`
`COMMUNICATIONS
`
`Fundamentals and Applications
`
`BERNARD SKLAR
`
`The Aerospace Corporation, El Segundo, California
`and
`
`University of California, Los Angeles
`
`P T R Prentice Hall
`
`Englewood Cliffs, New Jersey 07632
`
`ERIC-1033
`Ericsson v. IV, IPR2014-00919
`Page 1 of 3
`
`
`
`
`
`Library of Congress Cataloging-in-Publication Data
`
`SKLAR, BERNARD (date)
`Digital communications.
`
`Bibliography: p.
`Includes index.
`I. Title.
`1. Digital communications.
`TK5l03.7.S55
`1988
`62l.38'0413
`ISBN 0-13-211939-0
`
`87-1316
`
`
`
`Editorial/production supervision and
`interior design: Reynold Rieger
`Cover design: Wanda Lubelska Design
`Manufacturing buyers: Gordon Osbourne and Paula Benevento
`
`© 1988 by P T R Prentice-Hall, Inc.
`A Simon & Schuster Company
`Englewood Cliffs, New Jersey 07632
`
`All rights reserved. No part of this book may be
`reproduced, in any form or by any means,
`without permission in writing from the publisher.
`
`Printed in the United States of America
`
`10
`
`ISBN IJ-13-E].l‘lEl"l-El
`
`DE5
`
`Prentice-Hall International (UK).Limited, London
`Prentice-Hall of Australia Pty. Limited, Sydney
`Prentice-Hall Canada Inc., Toronto
`Prentice-Hall Hispanoamericana, S.A., Mexico
`Prentice-Hall of India Private Limited, New Delhi
`Prentice-Hall of Japan, Inc., Tokyo
`Simon & Schuster Asia Pte. Ltd., Singapore
`Editora Prentice-Hall do Brasil, Ltda., Rio de Janeira
`
`
`
` ERIC-1033
`
`Page 2 of 3
`
`ERIC-1033
`Page 2 of 3
`
`
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`
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`3.1 WHY MODULATE?
`
`
`
`Digital modulation is the process by which digital symbols are transformed into
`waveforms that are compatible with the characteristics of the channel. In the case
`0 of baseband modulation, these waveforms are pulses, but in the case of bandpass
`modulation the desired information signal modulates a sinusoid called a carrier
`wave, or simply a carrier; for radio transmission the carrier is converted to an
`electromagnetic (EM) field for propagation to the desired destination. One might
`ask why it is necessary to use a carrier for the radio transmission of baseband
`signals. The answer is as follows. The transmission of EM fields through space
`is accomplished with the use of antennas. To efficiently couple the transmitted
`EM energy into space, the dimensions of the antenna aperture should be at least
`as large as the wavelength being transmitted. Wavelength, A , is equal to c/f , where
`c, the speed of light, is 3 X 103 m/s. For a baseband signal with frequency f =
`3000 Hz, A = 105 m = 60 miles. To efficiently transmit a 3000-Hz signal through
`space without carrier-wave modulation, an antenna that spans at least 60 miles
`would be required. Even if we were willing to inefficiently transmit the EM energy
`with an antenna measuring one-tenth of a wavelength, we are faced with an im-
`possible antenna size. However, if the information to be transmitted is first mod-
`ulated on a higher frequency carrier, for example a 30-GHZ carrier, the equivalent
`antenna diameter is then less than % in. For this reason, carrier-wave or bandpass
`modulation is an essential step for all systems involving radio transmission.
`Bandpass modulation can provide other important benefits in signal trans-
`mission. If more than one signal utilizes a single channel, modulation may be used
`
`118
`
`
`
`Bandpass Modulation and Demodulation Chap. 3
`
`ERIC-1033
`
`Page 3 of 3
`
`ERIC-1033
`Page 3 of 3
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