CDMA: Principles of Spread Spectrum Communication

About

Features

• Presents an introduction to digital communications and covers all aspects of commercial direct-sequence spread spectrum technology.

• Incorporates both physical-level principles and network concepts.

• Includes detailed information on signal generation, synchronization, modulation, and coding of direct-sequence spread spectrum signals.

• Illustrates how these physical layer functions relate to link and network properties involving cellular coverage, Erlang capacity, and network control.

Description

Spread spectrum multiple access communication, known commercially as CDMA (Code Division Multiple Access), is a driving technology behind the rapidly advancing personal communications industry. Its greater bandwidth efficiency and multiple access capabilities make it the leading technology for relieving spectrum congestion caused by the explosion in popularity of cellular mobile and fixed wireless telephones and wireless data terminals.

CDMA has been adopted by the Telecommunications Industry Association (TIA) as a wireless standard. As an electrical or communications engineer, you must acquire a thorough grasp of CDMA fundamentals in order to develop systems, products, and services for this demanding but rewarding market.

Written by a leader in the creation of CDMA and an internationally recognized authority on wireless digital communication, this book gives you the technical information you need. It presents the fundamentals of digital communications and covers all aspects of commercial direct-sequence spread spectrum technology, incorporating both physical-level principles and network concepts. You will find detailed information on signal generation, synchronization, modulation, and coding of direct-sequence spread spectrum signals. In addition, the book shows how these physical layer functions relate to link and network properties involving cellular coverage, Erlang capacity, and network control.

With this book, you will attain a deeper understanding of personal communications system concepts and will be better equipped to develop systems and products at the forefront of the personal wireless communications market.

Sample Content

Table of Contents

1. Introduction.

Definition and Purpose. Basic Limitations of the Conventional Approach. Spread Spectrum Principles. Organization of the Book.

Purpose. Pseudorandom Sequences. Maximal Length Linear Shift Register Sequences. Randomness Properties of MLSR Sequences. Conclusion. Generating Pseudorandom Signals (Pseudonoise) from Pseudorandom Sequences. First- and Second-Order Statistics of Demodulator Output in Multiple Access Interference. Statistics for QPSK Modulation by Pseudorandom Sequences. Examples. Bound for Bandlimited Spectrum. Error Probability for BPSK or QPSK with Constant Signals in Additive Gaussian Noise and Interference. Appendix 2A: Optimum Receiver Filter for Bandlimited Spectrum.

Purpose. Acquisition of Pseudorandom Signal Timing. Hypothesis Testing for BPSK Spreading. Hypothesis Testing for QPSK Spreading. Effect of Frequency Error. Additional Degradation When N is Much Less Than One Period. Detection and False Alarm Probabilities. Fixed Signals in Gaussian Noise (L=1). Fixed Signals in Gaussian Noise with Postdetection Integration (L>1). Rayleigh Fading Signals (L>/=1). The Search Procedure and Acquisition Time. Single-Pass Serial Search (Simplified). Single-Pass Serial Search (Complete). Multiple Dwell Serial Search. Time Tracking of Pseudorandom Signals. Early-Late Gate Measurement Statistics. Time Tracking Loop. Carrier Synchronization. Appendix 3A: Likelihood Functions and Probability Expressions. Bayes and Neyman-Pearson Hypothesis Testing. Coherent Reception in Additive White Gaussian Noise. Noncoherent Reception in AWGN for Unfaded Signals. Noncoherent Reception of Multiple Independent Observations of Unfaded Signals in AWGN. Noncoherent Reception of Rayleigh-Faded Signals in AWGN.

Purpose. Chernoff and Battacharyya Bounds. Bounds for Gaussian Noise Channel. Chernoff Bound for Time-Synchronous Multiple Access Interference with BPSK Spreading. Chernoff Bound for Time-Synchronous Multiple Access Interference with QPSK Spreading. Improving the Chernoff Bound by a Factor of 2. Multipath Propagation: Signal Structure and Exploitation. Pilot-Aided Coherent Multipath Demodulation. Chernoff Bounds on Error Probability for Coherent Demodulation with Known Path Parameters. Rayleigh and Rician Fading Multipath Components. Noncoherent Reception. Quasi-optimum Noncoherent Multipath Reception for M-ary Orthogonal Modulation. Performance Bounds. Search Performance for Noncoherent Orthogonal M-ary Demodulators. Power Measurement and Control for Noncoherent Orthogonal M-ary Demodulators. Power Control Loop Performance. Power Control Implications. Appendix 4A: Chernoff Bound with Imperfect Parameter Estimates.

Purpose. Interleaving to Achieve Diversity. Forward Error Control Coding - Another Means to Exploit Redundancy. Convolutional Code Structure. Maximum Likelihood Decoder - Viterbi Algorithm. Generalization of the Preceding Example. Convolutional Code Performance Evaluation. Error Probability for Tailed-off Block. Bit Error Probability. Generalizations of Error Probability Computation. Catastrophic Codes. Generalization to Arbitrary Memoryless Channels - Coherent and Noncoherent. Error Bounds for Binary-Input, Output-Symmetric Channels with Integer Metrics. A Near-Optimal Class of Codes for Coherent Spread Spectrum Multiple Access. Implementation. Decoder Implementation. Generating Function and Performance. Performance Comparison and Applicability. Orthogonal Convolutional Codes for Noncoherent Demodulation of Rayleigh Fading Signals. Implementation. Performance for L-Path Rayleigh Fading. Conclusions and Caveats. Appendix 5A: Improved Bounds for Symmetric Memoryless Channels and the AWGN Channel. Appendix 5B: Upper Bound on Free Distance of Rate 1/n Convolutional Codes.

General. Reverse Link Power Control. Multiple Cell Pilot Tracking and Soft Handoff. Other-Cell Interference. Propagation Model. Single-Cell Reception - Hard Handoff. Soft Handoff Reception by the Better of the Two Nearest Cells. Soft Handoff Reception by the Best of Multiple Cells. Cell Coverage Issues with Hard and Soft Handoff. Hard Handoff. Soft Handoff. Erlang Capacity of Reverse Links. Erlang Capacity for Conventional Assigned-Slot Multiple Access. Spread Spectrum Multiple Access Outage - Single Cell and Perfect Power Control. Outage with Multiple-Cell Interference. Outage with Imperfect Power Control. An Approximate Explicit Formula for Capacity with Imperfect Power Control. Designing for Minimum Transmitted Power. Capacity Requirements for Initial Accesses. Erlang Capacity of Forward Links. Forward Link Power Allocation. Soft Handoff Impact on Forward Link. Orthogonal Signals for Same-Cell Users. Interference Reduction with Multisectored and Distributed Antennas. Interference Cancellation. Epilogue.

Preface

Spread spectrum communication technology has been used in military communications for over half a century, primarily for two purposes: to overcome the effects of strong intentional interference (jamming), and to hide the signal from the eavesdropper (covertness). Both goals can be achieved by spreading the signal's spectrum to make it virtually indistinguishable from background noise. Several texts, or portions of texts, on this subject have been published over the past twenty years. This book is the first to present spectrum technology specifically for commercial wireless applications.

In response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology has achieved much higher bandwidth efficiency for a given wireless spectrum allocation, and hence serves a far larger population of multiple access users, than analog or other digital technologies. While it is similar in implementation to its military predecessors, the spread spectrum wireless network achieves efficiency improvements by incorporating a number of unique features made possible by the benign noise-like characteristics of the signal waveform. Chief among these is universal frequency reuse (the fact that all users, whether communicating within a neighborhood, a metropolitan area, or even a nation, occupy a common frequency spectrum allocation). Besides increasing the efficiency of spectrum usage, this also eliminates the chore of planning for different frequency allocation for neighboring users or cells. Many other important multiple access system features are made possible through this universal frequency reuse by terminals employing wideband (spread) noise-like signal waveforms. Most important is fast and accurate power control, which ensures a high level of transmission quality while level for each terminal, and hence a low level of interference to other user terminals. Another is mitigation of faded transmission through the use of a Rake receiver, which constructively combines multipath components rather than allowing them to destructively combine as in narrowband transmission. A third major benefit is soft handoff among multiple cell base stations, which provides improved cell-boundary performance and prevents dropped calls.

In Chapters 2 to 5, this book covers all aspects of spread spectrum transmission over a physical multiple-access channel: signal generation, synchronization, modulation, and error-correcting coding of direct-sequence spread spectrum signals. Chapter 6 relates these physical layer functions to link and network layer properties involving cellular coverage, Erlang capacity, and network control. This outline is unusual in bringing together several wide-ranging technical disciplines, rarely covered in this sequence and in one text. However, the presentation is well integrated by a number of unifying threads. First, the entire text is devoted to the concept of universal frequency reuse by multiple users of multiple cells. Also, two fundamental techniques are used in a variety of different forms throughout the text. The first is the finite-state machine representation of both deterministic and random sequences; the second is the use of simple, elegant upper bounds on the probabilities of a wide range of events related to system performance.

However, given the focus on simultaneous wideband transmission for all users over a common frequency spectrum, the text purposely omits two important application areas: narrowband modulation and coding methods, including multipoint signal constellations and trellis codes; and frequency hopped multiple access, where modulation waveforms are instantaneously narrowband over the duration of each hop. It also generally avoids digressions into principles of information theory. In short, although the material covered through Chapter 5 mostly also applies to narrowband digital transmission systems, the book mainly covers topics that apply to wideband spread spectrum multiple access.

Three motivating forces drove me to write this book. It began with my three decades of teaching within the University of California system. There, keeping with the healthy trend in communication engineering courses, I tried to make theory continually more pertinent to applications. Then there was the fulfillment of a voluntary commission for the Marconi Foundation, which honored me with a Marconi Fellowship award in 1990. Most important was my participation in a significant technological achievement in communication system evolution: the implementation, demonstration, and standardization of a digital cellular spread spectrum code-division multiple access (CDMA) system. Adopted in 1993 by the Telecommunication Industry Association, the CDMA standard IS-95 is the embodiment of many of the principles presented in this text. Although this book is not meant solely for this purpose, it does explain and justify many of the techniques contained in the standard. I emphasize, however, that my goal is to present the principles underlying spread spectrum communication, most but not all of which apply to this standard. It is not to describe in detail how the principles were applied. This is left to the practicing engineer with the patience and commitment to delve into the details and correlate them with the principles presented here.

Which brings me to the question of prerequisites for a basic understanding. Several excellent texts on statistical communication and information theory have been available for almost four decades. Thus, I have not tried to provide all the fundamentals. The text is nevertheless self-contained: any significant results are derived either in the text or in appendices to the chapter where they are first used. Still, the reader should have at least an undergraduate electrical engineering background with some probability and communication engineering content. A first-year engineering graduate course in communication theory, stochastic processes, or detection and estimation theory would be preferable. As a text for a graduate-level course, the book can be covered in one semester, and with some compromises even in one quarter. It is equally suitable for a one- or two-week intensive short course.

This leaves only the pleasant task of thanking the many contributors to the creation of this text. First, from my superb group of colleagues at QUALCOMM Incorporated, running the gamut from mature and renowned engineers to newly minted graduates, have come the inventive system concepts and the innovative implementation approaches that turned the complex concepts into a useful reality. Among the major contributors, Klein Gilhousen, Irwin Jacobs, Roberto Padovani, Lindsay Weaver, and Charles Wheatley stand out. On the more focused aspects of the text, and the research which preceded it, I owe an enormous debt to Audrey Viterbi. She contributed not only ideas, but also considerable dedication to turn fluid concepts and derivations into firmer results with solid theoretical or simulation support. Finally, she was the first to read, critique, and error-correct the entire manuscript. Over a number of years, Ephraim Zehavi's many ideas and novel approaches have produced results included here. Jack Wolf, always a clear expositor, suggested several improvements. When it came to reviewing the final text and offering corrections and changes, I am indebted to more people than I can recall. Foremost among them are my collaborators at QUALCOMM, including Joseph Odenwalder, Yu-Cheun Jou, Paul Bender, Walid Hamdy, Samir Soliman, Matthew Grob, John Miller, and John McDonough. The last three served as experimental subjects among the first set of graduate students on which I class-tested the entire text. Very helpful outside reviews have come from Robert Gallager, Bijan Jabbari, Allen Levesque, James Mazo, Raymond Pickholtz, and Robert Scholtz. To all of the above, and especially to Deborah Casher, my infinitely patient and cooperative assistant, who processed all of my words and equations, I express my sincere thanks.

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