Principles of Spread-Spectrum Communication Systems, Second Edition

589 Pages·2011·4.969 MB·English

by  Torrieri, Don

Principles of Spread-Spectrum Communication
Systems

Don Torrieri
Principles of
Spread-Spectrum
Communication Systems
Second Edition
123
DonTorrieri
USArmy Research Laboratory
Adelphi, MD, USA
ISBN978-1-4419-9594-0 e-ISBN978-1-4419-9595-7
DOI10.1007/978-1-4419-9595-7
SpringerNewYorkDordrechtHeidelbergLondon
LibraryofCongressControlNumber:2011932168
c SpringerScience+BusinessMedia,LLC2011

Allrightsreserved.Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten
permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York,
NY10013, USA),except forbrief excerpts inconnection with reviews orscholarly analysis. Usein
connectionwithanyformofinformationstorageandretrieval,electronicadaptation,computersoftware,
orbysimilarordissimilarmethodologynowknownorhereafterdevelopedisforbidden.
Theuseinthispublicationoftradenames,trademarks,servicemarks,andsimilarterms,eveniftheyare
notidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyaresubject
toproprietaryrights.
Printedonacid-freepaper
SpringerispartofSpringerScience+BusinessMedia(www.springer.com)
To MyFamily

Preface
Spread-spectrum communication is a core area within the field of digital
communication. Originally used in military networks as countermeasures against
the threats of jammingand interception,spread-spectrumsystemsare now widely
used in commercial applications and are part of several wireless and mobile
communication standards. Although spread-spectrum communication is a staple
topicin textbookson digitalcommunication,its treatmentis usuallycursory.This
book is designed to provide a more intensive examination of the subject that is
suitable for graduate students and practicing engineers with a solid background
in the theory of digital communication. As the title indicates, this book stresses
principles rather than specific current or planned systems, which are described in
manyotherbooks.Mygoalinthisbookistoprovideaconcisebutlucidexplanation
of the fundamentals of spread-spectrum systems with an emphasis on theoretical
principles.Thechoiceofspecifictopicstoincludewastemperedbymyjudgment
oftheirpracticalsignificanceandinteresttobothresearchersandsystemdesigners.
Throughoutthebook,learningisfacilitatedbymanyneworstreamlinedderivations
of the classical theory. Problems at the end of each chapter are intended to assist
readers in consolidating their knowledge and to provide practice in analytical
techniques.The listed referencesare ones that I recommendfor furtherstudy and
assourcesofadditionalreferences.
A spread-spectrum signal is one with an extra modulation that expands the
signal bandwidth greatly beyond what is required by the underlying coded-data
modulation. Spread-spectrum communication systems are useful for suppressing
interference, making secure communications difficult to detect and process, ac-
commodating fading and multipath channels, and providing a multiple-access
capability. Spread-spectrum signals cause relatively minor interference to other
systems operating in the same spectral band. The most practical and dominant
spread-spectrumsystemsaredirect-sequence and frequencyhoppingsystems.
There is no fundamental theoretical barrier to the effectiveness of spread-
spectrumcommunications.Thatremarkablefactisnotimmediatelyapparentsince
the increased bandwidthof a spread-spectrumsignal mightrequirea receive filter
vii
viii Preface
that passes more noise power than necessary to the demodulator. However, when
anysignalandwhiteGaussiannoiseareappliedtoafiltermatchedtothesignal,the
sampledfilteroutputhasasignal-to-noiseratiothatdependssolelyontheenergy-
to-noise-density ratio. Thus, the bandwidth of the input signal is irrelevant, and
spread-spectrumsignalshavenoinherentlimitations.
Chapter1reviewsfundamentalresultsofcodingandmodulationtheorythatare
essentialtoafullunderstandingofspread-spectrumsystems.Channelcodes,which
arealsocallederror-correctionorerror-controlcodes,arevitalin fullyexploiting
the potential capabilities of spread-spectrum systems. Although direct-sequence
systemscangreatlysuppressinterference,practicalsystemsrequirechannelcodes
to deal with the residual interference and channel impairments such as fading.
Frequency-hoppingsystems are designed to avoid interference,but the possibility
of hopping into an unfavorablespectral region usually requires a channel code to
maintainadequateperformance.Inthischapter,codingandmodulationtheoryare
usedtoderivetherequiredreceivercomputationsandtheerrorprobabilitiesofthe
decoded information bits. The emphasis is on the types of codes and modulation
thathaveprovedmostusefulinspread-spectrumsystems.
Chapter 2 presents the fundamentals of direct-sequence systems. Direct-
sequence modulationentails the direct addition of a high-rate spreadingsequence
with a lower-rate data sequence,resulting in a transmitted signal with a relatively
wide bandwidth. The removal of the spreading sequence in the receiver causes
a contraction of the bandwidth that can be exploited by appropriate filtering to
remove a large portion of the interference. This chapter begins with a discussion
of spreading sequences and waveforms and then provides a detailed analysis of
howthedirect-sequencereceiversuppressesvariousformsofinterference.Several
methodsthatsupplementthe inherentabilityof a direct-sequencesystem to reject
narrowbandinterferenceareexplained.
Chapter 3 presentsthe fundamentalsof frequency-hoppingsystems. Frequency
hopping is the periodic changing of the carrier frequency of a transmitted signal.
Thistime-varyingcharacteristic potentiallyendowsa communicationsystem with
great strength against interference. Whereas a direct-sequence system relies on
spectralspreading,spectraldespreading,andfiltering to suppressinterference.the
basic mechanism of interference suppression in a frequency-hopping system is
that of avoidance. When the avoidance fails, it is only temporary because of the
periodicchangingofthecarrierfrequency.Theimpactoftheinterferenceisfurther
mitigated by the pervasive use of channel codes, which are more essential for
frequency-hoppingthan for direct-sequence systems. The basic concepts, spectral
andperformanceaspects,andcodingandmodulationissuesarepresentedinthefirst
five sections of this chapter. The effects of partial-bandinterference and jamming
areexamined,andthemostimportantissuesinthedesignoffrequencysynthesizers
aredescribed.
Chapter4focusesonsynchronization.Aspread-spectrumreceivermustgenerate
a spreading sequence or frequency-hoppingpattern that is synchronized with the
received sequence or pattern; that is, the corresponding chips or dwell intervals
Preface ix
must precisely or nearly coincide. Any misalignment causes the signal amplitude
at the demodulatoroutputto fall in accordancewith the autocorrelationor partial
autocorrelationfunction.Althoughtheuseofprecisionclocksinboththetransmitter
and the receiver limit the timing uncertainty in the receiver, clock drifts, range
uncertainty, and the Doppler shift may cause synchronization problems. Code
synchronization, which is either sequence or pattern synchronization, might be
obtained from separately transmitted pilot or timing signals. It may be aided or
enabled by feedback signals from the receiver to the transmitter. However, to
reduce the cost in power and overhead, most spread-spectrum receivers achieve
code synchronization by processing the received signal. Both acquisition, which
providescoarsesynchronization,andtracking,whichprovidesfinesynchronization,
aredescribedinthischapter.Theemphasisisontheacquisitionsystembecausethis
systemisalmostalwaysthedominantdesignissueandmostexpensivecomponent
ofacompletespread-spectrumsystem.
Chapter 5 provides a general description of the most important aspects of
fadingandtheroleofdiversitymethodsincounteractingit.Fadingisthevariation
in received signal strength due to a time-varying communications channel. It is
primarily caused by the interaction of multipath components of the transmitted
signal that are generated and altered by changing physical characteristics of the
propagation medium. The principal means of counteracting fading are diversity
methods, which are based on the exploitation of the latent redundancy in two or
moreindependentlyfadingcopiesofthesamesignal.Therakedemodulator,which
isofcentralimportanceinmostdirect-sequencesystems,isshowntobecapableof
exploitingundesiredmultipathsignalsratherthansimplyattemptingtorejectthem.
Themulticarrierdirect-sequencesystemisshowntobeaviablealternativemethod
ofexploitingmultipathsignalsthathaspracticaladvantages.
Chapter 6 presents the general characteristics of spreading sequences and
frequency-hopping patterns that are suitable for code-division multiple access
(CDMA) systems. Multiple access is the ability of many users to communicate
witheachotherwhilesharingacommontransmissionmedium.Wirelessmultiple-
access communicationsare facilitated if the transmitted signals are orthogonalor
separable in some sense. Signals may be separated in time, frequency, or code.
CDMAisrealizedbyusingspread-spectrummodulationwhiletransmittingsignals
from multiple users in the same frequencyband at the same time. All signals use
the entire allocated spectrum, but the spreading sequences or frequency-hopping
patternsdiffer.CDMA isadvantageousforcellularnetworksbecauseiteliminates
the need for frequency and time-slot coordination among cells, allows carrier-
frequency reuse in adjacent cells, and imposes no sharp upper bound on the
number of users. Another major CDMA advantage is the ease with which it can
becombinedwithmultibeamedantennaarraysthatareeitheradaptiveorhavefixed
patternscoveringcellsectors.Inactivesystemsinanetworkreducetheinterference
receivedby an active CDMA system. These generaladvantagesand its resistance
tointerference,interception,andfrequency-selectivefadingmakespread-spectrum
CDMAanattractivechoiceformanymobilecommunicationnetworks.Theimpact