31
`
`ND.WDEDN0CE1.
`
`Robert C.
`
`Dixon
`
`I
`
`PETITIONERS 1016-0001
`
`

`
`SPREAD SPECTRUM
`
`SYSTEMS
`
`Second Edition
`
`Robert C. Dixon
`
`A Wiley-Interscience Publication
`
`JOHN WILEY & SONS
`
`New York - Chichester - Brisbane - Toronto - Singapore
`
`I P
`
`ETITIONERS 1016-0002
`
`

`
`
`
`PREFACE TO
`FIRST EDITIC
`
`Spread spectrum systems encor_n1
`sion, message privacy, signal hidi:
`repertoire- These.systems are a uni
`digital disciplines. The coding me
`spread spectrum system’s capabili
`This_book represents an effort t(
`those"worki’ngt in the spread specl
`working engineers and students to
`of spread spectrum‘ technology. 1
`contained. Those who wish to dig:
`or investigate rigorous proofs wil
`listing of references in spread spe
`Most of the concepts of spread s
`' for many'yea‘r's‘, but the componer
`capable of reliable operationhave
`J. P. Costas concluded in 1959
`
`broadband systems appear to 0
`problem and a potentially higher
`band systems-.” At that time, how.
`were not available to build a reliz
`
`-A system.
`Todasy components have advan
`spread spectrum system can be co
`code generator, for instance, whi
`least 100 discrete transistors, can
`package. only slightly larger than
`complete subsystem may well be 2
`The point is that the use of spr
`constrained. by constituent electr.
`Spread spectrum applications 5
`set up a scheduled time to send an
`
`PETITIONERS 1016-0003
`
`Copyright © 1984 by John Wiley & Sons, Inc.
`
`All rights reserved. Published simultaneously in Canada.
`
`Reproduction or translation of any part of this work
`beyond that permitted by Section 107 or I08 of the
`1976 United States Copyright Act without the permission ‘
`of the copyright owner is unlawful. Requests for
`permission or. further information should be addressed to
`the Permissions Department, John Wiley & Sons, Inc.
`
`Library of Congress Cataloging in Publication Data:
`Dixon, Robert C. (Robert Clyde), 1932-
`Spread spectrum systems.
`
`“A Wiley-lnterscience publication."
`Bibliography: p.
`Includes index.
`
`I. Title.
`1. Spread spectrum communications.
`Tl(5I02.5.D55
`1984
`62l.38’043
`83-26080
`ISBN 0-471-88309-3
`
`Printed in the United States of America
`
`109876543
`
`

`
`;
`5
`
`S
`2
`
`l
`i
`
`Spread spectrum techniques, applied in recent years, have produced
`results in communications, navigation, and test systems that are not
`A possible with standard signal formats. In many applications the advent of
`high-speed transistors and/ or integrated circuits was the key to practical-
`sized—and-powered equipment based on spread spectrum modulation.
`But what is a spread spectrum system? What are the advantages of spread
`
`.
`
`are they?
`These questions and others are posed in this chapter and in those that
`
`spectrum modulation? To be sure, there are also disadvantages, but what
`follow. Itishopedthatthereaderwillfindtheanswersheorsheneedsina
`
`I
`
`Before we attempt to define a spread spectrum system, let us be sure that
`we,-understand what is meant by a spectrum. Every transmitting or
`modulating system has a characteristic signature that includes not only
`‘the frequency at which the signal is centered, but also the bandwidth of
`the signal when modulated by the intended signaling waveform. A
`spectrum, as we speak of it here,
`is the frequency-dornainlm repre-
`sentation of ‘the signal and, for our purposes, especially the modulated
`, signal. We most often see signals presented in the time domain (that is, as
`‘functions of time). Any signal, however, can also be presented in the
`frequency domain, and transforms (mathematical operators) are avail-
`able for converting frequency- or time-domain functions from one
`domain to the other and back again. The most basic of these operators is
`the Fourier transform, for which the relationship between the time and __
`
`WHAT AND WHYS
`OF SPREAD SPECTRUM
`SYSTEMS
`
`. useful form.
`
`1.1 WHAT IS A SPREAD SPECTRUM SYSTEM?
`
`PETITIONERS 1016-0004
`
`

`
`
`
`1.2 Why Bother?
`
`W
`
`5
`:1><3><105 _
`1.44 —2.08><l0 Hz
`
`f a channel to
`to-noise ratio
`transmit
`the
`
`anging bases,
`
`r1 an antijam
`
`7
`
`<.—<1)
`
`Incidentally, the information itself may be embedded in the spread
`spectrum signal by several methods. The most common is that of adding
`the information to the spectrum-spreading code before its use for
`spreading modulation. This technique is applicable to any spread
`spectrum _system that uses a code sequence -to determine its RF bandwidth
`(either direct sequence or frequency hopping systems are good candi-
`dates). Of course, the information to be sent must be in some digital form
`because addition to a code sequence involves modulo-2 addition to a
`binary code. Alternately, information may be used to modulate a carrier
`before spreading it. This is usually done by some form of angle
`modulation, for the need in spread spectrum systems is often to output a
`constant—power RF envelope.
`(1) the
`then, must meet two criteria:
`~ A spread spectrum system,
`transmitted bandwidth is much greater than the bandwidth or rate of the
`information being sent and, (2) some function other than the information
`being sent
`is employed to determine the resulting modulated RF
`bandwidth.
`_
`_
`This is the essence of spread spectrum communications—the art of
`if expanding the bandwidth of a‘ signal,—transmitting thatexpanded signal,
`and recovering the desired "signal by remapping the received spread
`spectrum into the original information bandwidth. Furthermore, in the
`process of carrying out this series of bandwidth trades the purpose is to
`allow the system to deliver error‘-free information in a noisy signal
`environment.
`
`'‘.9-/mini»;
`
`
`
`1.2 WHY BOTHER?
`
`The first reaction common ‘among those encountering spread spectrum
`techniques is “whfy._b other?” The answers are varied and s_e1dom the same.
`In a world beset by too littl-le'RF spectrum to satisfy the ever-growing
`demands of military, commercial, and private users the question “why
`spread spectrum” must certainly be considered valid, for spread spectrum
`systems have almost as many reasons for being as they have users.
`‘Some of the properties that may be cited are the following:
`
`1. Selective addressing capability.
`
`'
`
`2. Code division multiplexing is possible for multiple access.
`3. Low—density power spectra for signal hiding.
`
`have a low
`ransfer the
`nk in which
`~.l and our
`iformation
`
`
`4. Message screening from eavesdroppers.
`
`
`
`PETITIONERS 1016-0005
`
`

`
`
`
`NAVIGATING
`WITH SPREAD SPECTRUM
`
`SYSTEMS
`
`
`
`
`
`
`
`Certainly the best known applications of spread spectrum methods lie in
`the navigation area. Direct-sequence ranging has been applied in space
`exploration programs since the early 1960s at least, and the methods
`employed have been well described in the literature.
`0
`The coded modulation characteristic of spread spectrum systems
`uniquely qualifies them for navigation—from the standpoints of both
`_ range measurement and direction finding—enough to allow homing or
`navigation with respect to any other spread spectrum transmitter. Little
`. use has been made of spread spectrum techniques for direction finding,*
`but spread spectrum‘ ranging, ‘particularly direct sequence ranging, has
`been well exploited.
`This chapter first describes ranging methods for direct sequence sys-
`tems and for frequency hoppers. The second part of the chapter is a
`discussion of direction finding as applied in spread spectrum systems.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`1d, causes
`se level. If
`nce can be
`
`a spread
`
`cy and 10
`ew signal
`nodulated
`LlCS of this
`
`-.n filtering
`n to a 2.5
`
`having the
`
`the same
`
`interfering
`Lultaneous
`:he desired
`.nterferer‘?
`
`a 10 MHZ
`would the
`) dB noise
`
`the above
`
`spectrum
`
`ase direct-
`
`8.1 RANGING TECHNIQUES
`
`
`
`
`
`
`
`Any RF signal is subject to a fixed rate of propagation (approximately 6
`;usec/ mi). The signal reaching a receiver at any given instant left the
`transmitter that sent it some time before. Because signaling waveforms or
`modulations are also functions of time, the difference in a signaling
`waveform. as seen at a receiver, from that present at the transmitter can be
`related directly to distance between them and used to measure that
`distance.
`
`
`
`
`
`
`
`
`
`
`*Programs such as GPS (global positioning system) and PLSR (position location
`reporting system) are now being configured to employ multiple spread spectrum range
`measurements (multilateration) for position location.
`
`
`
`
`
`PETITIONERS 1016-0006
`
`

`
`
`
`
`292
`
`Navigating With Spread Spectrum Systems
`
`A common type of echo range measurement method is that used in
`radar systems. Many such systems simply transmit a pulse of RF energy
`and then wait for the return of a portion of the energy due to its being
`reflected from objects in the signal path. The radar marks time from the
`instant of the pulse transmission until its return. The time required for the
`signal to return is a function of the two-way range to the reflecting object,
`and because the signal propagation rate is known, the range is easily
`derived.
`
`Any signal used is subject to the same distance/ time relations. An
`unmodulated carrier, forinstance, is delayed precisely the same amount
`as a spread spectrum signal traveling the same distance. The spread
`spectrum signal has an advantage, however, in that its phase is easily
`resolvable. A direct-sequence signal and a simple CW carrier may be
`. likened to a pair of yardsticks, one with feet and inches marked on it and
`the other blank. With the marked yardstick we can easily resolve distance
`within about% in., but the unmarked ardstick, though 'ust as
`recise in
`Y
`J
`P
`length, would be of little use_ for resolving to better than perhaps 1 ft. The
`marks on a direct sequence signal, of course, are the code sequence
`modulation, and the basic resolution is one code chip; the higher the chip _
`rate, the better the measurement capability.
`Frequency hopping systems do not normally possess high-resolution
`properties, but this is simply because their hopping rates are not high
`enough to act as the fine marks on our yardstick. There is no reason, other
`than that of being less able to construct high—hopping-rate synthesizers,
`' for frequency hoppingzranging capability to be less than that of direct
`sequence.
`._
`~ -—
`Let us discuss the operation of spread spectrum ranging systems. In
`direct sequence ranging code modulation and demodulation are
`performed in exactly the same manner as in a system used for
`communications; that is, the code employed phase shift modulates the
`carrier. (For ranging, whether biphase or quadriphase modulation is used
`is usually of little consequence. We assume -biphase modulation.) The
`baseband or information channel is unaffected by the ranging operation
`and may be employed concurrently. It is possible, in fact, to enhance
`operation by employing a baseband channel in a ranging system.
`Figure 8.1 illustrates two simple direct sequence ranging systems. One
`we call simple, or transmit-then-receive. The other is called duplex,
`because it transmits and receives simultaneously. In both, the transmitter
`sends a pseudonoise code-modulated signal. Considering the duplex
`method,
`the signal at F;
`is simply translated to a different center
`frequency F2 (preserving the code modulation) and retransmitted. This
`signal
`then reaches the first
`transmitter’s site with a time delay .
`corresponding to the two-way signal propagation delay between the two
`units (see Figure 8.2).
`‘
`
`A receiver, located at thef1 transmitter location, then synchronizes to
`
` Transmitter
`
` Balanced
`
`modulator
`
` Carrier
`oscillator
`
`PN nanerator
`
`
`
`
`
`PETITIONERS 1016-0007
`
`counter
`
`n.
`
`BalancedSynchronizationmixer
`
`detector
`
`PNRange
`
`OH(UL.DI:OJU1
`
`

`
`
`
`_
`7':
`3
`
`£2
`«T
`
`translator
`F,toF2frequency
`
`
`
`
`
`
`
`
`
`
`
`
`
`Figure8.1Simpledirectsequencerangingsystems:(a)transmit—receiverangingsystem;(b)duplex
`
`rangingsystem.
`
`
`
`
`25
`.§
`EL‘?
`I:
`E
`
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`1;,"
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`u_=g
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`N
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`
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`
`
`Balanced
`generator
`
`is that used in
`e of RF energy
`lue to its being
`9 time from the
`equired for the
`fleeting object,
`range is easily
`
`- relations. An
`: same amount
`
`e. The spread
`phase is easily
`:arr_ier may be
`.rked on it and
`:solve distance
`
`at as precise in
`rhaps 1 ft. The
`zode sequence
`nigher the chip
`
`igh—reso1ution
`: are not high
`u reason, other
`: synthesizers,
`that of direct
`
`g systems. In
`dulation are
`em used for
`modulates the
`llation is used
`
`Transmitter F1
`
`L
`c—
`W’:
`11-3
`E
`3°
`
`Carrier
`
`oscillator
`
`L
`to
`2
`25
`um
`“*5
`
`generator
`
`Range
`
`counter
`
`RF stagesF2
`
`Balanced
`
`Synchronization
`
`(b)
`
`
`
`Transmitter
`
`ulation.) The
`ng operation-
`2, to enhance
`system.
`systems. One
`u.l1ed duplex,
`e transmitter
`
`Balanced
`
`
`modulator
`generator
`
`
`
`
`
`
`
`
`
`
`
`
`
`293
`
`PETITIONERS 1016-0008
`
`5 the duplex
`'erent center
`mitted. This
`time delay
`veen the two
`
`chronizes to
`
`Carrier
`
`oscillator
`
`(11)
`
`
`
`

`
`
`
`Code sequence at transmitter, frequency f,
`
`
`Navigating With Spread Spectrum Systems
`
`must
`
`signal
`the pi
`and i1
`measi
`
`
`
`PETITIONERS 1016-0009
`
`is the]
`error.
`
`of a c
`can 11
`
`Tone
`
`Atec’
`modt
`relati
`
`,
`
`(to). 1?
`less tl
`
`comp
`three
`of the
`measi
`
`' ratio .
`Th
`ifrequ
`.. cente;
`. woul<'
`
`frequ«
`becat
`futura
`line v '
`
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`
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`
`_—:._2=--.x._—~.,=_mu=.—4-.,_:_cn,3n,_<n_;c__,;;gwg__3_5'35‘£23’3'
`
`l~éTwo—way range delay9-‘
`
`______l—'|___|'l__l'|_l_"""
`
`Code sequence at receiver, frequency f2
`
`Figure 8.2 Code sequence comparison at transmitter and receiver.
`
`the return signal, and on measuring the number of chips of code delay
`between the signals being transmitted and received can determine
`uniquely the range from itself to the repeating station. This type of system
`requires that the receiving subsystem employ a code sequence generator
`that is independent of the one used to modulate the transmitter, because
`the code the receiver must use is by definition ‘out of phase with the
`transmitted code. The repeating terminal, on the other hand, is reduced in
`its complexity.
`‘
`In the simple time—sharing ranging systems transmissions are exactly
`the same as in the duplex systems, except that transmitters and receivers
`at the two cooperating terminals do not operate simultaneously and,
`therefore, only one operating’ frequency is necessary. First, the terminal
`. initiating the range measurement transmits a code sequence-foreaeperiod ‘
`long enough for the desired receiver to which a range measurement is to
`be made to synchronize itself. It is sufficient to say that the code sequences
`employed are long enough to support unambiguous range measurement
`over the distances in question and that an initial synchronization method
`is provided. The important point is that the receiver matches its internal
`code reference to the code modulation on the signal it is receiving and that
`its internal reference is then offset from the internal reference at the
`
`transmitter by the distance (propagation time) between them. The
`interrogator’s transmitter, then, is generating the same code sequence as
`the responder’s receiver but is some number of chips ahead of it.
`Once initial acquisition has occurred the transmitter (or interrogator)
`instructs the receiving system to respond to its transmission by switching
`to the transmit mode at the reception of a given signal. The interrrogating
`unit then sends a command to the responder to start transmitting, while it
`goes from the transmission to receiving mode. Both the interrogator and
`the responder continue to generate the same code without interruption or
`resetting throughout the range measurement. The responder (the original
`receiver) is locked to a coded signal, which was offset from the
`interrogator’s transmitted signal by the amount of the range delay, and
`transmits by using that same code reference to reply. The interrogator
`
`
`
`
`
`

`
`
`
`
`
`
`
`8.1 Ranging Techniques
`
`295
`
`must delay its code phase to match the twice-range-delayed responder’s
`signal. The amount of code phase change necessary is again equal to twice
`the propagation delay from the interrogator (who is measuring the range
`and initiates the measurement) to the responder. In both cases the range
`measurement is made by counting chips of offset or fractions of chips and
`is therefore a discrete measurement, inherently accurate within a 1/ 2 chip
`error. In practice measurements are commonly made to within a fraction
`of a chip period. The highest-resolution spread spectrum systems known
`can measure range to aproximately 1/ l00Oth of a chip period.
`
`Tone Ranging
`
`,
`
`-
`
`*
`
`
`
`A technique for range measurement using multiple, coherent tones that
`modulate a single carrier is also practical. Figure 8.3 shows the phase
`_ relationship for these tones. All tones are started at'a single point in time
`' (to). At a point distant from the point of origin, but within a range delay
`less than the peroid of the lowest tone, a receiver that demodulates and
`compares the phase of the tones can measure range. This is because the
`three tones have a unique phase relationship that depends on the position
`of the receiver. Range resolution is limited by the receiver’s ability to
`measure the tone’s phases_._T_hus it is limited in range b.y_Asigna1-to-noise
`’”*ra“°'
`.
`.
`.-
`.
`T
`5
`.
`.
`.
`.
`The point in discussing tone ranging is that it could be applied to a
`frequency hopping system, where the tone’s separation from the band
`center would correspond to a particular hop frequency. This capability
`would depend on having coherence between all of the hopping
`frequencies used for ranging. Such a method has not been implemented
`because of the need for coherence, but does offer possible promise for
`future frequency hopping systems to bring their range resolution more in
`line with the direct sequencesystems.
`
`
`
`
`
`(b)
`
`(3)
`
`At
`
`
`
`
`
`
`
`
`
`PETITIONERS 1016-0010
`
`
`
`
`
`
`
`low—frequency tone
`la)
`lb) medium—frequency tone
`(cl high—frequency tone
`
`to = starting point, all tones
`at zero phase
`At= distant point, tone phase
`relationship a function
`of distance
`Figure 8.3 Tbne-ranging technique.
`
`8 delay
`ermine
`System
`lerator
`Iecause
`ith the
`uced in
`
`ixactly
`ceivers
`y and’
`rminal
`period
`at ism
`uences
`ement
`16-thod
`ltemal
`id that
`at the
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`rice as
`
`gator)
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`gating
`7hliC it
`)r and
`.
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`n the
`I, and
`ngator