Principles of Spread-Spectrum Communication Systems

Transcription

1 Principles of Spread-Spectrum Communication Systems

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3 Don Torrieri Principles of Spread-Spectrum Communication Systems Second Edition 123

4 Don Torrieri US Army Research Laboratory Adelphi, MD, USA ISBN e-isbn DOI / Springer New York Dordrecht Heidelberg London Library of Congress Control Number: c Springer Science+Business Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (

5 To My Family

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7 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 jamming and interception, spread-spectrum systems are now widely used in commercial applications and are part of several wireless and mobile communication standards. Although spread-spectrum communication is a staple topic in textbooks on digital communication, its treatment is usually cursory. 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 many other books. My goal in this book is to provide a concise but lucid explanation of the fundamentals of spread-spectrum systems with an emphasis on theoretical principles. The choice of specific topics to include was tempered by my judgment of their practical significance and interest to both researchers and system designers. Throughout the book, learning is facilitated by many new or streamlined derivations 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 references are ones that I recommend for further study and as sources of additional references. 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, accommodating 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-spectrum systems are direct-sequence and frequency hopping systems. There is no fundamental theoretical barrier to the effectiveness of spreadspectrum communications. That remarkable fact is not immediately apparent since the increased bandwidth of a spread-spectrum signal might require a receive filter vii

8 viii Preface that passes more noise power than necessary to the demodulator. However, when any signal and white Gaussian noise are applied to a filter matched to the signal, the sampled filter output has a signal-to-noise ratio that depends solely on the energyto-noise-density ratio. Thus, the bandwidth of the input signal is irrelevant, and spread-spectrum signals have no inherent limitations. Chapter 1 reviews fundamental results of coding and modulation theory that are essential to a full understanding of spread-spectrum systems. Channel codes,which are also called error-correction or error-control codes, are vital in fully exploiting the potential capabilities of spread-spectrum systems. Although direct-sequence systems can greatly suppress interference, practical systems require channel codes to deal with the residual interference and channel impairments such as fading. Frequency-hopping systems are designed to avoid interference, but the possibility of hopping into an unfavorable spectral region usually requires a channel code to maintain adequate performance. In this chapter, coding and modulation theory are used to derive the required receiver computations and the error probabilities of the decoded information bits. The emphasis is on the types of codes and modulation that have proved most useful in spread-spectrum systems. Chapter 2 presents the fundamentals of direct-sequence systems. Directsequence modulation entails the direct addition of a high-rate spreading sequence 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 how the direct-sequence receiver suppresses various forms of interference. Several methods that supplement the inherent ability of a direct-sequence system to reject narrowband interference are explained. Chapter 3 presents the fundamentals of frequency-hopping systems. Frequency hopping is the periodic changing of the carrier frequency of a transmitted signal. This time-varying characteristic potentially endows a communication system with great strength against interference. Whereas a direct-sequence system relies on spectral spreading, spectral despreading, and filtering to suppress interference. 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 periodic changing of the carrier frequency. The impact of the interference is further mitigated by the pervasive use of channel codes, which are more essential for frequency-hopping than for direct-sequence systems. The basic concepts, spectral and performance aspects, and coding and modulation issues are presented in the first five sections of this chapter. The effects of partial-band interference and jamming are examined, and the most important issues in the design of frequency synthesizers are described. Chapter 4 focuses on synchronization. A spread-spectrum receiver must generate a spreading sequence or frequency-hopping pattern that is synchronized with the received sequence or pattern; that is, the corresponding chips or dwell intervals

9 Preface ix must precisely or nearly coincide. Any misalignment causes the signal amplitude at the demodulator output to fall in accordance with the autocorrelation or partial autocorrelation function. Although the use of precision clocks in both the transmitter 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 provides coarse synchronization, and tracking, which provides fine synchronization, are described in this chapter. The emphasis is on the acquisition system because this system is almost always the dominant design issue and most expensive component of a complete spread-spectrum system. Chapter 5 provides a general description of the most important aspects of fading and the role of diversity methods in counteracting it. Fading is the variation 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 more independently fading copies of the same signal. The rake demodulator, which is of central importance in most direct-sequence systems, is shown to be capable of exploiting undesired multipath signals rather than simply attempting to reject them. The multicarrier direct-sequence system is shown to be a viable alternative method of exploiting multipath signals that has practical advantages. 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 with each other while sharing a common transmission medium. Wireless multipleaccess communications are facilitated if the transmitted signals are orthogonal or separable in some sense. Signals may be separated in time, frequency, or code. CDMA is realized by using spread-spectrum modulation while transmitting signals from multiple users in the same frequency band at the same time. All signals use the entire allocated spectrum, but the spreading sequences or frequency-hopping patterns differ. CDMA is advantageous for cellular networks because it eliminates the need for frequency and time-slot coordination among cells, allows carrierfrequency 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 be combined with multibeamed antenna arrays that are either adaptive or have fixed patterns covering cell sectors. Inactive systems in a network reduce the interference received by an active CDMA system. These general advantages and its resistance to interference, interception, and frequency-selective fading make spread-spectrum CDMA an attractive choice for many mobile communication networks. The impact

10 x Preface of multiple-access interference in spread-spectrum CDMA systems and networks and the role of power control are analyzed. Multiuser detectors, which have great potential usefulness but are fraught with practical difficulties, are derived and explained. The ability to detect the presence of spread-spectrum signals is often required by cognitive radio, ultra-wideband, and military systems. Chapter 7 presents an analysis of the detection of spread-spectrum signals when the spreading sequence or the frequency-hopping pattern is unknown and cannot be accurately estimated by the detector. Thus, the detector cannot mimic the intended receiver, and alternative procedures are required. The goal is limited in that only detection is sought, not demodulation or decoding. Nevertheless, detection theory leads to impractical devices for the detection of spread-spectrum signals. An alternative procedure is to use a radiometer or energy detector, which relies solely on energy measurements to determine the presence of unknown signals. The radiometer has applications not only as a detector of spread-spectrum signals, but also as a sensing method in cognitive radio and ultra-wideband systems. Chapter 8 examines the role of iterative channel estimation in the design of advanced spread-spectrum systems. The estimation of channel parameters, such as the fading amplitude and the power spectral density of the interference plus noise, is essential to the effective use of soft-decision decoding. Channel estimation may be implemented by the transmission of pilot signals that are processed by the receiver, but pilot signals entail overhead costs, such as the loss of data throughput. Deriving maximum-likelihood channel estimates directly from the received data symbols is often prohibitively difficult. There is an effective alternative when turbo or low-density parity-check codes are used. The expectation-maximization algorithm provides an iterative approximate solution to the maximum-likelihood equations and is inherently compatible with iterative demodulation and decoding. Two examples of advanced spread-spectrum systems that apply the expectationmaximization algorithm for channel estimation are described and analyzed in this chapter. These systems provide good illustrations of the calculations required in the design of advanced systems. Three appendices contain mathematical details about bandpass processes, basic probability distributions, and the convergence of important adaptive algorithms. The evolution of spread spectrum communication systems and the prominence of new mathematical methods in their design provided the motivation to undertake this new edition of the book. This edition is intended to enable readers to understand the current state-of-the-art in this field. More than twenty percent of the material in this edition is new, including a chapter on systems with iterative channel estimation, and the remainder of the material has been thoroughly revised. In writing this book, I have relied heavily on notes and documents prepared and the perspectives gained during my work at the US Army Research Laboratory. I am thankful to my colleagues Matthew Valenti, Hyuck Kwon, and John Shea for their trenchant and excellent reviews of selected chapters of the original manuscript. I am grateful to my wife, Nancy, who provided me not only with her usual unwavering support but also with extensive editorial assistance.

11 Contents 1 Channel Codes and Modulation Block Codes Error Probabilities for Hard-Decision Decoding Soft-Decision Decoding and Code Metrics for Pulse Amplitude Modulation Code Metrics for Orthogonal Signals Metrics and Error Probabilities for Uncoded FSK Symbols Performance Examples Convolutional Codes and Trellis Codes Chernoff Bound Trellis-Coded Modulation Interleavers Classical Concatenated Codes Turbo Codes MAP Decoding Algorithm Turbo Codes with Parallel Concatenated Codes Serially Concatenated Turbo Codes Low-Density Parity-Check Codes Irregular Repeat-Accumulate Codes Iterative Demodulation and Decoding Bit-Interleaved Coded Modulation Simulation Examples References Direct-Sequence Systems Definitions and Concepts Spreading Sequences and Waveforms Random Binary Sequence Shift-Register Sequences Maximal Sequences Autocorrelations and Power Spectrums xi

12 xii Contents Characteristic Polynomials Long Nonlinear Sequences Chip Waveforms Systems with BPSK Modulation Tone Interference at Carrier Frequency General Tone Interference Gaussian Interference Quaternary Systems Systems with Channel Codes Pulsed Interference Despreading with Bandpass Matched Filters Noncoherent Systems Multipath-Resistant Coherent System Rejection of Narrowband Interference Adaptive Filters Time-Domain Adaptive Filtering Transform-Domain Processing Nonlinear Filtering Adaptive ACM Filter References Frequency-Hopping Systems Concepts and Characteristics Frequency Hopping with Orthogonal FSK Soft-Decision Decoding Multitone Jamming Frequency Hopping with CPM and DPSK Hybrid Systems Codes for Partial-Band Interference Reed Solomon Codes Trellis-Coded Modulation Turbo and LDPC Codes Frequency Synthesizers Direct Frequency Synthesizer Direct Digital Synthesizer Indirect Frequency Synthesizers References Code Synchronization Acquisition of Spreading Sequences Matched-Filter Acquisition Serial-Search Acquisition Uniform Search with Uniform Distribution Consecutive-Count Double-Dwell System Single-Dwell and Matched-Filter Systems Up-Down Double-Dwell System

13 Contents xiii Penalty Time Other Search Strategies Density Function of the Acquisition Time Alternative Analysis Nonconsecutive and Sequential Searches Acquisition Correlator Code Tracking Frequency-Hopping Patterns Matched-Filter Acquisition Serial-Search Acquisition Tracking System References Fading and Diversity Path Loss, Shadowing, and Fading Time-Selective Fading Fading Rate and Fade Duration Spatial Diversity and Fading Frequency-Selective Fading Channel Impulse Response Diversity for Fading Channels Optimal Array Maximal-Ratio Combining Coherent Binary Modulations and Metrics Equal-Gain Combining Selection Diversity Transmit Diversity Channel Codes Bit-Interleaved Coded Modulation Rake Demodulator Diversity and Spread Spectrum Multicarrier Direct-Sequence Systems MC-CDMA System DS-CDMA System with Frequency-Domain Equalization References Code-Division Multiple Access Spreading Sequences for DS/CDMA Synchronous Communications Asynchronous Communications Symbol Error Probability Complex-Valued Quaternary Sequences Systems with Random Spreading Sequences Jensen s Inequality Direct-Sequence Systems with BPSK Quadriphase Direct-Sequence Systems

14 xiv Contents 6.3 Cellular Networks and Power Control Intercell Interference of Uplink Outage Analysis Local-Mean Power Control Bit-Error-Probability Analysis Impact of Doppler Spread on Power-Control Accuracy Downlink Power Control and Outage Frequency-Hopping Multiple Access Asynchronous FH/CDMA Networks Ad Hoc and Cellular Mobile Networks Ad Hoc Networks Cellular Networks Multiuser Detectors Optimum Detectors Decorrelating Detector Minimum-Mean-Square-Error Detector Adaptive Multiuser Detection Interference Cancellers Multiuser Detector for Frequency Hopping References Detection of Spread-Spectrum Signals Detection of Direct-Sequence Signals Radiometer Estimation of Noise Power Other Implementation Issues Detection of Frequency-Hopping Signals Channelized Radiometer References Systems with Iterative Channel Estimation Expectation-Maximization Algorithm Fixed-Point Iteration Direct-Sequence Systems Encoding, Modulation, and Channel Estimation Iterative Receiver Structure EM Algorithm Perfect Phase Information at Receiver No Phase Information at Receiver Blind-PACE Estimation Trade-Offs Simulation Results Guidance from Information Theory Robust Frequency-Hopping Systems System Model Demodulator Metrics Channel Estimators

15 Contents xv Selection of Modulation Index Performance in Partial-Band Interference Asynchronous Multiple-Access Interference References Appendix A Signal Characteristics A.1 Bandpass Signals A.2 Stationary Stochastic Processes A.3 Direct-Conversion Receiver Appendix B Probability Distributions B.1 Chi-Square Distribution B.2 Central Chi-Square Distribution B.3 Rice Distribution B.4 Rayleigh Distribution B.5 Exponentially Distributed Random Variables Appendix C Convergence of Adaptive Algorithms C.1 LMS Algorithm C.1.1 Convergence of the Mean C.1.2 Misadjustment C.2 Frost Algorithm C.2.1 Convergence of the Mean Index

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