basics to gps

8/7/2019 basics to GPS

1/48

The Guide To

WirelessGPSDataLinks

Pacific Crest Corporation990 Richard Avenue Suite 110 Santa Clara California 95050 USATel 408 653 2070 Fax 408 748 9984 www.paccrst.com


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Notice

2000 by Pacific Crest Corporation.All rights reserved.

No part o f this book may be reprod uced inany form withou t pe rmission in writing fromPacific Crest Corp oration .

All Pacific Crest Corp oration produ cts are

trademarks or registered trademarks of Pacific Crest Corpo ration. O ther brand andprod uct names are tradem arks or r egisteredtrademarks of their respective holders.

Printed in the United States of America.

First Edition May 1995Second Edition O ctober 1995Third Edition September 2000

T h e G u i d e t o W i r e l e s s G P S D a t a L i n k si


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Notice i

Table of contents ii

Introduction iii

Maximize the performance of your DGPS RTK system 1

Basics 4Radio wave propagation 5Terminology 6General rules 6VLF, LF, and MF radio signal propagation 6HF radio signal propagation 7VHF rad io signal p rop agation 7UHF radio signal propagation 8Modulation 9

Data communication 9Antennas 10

Performance issues 13RF power 14Line and system losses 15Path loss 16Antenna gain 17Receiver sensitivity 18Fade margin and multi-path 19

Estimating system performance 20UHF/ VHF ran ge calculation s 21FEC and data scrambling 23Fade considerations 25

FCC licensing and regulations 26Licensing requiremen ts 27App lication forms 27Frequencies 27Technical issues 28How to get help 29

RFDC applications 30Point-to-point 30Digipeater 31Packet operation 32

Appendix A Glossary 33

Appendix B Bibliography 37

Appendix C Frequency coordinators 38Pub lic safety radio services 38Ind ustrial radio services 39

Indu strial radio groups 40Oth er land transportation radio services 40Oth er frequen cy pools 41

Index 42

c

on t en t s

T h e G u i d e t o W i r e l e s s G P S D a t a L i n k s

Table of

ii


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This Guide has beenwritten specifically for the sur veyor toprovide u seful inform ation abou t wirelesscommunication. The first section offerstips and techniques for maximizing theper forman ce of your DGPS RTK

(Differential Global Positioning SystemReal Time Kine matic).

The remainder of this Guide isdesigned to p rovide a basic nuts-and -boltsawaren ess of the issues an d tradeoffsconcerning r adio frequency datacommunication (RFDC). The objectiveis to provide the surveyor with the skills andknowledge required to successfully estimatethe d ata link requiremen ts, set up the d atalink, and to diagnose and fix problemsthat are common to these systems. Thoseinterested in further understanding of thetechn ical or academic aspects of RFDCshould refer to the bibliography inAppendix B.

Before beginning, a few simplepoints. Radio data communication is notdifficult or mysterious. Detailed knowledgeof data and radio commu nication th eory isnot re quired. With an un derstanding of afew basic principles, you can design and set

up an RFDC link that will provide reliabledata commun ication.

Naturally, the physical laws of natur e place boun ds on what can beexpected of an RFDC system. Recognizingthese boun ds and knowing some basicrules-of-thumb is the goal of this course.You will find thr oughout this book, textboxes which highlight things to do or thingsto avoid . Following these simple ru les willallow you to get the most out of your RFDCsystem.

Introduction

T h e G u i d e t o W i r e l e s s G P S D a t a L i n k siii


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Getting the most out of your

real-time differential GPS

system is a challenge.

While GPS technology

offers tremendous potentialand is increasingly easy to use,

there are still some aspects

that require a basic degree of

understanding.

This guide focuses on

the radio data link. The radio

data link passes correction

information from a stationary

GPS reference station to the

rover.

In this chapter we

present the best of what wevelearned through almost a decade

of providing radio links for

survey applications.

Maximizethe performanceof your DGPS RTK system

1

First things first

How many of us have arrived at a job site,on ly to find th at we h ave forgotten a cable,or battery, or o ther necessary compon ent?We waste valuable time trying to track down

the equipmen t, when this problem is bestsolved by being organized. Invest in acarrying case and system that assures all thenecessary equipmen t is accoun ted for. This isnot a r adio issue per se, but we want to saveyou the frustration of arriving at a remote

job site without th e pr oper equipmen t!

Maintenance

Cables, connectors, and antennas are subjectto stresses that ultimately lead to failure.Preventative maintenance is important toreduce down time. Inspect cables frequentlyand replace those that show wear. Its alwaysa good idea to h ave a spare set of cables tocover for loss or failure d ue to n orm al wear

and tear.



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Select the bestavailable channel

Sur veyors often ope rate o ver a wide are adepending on the type of firm. If you work in a fixed location, a coordinated frequency

is best used. For th ose who move about,the use of itine ran t frequen cies is calledfor. When you en ter a n ew location, itsimportant to select a channel that minimizesinterference with other users (an dsubsequen t comp lain ts) , as well as on e th atprovides the best operation . There are

techn ologies available to h elp select the b estchann el for o peration given cond itions atthe ou tset of th e sur vey Auto Base an dAutorover help you select an appr opriatechannel.

The table be low summar izes thepo ints mad e in th is chapter. You will improveyour level of success and satisfaction byfollowing these simple rules.

Batteries

Batteries are an oth er concern . Especially inareas wher e high p owered radio mod emsare used, the condition and degree of charge are critical for best operation. Only

use batteries that are d esigned for deepdischarge an d frequent charging. Keep inmind that batteries degrade over time.Depend ing on usage pattern s, you shou ldrep lace you r batte ry every one to th ree years.

Picking a locationRadio link ran ge is directly related toanten na h eight. The b est thing you can d oto improve your system range (a commoncomplaint) is to get your base station androver radio an ten nas as high as possible. If possible, select a base station location thattakes advantage of terrain, and make use of telescoping m asts to improve range in allcircumstances.

Use quality antennas

If you want superior per forman ce, demanda h igh-quality anten na. Next to anten naheight, having a good an tenn a is the m osteffective and inexpensive way to improvesystem performan ce. Cheap, rubber du ck antenn as are okay for short-ran ge sitesurveys, but if you want the best possibleper formance, invest in a h igh-per forman ceantenn a. Ou r tech n ical assistance staff canmake recommendations for you dependingon your application.

2 T h e G u i d e t o W i r e l e s s G P S D a t a L i n k s

1 . Keep your equipment organized a carrying case is helpful.

2 . Inspect and replace cables an dbatteries before they fail.

3 . Maintain spares of cables and otherequipment pron e to failure throu gh norm al

wear an d tear.

4 . Select a location that takesadvantage of terrain the higher th e

better. Make use of anten na masts to get

your antennas higher, especially in

challenging base station locations.

5 . Use high quality antennas. Rubberduck antennas are not for high

performance systems.

6 . Monitor and select an appropriatechannel to m inimize inte rferen ce with

other users.


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One last note

Surveyors doing RTK work are members of a greater commu nity that share th e radiospectrum. In order to maintain access to thisvital spectrum, its important that we all

understand and follow our obligations aspart of this commu nity.

Get a license

You are legally req uired to license you r

nar row-ban d radio mod em system. We canrecommen d a licensing service that will helpyou with the paper work and the applicationprocess. (See section on Licensing andRegulation s). If you operate witho ut alicense, you are subject to fines andpossible equipment confiscation.

Turn off your radiowhen not in use

Never leave your base station broadcastingwhen you are not using the signal.Continuous operation, especially on

itineran t app lication s, may lead toco-channel user complaints. Remember,data is secondar y to voice in th e itineran tban ds. This mean s that if ther es a conflict,you (the data user), are obligated to vacatethe frequen cy.

Limit output power

If youre doing a local area survey on aconstruction site or other shor t rangeapp lication , limit you r RF outp ut p owerby selecting low power setting for your

equipment.

Select a channel withthe least activity

This is common sense. Use AutoBase or

monitor the available channels prior tooper ation and select the chan nel with th eleast activity.

Get along withco-channel users

If theres a complaint from a co-channeluser, move to ano the r frequen cy. If youoperate out of a fixed location, thencoordinate a frequency specifically foryou r u se. You will not be given an exclusivefreque ncy, bu t you will be p laced on afrequency that is appropriate for your

activity.

T h e G u i d e t o W i r e l e s s G P S D a t a L i n k s 3


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Understanding the basics of

RFDC provides a foundation

for successful implementation

of radio data communication

systems.In this

section, the

fundamentals are

addressed in

laymans terms, beginning with

the concepts of radio signal

propagation.

At the end of this section,

you should be familiar with some

common radio terms, and have a

basic understanding of radio

transmission theory including

data communication.

Basics


Figure 1 Electromagnetic spectrum

1 x 10 -4 .01 1 100 10,000 1 x 10 6 1 x 10 8 1 x 10 10 1 x 10 12 1 x 10 14 1 x 10 16

CosmicRays

GammaRays

XRays

UltraViolet Infrared

Radio orHertzian Waves

PowerLight V

i s i b l e

I n d u c

t i o n

H e a t i n g

Wavelength in Angstrom Units

4


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at VHF frequen cies between 88 an d 108MHz (check you r rad io dial). AM radiosoper ate at a much lower frequen cy in theMF band from 530 to 1700 kHz. Television

station s operate at various frequ encies inthe VHF and UHF ban ds. Cellular ph on esoperate in the 800 MHz region of theUH F band . We live in a world bath ed inelectromagn etic ene rgy from television,rad io, cord less ph on es, microwaves, and

visible light.

Mostradio frequencybands aredivided intochannels, eachof which may beused to tran smitvoice, data,or signalinginformation.RFDC systemsmake use of discrete radiochann els wher einformation is

communicatedby modulationof the rad iocarrier. Thereare other

mediums andtechniques used to comm un icate data usingthe electromagnetic spectrum , includingfiber-optics, infra-red, spread-spectrum,and other s. These are beyond the scopeof this book and will only be mentionedin passing.

Radio wave propagation

Radio waves are p art of th e electrom agneticspectrum which encompasses visible light,x-rays, u ltra-violet r adiation and microwaves(see Figure 1). Electromagnetic waves travel

at roughly the speed of light and havewavelength s wh ich are r elated to thefrequency of the wave. Th e table b elowshows the spectru m of frequencies whichare considered radio waves and theirclassifications:

Were all familiar with portionsof the radio spectrum that many of us useon a daily basis as we communicate, work,or relax. You may have en joyed mu sic orlistened to n ews on your car radio on theway to work th is morn ing. FM radios oper ate


Frequency Wavelength Classification

10 - 30 kHz 30 km - 10 km Ver y Low Frequ en cy ( VLF)

30 - 300 kHz 10 km - 1km Low Frequency (LF)

300 - 3000 kHz 1km - 100 m Medium Frequency (MF)

3 - 30 MHz 100 m - 10 m High Frequency (HF)

30 - 300 MHz 10 m - 1m Very High Frequency (VHF)

300 - 3000 MHz 1m - 10 cm Ultra-High Frequency (UHF)

3 - 30 gHz 10 cm - 1cm Super-High Frequen cy (SHF)

30 - 300 gHz 1cm - . 1cm Extremely-High Frequency ( EHF)

Table 1 Radio spectrum classifications

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6

Terminology

Before d escribing th e characteristics of thedifferent radio bands, lets review some basicradio te rmin ology (r efer to th e glossary inAppendix A for an extensive listing of radio

terminology). The following terms will beused in the sections which follow and shou ldbe un derstood.

propagation : The path and m annerwhich a radio wave travels from its source(th e transmitter) to its destination ( the

receiver). The path (often called mode)of propagation d iffers dep end ing on th efrequency of the radio signal. Alsodepe nd ent o n frequency is the reflectionor r efraction of the radio signal as it passesthrou gh layers of the iono sph ere o r as itreflects off of objects in its path.

range : The d istance at which rad iocommun ication is adequate for aparticular task. Voice commu nicationis acceptable with noise or interferenceconditions which would make datacommun ication un reliable.

coverage : The ability of the radio signal tobe available within the expected range butwhe re th e signal is blocked by man -mad e ornatural structures.

General RulesA few generalities can be made about r adiocommunication. Lower frequencies providebetter range than higher frequen cies.Lower frequencies are more susceptible tointerference than higher frequen cies.Coverage and signal penetration is better athigher frequencies than at lower frequencies.Most reliable data commu nication depe nd son line -of-sight con ditions where the ran ge islimited by the radio horizon. Figure 2 belowillustrates a line-of-sight tr ansmission .

VLF, LF, and MF radio signalpropagation

This portion of the spectrum does no t offerchannels available for data communicationexcept in a few special cases. Signals in these


C u r v a tu r e o f th e E a r t h

Line of sightAntenna Antenna

Figure 2 Direct wave propagation "line-of-sight"


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Earth

Skip

Ionosphere

bands have exceptional range but sufferfrom man-made and environmen tal noisewhich wou ld limit data tr ansmission in m ostcircumstances to very low rates (< 300 bitsper second) . Some radio n avigation beaconsoperating in the 285 to 325 kHz rangeprovide 200 baud data DGPS correctionsfor marine based navigation. These beacontransmitters provide ran ges in the h un dred sof miles.

HF radio signal propagation

HF rad io signals provide excellent ran ge,but suffer low reliability for datacommun ication because o f co-chann elinterference from d istant radio stationsand susceptibility to man-mad e n oise. TheFCC has allocated channels in the upperportion o f the HF band starting at 25 MHz.Anoth er m ode of prop agation for H F signalsis via ionosphere reflection. At thesefrequencies, the radio signal will reflect off of the ionosph ere an d travel back to theearth . See Figure 3 above. This "sky-wave"

propagation from distant radio stations isdifficult to predict, and is often in fluen cedby the time of day and year, and the activityof sun-spots. Because of these variousphenomena, HF data communication islimited to low baud rates and questionablereliability.

The higher the frequen cy the lowerthe influence of man-made n oise and theco-channel interference caused by sky waveinter ference ( also called skip).

VHF radio signal propagation

While not entirely eliminated, problems of signal skip an d susceptibility to inter ferencefrom man -made and environmen tal noiseare minimized in the VHF spectrum. Goodsignal coverage is provided bu t at a some-what shorter range th an can be achieved inlower bands. VHF frequencies above 100MHz (high band) provide characteristics


Figure 3 HF signal propagation via "skip"

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which make them adequ ate for mo derateor h igh speed d ata commu nication. Theavailability of channels in the VHF highband ( 150 to 174 MHz ) for no n-voice radiocommun ication p rovide capabilities fortran smitting data in excess of 19,200 bitsper second.

VHF high band and higherfrequency radio signals are considered totravel in a line-of-sight (also called directwave) mode with minimal pro blems fromskip an d noise. Line -of-sight propagationmeans that the range of the r adio link islimited by the radio ho rizon, being th e po int

at which the curvature of the earth blocksthe signal between the transmitting andreceiving antennas.

On rare occasions, VHF signalsmay propagate in a mode called ducting.See Figure 4 above. This occur s when atemperature inversion provides atmosphericconditions such that th e radio signalbecomes trapped between layers in theatmosph ere. Th e rad io waves travel in theduct and may propagate for long distances.Naturally, th is is not a reliable mod e o f

prop agation and may be a source of interference with distant radio transmissionsinterfering beyond their nor mal range of influence.

UHF radio signal propagation

The UHF frequen cy band provides a goodcomprom ise for radio data comm un ication swhere ran ge and d ata throughp ut arerequired. Signal skip and ducting areminimal, and the susceptibility to n oise,including co-channel interference is much

easier to con trol. Radio coverage is excellen twith penetration into buildings and overterrain better than at lower frequencies.Propagation is direct wave limited by theradio ho rizon . Prob lems associated withman-made noise are minimal.

At UH F and higher frequencies,care m ust be taken to m inimize system losseswhich increase as the frequency increases.Also, at higher UHF frequencies andmicrowave frequencies, attenuation throughfoliage and caused by meteorologicalconditions must be considered .


Earth

NormalWave Range

Figure 4 VHF signal propagation via "ducting"

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Modulation

The m odu lation of the radio signalpro vides the ability to com mu nicateinformation. Modulation is addinginformation to a radio wave by modifying

on e of its fund amen tal characteristics.The fundamental characteristics of radiowaves are frequ en cy, ph ase, and amp litud e.

Differen t m odu lation schemeshave been devised which make use of varyingthe frequen cy (frequen cy mod ulation ),amplitude (amplitude modulation), and

phase (phase modulation). These termsshou ld be familiar to rad io listeners whotune in freque ncy mod ulated (FM) o ramplitude modulated (AM) radio stations.Figure 5 here shows thedifferen t m odu latingwaveforms of a basebandsignal. The basebandsignal is the actual datawhich is represented bya serial stream o f 0san d 1s.

The type o f modulation and suitabilityof a modu lation for a

particular application isbeyond the scope of thiscour se mater ial. Mostmodern high-speed datalinks use one of the formsof frequency modulationwhich is spectrally efficientand provides immun ityfrom variations in signalstrength known as fade(more on this later).

Data communication

Data comm un ication over rad io wavesprovides an efficient an d r eliable tran sferof information . Because of the limitedspectrum available for communication,

more and more digital systems are beingutilized. In comp arison to voicecommu nication, the amou nt of informationtransferred over a radio link via modulateddata is astound ing. Because of th is,trad ition al users of voice commun icationin th e p ub lic safety sector (po lice, fire,

search-and-rescue) are turning to radiodata communications to provide dispatchand other information.

Data communication is


1 0 1 0 1

Baseband

AmplitudeModulation

(AM)

PhaseModulation

(PM)

FrequencyModulation

(FM)

t

t

t

t

Figure 5 Modulation waveforms

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fundamentally similar to voicecommun ication in th e man ner in whichthe radio signal is modulated. Most modernvoice and data com mu nication systems inthe UHF and VHF commercial band s makeuse of frequency modulation, although

amplitude mod ulation is common in somemarin e an d aviation app lication s. For voicecommunication, the audio signal is pickedup by the m icrophone and changed into avarying voltage level. The voltage level isapplied to the rad io transmitter to mod ulatethe carrier frequ en cy. The radio r eceiverin a voice system takes the mod ulatedfrequency and produces a signal whichreplicates the signal from the transmitter,which is then applied to a speakerproducing audio output.

Data commu nication is performedby encoding the digital signal on an analogwaveform which is capable of being passed

via radio modulation. Digital signals arecomposed of groups of binary dataconsisting of 0s and 1s which rep resentnumbers, or characters (called bytes). Thestream of 0s and 1s wh ich make u p th edigital message are shifted through amod ulator which pr odu ces an an alog

waveform which repre sents the d ata. Thissignal, called the baseband modulation, isdesigned to p rovide a signal waveform thatcan be transmitted and received by the radiohardware. High speed data requires specialfiltering to assure that the data modulatedcarrier fits within th e chann el spacingmandated by the FCC.

A measure of how good acommun ication system can tr ansmit data is ameasuremen t of the Bit Error Rate (BER).A bit error occurs due to interferen ce or low


signal levels where a bit as sensed by thereceiver is no t corr ect. The BER is the ratioof bit error s to th e total num ber of bitstransmitted. For valid comp arison s of radiodata systems, the BER must be measure dwithin the context of a total system that

includes low signal levels and fade conditions.Sophisticated RFDC equipment uses

Forward Error Correction (FEC) protocolswhich allow erro rs in th e received data to becorrected. Because of the nature of RFDC,an FEC algorithm should be chosen whichworks well in the correction of bur st bit errorsas opposed to single bit errors.

Antennas

Selection and proper installation of theantenna system often makes the differencebetween a reliable and robu st or an

unreliable data communication system.The anten na is the radiating elemen t whichtakes the RF energy generated by the radioand begins its prop agation th rough space.Antennas come in a variety of sizes andshapes designed for specific uses.

The ability to focus the RF ener gy

in a specific pattern provides a method foroptimizing the coverage and range of thecommun ication network. Some an tenn asare highly directional and allow the use of relatively low power radio transmitters tosend data over long distances. Otheranten nas are d esigned for omn i-direction aluse wher e th e relation ship b etween thetran smitter an d r eceiver is constantlychanging. The natu re of the commu nicationactivity normally dictates what sort of antennato use (directional or omni-directional).

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The m ost impor tant activity insetting up a radio transmitter is determiningthe placement and type of the antenna.Where flexibility pe rm its, always place theanten na o n the highest point available andalways select an antenn a with a gain p attern

(more on this later) which optimizes thecoverage. In general, use a directionalgained antenn a such as a Yagi for apoin t-to-po int fixed location app licationand a gained omn i-directional anten na formob ile p oint-to-po int o r p oint-to-mu ltipo intcommun ication systems. See Figures 6an d 7 here.

Antenna

Horizontal Gain Vertical Gain

Antenna

Horizontal Gain

Vertical Gain

Figure 6 Omni-directional antenna gain pattern

Things to do

Be aware of power lines or otherobstacles that can inadvertentlycome in contact with the antennaand cause potentially lethal

conditions.

Guy-wire antenna masts higherthan 10 feet.

Use lightening arrestors forequipment and personal protection

if erecting an antenna in areasprone to lightening.

Installation of antennas on buildingsor other structures (towers, etc.)must be done in accordance withlocal building regulations. Contacta local antenna installer who is

familiar with building codes andproper antenna installation for anypermanent installation.

Figure 7 Directional antenna gain pattern

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In some app lications the anten nasystem mu st be moved from location tolocation. Mobile RFDC users often move anen tire rad io system from location to locationdepending on the requirements of the job.In these circumstances, it is difficult to

optimize th e rad io anten na setup. At aminimum, attempt to get the antenna atleast 10 feet above th e ter rain with anantenna mast. Make sure that the RF powercoming from th e tran smitter is attenuated aslittle as possible by making use of the highestquality coaxial cables with the minimumlength required between the radio and theantenna.

Anten nas pro vide the mosteconom ical method for impr oving theper formance of the r adio commun icationsystem.

Things to do

Maintain the antenna andinterconnecting cables in excellentcondition.

Tune the antenna as per the

instructions included to the properlength for the frequency of thetransmission.

Use a professional antenna installer forpermanent installations and make surethat the antenna is tuned for minimumreflected power.

Take advantage of any landform orstructure for higher placement of theantennas.

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Next, the attenuation

(reduction) of the signal as it propa-

gates between the transmitting and

receiving antenna is addressed.

Lastly, the signal is received

and processed by the radio receiver.

(See Figure 8 on the following page.)

At each stage of the RF signal,

performance should be optimized for

the requirements of the application.

An understanding of these

topics provides a basis for determining

the trade-offs in setting up an RFDC

system.

Performanceissues

Getting the best possible performance

from a radio communication system

requires attention to the fundamentals

as addressed in the previous chapter.

In this chapter we discuss in

greater detail the

component parts of a radio

system and

"rules-of-

thumb" as well

as specific recommendations which

will lead to good radio system

performance.

The order of topics in this

chapter follows the path of a

transmitted signal, beginning with

the transmitter, followed by the

antenna feed-line system and

transmitting antenna.

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Equation 2

P = .001 * 10 10

Where:

x = power as expressed in dBmP = power as expressed in watts

The outpu t power at the radio antenna portis the starting po int as the signal moves fromtran smitter to receiver. The p ower as seen bythe radio receiver is determined by all of thelosses and gains in th e system. RF outpu tpower minus th e system losses must exceed

x __

RF power

A radio mod em transmitterprovides RF power at th eantenna port which consists of a fundam ental carrier of a

specific frequency which ismod ulated with data. Thepower of the signal is gene rallyspecified in watts, but may alsobe specified dBm (dB withrespect to a 1 milli-watttran smitter) . Many calculationsare simplified by working inunits of "dB" so the conversionof watts to dBm is given inEquation 1 .

Figure 8 Radio data link

Equation 1

P X = 10 log 10 ( _____ ).001

To convert from dBm to watts, useEquation 2 .

RF Signal(Path Loss)

Antenna("Gain")

Antenna("Gain")

PDL Base Unit

PDL Rover

T h e G u i d e t o W i r e l e s s G P S D a t a L i n k s14

( )


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the sensitivity of the receiver for successfulcommun ication to occur.

The RF outpu t power of aRFDC link is fixed by the rad io system.The selection of the RF output powerappro priate for a system de pen ds on the

range requirements in the context of the frequen cy band , antenn a type an dplacemen t, terrain, and th e radioper formance param eters. The FCC placeslimits on the output power which can beused on a particular frequency or for aparticular application.

Some systems make u se of extern alRF power amplifiers to boost the signal levelof the transmitter. RF power amplifiers work by using high-speed RF power transistorswhich amplify the voltage swing of the RFsignal. RF power transistors are designed forstable operation at the frequency of theradio signal.

Power is calculated as the voltagesquared divided b y the impedan ce of theoutput.

Equation 3

V 2P = ______

R

Where:

P = power in wattsV = voltage across an imp edan ceR = imped ance

Solving the above equation for voltage givesyou Equation 4 next.


Equation 4_______

V = PR

Assuming an impedan ce of 50 oh ms wh ichis standard in m ost commercial RFDC

equipment, a 2-watt tran smitter voltageswing is 10 volts.

The voltage swing of a 35 -watt RF signal is 42volts. The increased voltage with power levelswith h igh p ower outp ut radio equipme ntsho uld be respected.

Line and system losses

After th e signal leaves the tran smitter, theprocess of signal attenu ation b egins. MostRFDC systems have th e r adio eq uipm en tconnected to the antenn a through a length

of coaxial cable of matched impedance tothe an tenn a and radio ports. This cable canbe a m ajor source of power attenu ation andsho uld th erefore be optimized for bestsystem performance.

Two types of system lossespred ominate when the tran smitter is

connected to the antenn a through afeed-line. First is VSWR ( frequ en tlypronounced as "vis-waur") which is theVoltage Standing Wave Ratio. The VSWRis measure of the reflection of the voltage(or power) as the signal passes across animpedan ce bound ary. The conn ection

_______( 2 x 50 )

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of a coaxial cable to the anten na p ortof the radio p resents such a bou nd ary.Its imp ortan t to u se coaxial cable andconn ectors wh ich will min imize th eimpedance mismatch which would otherwisecause part o f the RF ener gy to be reflected

back into the transmitter.The second type of system loss isthe attenuation of the signal as it propagatesalong th e cable length. The attenuation of the signal is a function of the frequency andthe p roper ties of the cable. The h igher thefrequency, the h igher the atten uation of thecable. Atten uation is caused b y the leakageof RF through imperfect shielding of thecable as well as resistance in the cableconductors. Table 2 shows some popu larcables and their attenuation across thefrequencies commonly used in RFDC.

As a rule of thumb, 3 dB of attenuation is equivalent to approximately

halving the output power of the transmitter.As an illustration of the losses whichcommonly occur with a coaxial feed-line,consider a 35 -watt power output at 460 MHzgoing through 33 feet of RG- 58 . Theeffective power d elivered to th e an ten na isonly 15.7 watts! Using RG- 8 cable, the p owerdelivered to the an tenn a is 25 watts.

Nominal attenuation (dB/ 100 feet)

RG-58 50 5.7 10.5 16

RG-8 50 2.3 4.3 7.6

RG-213 52 2.3 4.3 7.6

Heliax 50 .9 1.4 2.21/2- inch

Table 2 Common RF cable characteristics

Path lossPath loss is the loss in signal strength as thesignal passes through free space between thetran smitter an d receiver. The loss of poweris inversely proportional to the square of the d istance between the an tenn as. Theattenuation or weakening of the signal isdependent on factors including antennaheight, natur al obstruction s to the radiosignal such as foliage or terr ain, andman-made obstructions such as buildings,bridges, etc.

Path loss is also affected by aph enom ena known as multi-path where

reflections or refractions of the direct-pathsignal combine with th e or iginal signal


Things to do

Use coaxial cable and connectorswhich are impedance matched withthe radio equipment (generally 50

ohms).

Use an optimal (shortest) length of cable required to move the signal

from the transmitter to the antenna.

C a b l e

I m p e

d a n c

e

1 5 0 M

H z

4 5 0 M

H z

9 0 0 M

H z

16


21/48

prod ucing d estructive interference.The multi-path effect is often noticed ontelevision r eception where gh ost imagesapp ear as an airplane (RF reflectivesource) flies overhead. In a mobile datacommun ication system, mu lti-path may

cause a signal variation as the vehicle movesthr ough r oadways whe re b ridges, buildings,terrain and other objects cause signalreflections.

Things to do

Select antenna location to minimizeobstacles between the transmitting andreceiving antennas.

Elevate the antenna above the terrainto keep path loss at a minimum.

Antenna gain

All antennas focus RF energy in anon-isotropic manner. The focusing of theRF energy is called the antenna gain, and isgene rally rep resented in terms dB withrespect to either a theoretical isotropicantenna ( dBi), or a dipole antenna (d Bd).Anten na m anufacturer s often omit thisrelationship when placing a gain value ontheir anten nas. This is an imp ortant p oint.A dipole anten na h as a gain of 2.1 dBi. Anantenna reported to provide a gain of 5 dBmay be equivalent to 5 dBi or 7.1 dBi.Always compare an ten nas using either d Bi(comm only used for por table or mobilewhip an tenn as) o r dBd ( common ly used forhigher quality base station antennas).

Antenna design is the art of shap ing the rad iation pattern of anantenna to provide a signal density patternappr opriate for the ap plication . For m anyapp lication s, an omn i-directional gainpatter n is desirable. Mobile data

commu nication wher e th e po sition of thereceiver with respect to the transmitter mayvary calls for an omni-directional an ten na.Depen ding on the terrain, it is generallyadvisable to u se a high-gain an ten na for bestperformance. High-gain omni-directionalanten nas have an increased h orizon talradiation pattern with a decrease in thevertical radiation pattern.

For applications where thetransmitting and receiving antennas arefixed in location with respect to each other,directional antennas provide bestper formance. Both transmitter an d receiversho uld use the gained anten nas in a fixed

location app lication s. With careful de sign,mo st systems of th is type will be able tooperate on relatively low power o utp uts.

Most antennas require groundingin order to provide the focused en ergypattern for which they are designed. Atlower frequen cies whe re fractionalwavelength antennas are used, it is critical tohave a good grou nd conn ection. It is no tuncommon for HF frequency antennainstallations to make use of hundreds of feetof copper wire buried beneath the antennato provide th e groun ding required for bestoperation. As the frequency is increased andantennas can be constructed of 1/4 wave



22/48

length s or longer econ omically, therequirement of a ground plane connectionbecomes less critical, however, it is stillrecommended to ground th e antenna forbest performance.

Things to do

Use a gained omni-directional antenna(>3.5 dBd) over flat or hilly terrain.

Use a gained directional antenna(>6 dBd) in fixed location applications.

Ground the antenna for bothperformance and safety reasons.

Mobile and portable antennas arealso available with excellent gainpatterns. In some applications where

ground planes are not available,antennas designed for no-groundplane operation are available. 1/2wavelength antennas operate wellin a no - ground plane application andshould be used if antennas designedspecifically for no-ground planeoperation are not available.

Receiver sensitivity

Receiver sensitivity is a ch aracteristic of th eradio equipment which determines its abilityto receive low level signals. Traditionally,receiver sensitivity is measured in terms of

the signal inpu t level at the anten na port isrequired to provide a signal to noise anddistortion level of 12 dB. The RF signal ismodulated with a 1 kHz tone producing a+/ - 3 kHz deviation o f the carr ier. Themeasuremen t, called the 12 dB SINAD(signal to n oise an d distortion, common lypron oun ced "sign-ad") is often repor ted inthe specifications of the radio equipment.

The more sensitive the radioreceiver, the better the range. There is alimit to receiver sensitivity which is set bythe ambient RF noise in the signal band .This background noise level determines theminimu m level where th e RF signal can be

recognized.Most radio data communication

equipmen t relies on a carrier detect signalwhich is gene rated by circuitry that measure sthe power of the received signal. The carrierdetect lets the radio mo dem know that asignal is available which may contain data.

The setting of the carrier d etect shou ld bechosen to take full advantage o f the r eceiversensitivity, but not so sensitive that ambientRF energy causes false tr iggers.



23/48

Things to do

Select radio data communicationequipment with receiver sensitivitybetter than -116 dBm.

If available, select radio datacommunication equipment with anadjustable carrier detect level(sometimes called digisquelch).

Fade margin and multi-path

Variation in signal level as a result of mu lti-path or obstacles in th e RF signal pathresults in a con dition kn own as fade. H ighspeed data communication is especiallysusceptible to failure caused by fadeconditions. Radio eq uipmen t de signed forhigh speed d ata comm un ication must make

use of fade resistant modulation schemesand be read ily adap table to varying signallevels. For th is reason , most radio d atacommun ication makes use of frequen cymod ulation instead of amp litudemodulation where varying signal levels(amp litudes) d ue to fade con dition s can

corrupt the data.



24/48

RFCALC is a software program thats

distributed with this book and that

provides useful tools for estimating

system performance.It also provides a means for

optimizing the RFDC network by

testing the effects of different

antenna systems, cable types and

lengths, power outputs and radio

parameters.

RFCALC is best used as a

reference tool to see how adjusting

different parameters affects range.

Actual system performance over

varying terrain cannot be addressed

with RFCALC.

Estimatingsystem

performance



25/48

UHF / VHF range calculations

Estimating the ran ge over wh ich an RFdata communication system will work reliably is not a trivial matter. Indeed, themost often asked (and dodged) question of

designers and man ufacturers or rad io datacommunication equipment is "Whats therange?" To make a rough estimate, thefollowing factors must be con sider ed:

Transmitter power

Transmitter frequency

Antenna feed-line length and type

Antenna type and placement

Terrain relief

RF obstructions(buildings, foliage, etc.)

Receiver antenna type and placementReceiver antenna gain

Receiver carrier detect

Receiver sensitivity

Sometimes minor adjustmen ts

in an installation can make majorimprovemen ts in the range an d coverageof the radio system. Radio system designersand installers are well aware of theseparameters and are able to optimize th eran ge of a system in a given circum stance.

This section details the conceptswhich can b e used to determ ine rou ghly theran ge wh ich you may expect to achieve in alink o ver flat groun d, or water, or in aground-to-air situation. Range over varying

terrain is beyond the scope of this courseand gene rally requires soph isticated softwarewith digitized terrain data access.

The appro ach to ran ge calculationis simple. First, start with the output powerof the tran smitter. From th is, sub tract all of the system losses and gains as the signalpasses through the various feed-line s,antennas, and through free space. Theresulting po wer o f the signal at th e receivermust be above the level required for reliabledata com mu nication. Now lets get in to th ede tails. ( Don t worry, you can use RFCALCto do the actual math.)

Figure 9 on the n ext page showsthe calculations which are used to determinethe range of VHF and UHF radioprop agation. Note that these calculationsprovide line-of-sight values, and do notconsider the effects of ducting o r skip whichmay occur at lower VHF frequencies.

The first step is to calculate th evarious gain factors which are characteristicof the transmitter, receiver, and th e RF pathwhich is dependent on antenna height oralternatively free space. In general, air-to-airor air-to-grou nd comm un ication systemsuse th e free space gain while over-grou ndcommunication is dependen t on theantenna gain factor.

Next, the subjective in fluen ces mustbe evaluated, either thro ugh m easurem entor estimation. These include the noise floorlimits which affect th e u ltimate sensitivity of the radio receiver in th e context of theambien t RF levels at the frequen cy being

used. Also, estimation of multi-path fading



26/48

which is depend ent o n relative an tenn amovemen t, frequ ency, and terrain ( not anissue in air-to-air data commun ication) .RFCALC do esnt conside r these issues wh ichare environment dependent in itscalculations.

Fade mar gin factors are alsoconsidered at this stage in light of the


considered at this stage in light of thereliability of the radio d ata commun icationsystem. (See Table 3 for commonly used fademargin factors.) More sophisticated radiocoverage software would estimate multi-pathand fade effects looking at the true terrainprofile.

VHF / UHF range calculations

Step 1 Calculate system gain

Step 2 Select path model

Step 3 Equate system gain to selected path model and solve for d (miles)

Use th e value which

translates into th e

shortest range.

Tx Pwr - dBm, Feedline Loss - dB, Ant Gain - dBi, Rec Sens - dBm @ 12 dB SINADFrequency (f) - MHz, Distance (d) - Miles, Height of Antennas (Ht, Hr) - Feet

Figure 9 VHF / UHF radio range calculations

Transmitter System Gain

(TSG)

Tx Pwr - Feedline Loss + Ant gain

Receiver System Sensitivity

(RSS)

- Rec Sens - Feedline Loss + Ant gain

System Gain

(SG)+ =

Free Space Gain

(FSG)

37 + 20 log(f) + 20 log(d)

Atm Fade Margin

(AFM)

Estimate/ Calculate

Anten na Gain Factor

(AGF)

149 40 log(d) 20 log (HtHr)

Terrain Fade Margin

(TFM)

Estimate/ Calculate+ +or

Free Space Range

((SG 37 AFM 20 log(f) / 20 )d = 10

Over Ground Range

((SG 149 TFM + 20 log(HtHr) / 40 )d = 10

22


27/48

The final step is to calculate thesystem gain an d equate it with th e path loss.For reliable data communication to occur,the system gain m ust be greater th an thepath loss gain. Equ ating th e system gain tothe path loss gain an d solving for d istancegives the maximum range at which the radiocommunication network will operate.

VHF / UHF over-groundrange estimate equation

The following equation providesa range estimate for VHF and UHFover-groun d radio link (an tenn a gainfactor calculation) :

(( SG 149 + 20 log 10 (Ht x Hr)) / 40 ) ) D agf = 10

Where:SG = system gainHt = height in feet of the

transmitterHr = height in feet of the receiver

VHF / UHF air-to-air /line-of-sight ground-to-airrange estimate equation

For air-to-air and line-of-sight gr ound-to-air,the following equation provides a rangeestimate for a VHF and UHF radio link (freespace calculation) :

(( SG 37 20 log 10 ( f ) ) / 20 ) ) D fs = 10

Where:SG = system gainHt = height in feet of the

transmitterHr = height in feet of the receiver

FEC and data scrambling

When data is transmitted by any medium,as the rate increases the energy per bit of information decreases. This decrease inenergy per bit leads to increased difficulty indiscern ing th e bit inform ation in a system

where n oise is present. Noise is pr esent in allcommunication systems. Because of this,high-speed data communication is limited bynoise in the system.

To comp ensate for th e h igher biterror r ate with increasing data tran smissionspeed, communication system designers

often include forward error correctingalgorithms which allow the r eceiving m odemto recognize and correct errors in th ereceived d ata. All forward erro r cor rectingalgorithms require additional overhead bitsto be sent with the actual data bits. Theobjective in forward error correcting systemsis to send an optimal number of bits so thatthe effective data throughput is increased.

Radio data commu nication erro rstypically occur in bur sts and are cau sed byfade conditions or interference. Because of this, forward error correcting schemes forradio data commun ication shou ld bedesigned to detect and correct burst errors.



28/48

On e pop ular metho d for forward errorcorrection in m obile d ata environmen ts isa block coded shortened Hamming Code( 12,8 ). The ( 12,8 ) designation mean s that12 bits are sen t for each 8 bits or raw dataresulting in a 50 % overhead. This codeis interleaved to provide burst error

protection, with th e inter leave factordetermining th e minimum block size andthe maximum burst error correction size.Popular mobile data networks use a block size of 20 words, giving burst errorprotection for 1 to 20 bits.

Forward error correction ismandatory in high speed, high reliabilitydata links where fade conditions are present.The inclusion of forward error correctionallows much lower signal levels to p rovide

equivalent or lower bit error rate (aftercorrection). Figure 10 above demo nstratesthe theoretical gain for low signal levels in

the context of a system with forward errorcorrection turned on and turned off.

Modulation schemes for high-speeddata most often allow coherent signaldetection. In order for the demodulatorcircuit to kn ow when to sample th e signalto get the proper bit value, the zero-crossings

must be identified.Transmitting andreceiving radio modemclocks are neverperfectly synchronizedand therefore thereceiving circuit must

derive the transmittingcircuit clock from thedata. Th e circuit wh ichdoes this function,called a p hase lockedloop, requires that th esignal have a sufficien t

nu mber of transitions tomaintain bit alignm ent.

Because of this, high speed radio datamodems normally provide scrambling of thedata to assure th at sufficient signal tran si-tions are p resent.

After de scramb ling and forwarderror correction, th e d ata integrity must bechecked. Most high speed radio mod emscalculate and transmit error detectioninformation which is checked by the re ceiverto assure data validity. The checkinginformation can be a simple checksumor a more robust cyclic redundancy check (CRC). The popular 16-bit cyclic

redu nd ancy check provides exceptional


10 -1

10 -2

10 -3

10 -4

10 -5

10 -6

5 6 7 8 9 10 11 12 13 14 15

B i t - E r r o r R

a t e

S/N (dB) (Noise Bandwidth = Bit Rate)

FEC ON FEC OFF

Figure 10 Theoretical gain with FEC

24


29/48

per formance in detecting erro rs in th ereceived data. The following table illustrates

the error checking available with a 16-bitCRC.For r obust and reliable operation,

all h igh-speed data com mu nication systemsshou ld pro vide forward error detection, d atascrambling and 16 -bit CRC error detectionmechanisms.

Fade considerations

Its traditional for d esigne rs of radio d atacommun ication systems to use a fud ge factorcalled th e fade m argin wh ich pro vides amargin for naturally occurring fade condi-

tions. Most fading problems are caused bymu lti-path which increases with frequen cy


Type of error Detection Capabilities

Single-bit errors 100 %Double-bit er rors 100 %Odd-number errors 100 %Burst errors shorter than 16 bits 100 %Burst errors of exactly 17 bits 99.9969 %All othe r bu rst errors 99.9984 %

Table 3 16-bit CRC error detection capabilities

and path distance. Multi-path fading followsa Rayleigh probab ility distribution . Th efudge factor is sub tracted from the receiversensitivity in the range calculations. Thefollowing table provides a rule-of-thumbvalue for fade m argin as it relates to th ereliability of the data link.

Note that fade conditions are mor esevere in mobile environments. Expectdegraded per formance if you are in a m obileen vironmen t. If you are in a fixed locationand you are exper iencing multi-path, tr yadjusting the an tenn a position. O ften, mino radjustmen ts of the r adio anten na canimprove p er forman ce substantially.

25

Reliability Fade Margin (dB)

90 899 1899.9 2899.99 3899.999 48

Table 4 Fade margin for reliable data communication


30/48

The FCC regulates the portions of the

electromagnetic spectrum used for

radio communication.

The

entire set

of regulations

is contained in Title 47 of the Code of

Federal Regulations (CFR).

The sections of Title 47 which

are applicable to radio frequency data

communication are contained in Parts

90, 17 and 15 which contain the rulesand regulations for the operation of

land mobile radios for voice and non-

voice applications. Licensees of

narrow-band radio communication

equipment for use in commercial ornon-profit activities are required by

law to have a copy of these regulations.

Other parts of Title 47 con-

cern amateur (Part 47), Marine (Part

80), Aviation (Part 87), and other uses

of radio in voice and non-voice

communication.

FCClicensing andregulations

An excellent publication is

available from the Personal

Communications Industry Association

(PCIA). This publication contains a

detailed explanation of the licensing

process, as well as current copies of

Parts 90 and 17. PCIA can be reached by

calling (703) 739-0300. All titles of

the CFR are available at

government bookstores.

This section is not

intended to be a description of

how the various FCC forms need to

be filled out for a particular

circumstance. The FCC is continuously

amending the forms and theregulations are subject to change at

any time. Frequency coordinators and

businesses offering services for FCC

licensing issues should be consulted if

required for help in filling out theforms.

We recommend that you

contact Josie Lynch at Professional

Licensing Consultants Inc. Her phone

number is 301 309 2380.



31/48


Licensing requirements

Part 90 of Title 47 describes the rules,regulations and licensing requirements forprivate land mobile use of the radio spec-trum. In general, radio transmitters licensed

under the Part are for use in public safety,special emergen cy, ind ustrial, or landtransportation activities.

As part of the application p rocess,licensees will be asked to describe the activityfor which the radio system will be used, andalso to cite the regulation under whicheligibility is requ ired . (See CFR Title 47 ,Part 90.75 (a)( 1) as an example of aneligibility citation.)

Caution

Never operate a radio transmitter

without proper licensing (the FCC hasauthority to impose fines for severalthousands of dollars per day of illegaloperation).

Application forms

Section 90.119 outlines the applicationforms which are required for licensing aradio tran smitter. Note th at thro ugh April1995 , application was made using FCC Form574 . As of April 1995 , the FCC requiresapp lication be m ade with FCC form 600 .Copies of FCC form 600 can be obtained by

fax by calling 202 418 0177 .

Several m anu facturers p laceapplication forms and applicableinformation in boxes of radio equ ipment forsale. These forms may be outdated or mayreflect erroneous information causing delayin th e p rocessing of you r ap plication. Pleaseno te the curren t acceptable edition date(s)by referrin g to th e Private Radio Bureau FeeFiling Guide. Th e filing guide also pre sentsthe licensing fees and where to submit thelicense app lication s.

Contact your local FCC field officefor information o n curren t applicationforms or call the Private Radio Bureaus

Consumer Assistance staff in Gettysburg,Pen nsylvan ia at 1 888 CALL FCC. You mayalso con tact the ap prop riate FCC appointedfrequency coordinator an d re quest formsand filing information.

FrequenciesFrequency assignment for operation of aradio transmitter un der Part 90 is done incooperation with a frequency coordinator.The FCC has appointed groups, norm allytrade organizations, as frequencycoordinators to act as intermediariesbetween the license applicant and the FCC.Frequency coord inators assign and controlblocks of frequ encies set aside for par ticularuses. For example, the frequencycoordinator for police licensing is the

27


32/48


33/48


Anoth er p arameter of interest tothe FCC and also requ ired for theapp lication is the radio emissions de signato r.The em ission s designator is a techn icaldescriptor of the ban dwidth requ irementsand type o f modulation used by the radiotransmitter. Th is inform ation allows the FCCto judge the channel requirements in termsof channel width. Primary channels are gen-erally spaced at 25 kHz intervals across theband . In the UH F commercial band( 450 - 470 MHz), the FCC allows operationon offset channels which are spaced at12.5 kHz offsets from p rimary chan ne ls.

The em ission s designator is no rmallysupp lied with the rad io equipmen t, butmay also be ob tained by calling the rad ioman ufacturer directly.

For fixed location station s, thecoordinates of the base station transmittermust be supp lied. Th ese coord inates are

referenced to a particular datum (NAD 27 orNAD 83 or oth er) and are given in latitud e(d egrees, minu tes, secon ds), longitude(d egrees, minu tes, second s) an d grou ndelevation (meters). This information canbe determined using a 7.5 -minutetopograph ical quadrangle map of the area,or you may consult th e city or cou ntysurveyor in you r area. Topograph ic maps canbe purchased from the USGS inWashington , DC 20242 .

How to get help

Frequency coordin ators such as Josie Lynchat Pro fession al Licensing Con sultan ts Inc.are paid the coordination fee for th eirservices in aiding app lication s in ob taining

licensing. They are responsible for reviewingthe forms, and making sure the applicationprocess proceed s smoo thly. Con tact Josie at301 309 2380 or see Appendix C.

There are a nu mber of privatebusinesses who have access to the FCCdatabase and provide help in filling out theFCC form s. These businesses often add add i-tional supp ort in the form o f site sur veys andcoverage an alysis. Look in th e Yellow Pagesfor Communication Consultants in yourarea.

Another resource for licensing helpis the manu facturer or reseller of the r adio

equipment. Technical characteristics andemissions designator information which arerequired for the license application are bestobtained directly from the manufacturer.Some manufacturers provide licensingassistance of which you can take advantage.

The FCC can be contacted directly

for access to forms and some technical infor-mation. The local FCC field office cananswer r egulatory question s, bu t is notequippe d to help with the licenseapp lication process.

As mention ed earlier, the PCIAResource book is an invaluable aid to h elpwith th e license app lication process and alsomeets the regulatory requiremen t that thelicensee maintain a set of the rules and regu-lations.

The licensee is ultimately responsi-ble for compliance to the regulations.Informed use of the radio spectrum will pro-tect you from being subject to fines, and will

also provide you with knowledge of yourrights as a radio station operator.

29


34/48

Applications for radio frequency data

communication are increasing in number

as the requirements for real-time

data grows.Getting data from one location

to another using traditional wire-line

or telephone communication can

be overly expensive or impossible.

Radio data communication is growing

fast in areas such as computer-

automated dispatch, remote monitoring

and control, automatic vehicle

monitoring, differential GPS, and

others.

This section shows a small

sampling of applications and common

radio data communication topologies.

RDFCapplications

Point-to-point

Simple point-to-point RFDC links are oftenused to replace wired communication linkswhere the cost or difficulty of wiring makesit appropriate. An example of this type of system is a rem otely mon itored weatherstation making use of low-powered radiomod ems and directional anten nas to meetthe link r ange r equiremen t. See Figure 11 .

Th is system illustra tes a verycommon topo logy, with a computerconnected to the radio modem to broadcastcommand s and receive information from theremote site which consists of various sensorsconn ected to a data logger or remoteterminal unit (RTU). The computer controllocation is typically called the base station,while the instrumen tation and eq uipmen tat the weather station sight is called theremote station. Note the use of low power

transmitters with highly gained antennaspointing at each other. Many applicationssuch as this must rely on battery power atthe remote station, and therefore requireefficient use of the power resource. Antennaselection an d p lacement is critical in thistype of application where it is desirous tominimize power consumption.

Ano the r point-to-poin t app licationwhere the receiver station is mobile and thetransmitting station is stationary is a DGPScorrection link. In a DGPS correction link application, a DGPS base station is placed ina fixed kno wn location and mon itors errorsin the system. Correction factors are then

broadcast to the mob ile DGPS receiversbeing used in a survey or navigation activity.



35/48


The com mon terminology for the mo bileDGPS station is the rover.

Note th at in th is app lication , its

common to use omn i-directional anten nasfor th e base and rover station s. This allowsfor op eration in a 360 -degree p attern abou tthe base station. If the u se of the rovers islimited to on e direction from the basestation , output p ower can be red uced and aun idirection al anten na such as a Yagi can be

used. DGPS operations are often itinerant,and may make use of itinerant frequencies asrequired and in accordance with the FCCregu lation s. Because of th e varying con di-tions encoun tered in d ifferen t DGPS sites, itis difficult to provide a single configurationwhich can b e u sed in all situations. DGPSusers must have a basic understanding of radio data commun ication in o rder to m akechoices in setting up the communicationlink for b est per formance.

Figure 11 Point-to-point application

Digipeater

Figure 12 belowillustrates the use of a d igipeater. Adigipeater, also called astore-and-forwardrepeater, receives adata bro adcast andthen retransmits it onthe same or a differen tfreque ncy. Therebroadcast allows forlong distance radiocommunication andalso p rovides a meansfor getting coverage in

areas which may be shadowed from thedirect radio broadcast.

Figure 12 Digipeater application

Base

Station

DirectionalAntenna

RemoteWeatherStation

RF CarrierModulatedwith Data

BaseStation

Low PowerDirectionalAntenna

High PowerOmnidirectionalAntenna

Digipeater

31


36/48

In amateur packet radio, digipeater n etworkscan be taken advantage of to allow datacommun ication across the coun try. Incommercial activities, ther e is no digipeaterinfrastructure in place, so digipeatersare installed in a system for a particularapplication. Note that digipeating functionsare often designed into high quality radiodata commun ication prod ucts.

Packet operation

Sophisticated radio data communicationmakes use of packet operation to allow foraddressed, point-to-multipoint operation.In packet operation, each n ode in a n etwork

is assigned a un ique add ress. Packets of information are broadcast to specific notes

Digipeater

MobileAddress 1

Mobile

Address 2BaseAddress 0

in the network an d acknowledgments arereturned to the transmitting node. Thisallows for a reliable data network withmu ltiple n odes sharing a specific frequency.

Figure 13 shows a typical packetoperation network used in a taxi dispatchsystem. The dispatcher needs to sendspecific information to one of the fleet of taxis, and may also requ ire a broadcastcapability to send information to all taxisin the fleet.

Virtually all packet network app lication s requ ire application specificsoftware. The management of information is

gene rally adm inistered at a b ase station sitewhich controls the flow of information via apolled or Time Division Multiple Access(TDMA) algorithm. H igh qu ality radiodata commun ication equipmen t oftenprovides much of the packet protocolope ration which facilitates the design and

implementation of these systems. At aminimum, th e RFDC equipmen t sho uldprovide unique addressing capabilities andautom atic ackno wledgmen t capab ilities aspart of the modem protocol.


Figure 13 Packet switched application

32


37/48


antenna : The radiating/ receiving elementin a r adio system. Antennas are d esignedfor the efficient transmission and receptionof a radio signals and vary in length andelectrical configuration to match thefrequency and impedan ce of the r adiosystem.

antenna gain : A shaping of the pattern of an an tenn a to concentrate radiated energy,or received signal pickup , in some d irectionat the expen se of oth ers. All anten nasexh ibit gain over an isotropic radiator.

amplitude modulation (AM) : A modu lationof a carrier making use changes in signalvoltage levels to encode the signal.

attenuation : Redu ction of energy or signallevel.

baseband: A digital signal which containsthe binary information which is used to

modulate a carrier.

baud : A measure of the symbol rate withthe symbol being th e shortest elemen t in thedata encoding scheme. Each symbol mayencode one or more bits.

bit error rate (BER) : A ratio of the numberof bits foun d to b e in erro r to th e totalnumber of bits transmitted. Commonly usedto compare the quality of a data link.

bits per second (BPS) : A measure of the

number of bits (binary digits) transferredper second.

Appendix AGlossary

33


38/48

burst error : A sequence of consecutive bitswh ich are r eceived in er ror.

byte : A grou ping of bits wh ich constitute adiscrete item of information, normally 8 bitsin length.

carrier : A signal of fixed frequency oramplitude which is modulated with aninformation bearing signal.

carrier detect : A signal passed from theradio to an external device that indicatesthat a carrier of pred etermined strength is

present.

channel : A data communication path whichmay consist of a discrete frequency (FDMA),time-slot ( TDMA) or spreadin g code(CSMA).

code sense multiple access (CSMA) :A commun ication chann el making u se of spread-spectru m techn ology which uses aspreading code to mod ulate the widebandcarrier.

coverage : A measure of the p ropo rtion of agiven error which meets the communicationchannel requirements.

decibel (dB) : A relative unit of measureoften used to describe power or voltage.

demodulator : A circuit which takes amodulated signal and converts it to the

baseband information.

differential GPS (DGPS) : A techn iqueusing standard GPS information alongwith correction information broadcastfrom a base station which monitors thepseud o-ran ge errors in th e GPS signals.Allows for accuracy measures in metersand centimeters.

filter : An electron ic circuit which chan gesthe properties of a signal passing through it.

firmware : A program which controlsthe operating characteristics of amicrocon troller based d evice. Normally

stored in n on -volatile m emory such as aPROM (programmable read-only memory).

forward error correction (FEC) : Atechnique used to improve the effectivedata throughput in a communication link wher e error s due to n oise are pr esent.

A transmission of extra data which isencoded with inform ation which allowsthe receiving de vice to cor rect inform ationwhich is corrupted.

four-level minimum shift keying (4LFSK) :A very efficient variant on minimum shiftkeying mod ulation that en codes additionalbits per symbol by looking for zero crossingsat different levels.

frequency division multiple access(FDMA) : A division of the radio spectruminto different frequency channels to allowconcurrent simultaneous transmissions to

occur.



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frequency modulation (FM) : A modu lationof the carrier by varying its frequency withtime.

frequency shift keying : A modu lationtechn ique with th e d igital signal states(generally 1s and 0s) be ing tr anslatedinto different frequencies which arecapable of being tran smitted thr oughthe communication medium.

full duplex : A communication systemcapable of simultaneous transmission andreception of data.

gain : An increase of power or voltage onthe output of a circuit which is proportionalto the input.

Gaussian minimum shift keying : A varianton minimum shift keying modulation which

uses a baseband gaussian filter to shape themodulation output. GMSK is commonlyused in high speed data applications becauseof its ban dwidth conservative n ature an dgood immunity to fade conditions.

global positioning system (GPS) :A system of satellites wh ich p rovide timingand other information via spread spectrumradio tran smissions allowing th e p recisedete rmin ation o f position u sing r elativelyinexpensive r eceivers. GPS satellites aremanaged by the US Departmen t of Defenseand the US Department o f Transportation.

half duplex : A communication systemcapable of transmission and receptionof data in a mu tually exclusive mann er.(Cann ot transmit and r eceive at the sametime.)

handshaking : A hardware or softwaremech anism which allows for th e con trol of data flow.

isotropic radiator : A theoretical pointsource of rad iation wh ich rad iates equally inall directions.

modem : A circuit o r d evice wh ich isdesigned to modulate and demodulate asignal from its digital repr esentation to awaveform wh ich is app rop riate for th etransmission medium.

noise : Unwanted signals caused by sou rces

external and internal to an y circuit. Thelimiting factor in most communicationsystems.

parity : An add ition al bit sen t in a serial datastream which is depend ent o n the byte beingsent and is used to check for error s in thedata. Even par ity mean s that the parity bitwill be set such that there is an even numberof 1s in the data stream. Odd parity bit willbe set such that there is an odd number of 1s in the data stream. Parity is a poor erro rdetection because it does not catch erro rswhere an even number of bits are affected.

phase modulation (PM) : A modulationmeth od wher e the ph ase of the signal isvaried to provide information conten t.

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point-to-multipoint : A comm un icationsystem where a single point is capable of addressing multiple points using a packetswitched mechanism of addressed datadelivery.

point-to-point : A fixed data pathcommunication system where informationflows between two points.

propagation : The p ath or m ethod whicha radio wave travels from its source to itsdestination. The mod e of prop agationdiffers dep end ing on the freque ncy of the

signal.

range : The d istance at which rad iocommunication is adequate for a particulartask.

signal-to-noise (S / N) : A measure of the

ratio of signal to noise in a received signal.Signal-to-noise is often expressed in termsof dB.

signal-to-noise and distortion (SINAD) :A measure of the ratio of signal to n oiseand distortion. SINAD is a comm on test forradio receiver sensitivity where an RF signalwh ich is mod ulated with a 1 kHz audio toneat +/ - 3 kHz deviation is applied to the radioreceiver. A notch filter circuit is used toanalyze the audio output signal to noise anddistortion. Th e RF signal is lowered un til theaudio signal is measured to be 12 dB. Thepower of the RF is the 12 dB SINAD figure

used to indicate radio receiver sensitivity.

synchronous modem : A modem whichmakes use of synchro no us clock extractionwhich relies on a continuous stream of data.

time division multiple access (TDMA) :A division o f a single chan nel into a n um berof time slots where devices are assignedspecific time slots when they can transmitdata. Th is mu ltiplexing o f a frequen cy allowsmu ltiple u sers to shar e a single freque ncy.

telemetry : The transmission of non-voicesignals for th e purpose of automaticallyindicating or record ing measurem ents at a

distance from the measuring instrumen t.

turnaround time : The time required toswitch from a tran smit to a r eceive fun ctionin a half duplex data link.

watt : A unit of measure of power being

equ ivalent to 1 joule/ second.



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Appendix BBibliography

The ARRL Handbook for Radio Amateurs 69th Edition , American Radio Relay League,1991

Balanis, Constantine A., Antenna Theory Analysis and Design , Harp er an d Row, NewYork City, New York, 1982

Bingham , John A.C., The Theory and Practice of Modem Design , John Wiley & Sons, NewYork City, New York, 1988

Fehe r, Dr. Kamilo, Wireless Digital Communications , Pren tice H all, New Jersey,

1995

Horowitz, Paul, The Art of Electronics Second Edition , Cambridge University Press,1989

Intern ational Telephon e and Telegraph

Corporation, Reference Data for Radio Engineers 6th Edition , Howard Sams,New York City, New York, 1981

Lath i, B.P., Modern Digital and Analog Communication Systems , Holt, Rinehart & Winsto n , New Yor k City, New Yor k, 1983

Techo, Robert, Data Communications An Introduction to Concepts and Design ,Plenum Pre ss, New Yor k City, New Yor k,1981

Wells, David, Guide to GPS Positioning ,Canadian GPS Associates, 1987

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Public safety radio services

Fire

International Association of Fire Chiefs/ International Municipal Signal Association( IAFC/ IMSA)c/ o IMSAPost Office Box 1513Providence, Rhode Island 02901-1513Tel 401 738 2220Fax 401 738 7336

Highway maintenance

American Association of State Highwayand Tran sportation Officials (AASHTO)444 North Capitol Street NorthwestSuite 249Washington , DC 20001Tel 202 624 5800Fax 202 624 5806

Forestry conservation

Forestry Conservation Communications

Association (FCCA)Hall of the States444 North Capitol Street NorthwestSuite 540Washington , DC 20001Tel 202 624 5416Fax 202 624 5407

AppendixC Frequencycoordinators

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Special industrial

Industrial TelecommunicationsAssociation Inc. ( ITA)1110 North Glebe RoadSuite 500Arlington, Virginia 22201Tel 703 528 5115Fax 703 524 1074Business

Personal Commun icationsIndustry Association (PCIA)1501 Duke Street

Alexandria, Virginia 22314Tel 703 739 0300Fax 703 836 1608

Manufacturers

Manufacturers Radio FrequencyAdvisory Committee (MRFAC)899 A Har rison Street South eastLeesburg, Virginia 20175Tel 703 318 9206Fax 703 669 0322

Industrial radio services

Power

Utilities Telecomm un ications Cou ncil(UTC)1140 Conn ecticut Aven ue North westSuite 1140Washington , DC 20036Tel 202 872 0030Fax 202 872 1331

Mail application to:

UTCDepartment 79307Baltimore, Maryland 21279

Petroleum

Petroleum Frequency Coord inatingCommittee (PFCC)c/ o Ind ustrial Telecomm un ication sAssociation Inc. ( ITA)1110 North Glebe RoadSuite 500Arlington, Virginia 22201Tel 703 528 5115Fax 703 524 1074

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Offshore zone frequencies

Petroleum FrequencyCoordinating Committee (PFCC)c/ o Ind ustrial Telecomm un ication sAssociation Inc. ( ITA)1110 North Glebe RoadSuite 500Arlington, Virginia 22201Tel 703 528 5115Fax 703 524 1074

Other land transportation radio services

Motor carrier

American Trucking Association (ATA)Attention: Frequency Coordination2200 Mill RoadAlexandr ia, Virginia

22314Tel 703 838 1730Fax 703 683 1934

Taxicabs

International Taxicab andLivery Association (ITLA)3849 Farragut Aven ueKensington, Maryland 20895Tel 301 946 5702Fax 301 946 4641

Telephone maintenance

Telephon e Mainten ance Frequen cyAdvisory Com mittee (TELFAC)c/ o Ind ustrial Telecomm un ication sAssociation Inc. ( ITA)1110 North Glebe RoadSuite 500Arlington, Virginia 22201Tel 703 528 5115Fax 703 524 1074

Industrial radio groups

Airport terminal use frequencies

Personal Commun ication sIndustry Association (PCIA)1501 Duke StreetAlexandr ia, Virginia 22314Tel 703 739 0300Fax 703 836 1608

Alarm frequencies

Cen tral Station Alarm Association (CSAA)c/ o Supreme Security Systems140 Hillside Aven ueHillside, New Jersey 07205Tel 201 923 4600Fax 201 923 4535

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Original 800-MHzconventional business

Personal Comm un ications In dustryAssociation (PCIA)1501 Duke StreetAlexandria, Virginia 22314Tel 703 739 0300Fax 703 836 1608

Original 800-MHzconventional industrial/landtransportation

Industrial Telecommunications AssociationInc. (ITA)1110 North Glebe RoadSuite 500Arlington, Virginia 22201Tel 703 528 5115Fax 703 524 1074

Original 800-MHz trunked

None

Subject to change

For the most current address of any certifiedfrequency coordinating committee, callthe FCCs Consumer Assistance Branch at1 888 CALL FCC.

Automobile emergency

American Automobile Association (AAA)Freque ncy Coordination Departmen t1000 AAA DriveHeathr ow, Florida 32746-5063Tel 407 444 7786Fax 407 444 7749

Other frequency pools

MHz paging

Personal Comm un ication s Indu stryAssociation (PCIA)1501 Duke StreetAlexandr ia, Virginia 22314Tel 703 739 0300Fax 703 836 1608

MHz business

Personal Comm un ication s Indu stryAssociation (PCIA)1501 Duke StreetAlexandr ia, Virginia 22314Tel 703 739 0300Fax 703 836 1608

MHz trunked SMRs

None

41

T h G i d t W i l G P S D t L i k42


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Ddata descrambling 25data logger 30data scrambling 23, 24dBi 17dBm 14decibel 34demodulator 25, 34differential Global Positioning System

(DGPS) 1, 30, 34DGPS base station 30DGPS rover 31digipeater 31direct wave 8

ducting 8

Eelectromagnetic spectrum 5emissions designator 29

Ffade 9, 22, 25fade margin 19, 25FCC 26FCC forms 27feed-line 15fiber-optics 5filter 34firmware 34forward error corr ection (FEC) 24, 34frequ ency coordinator 26, 28, 38frequency division multiple access

(FDMA) 34frequency modulation 9, 35

frequency shift keying 35full duplex 35

Aamplitude mod ulation 9, 33antenna 10, 33attenna, d irection al 17attenna gain 17, 33attenna gain pattern 17attenn a grounding 17attenna, omni-directional 17, 30atten na, Yagi 31attenuation 33automatic vehicle monitoring (AVM) 33

Bbase station 30baseband 9, 33baud 33bit error rate (BER) 23, 33, 10bits per second (BPS) 7, 33burst error 9, 34byte 10, 34

Ccarrier 10, 34carrier detect 18, 34carrier-operated relay 11cellular ph on es 5channel 34channel offset 29channel primary 29co-chann el inter ference 2, 3Code of Federal Regulation s (CFR) 26code sense multiple access (CSMA) 34computer-automated dispatch 30coverage 6, 34

cyclic redun dan cy check (CRC) 25

I n

d e x

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Ggain 35gain factors, free space 21gain factors, over groun d 21gain factors, system 23Gaussian minimu m shift keying

(GMSK) 35global positioning system (GPS) 35

Hhalf duplex 35hamming code 24handshaking 35

Iimpedance 15infra-red 5interference 6ionosphere 6ionosperic reflection

7isotropic radiator 35itinerant frequencies 28

LLF 6line-of-sight 6, 23

MMF 6microwave 5

modem 35modulation 5multi-path 16, 17, 19

Nnoise 35

Ppacket radio 32parity 35path loss 16phase locked loop 24

ph ase mo dulation 36point-to-multipoint 32, 36point-to-point 30, 36power 15propagation 6, 7, 8, 36

Rradio channels 2radio h orizon 6radio spectrum 5rad io waves 5range 2, 6, 36receiver sensitivity 18remote mon itoring and control 30

remote station 30remote terminal unit 30RFCALC 21

Ssignal penetration 6

signal-to-no ise ( S/ N) 36signal-to-noise and distortion(SINAD) 18, 36

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Wwatts 14, 36

YYagi 11

Zzero-crossings 24

skip 7sky-wave 7spread-spectrum 5synchr on ous modem 36system losses 15

Ttelemetry 36television 5time division multiple access

(TDMA) 36topographic maps 29turnaroun d time 36

UUH F 8

VVHF 7VLF 6voltage, function of power 15voltage standing wave ratio (VSWR) 15

44

basics to gps

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