the powersupplied to the input of the given antenna to produce, in a given direction, the same field strength at the samedistance. Parabolic antenna gain can be represented by:


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2

4

eAG

= Equation 1

2

24

c

fAG

e=

Equation 2

Where:G directive gain of the antennaf wavelength of the radio wavec speed of light 3x10

8m/sec

wavelength of the radio waveAe antenna effective area

2 GHz 4 GHz 6 GHz 11 GHz

Antenna Size (foot) Gain

4 25 31 35 406 29 35 38 438 31 37 41 46

10 33 39 43 48

Table 4 Relation Between Antenna Gain and Both Frequency And Antenna Size

Antenna Effective Area: The functionally equivalent area from which an antenna directed toward the source ofthe received signal gathers or absorbs the energy of an incident electromagnetic wave. Antenna effective areais usually expressed in square meters. In the case of parabolic and horn-parabolic antennas, the antennaeffective area is about 0.35 to 0.55 of the geometric area of the antenna aperture.

Effective Radiated Power (ERP) (in a given direction): The power supplied to an antenna multiplied by theantenna gain in a given direction. If the direction is not specified, the direction of maximum gain is assumed.The type of reference antenna must be specified.

Front-to-Back Ratio: Of an antenna, the gain in a specified direction, i.e., azimuth, usually that of maximumgain, compared to the gain in a direction 180 from the specified azimuth. Front-to-back ratio is usuallyexpressed in dB.

Isotropic Antenna: A hypothetical antenna that radiates or receives equally in all directions. Isotropic antennasdo not exist physically but represent convenient reference antennas for expressing directional properties ofphysical antennas.

Reference Antenna: An antenna that may be real, virtual, or theoretical, and has a radiation pattern that can beused as a basis of comparison with other antenna radiation patterns.

Effective Height: The height of the center of radiation of an antenna above the effective ground level.


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6 Modes of operation

6.1 Non Protected (NP)

The non-protected configuration is a single standalone terminal.

6.2 Monitored Hot StandBy (MHSB)

In communications systems operations MHSB is used to pertain to a power-saving condition or status ofoperation of equipment that is ready for use but not in use. An example of a standby condition is a radio stationoperating condition in which the operator can receive but is not transmitting. Pertaining to spare equipment thatis placed in operation only when other, in-use equipment becomes inoperative. Hot standby equipment, which iswarmed up, i.e., powered and ready for immediate service, and which may be switched into serviceautomatically upon detection of a failure in the regular equipment.

Some models with an MHSB configuration provide the option to use an equal power splitter (3dB) or unequalcoupler (10 dB, used to increase the system gain) at the receiver side. Some models also include the option touse a dual antenna set, in MHSB configuration, in order to reduce branching losses to nearly zero.

6.3 Diversity Systems

6.3.1 Definition

Diversity: The property of being made up of two or more different elements, media, or methods. Incommunications, diversity is usually used to provide robustness, reliability, or security.

6.3.2 Overview

A diversity scheme is a method that is used to develop information from several signals transmitted overindependent fading paths. This means that the diversity method requires that a number of transmission paths

be available, all carrying the same message but having independent fading statistics. The mean signalstrengths of the paths should also be approximately the same. The basic requirement of the independentfading is received signals are uncorrelated. Therefore, the success of diversity schemes depends on the degreeto which the signals on the different diversity branches are uncorrelated.

Proper combining the multiple signals will greatly reduce severity of fading and improve reliability oftransmission. Because deep fades seldom occur simultaneously during the same time intervals on two or morepaths. The simplest combining scheme is selection combining, which is based on the principle of selecting thebest signal among all of the signals received from different branches.

6.3.3 Space Diversity

For improvements in propagation reliability, a space diversity antenna arrangement can be used. In a spacediversity system, one transmitter and its associated antenna radiates on a transmit frequency. This signal isreceived by two receivers, which are tuned to the same frequency but connected to separate antennas locatedat different positions on the tower. The receiver output signals can be combined to give a composite output, orswitching can be done between receivers, keeping the receiver with the best Bit Error Ratio (BER) (in the caseof a digital radio system) connected to the line. Vertical spacing between the two receiving antennas should beapproximately 60 to 80 feet at 2 GHz, 30 to 40 feet at 6 GHz and 25 to 30 feet at 11 GHz. Space diversityprovides a substantial increase in reliability, especially over highly reflective surfaces such as water or desert.The necessity of two receiving antennas, two receiving waveguide runs, strong towers because of the two


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antennas and a taller tower required to give the necessary antenna spacing tends to make space diversity amore expensive means of increased path reliability.

The criteria that is followed to decide the height of the two antennas is: While one of the antennas is having amaximum received signal the other one should have a minimum received signal and vice versa. This issimulated using a Microwave Link Design software (Pathloss).

6.3.4 Frequency Diversity

A frequency diversity arrangement can be used at microwave frequencies above 2 GHz when equipment andpropagation reliability is desired and required communications cannot practically be achieved by other means.This method increases the total system reliability by providing both path and equipment duplication. Twotransmitters are on the air simultaneously and both are modulated with the same baseband signal but are tunedto different radio frequencies. The different frequencies can be either within the same operating frequency band,or in two different operating frequency bands. Both transmitters are connected to the same antenna, whichradiates the signals to the far-end of the path. At the far-end of the path there are two receivers and eachreceiver accepts the one incoming signal to which it is tuned. Each receiver then provides as an output thesignal, which modulated the transmitters. The two outputs are then combined using a combiner device toprovide one output signal to the multiplex.

Either space or frequency diversity can be used to overcome the multipath fading, but because of the spectrumlimitations and to reduce the interference in the network, we use space diversity.

Note: space and frequency diversity are NOTused to overcome the Rain fading. We overcome Rain fading byusing Path diversity (have two different routes that connect the two sites, one of these routes can be fiberoptics) or by increasing the Fade Margin.


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7 Repeaters

7.1 Active Repeater

A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on adigital input signal for retransmission. The distance covered by a microwave link can be increased through theuse of active repeaters, which AMPLIFY a microwave signal. In some cases they use Solar system as a powersupply.

There are basically two types of active repeaters: baseband and IF. The baseband repeater must receive thesignal and convert it all the way back to baseband before retransmission, where an IF repeater only convertsthe signal to IF where it is amplified (Figure 7). IF repeater advantages: low cost and simple hardware.Baseband repeater advantages: regenerate digital signals.

Figure 7 Active Repeaters

7.2 Passive Repeater

An unpowered device used to route a microwave beam over or around an obstruction. Examples of passiverepeaters are (a) two parabolic antennas connected back-to-back and connected by a short feeder cable, and(b) a flat metal reflector used as a mirror (Figure 8). A passive repeater is sometimes required when there is anobstacle such as a high mountain in the line-of-sight microwave path, where the cost, maintenance and powerrequirements for an active repeater would be prohibitive. The passive repeater is located in such a position asto act as a microwave mirror, reflecting the microwave signal as a mirror reflects a light beam, to bypass theobstruction. The passive repeater is used to RE-DIRECT a microwave signal.

Microwave

Receiver

Microwave

Transmitter

RF Amplifierand

EqualizerIF Repeater

MicrowaveReceiver

RF Amplifierand

Equalizer

Demodulate

MicrowaveTransmitter

RF Amplifierand

Equalizer

ModulateMultiplex

Equipment

Base band

Repeater


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Figure 8 Passive Repeater: Flat Reflector

The passive repeater supports any frequency band because it is a wideband device. The passive repeater is100% efficient compared to back-back repeaters that are typically only 55% efficient.

The reflector gain increases with reflector size. Reflectors as big as 12 m by 18 m are readily available. Passiverepeaters have the following advantages over active repeaters:

1. No power is required2. No regular road access is required3. No equipment housing is needed4. They are environmentally friendly5. Little or no maintenance is required.


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8 Microwave Radio Propagation

8.1 Characteristics Of Microwave Transmission

Microwave frequencies are generally defined as those frequencies, which have a wavelength short enough todisplay many of the properties of light waves. A wavelength of 30 centimeters (approximately a foot) or less isconsidered to be in the microwave region. Microwave energy may be refracted, diffracted, reflected, orabsorbed. The direct rays of the radiated energy travel essentially in a straight line and there is little reflectionfrom the ionospheric layers in the upper atmosphere. Because of the short wavelength of microwaves, theradiated energy can be concentrated by relatively small antennas into a narrow beam similar to that of asearchlight. Microwave energy can be obstructed or attenuated by solid objects such as trees, buildings, andmountains. It is for these reasons that microwave communication is almost always limited to unobstructed line-of-sight paths.

8.2 Refraction

Refraction

Retardation, and--in the general case-- redirection, of a wavefront passing through (a) aboundary between two dissimilar media or (b) a medium having a refractive index that isa continuous function of position. For two media of different refractive indices, the angleof refraction is closely approximated by Snell's Law.

Refraction is one of the factors that must be considered when determining microwave path clearance. Undernormal propagation conditions refraction results in the bending of the microwave beam beyond the opticalhorizon in the direction of the earth's curvature.

As a radio wave front moves forward, it will travel in a straight line if all points on the front travel at the samevelocity. In air of uniform pressure, temperature and relative humidity all points on a wave front would travel at

the same velocity. Since the pressure, temperature and relative humidity of the atmosphere are not uniform, butnormally decrease with height, the upper portion of the wave front travels slightly faster than the lower portionas it moves forward. The difference in velocity causes the wave, under normal conditions, to be bent orrefracted toward the earth. This is the reason that when a path profile is plotted, the radius of the earth must becorrected for refraction by the appropriate k-factor.

The greater the difference in velocity between the upper and lower portions of the wave front, the more a wavewill be bent toward the air having the highest index of refraction. The amount of bending thus depends upon theindex of refraction of the air through which the wave front passes. The index of refraction varies with relativehumidity, temperature, pressure, movement of air and other factors. The variation of these factors from minute-to-minute and day-to-day causes the amount of bending of a wave front to fluctuate.

Since normal atmospheric refraction results in the microwave beam being bent downward, this effect is thesame as a change in the earth's radius and is expressed in terms of an equivalent earth radius factor k. Theactual earth's radius multiplied by the k factor represents a fictitious earth with a radius, which accounts for therefractive index. A factor of k = 1 would be the case where the curvatures of the actual earth and the effectiveearth are equal. A factor greater than k = 1, for example k = 4/3, would indicate that the effective earth has lesscurvature or is flatter than the true earth. It is also possible when abnormal propagation conditions exist for thebeam to be bent upward, which would indicate a k factor less than 1. The k factor varies for differentatmospheric conditions, but at microwave frequencies in the 4 GHz, 6 GHz, and 11 GHz common carrier bandsa factor of k = 1 or k = 2/3 is used for most areas of the United States. For frequencies in the 2 GHz band afactor of k = 4/3 is normally used. The selection of the k factor is dependent on path location and path length.


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8.3 Atmospheric Duct

A horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radiosignals (a) are guided or focused within the duct, (b) tend to follow the curvature of the Earth, and (c)experience less attenuation in the ducts than they would if the ducts were not present. See Figure 9.

The reduced refractive index at the higher altitudes bends the signals back toward the Earth. Signals in a higherrefractive index layer, i.e., duct, tend to remain in that layer because of the reflection and refraction encounteredat the boundary with a lower refractive index material.

Figure 9 Atmospheric Ducting

8.4 Multipath

8.4.1 Overview

Multipath

The propagation phenomenon that results in radio signals' reaching the receivingantenna by two or more paths. Causes of multipath include atmospheric ducting andreflection from terrestrial objects, such as mountains and buildings. The effects ofmultipath include constructive and destructive interference, and phase shifting of thesignal.


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Figure 10 Multipath As a Result Of Reflection From The Ground Surface And NOT FromDucting

Multipath propagation is a phenomenon that can be caused by irregular changes in the index of refraction. Thismay occur on a still night after a hot, humid day when there are both temperature and humidity inversions.

Temperature inversion (increase in temperature with height) is caused by the earth and the air adjacent to itcooling faster than the warm air above it. To cause a higher index of refraction in the air at some height abovethe ground rather than near it, the absolute humidity or vapor pressure at that point must be higher than nearthe ground. This is called humidity inversion and usually occurs when the air is super-saturated and the excessmoisture appears as fog or dew.

Multipath propagation occurs when there exists a layer of air some distance above the ground, which has ahigher index of refraction than the air above or below it. Horizontal and vertical variations in temperature,pressure, and humidity cause more than one propagation path to exist between transmitter and receiver. Forexample, beam 1 might be close to a layer with a high index of refraction. While beam 2 could cross the layer ata greater angle and it would be bent enough by the low-density air near the earth to also arrive at the receiver.

Initially the separation would be slight and the electrical path lengths equal. The signal via each path wouldarrive in phase and the total signal received at the antenna would be doubled. However, as the paths becomemore divergent with cooling in the lower atmosphere, the signal will decrease until the energy received via bothpaths practically cancel. Thus the signal will fluctuate depending upon the difference in the electrical length ofthe paths. By morning, the temperature in the layer will become cooler and the two paths will come closertogether until only one path remains. If the two paths still exist by morning, the sun will warm the earth and airadjacent to it faster than the air in the upper layer so that the temperature inversion will soon cease to exist.

Frequency diversity and fade margin may be used to substantially reduce the adverse effects of multipathfading. Space diversity may also be used to provide increased propagation reliability.

8.4.2 How to overcome multipath?

Depending on the design at least one of the following methods is used to overcome the multipath fading:

1. Space Diversity2. Install the antenna so that the path to reflecting surface is obstructed


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3. Tilt the antenna up4. Change topology avoiding reflective surfaces e.g. water.

8.5 Diffraction

After correcting the profile of the path to take into consideration the bending of the radio rays by refraction, it is

generally necessary that the rays clear the earth and obstacles along the path by a certain amount to preventexcessive attenuation of the signal by diffraction.

Diffraction may be considered as a modification which waves undergo as they graze the surface of the earth,hills, or the edges of any opaque body by which the rays are apparently deflected or bent. The energy diffractedbeyond a given hill will increase as the frequency is decreased.

It was discussed in the previous chapter that the areas or zones around the axial between the transmitter andreceiver that contribute energy either in- or out-of-phase are called Fresnel zones. The first Fresnel zone isbounded by points through which the distance between the transmitter and receiver is 1/2 wavelength longerthan the direct ray. The second Fresnel zone is bounded by points through which the distance is onewavelength longer than the direct ray.

The areas of all zones are equal. The energy received from each zone, however, decreases with distance fromthe primary ray. About one-fourth of the energy received from an unobstructed wave is in the first Fresnel zone.The energy received from the second and other even-numbered Fresnel zones is negative with respect toenergy received from the odd Fresnel zones. About half of the total energy received from an unobstructed waveis cancelled by the waves received from the even numbered Fresnel zones. A sharp obstruction such as asharply pointed hill, which cuts off most of the energy below the first Fresnel zone, would permit more energy tobe received than if the obstruction were not there since part of the out-of-phase energy would be cut off by theobstruction.

If an obstacle cuts off the first Fresnel zone radius (non-line-ofsight path), some energy will be diffractedaround and over the obstacle and will be received in the shadow portion of the radio beam. It is for this reasonthat a certain amount of radio energy is present beyond true radio (refracted) line-of-sight when the path isintercepted by the earth. In general, the lower the frequency, the farther the signal is diffracted beyond the pointof interception.

The radius of the first Fresnel zone varies along the radio path. It is maximum at the midpoint between thetransmitter and receiver and can be calculated by the formula:

21

1140

=

f

dF Equation 3

Where:F radius in feet of the first Fresnel zone at the midpoint of the path,d distance in miles between the receiver and transmitter,f frequency in MHz.

The first Fresnel zone radius at any point x miles from one end of the path is:

( ) 21

2280

=

df

xdxF Equation 4

We use the above formula to determine the necessary antennas height for proper Fresnel zone clearance. It isdesirable to receive as much of the first Fresnel zone energy as possible and still keep the cost of towers as lowas possible. At frequencies of 4 GHz and higher, a clearance of 0.6 first Fresnel zone radius with k = 1 is a good


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design objective for most areas of the United States with hops of 25 miles or less. A more conservative designfor hops greater than 25 miles to account for possible upward beam bending (earth bulging) would be 0.3 firstFresnel zone with k = 2/3. At 2 GHz, a value of k = 4/3 with a 0.6 first Fresnel zone clearance is a typicaldesign. A summary is given in Table 5.

)()( milesdGHzf 25 > 25

4 Fm = 0.6, k = 1 Fm = 0.3, k = 2/32 Fm = 0.6, k = 4/3

Table 5 Values Taken for Clearance % and k-factor According To Frequency And Distance

8.6 Reflection

Reflection

The abrupt change in direction of a wave front at an interface between two dissimilarmedia so that the wave front returns into the medium from which it originated. Reflection

may be specular (i.e., mirror-like) or diffuse (i.e., not retaining the image, only the energy)according to the nature of the interface.

If the terrain between the antennas reflects radio waves efficiently, it is possible to receive strong reflectedwaves, either in- or out-of-phase with the direct wave, depending on the difference in the lengths of the directand reflected wave paths. If we assume complete reflection with the reflected wave equal in magnitude to thedirect wave, the resultant energy received would vary, depending on the location of the point of reflection,between zero and twice that of the direct path energy according to the following formula:

2

2sin2

*dd

EE d

=

Equation 5

Ed direct ray field strengthd* geometric length difference between the direct and reflected wave paths.E & Ed are in the same units, such as microvolts/meter.

Wavelength, is in the same units as d*.

If only the first Fresnel zone reflected signal is received (the reflecting plane is in the first Fresnel zone), thereflected signal would then be added to the direct signal. The path difference d* is approximately:

=

d

hhd

tr2* Equation 6

Where:

hrand ht are the heights of the receiving and transmitting antennas above the REFLECTING PLANE.d distance between transmitter and receiver in the same units as hrand ht.

If one of the heights such as hris varied so that E goes through a maximum and minimum, the difference in thetwo values of hris sometimes used as the spacing between two receiving antennas on a tower.

When two antennas are placed on a tower with this separation, the reception of the two signals and theselection of the stronger of the two is called space diversity reception. If one antenna is receiving a minimum


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signal, the other in all probability will be receiving a stronger signal. A particularly difficult problem exists whenthe reflection point is over tidewater, which causes variations in the length of the reflected wave path contingenton the tidal change in water level. The amount of separation used on a space diversity system should bedetermined by someone very familiar with this type of design.

Reflections are greatest when the point of reflection is over calm water, level moist earth, desert sand, andother types of smooth terrain. It is desirable to adjust the tower heights or to reroute the radio path so thereflection point will be over rough terrain. Radio energy striking rough terrain will be either absorbed orscattered. Thus the amount of reflected energy reaching the receiver will be only a small percent of the totalenergy. The point of reflection can be determined by trial and error from the path plot. At the point of reflectionthe angle of incidence will equal the angle of reflection.

Note: In some cases we take advantage of the terrain to reduce the multipath fading. The Figure below showshow the terrain can reduce multipath fading:

Figure 11 Terrain Shielding To Cancel Multipath

With zero clearance over a non-reflecting obstacle such as a hill covered with trees or brush, the signal will be 6or 7 dB below the loss that would exist between two antennas in free space. If, however, the top of the hill werebroad and barren and the soil had good reflection characteristics, the loss could be more than 16 dB greaterthan free space loss.

Coupling Loss

The loss that occurs when energy is transferred from one medium to another.

Absorption

In the transmission of electrical, electromagnetic, or acoustic signals, the conversion ofthe transmitted energy into another form, usually thermal. The conversion takes place asa result of interaction between the incident energy and the material medium, at the


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molecular or atomic level.

Scattering

Of a wave propagating in a material medium, a phenomenon in which the direction,frequency, or polarization of the wave is changed when the wave encountersdiscontinuities in the medium, or interacts with the material at the atomic or molecularlevel. Scattering results in a disordered or random change in the incident energydistribution.

8.7 Fading

Fading is a condition, which occurs during propagation of radio frequency energy that causes a reduction in thepower being received. It may be caused by refraction, diffraction or reflection or by a combination of theseconditions. This combination is usually referred to as multipath fading. In addition to multipath fading a verysevere form of fading may also be caused by rainfall. The effect on the 4 GHz and 6 GHz bands is quite small

and is usually ignored. The attenuation of the microwave radio signals increases substantially as the radiofrequency carrier is increased. This attenuation is definitely noticeable at the 10.5 GHz and 11 GHz bands. Itbecomes extremely severe at 18 GHz and 23 GHz.

In the absence of rain, variations in received signals due to multipath conditions are greater in summer than inwinter and greater during nighttime than daytime. These variations are smallest when the air is in a state ofturbulence, which prevents the formation of stratified layers of air.

Radio energy is absorbed and scattered by raindrops. These effects become more pronounced as thewavelength approaches the diameter of the raindrops. When the size of the drops becomes large enough andthe drops are sufficiently concentrated, this scattering and absorption will attenuate the signal appreciably. Inaddition, since the drops represent a lossy dielectric, energy will be absorbed from the signal and converted intoheat. These phenomena are entirely negligible below 3 GHz but will place a limitation on transmission throughrain over appreciable distances at frequencies above 10 GHz.

The amount of attenuation caused by rain depends on the intensity of the rainstorm. The rate of rainfall and nottotal rainfall is the determining factor. Areas with high annual rainfall accumulations may seldom experiencerainfall of a rate sufficient to interrupt service. Heavy rainfall rates are likely to accompany thunderstorms, whichmay be confined, to an area with a diameter of 1 to 2 miles. The fading margin may not be exceeded unless theheavy rainfall extends a sufficient distance along the radio path. System outages should be treated on aprobabilistic basis by geographical area. The worst areas occur near the Gulf of Mexico and the part of theAtlantic Ocean adjacent to the southeastern area of the United States.

At low microwave frequencies, polarization has little or no effect on signal propagation. At frequencies above 10GHz, rain attenuation becomes the controlling factor, and vertically polarized signals are less subject to rainattenuation than horizontally polarized signals. A falling drop of rain is not spherical but flattened in thehorizontal plane. This results in a greater attenuation of horizontally polarized waves. Rain attenuation over a

given microwave band is almost independent of frequency.

Table 6 below shows the relation between the frequency and the type of fading multipath/rain that is affectingthe wave:

Frequency Capacity Available Hop Distance Fading


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7GHz10GHz13GHz18GHz23GHz

38GHz

Med-HighLow-Med-HighLow-Med-HighLow-Med-High

All

All

>30km15 30km15 30km15 30km5 15km

Within short ranges Up to 5km

MultipathMultipathMultipath

Rain and MultipathRain

Rain

Table 6 Frequency And Fading

Polarization

Polarization of an electromagnetic wave, the property that describes the orientation, i.e.,time-varying direction and amplitude, of the electric field vector. Polarization is importantin frequency interference analysis.

Figure 12 Antenna Polarization

Since vertical space diversity and frequency diversity do not protect a system against rain fading it is importantthat good fading margins be obtained in the system design. Reducing the hop length not only reduces theprobability of severe rain fading but also increases the available fade margin.

A digital microwave system always uses regeneration at each intermediate terminal and each active repeater.

Consequently, fading is NOT accumulative over a multi-hop system. Each hop stands alone. This, of course istrue for noise, interference and other physical phenomena as well. Adaptive equalization reduces the multipathdepressive effect. In the absence of specific route information during design, one may allot 50% of the totaloutage time to propagation fading (multipath, rain, up fades), 25% to obstruction fading and 25% to equipmentand man-made failures. Up fading refers to the constructive (rather destructive) effects of multipath andintroduces some additional problems.

The magnitude of the received signal varies continuously. This variation could cause a higher BER however ifthe system is designed with an adequate fade margin, this will be infrequent and not objectionable. Under


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extreme conditions a fade can cause service failure, but transmission usually returns to normal in a short time.The adverse effects of fading can be reduced through the use of horizontal space diversity and by provision ofadequate fade margin to protect against both multipath and rain fades.


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9 Microwave Path Losses And Gains

9.1 Definition

Path loss: In a communication system, the attenuation undergone by an electromagnetic wave in transitbetween a transmitter and a receiver. Path loss may be due to many effects such as free-space loss, refraction,

reflection, aperture-medium coupling loss, and absorption. Path loss is usually expressed in dB.

9.2 Free Space Loss

The signal attenuation that would result if all absorbing, diffracting, obstructing, refracting, scattering, andreflecting influences were sufficiently removed so as to have no effect on propagation. Free-space loss isprimarily caused by beam divergence, i.e. signal energy spreading over larger areas at increased distancesfrom the source.

Typically when calculating free space loss it is assumed that the signal is being transmitted from a type ofantenna called an isotropic radiator. A simple way to visualize this is to imagine that a signal is beingtransmitted from a single point. Since free space loss is distance dependent, the power of the signal is equal atall points equi-distant from the transmitting point. This can be visualized as a sphere whose surface represents

equal signal powers. Figure 1 illustrates this spherical radiation of the transmitted signal's energy.

Figure 13 Received Power From An Isotropic Antenna

The density of signal power distance d from the transmission point is given by the equation:

2

24)( mWatts

d

PdP

t

=

Equation 7

Where:Pt transmitted power in Watts,

d distance in meters

From this equation it can be seen that the density of the transmitted signal's power drops with the square of thedistance.

Equation 7 represents transmitted signal power as a density, much like flux in electromagnetics.In order to compute the received power we must know the area of the receiving antenna. Representing antennaeffective area by Ae the received power is given by the following equation:


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Wattsd

AP et2

r

4P

=

Equation 8

It is evident from the previous section that an isotropic radiator is very inefficient for transmitting signals. Theproblem is that most of the transmitted signal's energy is not radiated towards the receiver. If, somehow, thetransmitted signal's energy could be focused in the direction of the receiver, the power of the received signal

should be increased dramatically. In fact, this is what is done in microwave we use directional antenna.

)4( =

yerIntensitAveragePow

yerIntensitMaximunPowG Equation 9

Where G directive gain of the antenna. his gives a measure for how well the antenna collects andfocuses the transmitted signal's energy.

Because the antenna amplifies the transmitted signal, the power of the signal radiating from the antenna iscalled Effective Isotropic Radiated Power (EIRP).

ttGPEIRP = Watts Equation 10

Figure 14 Antenna Radiation Pattern

Because EIRP represents the power of the signal being emitted by the transmitting antenna it can replace Pt inEquation 8:

Wattsd

AEIRP e2

r

4P

=

Equation 11

Note that G is given by:

2

4

eAG

= Equation 12

Where wavelength of the radio wave.

By assuming that the receiving antenna is isotropic (G=1) we can replace the A e term in Equation 11. Doing thisyields the following equation:


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Wattsd

EIRP2

2

r

)4(P

=

Equation 13

If the receiving antenna isn't isotropic then the amount of power received by the antenna can be found bymultiplying Equation 12 by the receiving antenna's gain, Gr. Equation 13 becomes:

Wattsd

GEIRP r2

2

r

)4(P

=

Equation 14

or

Wattsd

GGP rtt2

2

r

)4(P

=

Equation 15

Expressing Equation 14 and Equation 15 in dB:

+=

ddBGdBEIRPdB r

4log20)()()(Pr Equation 16

or

++=

ddBGdBGdBPdB rtt

4log20)()()()(Pr

Equation 17

The collection of terms

2

4

d

is called Free Space Loss.

dB

Abbreviation for decibel(s). One tenth of the common logarithm of the ratio of relativepowers. The ratio in dB is given by:

where P 1 and P 2 are the actual powers.

dBm

Abbreviation for dB referenced to one mill watt.

9.3 Link Budget


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A link budget is a term used to identify the gains and losses in a wireless communication system. The goal of alink budget is to balance the system gains and losses to achieve a desired performance objective at thereceiver. One method of performing a link budget is to view the gains and losses of a communication system asa single system gain.

Mathematically, system gain is:

minCPG ts = dB Equation 18

Where:Gs is system gain in dBPt is the transmitter output power in dBmCmin is the minimum receiver input power (dBm) for a given quality objective.

9.4 System Gain

In its simplest form, system gain is the difference between the nominal output power of a transmitter and the

minimum input power required by a receiver. System gain must be greater than or equal to the sum of all thegains and losses incurred by a signal as it propagates from a signal source to a receiver. The system gains andlosses are shown below:

rtbfpmts GGLLLFCPG +++= min dB Equation 19

Where:

Gains:Gt transmit antenna gain relative to an isotropic radiator [dBi]Gr receive antenna gain relative to an isotropic radiator [dBi]

Losses:Lp free-space path loss between antennas [dB]

Lf waveguide feeder loss between the distribution network and antenna [dB]Lb total coupling or branching loss in the coupler and filters [dB]Fm fade margin for a given reliability objective

9.4.1 Plane Reflector Insertion Loss

When using passive repeater in the Microwave link the required system gain should be obtained by acombination of increasing passive gain and the gain of the two antennas at the end of the link. The PlaneReflector insertion loss can be calculated as:

( ) GLLLIL ppp ++= 21 dB Equation 20

Where:IL Insertion Loss [dB]Lp overall free-space path loss [dB]Lp1 free-space path loss of the hope from site 1 to the passive site [dB]Lp2 free-space path loss of the hope from site 2 to the passive site [dB]G Reflector Gain relative to an isotropic radiator [dBi]

( )2coslog20log20log408.42 +++= aAfG dB Equation 21


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Where:f frequency in GHzAa Area of the reflector in m

2

True angel between the paths

9.4.2 Back-to-Back Antenna Insertion Loss

The Back-to-Back insertion loss can be calculated as:

( ) GLLLIL ppp ++= 221 dB Equation 22

G Antenna Gain relative to an isotropic radiator [dBi]

Transmission Lines

Transmission lines provide the means of coupling the transmitter and receiver to theantenna. There are two types currently available: waveguide and coaxial cable. Theradiated output power of the transmitter will be substantially reduced if the transmissionline is incorrectly used or if its length is too long, so precautions should be taken to usethe correct type of line for the radio equipment used, and to keep all transmission linelengths short.

Waveguide

A waveguide is a hollow metal duct, which conducts electromagnetic energy. This type oftransmission line can be used for distances of a few feet up to several hundred feet. Atypical type of waveguide has a loss from about 1.7 dB per hundred feet at 6 Gigahertz(GHz) to about 3.0 dB per hundred feet at 11 GHz. It is used at microwave frequencies

above 2 GHz and can have either a rectangular, elliptical, or circular cross-section,depending upon the system operation requirements. The length of a waveguide run ismore critical at higher frequencies since attenuation increases with frequency.


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Figure 15 Different Types Of Waveguides

Coaxial Cable (coax)

At low microwave frequencies, 2 GHz or less, coaxial cable can be used as theconnecting facility between the transmitter, receiver and antenna instead of waveguide.The loss of coaxial cable depends on the type of conductor, the cable diameter, the type

of dielectric, and the operating frequency. Coaxial cable with a diameter of one inch ormore should be used for long cable runs; 7/8" diameter coax can be used satisfactorilyfor short runs. The coaxial cable can have either a pressurized air or expandedpolyethylene (foam) dielectric between conductors, however, the air dielectric coaxialcable has less attenuation for a given diameter. In general, pressurized air dielectriccoaxial cable is used with higher capacity systems because the return loss characteristicsof foam dielectric lines may be a significant distortion contributor in such systems. This isnot usually a consideration in systems of low channel capacity. The cost of coaxial cableis less than waveguide and should be used when possible. Extreme attenuation of radiosignals above 2 GHz in the coaxial cable generally prohibits its use at the highermicrowave frequency bands.

When we have Microwave outdoor units (ODU), coaxial cables are used to connect

Indoor Units (IDU) with outdoor units.

Directional Coupler

A transmission-coupling device for separately sampling (through a known coupling loss)either the forward (incident) or the backward (reflected) wave in a transmission line. Adirectional coupler may be used to sample either a forward or backward wave in atransmission line. A unidirectional coupler has available terminals or connections for


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sampling only one direction of transmission; a bi-directional coupler has availableterminals for sampling both directions.

Filter

A device that attenuates certain frequencies and usually has some inherent attenuationfor all frequencies. Filters cause a slight reduction in signal strength for desiredfrequencies, while producing a dramatic reduction in signal strength for undesiredfrequencies.

9.5 Fade Margin

Essentially, the fade margin is an estimate of the additional power required to meet performance requirementsin a wireless link. The Fade margin is a fudge factor included in the system gain that considers the nonidealand less predictable characteristics of radio wave propagation. Multipath, terrain sensitivity, and abnormalatmospheric conditions alter the free-space path loss and are usually detrimental to the overall system

performance. For this reason, an additional margin in added to the system loss. One method for estimating theamount of required fade margin is called the Barnett-Vignant reliability equation.

Barnett-Vignant reliability equation for calculating the fade margin is:

( ) ( ) ( ) 701log106log0log30101010

+= RfBADFm 1 dB Equation 23

Where:D distance in kmf frequency in GHzR reliability expressed as a decimal (i.e. 99.999 %)A roughness factor (smooth = 4, average = 1, very rough = 0.25)B weather conversion factor (worst month case = 1, hot humid area = 0.5, average inland area = 0.25,very dry or mountainous area = 0.125)

Miscellaneous losses from circulators, radomes, and antenna system misalignment should be accounted for inmaking fade margin calculations.

Field Margin

This is a safety factor, which represents the long-term degradation of antenna orientationin a practical installation. A typical value would be in the order of 1 dB.

The result of subtracting the losses and adding the gains between the transmitter and receiver will give thesignal power in dBm at the receiver input. The fade margin is the difference between the received power leveland the power level required to produce a given Bit Error Rate (BER). Fade margins for digital radios are basedon Receiver Sensitivity specified in dBm for a particular BER; for example, a certain radio may have a receiver

sensitivity of -74 dBm at a BER of610 .


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Flat or Thermal Fade Margin

Is the difference between the threshold (for a given BER) and the receive level undernormal conditions.

Depressive Fade Margin

Is a number that reflects the response of the radio to multipath interference phenomena.Depressive fade margin is equipment dependent and is provided by the equipmentmanufacturer. A radio may have a Depressive Fade Margin of >49 dB at a BER of 10

-6.

Composite Fade Margin

Is used for calculations of availability. Composite Fade Margin is the power summation of

the flat fade margin and the depressive fade margin.

9.6 Reliability or Availability

Reliability or Availability of a digital microwave radio path relates to the time a given microwave link isoperational during a specified period of time, typically a year. One should expect to achieve a reliability (oravailability) of 99.999% or better. The following table illustrates the relationship between outage time andreliability:

Outage time per

Reliability Outage time Year 3 month Month (Avg.) Day (Avg.)

99.9 0.1 8.8 hours 2.2 hours 43 minutes 1.44 minutes

99.99 0.01 53 minutes 13 minutes 4.3 minutes 8.6 seconds99.999 0.001 5.3 minutes 1.3 minutes 26 seconds 0.86 seconds

99.9999 0.0001 32 seconds 38 seconds 2.6 seconds 0.086 seconds

9.6.1 Unavailability Standards

Unavailability has a special meaning in the ITU standard. According to the ITU-R, the period of unavailable timebegins when, in at least one direction of transmission, one or both of the following conditions occur for 10consecutive seconds: either the digital signal is interrupted or the BER in each second is worse than 1x10

-3.

These 10 sec are considered part of the unavailable time.

9.6.2 Causes Unavailability

Unavailability is caused by:

1. Propagation problems2. Human errors, and3. Equipment faults.

The Availability of the equipment is give by:


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( )( ) %100+= MTTRMTBFMTBFA Equation 24

WhereA AvailabilityMTBF Mean time before FailureMTTR Mean Time to Restore

The Availability of a protected equipment is give by:

( )( ) ( )( )BBBAAAHSB

MTTRMTBFMTBFMTTRMTBFMTBFA ++= Equation 25

9.6.3 Unavailability calculation for loop protected network

Unavailability in a loop can be approximated by the formula:

= +==

N

Mii

M

iiM ppP

11Equation 26

WherePM Unavailability of stationN Number of hops in loopM Consecutive number of hop from the hubP Probability of outage or unavailability of a hop in absolute terms (Propagation, human errors andequipment faults).

9.7 Report

Here is a report for the design of a Microwave Link:

INPUT DATA


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CALCULATED DATA:

9.8 Design Optimization

After using a software to estimate the performance of a path under consideration, you should compare yourresults with the predefined quality objectives to determine whether path optimization is required. If you need tooptimize the output, you can adjust antenna sizes and/or power output, or implement the use of space orfrequency diversity.


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10 Network Topologies

The specific physical, i.e., real, or logical, i.e., virtual, arrangement of the elements of a network. Two networkshave the same topology if the connection configuration is the same, although the networks may differ inphysical interconnections, distances between nodes, transmission rates, and/or signal types. See Figure 16.

10.1 Star Topology

The Star Network's main building blocks are multiple hub sites positioned in strategic locations. The hub siteshould usually be limited to serving a maximum of six or seven cell sites to maintain good network reliability.

10.2 Ring Topology

Ring in a network infrastructure provide diverse routing, so increasing the transport system's reliability. As thenetwork's number of cell sites grows, a system of rings should be established between major hub sites andswitches to increase survivability and reliability. This is part of the SDH/SONET Networks.

Figure 16 Network Topologies

10.3 Implications to topology

The following rules should be followed in the Microwave network design:

1. Even number of sites in a loop

2. The angle between hops > 603. Avoid several consecutive hops in one direction.

10.4 Implications of rain on loop topology

The following rules should be followed in order of avoiding the implications of rain fading:


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1. The diameter of a loop > 3 km

2. The angel between hops > 603. Do not cross hops belonging to one loop.


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11 Frequency Planning1

11.1 Definition

Interference is any unwanted signal that would present itself to the receiver section of a radio for demodulation.It can be a delayed copy of the radio links own signal, an adjacent channels signal traveling over the same link,

or a signal from another radio link or RF source.

11.2 Causes of Interference

11.2.1 Internal Causes

Internal causes are those causes that relate to the equipment at the site itself. This includes radio equipmentparameters such as the transmit and receive local oscillator, filter selectivities etc.

11.2.2 External Causes

External causes result from sources from that are seldom under the designers control. These include

interference from other systems that are already installed. It also includes interference from a distance site thatforms part of the same route this aspect can be controlled by the system designer.

11.3 Effects of Interference

In un-faded condition, digital receivers are very robust against interference mechanisms. Unlike analogsystems, however, the main interference problem occurs in a faded condition where the signal levels approachthe receiver threshold values. The interference effect is thus not in terms of its absolute signal amplitude but interms of the ratio between the wanted (carrier) signal and the unwanted (interference) signal, expressed as C/I.

11.3.1 Co-channel Interference

Interference resulting from two or more simultaneous transmissions on the same channel. In a digital system

there is a certain minimum C/I ratio (C/Imin) above which the BER is constant and below which the performancequickly becomes unacceptable. This depends very much on the modulation scheme: A simple example 4 PSKsystem requires only 15 dB whereas a 128 QAM system requires at least 30 dB.

11.3.2 Adjacent-channel interference

Extraneous power from a signal in an adjacent channel. Adjacent channel interference may be caused byinadequate filtering, such as incomplete filtering of unwanted modulation products, improper tuning, or poorfrequency control, in either the reference channel or the interfering channel, or both.

11.4 Frequency Channel Planning

11.4.1 Basic ITU Arrangements

The ITU-R Recommendations specify the center frequency of the band, the T/R spacing, the adjacent channelspacing and the number of channels.

The center frequency (f0) is the mid-band frequency around which the channels are arranged. A number ofchannels with a specific channel spacing are identified across the frequency band of that specific plan. A simpleexample in the frequency band 7425 MHz to 7725 MHz using f0 of 7575 MHz is calculated as follows:

1The book Microwave Radio transmission design Guide was used to write this chapter.


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nffn

+= 71540 and nffn ++= 77' 0 Equation 27

It can be seen that the channel spacing is 7 MHz and the T/R spacing is 161 MHz. Radio equipments duplexerwill usually only support one T/R spacing.

11.4.2 High/Low Arrangements

For Radio links one is always working with pairs of channels. A signal is transmitted from Site a to Site b with acertain transmit frequency and Site As receive frequency is the transmit frequency from Site B. These aretermed go and return channels. The go channels transmit in the lower half of the plan and are sometimesreferred to as transmit low and are designated as fn, where n is the channel number. The return frequenciestransmit high, and are designed as fn.

This is very important for the frequency plan. For a specific frequency band all the kinds must transmit eitherhigh or low.

11.4.3 Alternate Polarization

In fully developed routes, alternate polarizations are in the Microwave Network design.

11.5 Frequency Re-Use

Frequency re-use refers to a situation where the same frequency pair is re-used in a route.

11.5.1 Two-Frequency (One-pair) Plan

One needs to consider the interference, which results from using a single of frequencies is used throughout aroute. Two types of interference are there: the interference at the repeater (nodal) site and the problem at sitesfurther down the route (overshoot).

11.5.1.1 Nodal Sites

The only way to achieve frequency re-use is to use high-performance antennas with a good Front/Back ratio.

11.5.1.2 Overshoot

Overshoot problems are solved by alternating the polarization every two hops. This plan is shown in Figure 17.

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