Aviat StarLink 5_1 User Guide

OCTOBER 2011

STARLINK 5.1 User’s Guide

2 AVIAT NETWORKS OCTOBER 2011

STARLINK 5.1 USER GUIDE

COPYRIGHT Copyright © 2011 by Aviat Networks, Inc.

All rights reserved. No part of this publication may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any language or computer language, in any form or by any means, electronic, magnetic, optical, chemical, manual or otherwise, without the prior written permission of Aviat Networks Inc. To request permission, contact [email protected].

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Aviat Networks makes no representation or warranties with respect to the contents hereof and specifically disclaims any implied warranties or merchantability or fitness for any particular purpose. Further, Aviat Networks reserves the right to revise this publication and to make changes from time to time in the content hereof without obligation of Aviat Networks to notify any person of such revision or changes.

Trademarks

All trademarks are the property of their respective owners.

CONDITIONS OF SOFTWARE USE By executing this program, users agree to the following conditions:

Aviat Networks grants the user a non-exclusive license to use the software free of charge on an "as-is" basis, without warranty of any kind. The entire risk as to software and the quality and accuracy of its results are borne by the user. Because of natural geoclimatic and other factors associated with microwave path engineering not under Aviat Networks' control,, its results and analyses do not constitute a guarantee on the part of Aviat Networks for the performance of its manufactured equipment or of other manufacturers products it resells. All results of the computations are the responsibility of the user who will not hold Aviat Networks at fault for any direct or indirect consequences of the program's use.

The program is the property of Aviat Networks wherein title, ownership rights, and intellectual property rights in the software remain with Aviat Networks. A license is given for its use that will terminate on failure to comply with any of these described limitations. No part of the program may be copied for sale or resale, translated, modified, reverse engineered, or transferred to any printed or electronic medium without the prior consent of Aviat Networks.

It may only be copied for archival use and given in its entirety for use by third parties. Proprietary notices may not be removed.



TABLE OF CONTENTS

INTRODUCTION ....................................................................................................................................... 5

FEATURES ................................................................................................................................................ 6

MAIN FEATURES ................................................................................................................................................... 6

PARAMETERS MOST AFFECTING THE MICROWAVE HOP’S PERFORMANCE ............................................. 7

ADDITIONAL FEATURES ...................................................................................................................................... 8

AUTOMATIC UPDATE VIA THE INTERNET ...................................................................................................................................... 8

FASTER SPECIFICATIONS OF A NEW PROJECT .......................................................................................................................... 8

DEFAULT VALUES FOR NEW PATH CALCULATIONS WORKSHEETS ....................................................................................... 8

THE STARLINK WORKSPACE ................................................................................................................ 9

MENU BAR ............................................................................................................................................................. 9

CURRENT PROJECT SCREEN .......................................................................................................................... 10

USING STARLINK ................................................................................................................................... 11

DEFINING A NEW PROJECT .............................................................................................................................. 11

OPENING AN EXISTING STARLINK 5 PROJECT .............................................................................................. 12

EDITING PROJECT SETTINGS........................................................................................................................... 12

EDITING EXISTING PATH DATA ........................................................................................................................ 13

REMOVING A PATH FROM A PROJECT ........................................................................................................... 13

PERFORMING PATH CALCULATIONS .............................................................................................................. 13

PRINTING A PATH CALCULATION SHEET ....................................................................................................... 15

BROWSING RADIOS AND ANTENNAS DATABASES ....................................................................................... 15

USING THE RADIO FINDER WIZARD ................................................................................................................ 15

QUICK START ......................................................................................................................................... 16

VIGANTS C-FACTOR PATH CALCULATIONS “WHAT IF?” SCREEN ............................................................... 22

ITU-R P.530-13 PATH CALCULATIONS SCREEN ............................................................................................. 24

REFERENCES ........................................................................................................................................ 29

TUTORIAL ............................................................................................................................................... 30

MICROWAVE RADIO PATH CALCULATION OVERVIEW ................................................................................. 30

AZIMUTHS & PATH LENGTHS............................................................................................................................ 31

FREE-SPACE PATH LOSS .................................................................................................................................. 32

PATH LOSS (ENGLISH UNITS) ......................................................................................................................................................... 32



PATH LOSS (METRIC UNITS) ........................................................................................................................................................... 32

UNFADED (“FREE SPACE”) RECEIVE SIGNAL LEVEL .................................................................................... 32

DESIGN FADE MARGIN ...................................................................................................................................... 33

DISPERSIVE FADE MARGIN .............................................................................................................................. 33

SHORT-TERM OUTAGE CAUSED BY MULTIPATH FADES ............................................................................. 35

PROBABILITY OF OUTAGE CALCULATIONS (VIGANTS K•Q FACTOR, METRIC UNITS) ...................................................... 37

PROBABILITY OF OUTAGE CALCULATIONS (ITU-R P.530-13 DETAILED LINK DESIGN, K GEOCLIMATIC FACTOR,

METRIC UNITS) .................................................................................................................................................................................. 38

DIVERSITY IMPROVEMENT COMPUTATIONS FOR VIGANTS AND ITU-R P.530-13 PATH CALCULATIONS

.............................................................................................................................................................................. 39

SPACE DIVERSITY IMPROVEMENT FACTOR ISD (ENGLISH UNITS)......................................................................................... 39

SPACE DIVERSITY IMPROVEMENT FACTOR ISD (METRIC UNITS) ........................................................................................... 40

ANGLE DIVERSITY IMPROVEMENT FACTOR, IAD ........................................................................................................................ 40

SELF-HEALING RING (“ROUTE DIVERSITY”) IMPROVEMENT FACTOR, ISR ................................................. 40

FREQUENCY AND HYBRID DIVERSITY IMPROVEMENT FACTORS IFD, IHD .................................................. 40

REQUENCY DIVERSITY IMPROVEMENT FACTOR IFD (ENGLISH UNITS) ................................................................................ 41

FREQUENCY DIVERSITY IMPROVEMENT FACTOR IFD (METRIC UNITS) ................................................................................ 41

EFFECTIVE FREQUENCY DIVERSITY SPACINGS IN N+1 HOPS, ΔFEF .......................................................... 41

MULTIPATH PERFORMANCE OUTAGE OBJECTIVES ..................................................................................... 42

NORTH AMERICAN OUTAGE OBJECTIVES, BELL .......................................................................................... 42

ITU-R F.634 OUTAGE OBJECTIVE, PDH SYSTEMS ......................................................................................... 43

ITU-R F.1668 OUTAGE CALCULATIONS, SDH SYSTEMS ............................................................................... 44

RELIABILITY AND AVAILABILITY OVERVIEW ................................................................................................... 44

ATTENUATION BY ATMOSPHERIC GASES ...................................................................................................... 45

RAIN OUTAGE ..................................................................................................................................................... 46

RAIN ATTENUATION OVERVIEW ...................................................................................................................... 47



INTRODUCTION Starlink 5 is a major update of the Aviat Networks Starlink 4 program that adds MS Windows 7 compatibility, improves user and database functionality, reintroduces User-Selected Antennas and Feeders and hybrid diversity functionality, eliminates known installation difficulties, and corrects a number of computational errors. It is an intuitive, easy-to-use path engineering tool for estimating the multipath performance and rain availability for line-of-sight 5-38 GHz digital microwave radio hops.

Over 2000 radio, antenna feeder systems, and protection/diversity arrangements are quickly accessed from Starlink’s unique single “What If?” Path Calculations screen that tab-accesses full color North America and Worldwide geoclimatic factors, average annual temperatures, and rain regions pull-down maps. Starlink 5 accommodates North American (ANSI T1) and CEPT (ETSI E1) TDM and Ethernet/IP transport standards, English and Metric measurements, and Vigants and ITU-R P.530-13 “detailed link design” performance models.

Although Starlink 5 does not have the graphics capability necessary to construct profiles or do path geometry calculations, as an important feature all paths (or an individual path) are clearly displayed on a full color Google map.

In Starlink 5 you can select either the Vigants-Barnett (“Vigants”) computational model with its geoclimatic c-factor or ITU-R K●Q factor, or the ITU-R P.530-13 “detailed link design” model with its geoclimatic factor K for annual and worst-month multipath outage time, annual path reliability, and other short-term performance calculations for hops with and without diversity improvements.

Starlink 5 uses the R. F. Crane model to compute long-term rain outage for millimeterwave band hops, and from that the hop’s rain availability (“traffic uptime”). Starlink 5 can be expected to provide an estimate of the hop’s median free-space receive signal levels and fade margins within 2 dB, providing the path is line-of-sight and non-reflective with optimally sized and aligned antennas and not heavily affected by signal entrapments in ABL (atmospheric boundary layer) ground-based ducts.

Starlink is intended for initial path design and estimates of its performance and rain availability. It is not a substitute for a path survey and engineering propagation analysis as performed by Aviat Networks.

Starlink 5 runs under Windows 2000, Windows XP, Windows Vista, and Windows 7, and can print path data sheets on a Windows compatible printer. Starlink 5 is best obtained by downloading directly from the Aviat Networks Worldwide Web home page at http://www.aviatnetworks.com/products/tools/wireless-engineering-starlink/ to ensure that the current program and its updated product database are used. The Aviat Networks product database is in a separate file that is not accessible for user changes or modifications, but which along with the Starlink program its database is updated regularly and posted on Aviat Networks’ FTP site for user access (tab-selectable on the opening screen) as product additions or specification changes are made.

Please note however that the original Starlink 4 program should first be uninstalled as it is not supported by Windows 7and cannot be updated to the new Starlink 5 on the Aviat Networks FTP site.

Inquiries regarding this program can be made by e-mail to [email protected] or inquiries to your Aviat Networks sales engineer.



FEATURES

MAIN FEATURES Starlink has been designed to help you determine the most cost-effective, reliable microwave path arrangement appropriate to your requirements.

With Starlink, you can perform the following operations:

1. Define a project 2. Add paths to the project by defining their site latitude/longitude coordinates or by entering their

path lengths. 3. Display the sites and path(s) on a full color Google map for single or all paths established by their

site coordinates 4. Automatically compute the appropriate Vigants or ITU-R P.530-13 “detailed link design”

geoclimatic factors for the region. 5. Select the radio with its protection scheme, diversity arrangement, and transport capacity, and

antennas and feeders, appropriate to the hop’s 5-38 GHz frequency band. 6. Perform multipath performance (SESR outage probability, worst month and annual outage time,

annual path reliability, etc.) and for millimeterwave hops rain availability calculations. 7. Meet your multipath performance and rain availability objectives by optimally adjusting the radio,

antenna feeder system and path parameters on a single “What If?” Path Calculations screen. 8. Preview and Print the Path Calculation Sheets.

Starlink incorporates the Aviat Networks’ radio equipment database for North American TDM, CEPT TDM, and Ethernet/IP data rates, as well as adaptable antenna and feeder databases. A Radio Finder Wizard helps you select the appropriate Aviat Networks radio corresponding to your specifications such as frequency, standard T1 or E1 capacity or for Ethernet/IP transport, power level, and protection configuration.

Starlink has an AutoCalc that allows you to perform instantaneous “What If” changes to your path calculations, including the radio frequency, transport capacity, transmitter power level, antenna gains, path length, rain zone and path polarization, and any other input parameter.

When the value of the input parameter is modified you will instantly see the impact on path outage, reliability, and rain availability predictions on Starlink’s Path Calculations screen.

Starlink incorporates the latest methods and techniques for path calculations, including:

1. Inverse Position Azimuth and Path Distance calculations, of accuracies acceptable to the U.S. FCC and all other regulatory bodies, in both metric and English units.

2. Absorption (attenuation) caused by atmospheric humidity and gases. 3. Rain attenuation computed by the Crane algorithm using either Crane or ITU-R rain region tables

for annual rain outage time and path availability. 4. Multipath outage probability (SESR) and Worst Month outage predictions using either a Vigants

model or the ITU-R P.530-13 model for ITU-R regions. 5. Annual multipath outage predictions and path reliability calculations using either the Vigants c-

factor model, or the ITU-R P.530-13 model based on Fade Season, a function of the hop’s Annual Average Temperature, for North American and ITU-R regions.



PARAMETERS MOST AFFECTING THE MICROWAVE HOP’S PERFORMANCE

Typical Fixed Wireless Hop – a Checklist

In order of their importance to its performance, the following are essential to the deployment of a well performing digital microwave hop:

1. Adequate path clearance that respects the area’s geoclimatic conditions in order to mitigate long-term losses of fade margins due to signal entrapments in ground-based ABL (atmospheric boundary layer) ducts that greatly increase multipath fade outages.

2. Optimum diversity separations (space, frequency) to reduce or eliminate short-term multipath outages on paths with exposed specular ground or water reflections.

3. Optimum antenna sizes and exacting alignments of larger antennas, to best provide discrimination to multipath signals and to mitigate antenna decoupling (k-factor angle-of-arrival) power fades on longer hops that greatly increase multipath fade outages.

4. Adequate fade margin (system gain – Tx power out + Rx threshold – and antenna sizes) 5. Interference-free hop. Starlink does not consider the effect of threshold (fade margin)

degradations caused by external RF interference on hop performance. If intrastation or other known interference is predicted, a value (e.g. 2 dB) may be entered as a Miscellaneous loss on Starlink’s Path Calculation screen which lowers the hop’s fade margin to accommodate this interference.

Starlink is highly effective in computing the values listed in numbers 2-5 above for most digital radio hops, but separate graphical analyses would be required from the path profiles to establish path clearances and optimum diversity antenna separations and sizes in hops deployed in difficult geoclimatic regions with strong ground-based ABL (atmospheric boundary layer) duct formations, as well as in hops with exposed specular ground or seawater reflections that require optimally spaced main-diversity antennas or frequencies.

THIS PROGRAM AND ITS OUTPUT SHOULD NOT BE VIEWED AS A SUBSTITUTE FOR PROFESSIONAL MICROWAVE PATH ENGINEERING.



Starlink provides you with a valuable tool and the knowledge not only to help you with the preliminary analysis of your microwave paths on your own, but also to enable you to work with your microwave equipment supplier more effectively.

ADDITIONAL FEATURES

AUTOMATIC UPDATE VIA THE INTERNET When Starlink 5 is launched, it can automatically connect to the Starlink FTP site and check if there are newer versions of the program as well as the following files:

• Database (radios, antennas, feeders)

• Help file

• Reports templates

• Offices addresses

• Executable file

Any newer file is automatically downloaded to your PC; the old files are renamed with “backup” appended to their filename. The next time you launch Starlink it will automatically use the newest files.

From Starlink v.2.2 on, if you access the Internet via a proxy server, you simply have to specify the proxy server name before attempting to connect to the Starlink FTP site. Starlink will save your proxy name, and you will not have to specify it again unless you want to change the proxy server name. See PRINTING A PATH CALCULATION SHEET under THE STARLINK WORKSPACE section.

FASTER SPECIFICATIONS OF A NEW PROJECT If you design several projects, they are likely to have some characteristics in common, such as the equipment standard (T1/E1, Ethernet/IP), the measurement system, the reliability calculation method, and so on.

When you define a new project, you can import all these values from the last project you defined. Just check the Import settings from the last project checkbox before opening a new project.

DEFAULT VALUES FOR NEW PATH CALCULATIONS W ORKSHEETS On a given project, it is probable that you will select the same type of radio, antennas, feeders, and other like characteristics for all paths. Starlink avoids having to repeat the same information multiple times.

In Starlink 5, the microwave site’s latitude and longitude coordinates and ground elevation are entered on the Edit Site Data screen once. Hop data are automatically populated on the Edit Path Data screen by selecting the appropriate sites listed. Once you tell Starlink which path to use as a “template”, Starlink will fill any new path worksheet with values from this path.



THE STARLINK WORKSPACE

Opening Screen

MENU BAR The File menu includes a command you can use to convert some earlier Starlink .mln files to the Starlink 5 .mlnx format and to exit Starlink. Not all earlier Starlink projects can be converted to new Starlink 5 format, however, so if these legacy projects are important the retention of the earlier Starlink 3.10 (or other) program is highly recommended for this purpose.

The New Project menu provides commands that enable you to define and design a project

The Open Project menu includes commands you can use to open an existing Starlink 5 project.

The Databases menu includes commands you can use to browse through the radio equipment and the antenna/feeders databases as selected by Starlink. It includes the Radio Finder Wizard command which enables you to view the characteristics of a radio and lets the Wizard find the appropriate Aviat Networks radio(s) for you. In addition, access is provided under the Databases tab to the User defined antennas and User defined feeders directories where the Starlink user can assign new antennas and new waveguide or coax feeders not in the Starlink databases that will appear on the Path Calculations screen for deployment in his system.

FTP Update will access the Aviat Networks.ftp site to inform you if your Starlink 5 program or its database requires updating, and if so will download the necessary files into your computer.

The Help menu displays this Starlink Help file and tutorial.



CURRENT PROJECT SCREEN The opening window changes, as shown below, when a Current Project (under that tab) is selected. The Current Project screen appears. This window gives a snapshot of the current project including number of paths, site names, and path calculations if done and when. Sites and paths are added to the project or edited on this screen, which also provides tabs to View Paths on a Map and access to the Path Calculations “What If” screen.

Current Project Screen



USING STARLINK

DEFINING A NEW PROJECT To define a new project of any number of hops, click on the New Project icon on the main menu bar and the New Product screen appears. You will be asked to give a file name for your project. The name you give at this point will be the name of the file that will contain all the characteristics and data of your project.

To complete the definition of your project, you will enter the following data on Starlink’s Project Definition screen:

• Customer name

• Project name

• Network type (T1 or E1, both accommodate Ethernet/IP)

• Bit Error Rate for the receiver’s outage threshold. 10-3 BER (or LOF – Loss-of-Frame synchronization) is the internationally-defined outage point in a PDH hop, which equates to a 2x10-5 RBER LOP – Loss of Pointer synchronization in SDH hops, although a non-standard 10-6 RBER is now commonly assigned as the receiver’s outage point by many telecom users.

• Temperature unit (oF or oC)

• Distance units (English feet/miles or metric m/km)

• Performance calculation method (Vigants North America c-factor model, Vigants CCIR Rep. 338 K●Q model, or ITU-R P.530-13 model)

• Rain zones table (Crane or ITU-R) for Crane rain outage calculations

• Path polarization

• Rain unavailability or rain outage objective (optional)

• Multipath reliability objective calculation method (Bellcore, ITU-R, etc.)

The rain zone, rain unavailability objective (optional), and path polarization are meaningful only for hops deployed in millimeterwave bands above about 10 GHz. The rain outage objective is specified as sec/year per path. This objective will be shown in the Path Calculations screen, along with the computed rain outage and availability, e.g. 1600 SES/yr = 99.995% rain availability is a typical objective for a millimeterwave hop.

The choices for the multipath performance (path reliability) objective calculation method are:

• Bell Short Haul

• ITU-R Rec. F.634

• ITU-R Rec. F.1668

• User-defined

• None

User-defined objective is specified per path, either as %path reliability (e.g. 99.9995%) or outages/year (e.g. 160 sec/yr) as a typical per-hop short-term multipath outage objective for a short-haul multi-hop systems, i.e. those routes to some 250 miles (400km) in length of up to about 10 tandem hops . A more demanding outage objective, e.g. 99.9998% or 60 sec/yr per hop or even less, is more appropriate for a long-haul system.



Although you can change the project settings later, you should normally keep these initial settings. Some changes may invalidate path calculations already done, as explained in the “Editing Project Settings” topic.

OPENING AN EXISTING STARLINK 5 PROJECT You can open an existing project by clicking on the Open Project icon on the main menu bar.

An open file dialog box is opened and all project file names are displayed. By default, Starlink project .mlnx files are stored in the subdirectory named Projects in the Starlink directory, but additional user-assigned Project folders (subdirectories), e.g. North Region Projects, are also easily accessed in Starlink. The dialog box displays files in the default projects directory. If you saved the .mlnx project file outside of the Starlink folder you will have to give the directory where your project is located.

Once the project is successfully opened, you will see the project Snapshot window, which gives the number of paths in the project, the site names, and an indication if path calculations have been done and when. In addition to its View Edit Site tab, the Snapshot window has other command buttons you can use to quickly perform operations on your project:

• View/Edit Project Settings

• Add Site or Path to Project

• View/Edit Site or Path data

• Path Calculations

• See Paths on a Map

• Remove Site or Path

EDITING PROJECT SETTINGS • To edit Project Settings, click the View/Edit Project settings button on the Current Project screen

Although you can change the project settings anytime, you should not do so if you have already performed calculations on some paths. If you do so, you may need to perform those calculations again, if some of the settings have been changed, as explained below.

Settings that may require calculations to be performed again after project edits include:

• Network type

• Bit Error Rate threshold

• Multipath calculation method

• Rain outage (availability) objective

• Reliability (performance) objective

Starlink will issue a warning if any of these settings are changed when there are paths with calculations already performed.

Starlink will automatically handle changes in the temperature or distance units, but it is strongly recommended that you review these calculations on the paths again to ensure consistency.



EDITING EXISTING PATH DATA To edit existing path data, you can

• Click the View/Edit Path Data button on the Current Project form.

You can change any of the path data. If you have already performed calculations on the path, note that if you change the path length or site coordinates, the path reliability and availability calculations are automatically redone and you may have to make antenna or other changes to meet your path reliability or rain availability objective. Starlink will warn you upon exiting the path editing form, in addition to the next time you open the path for calculations.

REMOVING A PATH FROM A PROJECT You can remove a path from a project by clicking on the Remove Path button, which appears on the snapshot form. Starlink will ask for a confirmation before removing the path. Once removed, a path will no longer be available. If any of the removed path’s sites is not a terminal site for other path(s), it will also be removed.

PERFORMING PATH CALCULATIONS You can access the Path Calculations form by selecting a path on the Current Project form and clicking the Path Calculations button. Path reliability and rain availability calculations are the main features of Starlink. In order to proceed, you will be required to make some choices and input some general and specific information.

All required Inputs appear in white on the form. They are grouped into logical units, within frames that are labeled for easy navigation within the form. Below is a summary list of the inputs:

• Radio Equipment

• Type (modulation, etc.)

• Capacity in T1s, E1s, Mbps data throughput, etc.

• Frequency band

• Protection configuration or non-protected

• Diversity arrangement

• Transmit power level

• Antennas available for the selected frequency band

• Size and gain

• Centerline

• Transmission lines available for the selected frequency band

• Type and loss

• Length



• Climate-related data (all automatically computed in Starlink)

• Vigants climate-terrain factor, c-factor with or without “w” (tab choice) terrain roughness, in North American and International hops (in Starlink pull-down maps); and, optionally, K●Q with “S”, terrain roughness, in ITU-R regions

• ITU-R P.530-13 geoclimatic factor “K” for ITU-R regions, requiring 110x110km area roughness (standard deviation of terrain heights), Sa, and dN1, refractive index exceeded 1% of the year, values computed at the hop’s middle coordinates and inputted in Starlink 5.

• Average annual temperature (in Starlink pull-down map) for annual outage and path reliability calculations in North American or International hops, to determine the length of the fade season and therefore the annual outage time and path reliability

• Rain zone (Crane or ITU-R rain rate tables in Starlink pull-down maps)

• Path Polarization which affects the rain outage (V-pol reduces rain outage by over 50%)

• Losses

• Miscellaneous losses

• Negative losses are entered for higher fade margin or system gain in “What If” studies

• Space diversity improvement factors

• Main and diversity antenna centerlines

• Main and diversity antenna sizes and gains

• Frequency and hybrid (space + frequency) diversity improvement factors

• Frequency spacing

• Hybrid (space + frequency) spacings

Hybrid diversity, a very effective and economical two-way three- or four-antennas/path diversity arrangement, is also available in Starlink for Aviat Networks medium and high capacity radios. Hybrid diversity requires both space diversity and frequency diversity inputs. For indoor radios, e.g. Eclipse IRU 600 and Constellation, the dual antenna end of the hop selected on this Path Calculations screen will appear on the Path Calculations printout. Hybrid diversity is not available in Starlink for tower-mounted (ODU) radios, e.g. Eclipse E300hp, which use the nearly as effective HS+SD protection scheme by assigning dual (vertically spaced) TX/RX RFUs and antennas at both ends of a hop.

All the inputs are not required for all paths. For example, rain-related data (rain region, path polarization, etc.) are required only if transmission frequency is above 10 GHz. Also, all diversity configurations (space, frequency, or hybrid) may not be available for all Aviat Networks radios, or necessary on all paths.

Starlink has a built-in AutoCalc feature; SESR outage probability, path reliability, and rain availability calculations are done as soon as all required data is entered. If any required data are either missing or invalid, a warning is displayed on the form. You may click on the warning button to display what is missing or invalid (if there are many items missing, the first 2 or 3 will be displayed).

If you change the radio equipment or the transmission frequency, some previously entered data may no longer be valid (such as antennas and feeders at the new frequency). Starlink automatically detects this condition, and requires the new input(s).



PRINTING A PATH CALCULATION SHEET Starting in Starlink v.3.7, a new printout module has been released. Starlink now prints through HTML so it is necessary to have access to Internet Explorer for this.

Some configuration is required, please follow the instructions below.

In Internet Explorer, you need to:

» Select the File menu, then Page Setup

» In the Headers and Footers section, remove any entries in the boxes

» In the Orientation section, select Portrait

» In the Margins section, set all margins to 0.5 (inches)

» Click OK to apply changes

Print a Path Calculation Sheet by clicking the Print Preview button on the Path Calculations screen. From this Path Calculations form, you can print the current path calculation sheet. If you need to setup your printer, you can do it from the main menu: select File, then Printer Setup, to access standard Windows printer setup dialog.

The project and path information and calculations are automatically saved until deleted by Starlink.

BROWSING RADIOS AND ANTENNAS DATABASES The Starlink program includes two databases of Aviat Networks digital microwave equipment: the radios database and the antennas database. The antennas database also includes the coax or waveguide feeders that can be used in conjunction with the antennas.

You can browse through these databases by selecting Databases from the main menu, then either the Radio, Antenna, or Feeder submenu. For the radio equipment, using the Radio Finder Wizard is the most effective way of searching.

USING THE RADIO FINDER WIZARD The Radio Finder Wizard is designed to help the user in selecting the appropriate Aviat Networks radio equipment.

To use it, you can either select Databases from the main menu, then Radio Finder Wizard, or click on the Find icon on the main menu bar. The Radio Finder Wizard is also available from the Path Calculations form.

The Radio Finder allows you to specify the selection criteria, and returns the radio equipment meeting these criteria. The criteria are:

• Radio hierarchy (T1 or E1)

• Frequency range

• Capacity and Power level

• Protection configuration or diversity arrangement

For capacity, power level and protection configuration, you can choose not to apply any restriction. Once the criteria are given, Radio Wizard gives a list of all radios matching the criteria. If there is no radio equipment that matches all the criteria, you may relax your constraints: for example, remove the constraint on the power level, capacity or protection configuration.



If you use the Radio Wizard on the Path Calculations window, you can select a radio from the Finder’s list, and export it to the path calculation form.

QUICK START If you are a Windows user with prior experience in designing microwave paths, you can follow these steps to perform calculations on a network project. For others perhaps less familiar with “microwave path engineering” in North America and/or internationally, the tutorial provided on pp. 30-49 would be of great help.

1. Start the Starlink 5 program.

Opening Screen

2. Click on the Databases tab at the top of screen to view the exacting characteristics of all Starlink defined Aviat Networks radios, as well as Starlink defined antennas and waveguide/coax feeders available by microwave band (5-38 GHz) in Starlink’s database. While the Starlink user cannot alter these databases, new antennas and feeders can be easily added by accessing the User defined antennas (below) and User defined feeders tabs. For example, three new antennas with their descriptions and gains have been added by the Starlink user for the 10.7-11.7 GHz band in the following screen:

User Defined Antennas Screen



As seen in the Path Calculations screen extract below, these three new user-defined antennas are now available for deployment in 11 GHz hops:

Path Calculations Screen (extract) with its New User-Defined Antennas

3. Click the New Project button and give a name to your project in the dialog box; this name will be used as your project file name, so it should conform to your system file naming conventions.

New Project Screen

4. Provide the project settings in the Project Definition screen below which, after selecting the Calculation Method - Vigants (c-factor), ITU-R Vigants (K ●Q factor), or ITU-R P.530-13 model, typically (selected inputs may be mixed) encompasses either:

• Typical North American Vigants Model, ANSI standards: T1 trunk, English feet/miles, Fahrenheit degrees, terrain roughness “w”, geoclimatic c-factor for multipath outage, Bell short-haul performance objectives, Crane rain zones, Bell rain availability objective, or

• Typical ITU-R P.530-13 Model, CEPT standards: E1 trunk, metric meter/kilometers, Celsius degrees, terrain roughness S or Sa, geoclimatic K factor for the multipath outage model, ITU-R F.634 or F.1668 performance objectives, ITU-R rain zones, rain availability objective.

• The CCIR Rep. 338 1990 (now ITU-R) Vigants K•Q Factor model can also be selected in Starlink, although with this new P.530-13 model Vigants’ international Rep. 338 model that gives results similar to the Vigants North American model is seldom used.



• Any of these Vigants and ITU models will accept either English or metric inputs

Project Definition Screen

Note that Starlink uses the R. F. Crane rain outage model for all Vigants and ITU-R rain outage and path availability calculations. However, either the Crane North America or the ITU-R worldwide rain rate zones may be selected in this Project Definition screen for these calculations.

Once you have entered the mandatory data on this Project Definition screen, the OK button is enabled; click OK to save. You will notice a new window with the following title Current Project: <Your Project Name>; this window gives you all path and site names, the status of path calculations, and a View/Edit Project settings tab for changes.



Current Project Screen

5. On this Current Project window, click on the Add Site button to define your project's first path. At this step, you can also review and edit the data you provided in a previous Project Definition window by clicking on the View/Edit Project settings button.

6. Press the Add Site (or, for existing sites, View/Edit Site) button on the Current Project screen, and enter the site name, site latitude and longitude coordinates, and ground elevation, if known (required for P.530-13 path calculations). Repeat this for all other sites in the project.

7. While Starlink will compute Vigants path performance with some of this data omitted, e.g. ground elevations and site coordinates with Distance entered on the Edit Path Data screen on the Path Calculations screen, the ITU-R P.530-13 model requires all of these inputs.

Edit Site Data Screen



8. On the Current Project screen above, click on the Add Path tab. The following Edit Path Data screen then appears, which is populated upon selecting the desired two sites on the hop..

Edit Path Data Screen

9. All of your sites are available on this Edit Path Data screen under each of the two windows for hop selection, which is especially convenient at a hub site where a number of hops converge. In this star configuration, the hub site is selected only once. In tandem hops, the sites are simply moved from one window to the next as the route is configured.

10. For each site selected, its site coordinates and ground elevation will appear. For each valid hop, click on the Compute Distance and Azimuth tab and then Save the hop to the Current Project window shown above.

11. Or, you can chose to enter a path length rather than site coordinates to complete the Path Calculations, which is convenient as the Path Length can then be changed on the Path Calculations screen to do quick “What If” calculations.

12. For Vigants c-factor or K●Q calculations, if site coordinates are missing, a direct Distance (path length) input is useful because it can be easily changed along with antenna sizes, frequency band, radio type, and diversity protection, etc. on the Path Calculations screen for “What If” studies.

13. Although not needed in Vigants calculations, Ground Elevation (required for P.530-13 calculations) if entered will appear on the Path Calculations printouts. When you are done, click the OK button to save path data.

14. Repeat Step 8 above to include all other paths in the project. (You may perform calculations for defined paths, and add other paths later, if you wish.)

15. On the Current Project window, you should now see the site names of the defined paths, as well as the text NOT DONE under the column Calculations, which will change to DONE later along with the date of the calculations. Select a path by clicking in any cell on the path's row.

16. A useful new feature in Starlink 5 is its See Paths on a Map tab on the Current Project screen. If on-line, upon pressing this button a map showing all of the hops in the Current Project list appears on a map, as seen below. A map for an individual hop that also shows the two site names may be selected from the See Path on a Map tab on the Path Calculations screen.



Path(s) on a Map Screen

17. Using the Print Screen tab on your computer, the Current Project screen’s See Paths on a Map

screen display may be copied and then pasted into Excel or Word to add such annotation as site names and path lengths, then printed as a .pdf document for pasting into reports or attaching to an e-mail.

In the Current Project screen, highlight the path and then click on its Path Calculations button; the appropriate Path Calculations screen, described below, then appears. 18. Provide the data required (white textboxes, drop-down maps and lists, optional calculations, and

maps). A natural way to enter data is to fill the left half of the form first, beginning with the radio equipment (frequency band, transport capacity, modulation type), its transmit power, and then its desired protection and/or diversity scheme.



At this point, you may want to click on the Radio Finder Wizard tab to help you select the radio equipment.

VIGANTS C-FACTOR PATH CALCULATIONS “WHAT IF?” SCREEN Starlink’s ease of operation and powerful computational ability should be evident during the use of this Path Calculations screen.

It is on this screen that all path parameters are selected and changed as needed to meet the user’s performance and rain availability objectives - radio type, transport capacity, frequency band, protection scheme or diversity arrangement, high or low transmitter power output, antenna sizes and heights, diversity spacing, geoclimatic factors, annual average temperature, rain region, path polarization, feeder types and lengths, etc. New “What If?” inputs should immediately change the results shown on this screen; however if not press the Compute button.

Hybrid Diversity Path Calculations Screen Using Vigants Geoclimatic c-Factor

Other input path parameters include miscellaneous losses where, for example, a negative (-) value may be inputted for less loss to emulate a higher transmitter power (perhaps for another radio) or fade margin, as well as changing the hop’s path length if it is not already computed from the site coordinates entered on the View/Edit Site Data Screen and shown on the Edit Path Data Screen.



North American and worldwide pull-down maps are provided under the Maps button for access to Annual Average Temperatures (required for annual outage and path reliability calculations), Geoclimatic c-Factors, and Rain Zones maps. Tabs below the maps select the values inputted to Starlink 5 Path Calculations screen.

. U.S. and International Geoclimatic c-Factor Pull-Down Maps

Optional for Vigants c-factor but required for Vigants K ●Q calculations, C-Factor details and View K●Q Factors tabs (as appropriate to the calculation) provide access to tables wherein Terrain Roughness (w = 20-140ft or S = 6-42m range) from the path’s profile is entered. This, along with the region’s climate factor (0.5, 1, or 2 for c-factor calculations; 1x10-5 to 4.1x10-5 in the K●Q table), automatically derives a more accurate geoclimatic c-factor or K ●Q factor specific to the hop.

For annual two-way rain outage calculations for millimeterwave hops above about 10 GHz, the rain rate is easily selected (and automatically inputted into the Path Calculations screen) from the following North American Rain Region (left) and World Rain Region pull-down Maps:

U.S. and International Rain Rate Regions Maps



ITU-R P.530-13 PATH CALCULATIONS SCREEN Although Starlink’s ITU-R P.530-13 Path Calculations screen below looks somewhat similar to the Vigants Path Calculations screen above, P.530-13’s geoclimatic factor K for its “detailed link design” calculation method are more complex and thus quite different.

Also, while intended primarily for hops using metric values deployed internationally, Starlink’s ITU-R P.530-13 calculations may also be applied to hops deployed in North America as well as in other venues using English units.

Path Calculations Screen Using ITU-R P.530-13 Geoclimatic Factor K for “Detailed Link Design”

As seen below, the ITU-R geoclimatic factor K for ITU-R P.530-13 calculations is automatically computed under the above Path Calculations screen dN1, Roughness tab for access to Starlink’s regional dN1 (dN/dh refractive index, atmospheric density gradient in N-units/km exceeded 1% of the time) and to the on-line worldwide USGS GTOPO30 Sa 110x110 km terrain databases screen shown below.



ITU-R P.530-13 dN1 Refractivity Gradient and Sa 110x110km Terrain Roughness Screen

The hop’s dN1 Point Refractivity Gradient exceeded 1% of the time worldwide database accessed from the hop’s mid latitude and longitude computed from site coordinates as entered in the hop’s Edit Path Data screen is integrated into the Starlink program.

The worldwide terrain database needed for the P.530-13 “detailed link design” algorithm’s 110x110km Terrain Roughness Sa is automatically accessed from the USGS (United States Geological Survey) site by pressing the Go to GTOPO30 data download page tab in the above screen, and then on this screen further selecting the geographical area (“tile”) of interest:

USGS Worldwide GTOPO30 Terrain Data Selection Screen



The GTOPO30 terrain database selected from amongst the 27 quadrants (“tiles”) shown in the above USGS screen is in a .zip file that is imported into the Starlink 5 program’s GTOPO30 folder. After downloading the required tile, click on the Geoclimatic Factors screen’s Uncompress a GTOPO30 File button and double-click the zipped file (in your GTOPO30 folder in the Program Directory), e.g. W140N40.tar, to extract the data. The Sa value, terrain roughness over a 110x110km area, m, will then appear on the Geoclimatic Factors screen - as will the P530-13 geoclimatic K-factor% value on the above Path Calculations screen. 19. For annual multipath performance (outage seconds, path reliability) calculations using either the

Vigants or ITU-R P.530-13 model, the hop’s Average Annual Temperature input is always required to convert Vigants’ Fade Season or ITU-R’s Worst Month multipath outage time to an annual value, and from that the hop’s path reliability, %, and annual outage seconds. Starlink provides the pull-down U. S. and worldwide Annual Average Temperature maps shown below to select the 0F or 0C tab values required for this calculation:

U.S. and Worldwide Annual Average Temperature Pull-Down Maps for Annual Performance Calculations

20. Once all required data are entered, Starlink computes the hop’s median received signal level, fade margin, space, frequency, or hybrid diversity improvements, SESR (outage probability), worst month multipath outage (and with an Annual Average Temperature input) annual multipath fade outage and path reliability, and rain outage and availability in higher bands, as appropriate to the calculation method selected in the Project Definition screen.

21. Input data on the Path Calculations “What If?” screen - antenna size and height, diversity spacing, feeder type and length, path polarization, radio frequency band, transport type and capacity, transmitter power output, etc. - are easily changed to quickly optimize the hop’s performance and rain availability.

22. If the Required Input(s) Missing or Not Valid warning window appears at the lower right, click on the Click Here for Details button to see the appropriate message.



23. You can change any of the path parameters, and see instantly the changes in the hop’s path performance and rain availability. When you are satisfied with the path’s performance and rain availability values, you can click on the Print Preview to view, and then Print or save as a .pdf file, the completed path calculation sheet.

24. Before printing, however, Starlink now (with Starlink v.3.7 and later) requires that you set your Iexplorer.exe (Internet Explorer) or Netscape.exe browser in your Program Directory to display and print html files as described under Help:

A new printout module has been released

Some configuration is required, so please follow the instructions below:

In Internet Explorer, you need to:

» Select the File menu, then Page Setup

» In the Headers and Footers section, remove any entries in the boxes

» In the Orientation section, select Portrait

» In the Margins section, set all margins to 0.5 (inches)

» Click OK to apply changes

25. Click the Done button to save the path calculations and return to the main Starlink window. You can also print the path calculation sheet(s), as shown below, at this level after clicking the Print Preview tab.

26. You can save, export, or send as an e-mail attachment to another Starlink 5 user completed .mlnx files in the Starlink 5 program directory’s Projects (or a user-assigned) folder.





REFERENCES 1. ITU-R Rec. F.557-4, “Availability Objective for Radio-Relay Systems”, Geneva, 1997. 2. ITU-R Rec. F.634-4, “Error Performance Objectives for Radio-Relay Hops Forming Part of a High-

Grade Circuit”, Geneva, 1997. 3. ITU-R Rec P.676-3, “Attenuation by Atmosphere Gases “, Geneva, 1997. 4. ITU-R Rec. P.837-1, “Characteristics of Precipitation for Propagation Modeling”, Geneva, 1997. 5. ITU-R Rec. 838, Specific Attenuation Model for Rain for Use in Prediction Methods, Geneva, 1992 6. CCIR (ITU-R) Rep. 338-6, “Propagation Data and Prediction Methods Required for Terrestrial

Line-of-Sight Radio-Relay Systems”, Geneva, 1990. 7. U.S. Department of Commerce Special Publication No. 8, “Formulas and Tables for the

Computation of Geodetic Positions”, C&GS, Washington, DC, 1963. 8. R. K. Crane, “Prediction of Attenuation by Rain”, Proc. IEEE Trans. on Communications,

September, 1980. 9. S. H. Lin, “Measured Relative Performance of Antenna Pattern Diversity, Antenna Angle Diversity,

and Vertical Space Diversity in Mississippi”, IEEE Globecom ‘88, Hollywood, FL, November 1988 10. W. D. Rummler, “Advances in Microwave Radio Route Engineering for Rain”, ICC 87, Seattle,

WA, June 1987. 11. A. Vigants, ”Space Diversity Engineering”, Bell System Technical Journal, Vol. 54, No. 1, January

1975. 12. ITU-R Rec. P.530-13, “Propagation Data and Prediction Methods Required for the Design of

Terrestrial Line-of-Sight Systems”, Geneva, October 2009. 13. ITU-R Rec. F.1668, “Error Performance Objectives for Real Digital Fixed Wireless Links Used in

the 27500km Hypothetical Reference Paths and Connections”, Geneva, 2006.



TUTORIAL

MICROWAVE RADIO PATH CALCULATION OVERVIEW The basic calculations associated with a Digital Microwave Radio Path Data Sheet are simply the addition and subtraction of relative gains and losses in dB from/to power levels in dBm, to arrive at an expected or desired Composite Fade Margin. The gathering of those numbers and some of the preliminary calculations associated with path length and azimuths is a bit more difficult, depending on access to the correct tables, use of hand-held calculators, and the like. The conversion of the expected or desired Composite Fade Margin into predictable short-term (multipath fade) and long-term (rain fade) Outage Time, SESR, Path Reliability, and Rain Availability requires even more sophistication.

This Starlink Tutorial is intended to provide you a limited overview of the basic calculations associated with the Digital Microwave Radio Path Data Sheet and to compare these results with recognized performance and availability objectives.

Starlink’s short-term (multipath fade) Probability of Outage (SESR or Severely Errored Second - SES - Ratio) and Annual Outage Time computations are based upon Vigants’ widely-used field-verified model as defined in [Ref. 11], most often used for designing microwave links in North American hops with c-factors, but also widely used internationally, and in [Ref. 6] for International CCIR (ITU-R) Rep. 338 hops with K ●Q geoclimatic factors. Both Vigants models provide similar performance results for a given hop.

Starlink’s ITU-R Rec. P.530-13 [Ref. 12] multipath outage prediction method produces results that could divergent a bit from these two Vigants’ outage models, especially for flatland paths deployed in difficult geoclimatic regions.

Starlink’s long-term (rain fade) outage time computations use the Crane rain model, and access Crane’s rain rate tables [Ref. 8] for North American radio-relay hops which are derived from ITU-R but optimized for this region. This Crane model also accesses ITU-R rain regions and rain rate tables [Ref. 4] for international microwave path rain outage calculations.

The ITU-R Rec. P.530-13 rain availability model is not used in Starlink rain outage computations. Although the ITU-R P.530-13 and Crane models are dissimilar, both use similar rain coefficients (for the frequency band and hop polarization) and rain rates so the two models most often do yield somewhat comparable results. Starlink applies the Crane rain model to all rain outage and hop availability calculations for both North American and digital microwave hops deployed worldwide.



AZIMUTHS & PATH LENGTHS This section describes the method used in Starlink for the precise calculation of true north azimuths at each end of a path and its microwave path length when end coordinates (latitudes and longitudes) are accurately known.

If these site coordinates are not known or not entered on the View/Edit Site Data screen and you wish to add or change the path length along with other parameters such as frequency band, radio type, antenna sizes, etc. to exploit Starlink’s powerful “What If” feature in the Path Calculations screen, Starlink will accept a specific path length, in kilometers/miles, as a direct Path Calculations screen input to observe its immediate results. You must be prepared to provide either path length or site coordinates, along with Site Elevations for ITU-R calculations, to successfully complete the Path Data Sheet.

The Azimuths and Path Length calculation method used in Starlink is an adaptation of the "Inverse Position Computation" on page 14 of “Formulas and Tables for the Computation of Geodetic Positions, Special Publication No. 8, Coast and Geodetic Survey [Ref. 7]. Factors (log Bm and log Am) from tables in that publication are used in the Starlink computation. The tables list these factors for every minute of North or South Latitude from 00 to 720, but this degree of accuracy while required of most Regulatory bodies is not needed for microwave path calculations. Following the United States Federal Communications Commission’s approach, we have extracted from the table only those values of log Bm and log Am corresponding to integral degrees from 00 to 720.

The errors introduced by using these factors to the nearest degree, rather than to the nearest minute, of latitude will not exceed more than a few seconds in azimuth and substantially less than a tenth of a kilometer/mile in distance. This degree of accuracy is more than adequate for path calculation purposes.

Since the Inverse Position Method takes into account the oblateness of the earth, it gives more precise values than an uncorrected great-circle calculation method. For paths longer than about 120 kilometers/75 miles, however, the great-circle method (not in Starlink) could be used.

These methods will fail when the two stations have exactly the same longitude. In this case, one station will have an azimuth of 00 while the other an azimuth of 1800. Also, when the two stations have exactly the same latitude, angle W is equal to 00 and need not be calculated. Starlink has introduced a small adjustment into the formulas to allow these calculations to be made under the conditions of equal longitude or equal latitude.



FREE-SPACE PATH LOSS Determining the Free Space Path Loss is the next calculation in the preparation of a Path Data Sheet. Free-space loss is defined as the loss that exists between two isotropic antennas in free space where there are no ground influences or obstructions; in other words, where losses due to refraction, reflection, and diffraction fade activity and atmospheric absorption do not exist. Radio energy is lost with distance because of the triangular spreading of wavefront energy as it travels through space. In accordance with the “inverse-square law”, wavefront area quadruples for each doubling of distance, for a 10 log 4 = 6 dB increase in path loss.

The derivation of the formula for free-space loss based upon spherical geometry is presented in many other references and will not be repeated here. The formula itself, however, is shown below:

PATH LOSS (ENGLISH UNITS) A = 96.6 + 20 log f + 20 log D Where

A = free space attenuation between antennas, dB

f = frequency, GHz

D = path length, miles

PATH LOSS (METRIC UNITS) A = 92.4 + 20 log f + 20 log D Where

D = path length, kilometers

UNFADED (“FREE SPACE”) RECEIVE SIGNAL LEVEL The Unfaded Receive Signal Level is computed by introducing the Net Path Loss (NPL), dB, between a radio transmitter output and the following radio receiver’s input. The microwave radio transmit power is reduced by the NPL as determined by free space and atmospheric absorption losses, antenna gains, and all the feeder and network losses at both ends of the single radio path, link, or hop (preferred). “Hop” is the ITU-R’s definition for a single complete (with antenna feeder systems and radios) microwave radio path, with “Link” defined in ITU-R recommendations as multiple tandemly interconnected hops.

RSL = Pt - NPL

= Pt + Gt - Lfs - Labs + Gr - Lf - Lacu

Where

Pt = transmit power, dBm

Gt = transmit antenna gain, dB

Lfs = free space loss, dB

Labs = absorption loss, dB

Gr = receive antenna gain, dB

Lf = coax or waveguide feeder loss, dB

Lacu = antenna coupling unit (switch, coupler, etc.) and other radio RF losses, dB



DESIGN FADE MARGIN The Thermal (or Flat) Fade Margin (TFM) is the difference between the Unfaded Receive Signal Level and the receiver’s Static 10-6 (factory or field test) Threshold or Dynamic 10-3 (or LOF – loss of frame synchronization point) Outage (as defined by ITU) Threshold, dBm, as measured with back-to-back digital radios devoid of interference and spectrum distortion (dispersion), at this given Bit Error Rate.

Starlink provides you with the ability to select either the radio’s 10-3 BER Dynamic Threshold for outage computations, or the 10-6 BER Static Threshold for other performance and availability computations. This selection adjusts the receiver’s threshold to correspond with the selected BER. The threshold RSL values used in the Starlink template are based on Aviat Networks Digital Microwave Radios. The values are located in a table and are accessed via Windows TM commands based on the data you enter, e.g. threshold BER, frequency, capacity, protection type, etc.

Note that the internationally defined, field-verifiable outage in a PDH (asynchronous) digital radio hop corresponds to a 10-3 BER severely-errored second (SES) or LOF loss-of-frame synchronization event at the receiver’s Dynamic Threshold. This threshold is very near the initiation of a T1 or E1 AIS alarm (loss of frame synchronization) condition in the connected PABX, channel bank, etc. trunk port. The digital radio’s 10-6 BER Static Threshold or operating point is used for in-service radio and interference measurements (with attenuators) and as a measurement of circuit quality, not outage, although it is now often assigned as an “outage” threshold by many users, even though such errored second events are not field-verifiable during dynamic fade activity.

The outage threshold of a SONET or SDH (synchronous) digital hop is defined at its SONET/SDH multiplexer’s loss-of-pointer synchronization (LOP) point coinciding with a 2x10-5 BER for STM-1 and SONET (STS-3) radios. However, since an LOP event in SONET and SDH hops will cause an LOF event in its derived T1 or E1 trunks, digital microwave radio specifications show no difference between this LOP point and a 10-3 BER or LOF threshold point in a radio hop.

DISPERSIVE FADE MARGIN High-clearance digital microwave radio hops with adverse path geometry and inappropriate (too small or misaligned) antennas may be susceptible to Dispersive (spectrum-distorting) fade outages.

Dispersive fading may cause the loss of the digital radio receiver’s quadrature lock (synchronization) even during high RF signal level periods due to amplitude or group delay slope and notch effects resulting in degraded error performance, including short- or long-term outages. Powerful corrective measures that now combat dispersive fading include Space Diversity, Forward Error Correction (FEC), Adaptive Time Domain Equalization (ATDE), and Adaptive IF Slope Amplitude Equalization (ASAE). As the RF bandwidth or the number of signaling states in the RF bandwidth increase, the effect of dispersive fading increases, thus requiring the more sophisticated countermeasures typical of today’s digital microwave radios.

The robustness of a digital radio to dispersive fading is defined by its Dispersive Fade Margin (DFM), a value found in the radio’s data sheets. The DFM for a digital radio is derived from its Signature Curve, constructed by moving a high-level delayed multipath notch through the receiver’s IF passband at depths increased to cause an outage. The DFM values in the radio’s data sheet result from the canceling null generated by the combining of the transmitter’s direct signal with its multipath signal delayed 6.3 nsec (6 feet or 2m).



Starlink adds TFM + DFM (power addition, e.g. 40 dB TFM + 50 dB DFM = 39.6 dB CFM) to derive the hop’s Composite Fade Margin (CFM) used for multipath outage calculations. Unless very high thermal fade margins result with a short path and/or large antennas, the affect of a modern digital radio’s DFM upon CFM, and therefore upon the hop design, should be negligible. However, the radio’s DFM could be heavily degraded on some high clearance paths with inadequate antenna discriminations to the resulting very long-delayed (>10 nsec) multipath signal. The radio’s DFM could degrade from over 50 dB to 40 dB or even much lower if exposed to this long-delayed high amplitude multipath signal.

It is essential that the antenna discriminations (antenna’s frequency band, sizes, and uptilt) to the long-delayed specular reflection lower the echo’s amplitude adequately to meet a minimum 50 dB hop DFM objective for no degradation in the hop’s error performance:

Hop DFM = Radio DFM [related to the multipath signal’s delay time] + Antenna Discriminations

= >50 dB for good performance

Intra-station, intra-system, and inter-hop Interference from other microwave transmitters may also degrade the digital radio’s threshold and, therefore, the hop’s fade margin. Although usually ignored in initial path design, interference is typically limited to that level allowed by the FCC introducing less than 1 dB threshold (and fade margin) degradation into a hop.

Once the Composite Fade Margin has been determined, it is used to compute an expected probability of outage or severely errored second ratio (SESR) and, in North America and many international hop other hops (although not required in ITU-R calculations), the hop’s one-way path reliability based upon annual outage time. Annual short-term outage time from which path reliability is computed is influenced by the effects of:

• Multipath fading, defined by geographical propagation characteristics (climate and terrain) for given locales, and

• Average Annual Temperature defining the 2.1-4.5 month fade season during which the Vigants model assumes that all fade outage event occur for the same geographical region.

CFM is also an input for computing expected the annual two-way Rain Availability in high-frequency hops which accommodates all long-term (>10 CSES - continuous SES - traffic disconnect) rain outage events. Rain availability is computed from:

• Tables which assign multiplier and exponent coefficients based upon frequency and polarization, and

• Rain rate tables based upon thunderstorm and similar high rain rate activity in North America and all worldwide regions.

Of course, equipment, antenna system, and infrastructure failures and manual intervention (switching, etc.) can also cause long-term outage, but the calculation of unavailability (traffic “downtime”) caused by such failures requires MTBF (mean time between failures) and MTTR (supplier) or MTR (user, mean time to restore the traffic) values not in Starlink so are not a part of these Starlink two-way availability calculations.

Rain outage is a “self-healing” unavailability traffic disconnect event; thus unlike long-term outages caused by equipment and infrastructure failures, rain outage is predictable from North American and worldwide rain rate tables in Starlink’s pull-down maps.



Each of these factors has been the topic of numerous studies and papers, and it is not the intent of this tutorial to provide an in-depth discussion of them. If you desire more information than is presented here, please refer to the References section.

SHORT-TERM OUTAGE CAUSED BY MULTIPATH FADES The fade margin of a digital microwave radio hop provides a "safety margin" to protect the microwave signal from the adverse effects (carrier-to-noise degradations) of multipath fading, interference, and rain attenuation. Digital microwave hop fade margins are typically much smaller than for analog radios whose fade margins are often increased to provide baseband quieting (low kTb thermal noise) even in short and other non-fading hops.

The provision of an adequate fade margin and sufficient path clearance to protect against CFM degradations and outage due to surface ducting and earth blocking, plus diversity on longer or otherwise vulnerable paths, makes it possible to achieve per-hop propagation reliabilities with respect to Rayleigh-distributed multipath fading exceeding 99.999%.

Multipath outage and annual path reliability, along with quality (RBER, %EFS, etc.), define digital radio hop performance during traffic availability (“uptime”) periods.

Path availability relates to the time a given microwave hop is operational (traffic is not disconnected) for a specified period of time, typically a year. With the exception of millimeterwave (above 10 GHz) hops deployed in rain areas, the availability objective for users of most microwave hops is near 100%.

Predicted (and measured) only during available (traffic connected) periods, path reliability is a measure of annual short-term (<10 CSES) one-way multipath fade outages occurring over a 2.1-4.5 month fade season.

Table 1: Rain Availability, Multipath Reliability, and Outage Time Conversion Table

Annual Rain Availability

or Path Reliability

%

Outage Time, %

Outage Time per Period

Vigants Model ITU-R Model

Annual 3 Month

fade period

Fade Period, worst month

Fade period, one day in the worst

month

90.0 10.0 876 hrs 876 hrs 292 hrs 10 hrs

95.0 5.0 438 hrs 438 hrs 146 hrs 5 hrs

98.0 2.0 175 hrs 175 hrs 58 hrs 2 hrs

99.0 1.0 88 hrs 88 hrs 29 hrs 1 hr

99.9 0.1 52 min 52 min 18 min 35 sec

99.999 0.001 5 min 5 min 106 sec 4 sec

99.9999 0.0001 32 sec 32 sec 11 sec 1 sec

* Annual rain outage should be that averaged over a 10 or more year period as rain rates are far more intense in some years than others. Probability of multipath outage (SESR), and therefore multipath outage seconds are for a “worst month” (or Any Month) in ITU recommendations, or over a fade season (3 months @ 100C/500F, for North American path reliability calculations).



Method of Calculating Probability of Short-Term Outage In this section, the fractional probability of outage is the probability of an outage occurrence, known internationally as the Severely-Errored Second Ratio or SESR. For computational purposes microwave outages are considered to occur not over a tear but only during a “worst month” in ITU-R calculations, or over a 2.1-4.5 month “fade season” in North American calculations, not annually.

The numbers of multipath fade outages during the “worst month” is therefore SESR x 2.6x106 sec/month.

The numbers of multipath fade outages per year is therefore SESR x seconds in the fade season (2.5 to 4mo x 2.6x106 sec/month), from which annual (over 31.5x106 sec/yr) path reliability is determined:

Path reliability = 100-100x (outage sec/yr)/31.5x106 sec/yr, %.

Path “reliability” is never computed for a “worst month” or any period other than annual.

The term path or hop “availability” is sometimes incorrectly expressed as a percentage synonymous with path “reliability”. ITU-R defines a microwave hop as “unavailable” or “failed” (disconnects traffic) only after ten seconds of continuous outage (>10 CSES), such as with a typical rain outage event, whereas each second of multipath outage (up to ten continuous SES) only lowers path reliability.

Message (VF or data) traffic is lost or disconnected only with long-term unavailability events, not short-term outage events.

PROBABILITY OF OUTAGE (VIGANTS C-FACTOR, ENGLISH UNITS)

In North America, and very often internationally, the equation for the probability of short-term outage in a non-diversity microwave hop during the fade season using English units is as follows [10]:

SESR = c f/4 10-5 D3 10-CFM/10

Where

SESR = One-way probability of outage for a non- diversity hop

f = frequency, GHz


CFM = composite or thermal (flat) fade margin, dB

c = climate/terrain factor (from Starlink’s c-factor pull-down maps)

= 6: influenced by surface ducting (flat, very humid)

= 4: average terrain, very humid climate

= 2: average terrain, humid climate

= 1: average terrain, average climate

= 0.25: mountainous terrain and dry climate

Or

c = x/(w/50)1.3 Where

x = climate factor: 2, 1.4, 1, or 0.5 for very humid, humid, average, and dry climates

= c0.5 from the pull-down map (exception: when c = 6, x = 2)



w = terrain roughness over a 20-140 ft range (50ft = average terrain) from the path profile. The tilting ground of a high/low path computes to a large terrain roughness “w” even if the terrain is smooth, for maximum c = 0.52 (low outage time) even in very humid areas with large climate factors.

PROBABILITY OF OUTAGE (VIGANTS C-FACTOR, METRIC UNITS)

SESR = c f 6x10-7 D3 10-CFM/10

Where

SESR = One-way probability of outage for a non-diversity hop


c = either from the above English Units, or computed as c = x /(S/15)1.3

S = terrain roughness, 6-42m range, (20m average terrain) from the path profile

x = climate factor: 2, 1.4, 1, or 0.5 for very humid, humid, average, and dry climates

= c0.5 from the pull-down map (exception: when c = 6, x = 2)

PROBABILITY OF OUTAGE CALCULATIONS (VIGANTS K•Q FACTOR, METRIC UNITS) Starlink computes the probability of outage (SESR) for international non-diversity radio-relay hops as follows:

SESR = K•Q f D3 10-CFM/10

Where

SESR = one-way probability of outage for a non-diversity hop

f = frequency, GHz



K•Q = geoclimatic factor (see K•Q factors pull-down maps)

= x/(S1.3)

x = climate factor

= 4.1x10-5: Maritime climate, coastal ducting (flat, very humid, high temperature)

= 3.1x10-5: Maritime sub-tropical

= 2.1x10-5: Continental temperate or mid-latitude inland

= 1x10-5: High, dry, mountainous

S = Terrain roughness, 6-42m (20m = average terrain), from the path profile. The tilting ground of a high/low path computes to a large terrain roughness S = 42m even if the terrain is smooth, for a maximum K•Q = 3.2x10-7 (low outage time) even in very humid areas with large climate factors.



Table 2: Vigants Geoclimatic K•Q to c-Factor Conversion Table

PROBABILITY OF OUTAGE CALCULATIONS (ITU-R P.530-13 DETAILED LINK DESIGN, K GEOCLIMATIC FACTOR, METRIC UNITS) For detailed link design applications the ITU-R P.530-13 probability of short-term outage, SESR, exceeded in the average worst month is computed as follows:

SESR = K/100 D3.4 (1 + ξP)-1.03 f 0.8 10 -- 0.000876 hL – CFM/10

Where

SESR = One-way probability of outage for a non-diversity hop

K = Geoclimatic factor (below)

f = frequency, GHz



ξP = Path inclination, mrad (milliradians); 10 = 17.3 mrad

= hH – hL / D

hL = low site antenna height AMSL, m

hH = high site antenna height AMSL, m

The P.530-13 Geoclimatic Factor for “detailed link design” calculations is:

K = 10-4.4 – 0.0027 dN1 (10 + Sa) -0.46

Where

dN1 = dN/dh point refractivity gradient in the lowest 65m of the atmosphere not exceeded for 1% of an average year (autoselection from the worldwide dN1 database in Starlink).



Sa = terrain roughness, standard deviation of terrain heights within a 110x110 kilometer area with 30 sec resolution, from GTOPO30 data. Tab selection in Starlink accesses the USGS worldwide GTOPO30 terrain database for sa calculations

The “K/100” in Starlink’s SESR computation above is necessary because the ITU-R P,530-13 model computes outage probability from the geoclimatic factor “K” expressed as a percentage (%), and SESR as a “probability” is never defined as a percentage. The absence of this “1/100” factor would otherwise increase worst month and annual multipath outage computed in this P.530-13 model by x100.

DIVERSITY IMPROVEMENT COMPUTATIONS FOR VIGANTS AND ITU-R P.530-13 PATH CALCULATIONS For microwave systems needing the added protection of diversity (e.g. space diversity) to meet outage objectives, the calculation for SESR, the non-diversity outage probability, is identical to that above. Diversity typically provides a significant improvement to the path’s reliability (greatly reduces traffic outage time).

The probability of outage (SESR) or "unreliability" for a diversity system is calculated based on the following formula:

SESR (diversity) = SESR (Non-Diversity) / I

Where

I = Diversity Improvement Factor

The diversity improvement factor differs for each type of diversity - space, frequency, hybrid, multiline, etc.

SPACE DIVERSITY IMPROVEMENT FACTOR IS D (ENGLISH UNITS) The space diversity improvement factor, Isd, in Starlink is shown below:

Isd = 7x10-5 f s2 v2 10CFM/10 / D

Where

Isd = space diversity improvement factor

f = frequency, GHz


s = vertical main-diversity antenna spacing, feet

v2 = factor for dissimilar main and diversity antenna gains

The multiplier v2 is computed when the main and diversity paths fade margins differ, as with different main-diversity antenna sizes. In such cases, Starlink will take CFM as the larger of the two fade margins used in calculating SESR for the path. CFM is then the smaller of the two fade margins used to calculate Isd.



SPACE DIVERSITY IMPROVEMENT FACTOR IS D (METRIC UNITS) Isd = 1.2x10-3 f s2 v2 10CFM/10 / D

Where

f = frequency, GHz


s = vertical main-diversity antenna spacing, meters

v2 = factor for dissimilar main and diversity antenna gains (see above)

ANGLE DIVERSITY IMPROVEMENT FACTOR, IA D Starlink diversity improvement calculations do not accommodate Angle Diversity, a method of improving path reliability with a special antenna that is now rarely used. The angle diversity antenna has two feeds vertically offset by about 10, the smaller the better, in a single antenna. Angle diversity is occasionally suggested as a substitute for the venerable two-dish space diversity scheme where tower loading, aesthetics, antenna heights, or tower or building space is limited. Angle diversity may be most effective when path outages are dominated by dispersive fade activity (dispersive fade outage greater than flat fade outage).

However, digital radios are now far more robust to dispersive fade activity, i.e. have DFMs some 20 dB higher, than the 1980s digital radios which inspired the wide use of angle diversity antennas. When used, an angle diversity improvement factor IAD may be assigned, but some extended period of exacting antenna alignment to accommodate the path’s geoclimatic conditions may be required for optimum IAD.

SELF-HEALING RING (“ROUTE DIVERSITY”) IMPROVEMENT FACTOR, ISR Ring (loop) systems will automatically route traffic away from fading microwave paths, and therefore improve the performance (reduce outage) in these networks. While “route diversity” improvements are not in Starlink as it is a “multi-hop” parameter, an Isr = 20 is typical for ring systems.

FREQUENCY AND HYBRID DIVERSITY IMPROVEMENT FACTORS IFD, IHD Frequency or hybrid (frequency+space) diversity is computed in Starlink for hops deployed in regions where regulatory rules so permit, e.g. in electrical utility hops deployed in Canada or NTIA federal hops in the U.S., and in cases where waivers for frequency diversity are granted by the regulatory body.

The hybrid diversity improvement factor, IHD, is derived from the space diversity improvement Isd shown above and the frequency diversity improvement factor Ifd described below:

IHD = ISD + IFD

Above 3 GHz, ISD is typically higher (better) than IFD unless the frequency spacing, Δf, exceeds about 4% (240 MHz or 0.24 GHz in the L6 GHz band, for example).

In hybrid diversity hops, the higher T/R frequencies should always be assigned to the upper antenna at the space diversity (usually the lower elevation) site for optimum performance since IFD would otherwise tend to cancel, rather than add, to the ISD space diversity improvement.

While hybrid diversity is not available in Starlink for ODU (tower-mounted) Eclipse radios, its HS+SD protection scheme that is nearly as effective as HD is selected in Starlink.



The frequency diversity improvement factor, Ifd, in English units is as follows:

REQUENCY DIVERSITY IMPROVEMENT FACTOR IF D (ENGLISH UNITS) IFD = 50 Δf 10CFM/10 / f2 D

Where

Δf = diversity spacing, GHz

f = frequency, GHz


CFM = flat or composite fade margin, dB

FREQUENCY DIVERSITY IMPROVEMENT FACTOR IF D (METRIC UNITS) IFD = 80 Δf 10CFM/10 / f 2 D

Where


f = frequency, GHz


CFM = flat or composite fade margin, dB

EFFECTIVE FREQUENCY DIVERSITY SPACINGS IN N+1 HOPS, ΔFEF If(N+1) = 80 ΔfEF 10CFM/10 / f2 D

Where


f = frequency, GHz


CFM = flat or composite fade margin. dB

ΔfEF = ___________N________________ , GHz N + N-1 + N-2 + … + _1_ Δf 2 Δf 3 Δf N Δf

Where, in N+1 multiline hops:

ΔfEF = Effective FD spacing in IfdN+1 calculations that are done outside Starlink, then entered in the Frequency Diversity Improvement Spacing, MHz (1000 x GHz), window on the Path Calculations screen

N = Number of bearer channels (3, in an N+3 hop)

Δf = Actual RF channel spacing (e.g., 0.160 GHz), starting with

the smallest channel separation if asymmetrical



MULTIPATH PERFORMANCE OUTAGE OBJECTIVES The performance objectives are defined as the SESR (severely-errored second ratio, the probability of a non-diversity short-term multipath fade outage) objective for ITU-R hops and systems. In ITU, no further calculation of actual outage time over a measurement period (e.g., month, year) is necessary. As will be discussed later, the ITU any- or worse-month objective [2] for a microwave system is:

SESR Objective = 0.00054 x Dkm / 2500

Where

Dkm = Path or system length, km

Although it is not required in ITU calculations, the actual outage time over a one-month period can be computed by multiplying the probability of outage, SESR, times the number of seconds in a month (2.6 x106). Annual outage in an ITU hop or system may be computed from the following North American procedure.

In North American calculation procedures, the annual outage time due to short-term multipath fade activity in a microwave hop is computed over a one-year period. In this calculation, actual outage is defined as that occurring over a 2-4.5 month “fade season”:

T = SESR To t / 50

Where

SESR = One-way probability of outage for a diversity or non-diversity hop

T = annual outage, SES/year

To = fade season taken as three months, for a combination of two severe and two moderate fade months, or 8x106 seconds

t = average annual temperature, 0F, extends the fade season in warmer areas and shortens it in cooler climates. Per the Vigants model, t is within a 350F to 750F (20C to 240C) range. Starlink automatically does the necessary 0C to 0F conversions for metric calculations of annual outage and path reliability.

NORTH AMERICAN OUTAGE OBJECTIVES, BELL The Bell one-way 1600 SES/yr end-to-end outage objective for a system may be over links making up a part or whole of a long-haul system, a short-haul system, or other distance. While the following computations are for hop(s) making up a full short- or long-haul system, the 1600 end-to-end SES/yr outage allocation may be proportioned only to the actual number of tandem hops in an actual system.

For example, 1600/20 = 80 SES/yr (99.99975% path reliability) per-hop objective for a 20-hop digital microwave system.



The Bell one-way short-haul T1/E1 trunk outage objective at its 10-3 BER or LOF SES outage point is defined as:

T (short-haul, English) = 1600 x Dmi/250, SES/yr

= 6.4 x Dmi

Where

Dmi = System (T1/E1 trunk) length, miles

T (short-haul, metric) = (1600 Dkm/400), SES/yr

= 4xDkm, SES/yr

Where Dkm = System (T1/E1 trunk) length, kilometers

The AT&T long-haul one-way outage is defined as:

T (long-haul, English) = 1600 Dmi/2000, SES/yr

= 0.8xDmi

Where

Dmi = System (T1/E1 trunk) length, miles

T (long-haul, metric) = 1600 x Dkm/3200 SES/yr

= 0.5xDkm

Where

Dkm = System (T1/E1 trunk) length, kilometers

ITU-R F.634 OUTAGE OBJECTIVE, PDH SYSTEMS The ITU-R probability of outage (SESR) objective [2] for a long-haul reference 2500km/1500mi system (E1 trunk) length is:

SESR = 0.00054 Dkm/2500

Where

SESR = Probability of outage

Dkm = System (E1 trunk) length, kilometers

Although only SESR, not worst month or annual outage seconds (SES/month or SES/year) is specified as an ITU-R objective, T (outage objective in seconds) is computed from SESR:

Tmo = SESR x 2.6x106

Tyr = Tmo 3 x t / 50

Where

Tmo = outage objective, SES/month

Tyr = outage objective, SES/yr

t = average annual temperature, 0F = 1.80C + 32



ITU-R F.1668 OUTAGE CALCULATIONS, SDH SYSTEMS ITU-R F.1668 long-haul SESR (probability of outage) objectives are for multiple SDH (155 Mbps) hops in a link (multiple hops) within a county (“National Portion”) based upon ITU-T Rec. G.828 and G.826 [Ref. 13]:

Per-hop average SESR = 0.002 (0.01 + 0.002) x Llink/100 (Llink <100 km)

Example:

SESR, Llink = 80 km (two 40 km hops), A1= 0.01 to 0.02 (1-2%) SESR objective

= 0.002 (0.01 + 0.002) x 80/100 = 0.0000192 (50 SES, 25 SES/hop (worst month)

SDH Short-Haul Objective based upon ITU-T Rec. G.828/G826:

Per-hop average SESR, E3 (34 Mbit/s) = 0.002B/hops, B = 0.075 to 0.085 (7.5-8.5%)

Example:

Per-hop SESR, Llink = 90km (three 30 km hops), Short-Haul route (length independent)

SESR/hop objective = 0.002 x 0.075 = 0.00015 (390 SES end-to-end, 130 SES/hop any month)

RELIABILITY AND AVAILABILITY OVERVIEW In considering how to establish realistic outage or reliability objectives, several things need to be kept in mind. A single overall design objective, for not more than X hours, minutes, or seconds’ outage, over some period such as a year is an over-simplification. The character of the particular kind of outage and its effect on the system should be taken into account, and perhaps there should even be different objectives for different types of outage.

For example, propagation outages due to multipath fading are usually of short duration. An outage of an hour per year due to multipath might represent 1,000 or more individual outages, each averaging 1 second or less (1 SES) on a properly engineered path.

On the other hand, long-term propagation outages totaling an hour per hop due to rain attenuation may consist of four or five individual outages averaging five to fifteen minutes in duration each. A 60 sec/yr rain outage computed in Starlink equates to one 10 minute (600sec) duration typical rain outage every 600/60 = 10 years.

The effects of long-term and short-term system outage on message trunks are very different. Short-term “unreliability” outage events do not disconnect circuits nor reduce (in most circuits) data or Ethernet packet throughout, while long-term “unavailability” events likely cause both traffic disconnect and loss of data throughput.

A distinction should be made between communication circuits for which an outage of a few seconds or a few minutes is just a nuisance or an inconvenience, and circuits for which such an outage might result in danger to life, great economic loss, or other catastrophic consequences. The suitability or unsuitability of deploying millimeterwave hops that are not loop (UPSR ring) protected a rain-affected band such as 18 or 38 GHz could differ widely for these two situations.



Even if the maximum possible reliability and availability objectives are established and a path or a system is engineered to the full limit of the state of the art, the probability of outage can never be eliminated but only reduced to a very low value.

It is imperative to make any ultra-important services such as public safety, homeland security, and electrical utility protection as fail-safe as possible against a loss of the communications or control channel. Therefore, regardless of the degree of reliability a system should be engineered so that if an outage does occur it can be tolerated or at least its effects at least kept within acceptable bounds.

It seems that in some cases, perhaps many cases, a more relaxed attitude might be taken toward rain-induced outages than toward multipath outages or even equipment outages. In several respects, rain outage is somewhat benign in nature. If the fade margins are kept high and the paths are not stretched out too much, even in less advantageous areas the number of outages per year should not be very large and the length of individual rain outages on a hop should only rarely exceed five to perhaps ten minutes.

Short (less than 2-second duration per event) microwave outages, common in a typical longer diversity or a shorter non-diversity digital microwave hop with adequate fade margin, will not drop any telephone or data lines. Such outages quickly clear (“self-healing”) with all circuits remaining connected and little note taken of these transient events. Critical real-time, non-repeatable control or data blocks are usually sent over data circuits that have X.25, X.35, etc. error detection which requests a resend of interrupted data from far-end buffers, and Ethernet/IP packets are typically resent as a result of the ARQ automatic retransmission of “lost” packets.

Longer outages associated with low fade margins, rain, etc. disconnect all subscribers and may block access to the digital hop for at least 10 seconds after each long-term outage event. Such traffic disconnects are unacceptable to most users. These more vulnerable hops clearly require diversity or ring protection. For high reliability hops, (usually in long-haul systems with many hops in tandem), the per-hop objective may approach or exceed 99.9999%, allowing only 20-30 short-term multipath outage seconds per-hop per year.

Short-haul systems, up to about ten hops, often have a per-hop design objective of about 99.9995% for 160 SES/yr outage. Spur legs or short systems with 2-5 hops may be designed for something on the order of 99.999% per-hop path reliability equating to 315 SES (5.3 minutes) outages (if the radio’s 10-3 BER/LOP outage point if used; 10-6 errored second events if to the radio’s 10-6 RBER static threshold - factory test point) per year.

Objectives such as these are typical of those used in telephone, utility and public safety networks. For other services even dramatically lowered path reliabilities may be acceptable, even approaching 99.99% or about 1 hour outage per year. It is important to note that Starlink path reliability formulas and methods are for calculating short-term one-way outage in accordance with all North American and International (ITU) standards organizations. Although no domestic or international standards exist for this to calculate two-way outage it is necessary to double the calculated multipath outage.

Unavailability outages due to rain fading do not have to be doubled since they occur simultaneously in both directions of transmission and are always two-way.

ATTENUATION BY ATMOSPHERIC GASES The principal gaseous absorption is by oxygen and water vapor. The attenuation due to oxygen is relatively constant in the 2 to 14 GHz frequency range. Water vapor absorption, on the other hand, is highly dependent on the frequency, as well as to the density of the water vapor (absolute humidity,



gm/m3) The following formulas from ITU-R Report 719-1 have been incorporated into Starlink for the purposes of calculating the attenuating effect of oxygen and water vapor within an accuracy of within + 0.2 dB when compared to later ITU-R models:

Aoxy = [6.6/(f2+0.33) + 9/((f-57)2 + 1.96)] f2 10-3, dB/km

Where

F = frequency, GHz

Awv = [0.067 + 2.4/((f-22.3)2 + 6.6) f2 p 10-4, dB/km

Where

f = frequency, GHz

p = water vapor density, gm/m3

= 7.5 (ITU-R typical value corresponding to 50% humidity at 160C, 75% at 100C, is used in Starlink)

RAIN OUTAGE Heavy rainfall, usually in cells accompanying thunderstorm activity, has a great impact on path availability above 10 GHz in some areas, and this long-term (5-15 min) outage time causes traffic disconnects and packet loss. Such long-term outage is never added to short-term multipath outage previously discussed. Rain outage increases dramatically with frequency, and then with path length. Increased outage at 23 GHz can require a 50% reduction in path length compared to 18 GHz for a given availability, for example. Extended 10-15 minute duration fades to over 50 dB have been recorded on a 5km/3mi 18 GHz path in Houston, for example. The predicted annual outage may not occur for years, and then accumulate over a single rainy season for a long-term average.

Early studies, both theoretical and experimental, resulted from the recognition of the importance of rain in designing microwave paths with availability objective in excess of 99.9%. In recent years the emphasis has been on establishing predictive techniques for the statistical estimation of the attenuation probability distribution for a particular path. R. K. Crane [8] has developed a model for determining the attenuation due to rain based on several factors, including path length, frequency, and point rain rates. These have been incorporated into Starlink, with Crane's formulas:

A(%) = α Rx%β [(eλβd-1)/λβ - bβecβd/cβ + bβecβD/cβ] , dB for d < D < 22.5 km

= α Rx%β [(eλβD-1)/λβ], dB for D < d, km

Where

A(%) = Rain attenuation, dB, exceeded (%) of the time

λ = [ln (becd)]/d

b = 2.3 Rx%-0.17

c = 0.026 – 0.03 ln Rx%

d = 3.8 – 0.6 ln Rx% (typically 1-2 km)

α, β = Regression coefficients, f(frequency & polarization), in Table 3 below

α = multiplier, function of frequency and polarization



β = exponent, function of frequency and polarization

D = Path length, km

Rx% = Rain rate, mm/hr exceeded x% of the time (from Crane or ITU-R rain region tables)

Table 3: Rain Attenuation Coefficients α, β, with V-pol and H-pol Polarizations.

A greatly simplified rain attenuation (A%) model has been provided in ITU-R Rec. P.530, with its latest change provided in P.530-8. Unlike the Crane model, the P.530 rain outage model lends itself to calculator methods of computation, and provides A (%) values comparable to Crane in most rain regions but somewhat different values in others depending upon the rain rate.

Starlink uses the above Crane rain outage model, not the ITU-R P.530 rain outage model.

RAIN ATTENUATION OVERVIEW Rain attenuation at the higher microwave frequencies (>10 GHz) has been under study for more than 50 years. Much is known about the qualitative aspects, but the problems faced by the microwave transmission engineer – who makes quantitative estimates of the probability distribution of the rainfall attenuation for a given frequency band, polarization, path length, and geographic (rain distribution rate) area – remains more difficult.

In order to estimate this probability distribution, instantaneous rainfall data is needed. Unfortunately, the available rainfall data is usually in the form of a statistical description of the amount of rain which falls at a given measurement point over various time periods, generally at least an hour in length.

The rain-induced attenuation along a given path at a given instant in time is a function of the integrated effect of the rainfall existing at all points along the path. Rain attenuation is affected not only by the total amount of water in the path at that instant, but also by its distribution along the path in volume and drop size. For heavy rain rates, the instantaneous distribution of volume and drop size along the path is highly variable and is difficult to predict with any sort of accuracy from the kind of rainfall data generally available.



One of the earliest and most comprehensive attempts at developing a workable prediction method was carried out by Bell Laboratories in the 1950s, and was described in a classic paper by Hathaway and Evans (1958). A method of predicting annual outages for microwave paths operating in the 11 GHz common carrier band as a function of path length, fade margin, and geographical area within North America was developed in this paper.

This study has proved to be a worthwhile prediction tool and, even considering its limitations, is still one of the best references available for microwave engineers working within the United States. Additional studies have been conducted in Europe and Asia. The combined information has been reviewed and published by R. K. Crane [Ref. 8] for North American paths and internationally in the ITU-R Recommendations [Ref. 4]

Increasing fade margins, shortening path lengths, and increasing antenna sizes are the most readily available tools for minimally reducing the rain outage in a given area. Route diversity (ring protection), vertical polarization, or migration to a lower, less vulnerable frequency band are far more effective in reducing outage (increasing rain availability (“uptime”) in microwave links. .

The total annual rainfall in an area has little relation to the rain attenuation for the area. Within the U.S., for example, the northwestern states have the greatest annual rainfall (in excess of 100 inches/2500 mm per year) produced, however, by long periods of steady rain of relatively low intensity at any given time. Other areas of the country with lower annual rates experience thunderstorms and frontal squalls that produce short duration rain rates of extreme intensity. It is the incidence of rainstorms of this type that determines the rain rates for an area and thus the high-frequency microwave hop’s long-term path outage (“unavailability”) characteristics.

Even the rain statistics for a day or an hour have little relationship to rain attenuation. A day with only a fraction of an inch/centimeter of total rainfall may have a path outage due to a short period of concentrated, extremely high intensity rain. Another path that has days with several inches/centimeters of total rainfall may experience little or no path attenuation because the rain is spread over a long time period or area.

The most common reason a preference for lower frequencies is the susceptibility of bands above 10



GHz to rainfall attenuation. Although the effect is present to some degree at lower frequencies, it increases rapidly with frequency. For example, a rain cell intensity causing only a few dB of attenuation at lower frequencies could be sufficient to cause a path outage at 18 GHz.

Although fades caused by rain cells are occasionally observed at lower frequencies (10-20 dB fades at 6 GHz have been recorded even in North America), this type of fade generally causes outages only on paths above 10 GHz. The outages are usually caused by blockage of the path by the passage of rain cells (thunderstorms, etc.), perhaps 4-8km/2.5-5mi in diameter and 5-15 minutes in duration on the path. Such fading exhibits fairly slow, erratic level changes, with rapid path failure as the rain cell intercepts the path. The fades are nonselective in that all main and diversity paths in both directions of a millimeterwave hop are affected simultaneously.

Vertical polarization is less susceptible to rainfall attenuation than horizontal polarized frequencies. Increased fade margin is of some help in rainfall attenuation fading; margins as high as 45 to 60 dB, some with ATPC, have been used in some highly vulnerable hops for increased availability. When permitted, seldom-used cross-band diversity is totally effective; the lower frequency path is stable (affected only by multipath fading) during periods when the upper frequency path is obstructed by rain cells.

Route diversity with ring-protected paths separated by more than about 8 km/5mi or by more than 600-800 in azimuth at hub repeater sites is nearly totally effective in preventing long-term traffic outage and subscriber disconnects with rain cell passage, thus also is often used successfully.

In summary, things to bear in mind in connection with rain attenuation fades are:

• Rain outage doubles in each higher millimeterwave band, e.g. 18 to 23 GHz.

• Rain outage is directly proportional to path length, assuming a constant fade margin.

• Rain outage increases X2 to X3 for H-pol millimeterwave hops as compared to V-pol.

• H-pol hops require ~6 dB more fade margin than V-pol hops for equal rain outage.

• Rain outage in tandemly connected short hops is the same as for a single long hop, assuming that all hops have the same fade margin.

• Rain outage durations are ~5-15min each. A Pathloss, Starlink, etc. path calculation that shows small outage, e.g. 1min/year, equates to typically one 10min duration rain outage in 10/1 = 10 years.

• Rain cells typical travel E-W, thus causing fewer but longer duration outages to E-W millimeterwave hops than to N-S hops. The total average annual rain outage in both hops are about the same.

• Traffic is 100% protected from subscriber disconnect caused by a long-term rain outage in a millimeterwave hop with ring (“route diversity”) protection as two hops out of a repeater site will not both be affected (exhibit simultaneous rain outage) if separated in azimuth by at least 800.

• Multipath fading in millimeterwave hops does not occur during periods of heavy rainfall, so the entire path fade margin is available to combat rain attenuation fades.

• Neither space diversity nor in-band frequency diversity provides any improvement against rain attenuation fade outage.



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