Wireless Communication Technology

590 Pages·2000·11.612 MB·English

by  Roy Blake, Leo Chartrand

1
Fundamental Concepts
Objectives
After studying this chapter, you should be able to:
( Explain the nature and importance of wireless communication.
( Outline the history of wireless communication.
( Explain the necessity for modulation in a radio communication system.
( Outline the roles of the transmitter, receiver, and channel in a radio
communication system.
( Describe and explain the differences among simplex, half-duplex, and
full-duplex communication systems.
( Describe the need for wireless networks and explain the use of repeaters.
( List and briefly describe the major types of modulation.
( State the relationship between bandwidth and information rate for any
communication system.
( Calculate thermal noise power in a given bandwidth at a given
temperature.
( Explain the concept of signal-to-noise ratio and its importance to
communication systems.
( Describe the radio-frequency spectrum and convert between frequency and
wavelength.
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2 CHAPTER 1
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1.1 Introduction
Thisisabookonwirelesscommunication.Thatusuallymeanscommunica-
tion by radio, though ultrasound and infrared light are also used occasion-
ally. The term “wireless” has come to mean nonbroadcast communication,
usually between individuals who very often use portable or mobile equip-
ment.Thetermisrathervague,ofcourse,andtherearecertainlyborderline
applications that are called wireless without falling exactly into the above
definition.
Wirelesscommunicationisthefastest-growingpartoftheverydynamic
field of electronic communication. It is an area with many jobs that go un-
filledduetoashortageofknowledgeablepeople.Itistheauthor’shopethat
thisbookwillhelptoremedythatsituation.
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1.2 Brief History of Wireless Telecommunication
Mostofthisbookisconcernedwiththepresentstateofwirelesscommunica-
tion, with some speculation as to the future. However, in order to under-
stand the present state of the art, a brief glimpse of the past will be useful.
Present-day systems have evolved from their predecessors, some of which
arestillverymuchwithus.Similarly,wecanexpectthatfuturesystemswill
bedevelopedfromcurrentones.
The Beginning Wirelesstelecommunicationbeganonlyalittlelaterthanthewiredvariety.
Morse’s telegraph (1837) and Bell’s telephone (1876) were soon followed
by Hertz’s first experiments with radio (1887). Hertz’s system was a labora-
tory curiosity, but Marconi communicated across the English Channel in
1899andacrosstheAtlanticOceanin1901.Thesesuccessesledtothewide-
spreaduseofradioforship-to-shipandship-to-shorecommunicationusing
Morsecode.
Early wireless systems used crude, though often quite powerful, spark-
gaptransmitters,andweresuitableonlyforradiotelegraphy.Theinvention
of the triode vacuum tube by De Forest in 1906 allowed for the modula-
tion of a continuous-wave signal and made voice transmission practical.
Thereissomedisputeaboutexactlywhodidwhatfirst,butitappearslikely
thatReginaldFessendenmadethefirstpublicbroadcastofvoiceandmusic
in late 1906. Commercial radio broadcasting in both the United States and
Canadabeganin1920.
Earlyradiotransmittersweretoocumbersometobeinstalledinvehicles.
Infact,thefirstmobileradiosystems,forpolicedepartments,wereone-way,
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FUNDAMENTAL CONCEPTS 3
withonlyareceiverinthepolicecar.Thefirstsuchsystemtobeconsidered
practical was installed in Detroit in 1928. Two-way police radio, with the
equipmentoccupyingmostofthecartrunk,beganinthemid-1930s.Ampli-
tudemodulation(AM)wasuseduntilthelate1930s,whenfrequencymodu-
lation(FM)begantodisplaceit.
WorldWarIIprovidedamajorincentiveforthedevelopmentofmobile
and portable radio systems, including two-way systems known as “walkie-
talkies” that could be carried in the field and might be considered the dis-
tant ancestors of today’s cell phones. FM proved its advantages over AM
inthewar.
Postwar Soon after the end of World War II, two systems were developed that pres-
Expansion aged modern wireless communication. AT&T introduced its Improved
MobileTelephoneService(IMTS)in1946,featuringautomaticconnection
of mobile subscribers to the public switched telephone network (PSTN).
Thiswasanexpensiveservicewithlimitedcapacity,butitdidallowtruemo-
bile telephone service. This system is still in use in some remote areas,
where,forinstance,itallowsaccesstothePSTNfromsummercottages.
The next year, in 1947, the American government set up the Citizens’
Band (CB) radio service. Initially it used frequencies near 460 MHz, but in
thatrespectitwasaheadofitstime,sinceequipmentfortheUHFrangewas
prohibitively expensive. Frequencies in the 27-MHz band were allocated in
1958,andCBradioimmediatelybecameverypopular.Theservicewasshort-
range, had no connection to the PSTN, and offered users no privacy, but it
was(andstillis)cheapandeasytosetup.ThepopularityofCBradiohasde-
clined in recent years but it is still useful in applications where its short
rangeandlackofconnectivitytotherestoftheworldarenotdisadvantages.
For example, it serves very well to disseminate information about traffic
problemsonthehighway.
Meanwhile another rather humble-appearing appliance has become
ubiquitous:thecordlessphone.Usuallyintendedforveryshort-rangecom-
munication within a dwelling and its grounds, the system certainly lacks
rangeanddrama,butitdoeshaveconnectivitywiththePSTN.Mostcordless
phones use analog FM in the 46- and 49-MHz bands, but some of the latest
models are digital and operate at either 900 MHz or 2.4 GHz. Cordless
phonesarecheapandsimpletouse,buttheirrangeislimitedand,exceptfor
thedigitalmodels,theyofferlittleprivacy.
Pagers were introduced in 1962. The first models merely signaled the
usertofindatelephoneandcallaprearrangednumber.Morerecentmodels
can deliver an alphanumeric message and even carry a reply. Though rela-
tivelylimitedinfunction,pagersremainverypopularduetotheirlowcost
andsmallsize.
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4 CHAPTER 1
The Cellular The world’s first cellular radio service was installed in Japan in 1979, fol-
Revolution lowedin1983byNorthAmericanservices.Cellularsystemsarequitediffer-
ent from previous radiotelephone services such as IMTS in that, instead of
usingasinglepowerfultransmitterlocatedonatalltowerforwidecoverage,
thepowerofeachtransmitterisdeliberatelykeptrelativelysmallsothatthe
coverage area, called a cell, will also be small. Many small cells are used so
that frequencies can be reused at short distances. Of course, a portable or
mobiletelephonemaymovefromonecelltoanothercellduringthecourse
of a conversation. In fact, this handoff may occur several times during a
conversation. Practical cellular systems had to await the development of
computers fast enough and cheap enough to keep track of all this activity.
Theoretically at least, the number of users in a cellular system can be in-
creasedindefinitely,simplybymakingthecellssmaller.
ThefirstcellularsystemsusedanalogFMtransmission,butdigitalmod-
ulation schemes, which provide greater privacy and can use bandwidth
moreefficiently,areusedinallthenewsystems.Thesepersonalcommuni-
cation systems (PCS) usually operate in a higher frequency range (about
1.9GHzcomparedwith800MHzforNorthAmericancellularservice).
Current cellular systems are optimized for voice but can also transmit
data.Inthenearfuture,high-speeddatatransmissionusingPCSisexpected
tobecomeareality.Atthispoint,however,thepastmergesintothefuture,
andwe’llresumethediscussionlaterinthisbook.
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1.3 Elements of a Wireless Communication System
Themostbasicpossiblewirelesssystemconsistsofatransmitter,areceiver,
andachannel,usuallyaradiolink,asshowninFigure1.1.Sinceradiocan-
notbeuseddirectlywithlowfrequenciessuchasthoseinahumanvoice,it
is necessary to superimpose the information content onto a higher fre-
quencycarriersignalatthetransmitter,usingaprocesscalledmodulation.
Theuseofmodulationalsoallowsmorethanoneinformationsignaltouse
FIGURE1.1 Elementsofacommunicationsystem
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FUNDAMENTAL CONCEPTS 5
theradiochannelbysimplyusingadifferentcarrierfrequencyforeach.The
inverse process, demodulation, is performed at the receiver in order to re-
covertheoriginalinformation.
The information signal is also sometimes called the intelligence, the
modulating signal, or the baseband signal. An ideal communication sys-
tem would reproduce the information signal exactly at the receiver, except
for the inevitable time delay as it travels between transmitter and receiver,
and except, possibly, for a change in amplitude. Any other changes consti-
tutedistortion.Anyrealsystemwillhavesomedistortion,ofcourse:partof
the design process is to decide how much distortion, and of what types, is
acceptable.
Simplex Figure 1.1 represents a simplex communication system. The communica-
and Duplex tionisonewayonly,fromtransmittertoreceiver.Broadcastingsystemsare
Communication likethis,exceptthattherearemanyreceiversforeachtransmitter.
Mostofthesystemswediscussinthisbookinvolvetwo-waycommuni-
cation.Sometimescommunicationcantakeplaceinbothdirectionsatonce.
Thisiscalledfull-duplexcommunication.Anordinarytelephonecallisan
exampleoffull-duplexcommunication.Itisquitepossible(thoughperhaps
not desirable) for both parties to talk at once, with each hearing the other.
Figure 1.2 shows full-duplex communication. Note that it simply doubles
the previous figure: we need two transmitters, two receivers, and, usually,
twochannels.
FIGURE1.2 Full-duplexcommunicationsystem
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6 CHAPTER 1
Some two-way communication systems do not require simultaneous
communicationinbothdirections.Anexampleofthishalf-duplextypeof
communicationisaconversationovercitizens’band(CB)radio.Theopera-
tor pushes a button to talk and releases it to listen. It is not possible to talk
andlistenatthesametime,asthereceiverisdisabledwhilethetransmitter
is activated. Half-duplex systems save bandwidth by allowing the same
channel to be used for communication in both directions. They can some-
timessavemoneyaswellbyallowingsomecircuitcomponentsinthetrans-
ceivertobeusedforbothtransmittingandreceiving.Theydosacrificesome
ofthenaturalnessoffull-duplexcommunication,however.Figure1.3shows
ahalf-duplexcommunicationsystem.
FIGURE1.3 Half-duplexcommunicationsystem
Wireless The full- and half-duplex communication systems shown so far involve
Networks communicationbetweenonlytwousers.Again,CBradioisagoodexample
ofthis.Whentherearemorethantwosimultaneoususers,orwhenthetwo
users are too far from each other for direct communication, some kind of
networkisrequired.Networkscantakemanyforms,andseveralwillbeex-
aminedinthisbook.Probablythemostcommonbasicstructureinwireless
communicationistheclassicstar network,showninFigure1.4.
Thecentralhubinaradionetworkislikelytobearepeater,whichcon-
sistsofatransmitterandreceiver,withtheirassociatedantennas,locatedin
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FUNDAMENTAL CONCEPTS 7
FIGURE1.4
Starnetwork
agoodpositionfromwhichtorelaytransmissionsfromandtomobileradio
equipment. The repeater may also be connected to wired telephone or data
networks.ThecellularandPCStelephonesystemsthatwelookatlaterinthe
bookhaveanelaboratenetworkofrepeaterstations.
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1.4 Signals and Noise
Thecommunicationsystemsdescribedinthisbookdifferinmanyways,but
theyallhavetwothingsincommon.Ineverycasewehaveasignal,whichis
usedtocarryusefulinformation;andineverycasethereisnoise,whichen-
ters the system from a variety of sources and degrades the signal, reducing
the quality of the communication. Keeping the ratio between signal and
noisesufficientlyhighisthebasisforagreatdealoftheworkthatgoesinto
thedesignofacommunicationsystem.Thissignal-to-noiseratio,abbrevi-
atedS/Nandalmostalwaysexpressedindecibels,isanimportantspecifica-
tionofvirtuallyallcommunicationsystems.Letusfirstconsidersignaland
noiseseparately,andthentakeapreliminarylookatS/N.
Modulated Given the necessity for modulating a higher-frequency signal with a
Signals lower-frequency baseband signal, it is useful to look at the equation for
a sine-wave carrier and consider what aspects of the signal can be varied. A
generalequationforasinewaveis:
e(t) = E sin(ω t + θ) (1.1)
c c
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8 CHAPTER 1
where
e(t) = instantaneous voltage as a function of time
E = peak voltage of the carrier wave
c
ω = carrier frequency in radians per second
c
t = time in seconds
θ = phase angle in radians
Itiscommontouseradiansandradianspersecond,ratherthandegrees
and hertz, in the equations dealing with modulation, because it makes the
mathematics simpler. Of course, practical equipment uses hertz for fre-
quencyindications.Theconversioniseasy.Justrememberfrombasicacthe-
orythat
ω = 2πƒ (1.2)
where
ω = frequency in radians per second
ƒ = frequency in hertz
AlookatEquation(1.1)showsusthatthereareonlythreeparametersof
a sine wave that can be varied: the amplitude E , the frequency ω, and the
c
phaseangleθ.Itisalsopossibletochangemorethanoneoftheseparameters
simultaneously;forexample,indigitalcommunicationitiscommontovary
boththeamplitudeandthephaseofthesignal.
Once we decide to vary, or modulate, a sine wave, it becomes a com-
plex waveform. This means that the signal will exist at more than one
frequency; that is, it will occupy bandwidth. Bandwidth is a concept that
will be explored in more detail later in this chapter and will recur often in
thisbook.
Noise It is not sufficient to transmit a signal from transmitter to receiver if the
noise that accompanies it is strong enough to prevent it from being under-
stood.Allelectronicsystemsareaffectedbynoise,whichhasmanysources.
In most of the systems discussed in this book, the most important noise
componentisthermalnoise,whichiscreatedbytherandommotionofmol-
ecules that occurs in all materials at any temperature above absolute zero
(0Kor−273°C).Weshallhaveagooddealtosayaboutnoiseandtheratio
between signal and noise power (S/N) in later chapters. For now let us just
notethatthermalnoisepowerisproportionaltothebandwidthoverwhich
asystemoperates.Theequationisverysimple:
P = kTB (1.3)
N
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FUNDAMENTAL CONCEPTS 9
where
P = noise power in watts
N
k = Boltzmann’s constant, 1.38 × 10−23 joules/kelvin (J/K)
T = temperature in kelvins
B = noise power bandwidth in hertz
Notetherecurrenceofthetermbandwidth.Hereitreferstotherangeoffre-
quencies over which the noise is observed. If we had a system with infinite
bandwidth, theoretically the noise power would be infinite. Of course, real
systemsneverhaveinfinitebandwidth.
A couple of other notes are in order. First, kelvins are equal to degrees
Celsius in size; only the zero point on the scale is different. Therefore, con-
vertingbetweendegreesCelsiusandkelvinsiseasy:
T(K) = T(°C) + 273 (1.4)
where
T(K) = absolute temperature in kelvins
T(°C) = temperature in degrees Celsius
Also, the official terminology is “degrees Celsius” or °C but just “kelvins”
orK.
EXAMPLE 1.1 Y
Aresistoratatemperatureof25°Cisconnectedacrosstheinputofanampli-
fierwithabandwidthof50kHz.Howmuchnoisedoestheresistorsupplyto
theinputoftheamplifier?
SOLUTION
Firstwehavetoconvertthetemperaturetokelvins.
FromEquation(1.4),
T(K) = T(°C) + 273
= 25 + 273
= 298 K
NowsubstituteintoEquation(1.3),
P = kTB
N
= 1.38 × 10−23 × 298 × 50 × 103
= 2.06 × 10−16 W
= 0.206 fW