Preface
Wireless communications is a broad and dynamic field that has spurred tremendous excitement  and technological advances over the last fewdecades. The goal of this book is to provide readers with a comprehensive understanding of the fundamental principles underlying wireless communications. These principles include the characteristics and performance limits of wireless systems, the techniques and mathematical tools needed to analyze them, and the insights and trade-offs associated with their design. Current and envisioned wireless systems are used to motivate and exemplify these fundamental principles. The book can be used as a senior- or graduate-level textbook and as a reference for engineers, academic and industrial researchers, and students working in the wireless field. 

ORGANIZATION OF THE BOOK 
Chapter 1 begins with an overview of wireless communications, including its history, a vision  for the future, and an overview of current systems and standards. Wireless channel characteristics, which drive many of the challenges in wireless system design, are described in Chapters 2 and 3. In particular, Chapter 2 covers path loss and shadowing in wireless channels, which vary over relatively large distances. Chapter 3 characterizes the flat and frequency-selective properties of multipath fading, which change over much smaller distances – on the order of the signal wavelength. Fundamental capacity limits of wireless channels along with the capacity-achieving transmission strategies are treated in Chapter 4. Although these techniques have unconstrained complexity and delay, they provide insight and motivation for many of the practical schemes discussed in later chapters. In Chapters 5 and 6 the focus shifts to digital modulation techniques and their performance in wireless channels. These chapters indicate that fading can significantly degrade performance. Thus, fading mitigation techniques are required for high-performance wireless systems. 
The next several chapters cover the primary mitigation techniques for flat and frequencyselective fading. Specifically, Chapter 7 covers the underlying principles of diversity techniques, including a newmathematical tool that greatly simplifies performance analysis. These techniques can remove most of the detrimental effects of flat fading. Chapter 8 provides comprehensive coverage of coding techniques, including mature methods for block, convolutional, and trellis coding as well as recent developments in concatenated, turbo, and LDPC codes. This chapter illustrates that, though coding techniques for noisy channels have near-optimal performance, many open issues remain in the design and performance analysis of codes for wireless systems. Chapter 9 treats adaptive modulation in flat fading, which enables robust and spectrally efficient communication by leveraging the time-varying nature of the wireless channel. This chapter also ties the techniques and performance of adaptive modulation to the fundamental capacity limits of flat fading channels. Multiple-antenna techniques and space-time communication systems are covered in Chapter 10: the additional spatial dimension enables high data rates and robustness to fading. Equalization, which exploits signal processing in the receiver to compensate for frequency-selective fading, is covered in Chapter 11. Multicarrier modulation, described in Chapter 12, is simpler and more flexible than equalization for frequency-selective fading mitigation. Single-user and multiuser spread-spectrum techniques are described in Chapter 13. These techniques not only mitigate frequency-selective fading, they also allow multiple users to share the same wireless spectrum. 
The last three chapters of the book focus on multiuser systems and networks. Chapter 14 treats multiple and random access techniques for sharing the wireless channel among many users with continuous or bursty data. Power control is also covered in this chapter as a mechanism to reduce interference between users while ensuring that all users meet their performance targets. The chapter closes by discussing the fundamental capacity limits of multiuser channels as well as the transmission and channel sharing techniques that achieve these limits. Chapter 15 covers the design, optimization, and performance analysis of cellular systems, along with advanced topics related to power control and fundamental limits in these systems. The last chapter, Chapter 16, discusses the fundamental principles and open research challenges associated with wireless ad hoc networks. 

REQUIRED BACKGROUND 
The only prerequisite knowledge for the book is a basic understanding of probability, random processes, and Fourier techniques for system and signal analysis. Background in digital  communications is helpful but not required, as the underlying principles from this field are covered in the text. Three appendices summarize key background material used in different chapters of the text. Specifically, Appendix A discusses the equivalent lowpass representation of bandpass signals and systems, which simplifies bandpass system analysis. Appendix B provides a summary of the main concepts in probability and random processes that are used throughout the book. Appendix C provides definitions, results, and properties related to matrices, which are widely used in Chapters 10 and 12. The last appendix, Appendix D, summarizes the main characteristics of current wireless systems and standards. 

BOOK FEATURES 
The tremendous research activity in the wireless field – coupled with the complexity of wireless system design – make it impossible to provide comprehensive details on all topics discussed in the book. Thus, each chapter contains a broad list of references that build and expand on what is covered in the text. The book also contains nearly a hundred worked examples to illustrate and highlight key principles and trade-offs. In addition, the book includes about 300 homework exercises. These exercises, which fall into several broad categories, are designed to enhance and reinforce the material in the main text. Some exercises are targeted to exemplify or provide more depth to key concepts, as well as to derive or illustrate properties of wireless systems using these concepts. Exercises are also used to prove results stated but not derived in the text. Another category of exercises obtains numerical results that give insight into operating parameters and performance of wireless systems in typical environments. Exercises also introduce new concepts or system designs that are not discussed in the text. A solutions manual is available that covers all the exercises. 

USING THIS BOOK IN COURSES 
The book is designed to provide much flexibility as a textbook, depending on the desired length of the course, student background, and course focus. The core of the book is in Chapters 1 through 6. Thereafter, each chapter covers a different stand-alone topic that can be omitted or may be covered in other courses. Necessary prerequisites for a course using this text are an undergraduate course in signals and systems (both analog and digital) and one in probability theory and random processes. It is also helpful if students have a prerequisite or corequisite course in digital communications, in which case the material in Chapter 5 (along with overlapping material in other chapters) can be covered quickly as a review. 
The book breaks down naturally into three segments: core material in Chapters 1–6, single-user wireless system design in Chapters 7–13, and multiuser wireless networks in Chapters 14–16. Most of the material in the book can be covered in two to three quarters or two semesters. A three-quarter sequence would follow the natural segmentation of the chapters, perhaps with an in-depth research project at the end. For a course sequence of two semesters or quarters, the first course could focus on Chapters 1–10 (single-user systems with flat fading) and the second course could focus on Chapters 11–16 (frequency-selective fading techniques, multiuser systems, and wireless networks). A one-quarter or semester course could focus on single-user wireless systems based on the core material in Chapters 1–6 and selected topics from Chapters 7–13. In this case a second optional quarter or semester could be offered covering multiuser systems and wireless networks (part of Chapter 13 and Chapters 14–16). I use this breakdown in a two-quarter sequence at Stanford, where the second quarter is offered every other year and includes additional reading material from the literature as well as an in-depth research project. Alternatively, a one-quarter or semester course could cover both single and multiuser systems based on Chapters 1–6 and Chapters 13–16, with some additional topics from Chapters 7–12 as time permits. 
A companion Web site (http: //www.cambridge.org/9780521837163) provides supplemental  material for the book, including lecture slides, additional exercises, and errata. 

ACKNOWLEDGMENTS 
It takes a village to complete a book, and I am deeply indebted to many people for their help during the multiple phases of this project. I first want to thank the ten generations of students at Caltech and Stanford who suffered through the annual revisions of my wireless course notes: their suggestions, insights, and experiences were extremely valuable in honing the topics, coverage, and tone of the book. John Proakis and several anonymous reviewers provided valuable and in-depth comments and suggestions on early book drafts, identifying omissions and weaknesses, which greatly strengthened the final manuscript. My current graduate students Rajiv Agrawal, Shuguang Cui,Yifan Liang, Xiangheng Liu, Chris Ng, and Taesang Yoo meticulously proofread many chapter drafts, providing new perspectives and insights, rederiving formulas, checking for typos, and catching my errors and omissions. My former graduate students Tim Holliday, Syed Jafar, Nihar Jindal, Neelesh Mehta, Stavros Toumpis, and Sriram Vishwanath carefully scrutinized one or more chapters and provided valuable input. In addition, all of my current and former students (those already mentioned as well as Mohamed-Slim Alouini, Soon-Ghee Chua, Lifang Li, and KevinYu) contributed to the content of the book through their research results, especially in Chapters 4, 7, 9, 10, 14, and 16. The solutions manual was developed by Rajiv Agrawal, Grace Gao, and Ankit Kumar. I am also indebted to many colleagues who took time from their busy schedules, sometimes on very short notice, to read and critique specific chapters. They were extremely gracious, generous, and honest with their comments and criticisms. Their deep and valuable insights not only greatly improved the book but also taught me a lot about wireless. For these efforts I am extremely grateful to Jeff Andrews, Tony Ephremides, Mike Fitz, Dennis Goeckel, Larry Greenstein, Ralf Koetter, 
P. R. Kumar, Muriel Médard, Larry Milstein, Sergio Servetto, Sergio Verdú, and Roy Yates. Don Cox was always available to share his infinite engineering wisdom and to enlighten me about many of the subtleties and assumptions associated with wireless systems. I am also grateful to my many collaborators over the years, as well as to my co-workers at Maxim Technologies and AT＆T Bell Laboratories, who have enriched my knowledge of wireless communications and related fields. I am indebted to the colleagues, students, and leadership at Stanford who created the dynamic, stimulating, and exciting research and teaching environment in which this book evolved. I am also grateful for funding support from ONR and NSF throughout the development of the book. Much gratitude is also due to my administrative assistants Joice DeBolt and Pat Oshiro for taking care of all matters big and small in support of my research and teaching, and for making sure I had enough food and caffeine to get through each day. I would also like to thank copy editor Matt Darnell for his skill and attention to detail throughout  the production process. My editor Phil Meyler has followed this book from its inception ten years ago until today. His encouragement and enthusiasm about the book never waned, and he has accommodated all of my changes and delays with grace and good humor. I cannot imagine a better editor with whom to embark on such a difficult, taxing, and rewarding undertaking. 
I would like to thank two people in particular for their early and ongoing support in this project and all my professional endeavors. Larry Greenstein ignited my initial interest in wireless through his deep insight and research experience. He has served as a great source of knowledge, mentoring, and friendship. Pravin Varaiya was deeply influential as a Ph.D. advisor and role model due to his breadth and depth of knowledge along with his amazing rigor, insight, and passion for excellence. He has been a constant source of encouragement, inspiration, and friendship. 
My friends and family have provided much love, support, and encouragement for which I am deeply grateful. I thank them for not abandoning me despite my long absences during the final stages of finishing the manuscript, and also for providing an incredible support network without which the book could not have been completed. I am especially grateful to Remy, Penny, and Lili for their love and support, and to my mother Adrienne for her love and for instilling in me her creativity and penchant for writing. My fatherWerner has profoundly influenced this book and my entire career both directly and indirectly. He was the senior Professor Goldsmith, a prolific researcher, author, and pioneer in many areas of mechanical and biological engineering. His suggestion to pursue engineering launched my career, for which he was my biggest cheerleader. His pride, love, and encouragement have been a constant source of support. I was fortunate to help him complete his final paper, and I have tried in this book to mimic his rigor, attention to detail, and obsession with typos that I experienced during that collaboration. 
Finally, no words are sufficient to express my gratitude and love for my husband Arturo and my children Daniel and Nicole. Arturo has provided infinite support for this book and every other aspect of my career, for which he has made many sacrifices. His pride, love, encouragement, and devotion have sustained me through the ups and downs of academic and family life. He is the best husband, father, and friend I could have dreamed of, and he enriches my life in every way. Daniel and Nicole are the sunshine in my universe – each day is brighter because of their love and sweetness. I am incredibly lucky to share my life with these three special people. This book is dedicated to them.