Introduction To Communication Systems by Upamanyu Madhow

Progress in telecommunications over the past two decades has been nothing short of revolutionary, with communications taken for granted in modern society to the same extent as electricity.

There is, therefore, a persistent need for engineers who are well-versed in the principles of communication systems. These principles apply to communication between points in space, as well as communication between points in time (i.e, storage). Digital systems are fast replacing analog
systems in both domains. This book has been written in response to the following core question:

what is the basic material that an undergraduate student with an interest in communications should learn, in order to be well prepared for either industry or graduate school? For example, a number of institutions only teach digital communication, assuming that analog communication is dead or dying. Is that the right approach? From a purely pedagogical viewpoint, there are critical questions related to mathematical preparation: how much mathematics must a student learn to become well-versed in system design, what should be assumed as background, and at what point should the mathematics that is not in the background be introduced? Classically, students learn probability and random processes and then tackle communication. This does not quite work today: students increasingly (and I believe, rightly) question the applicability of the material they learn and are less interested in abstraction for its own sake. On the other hand, I have found from my own teaching experience that students get truly excited about abstract concepts when they discover their power in applications, and it is possible to provide the means for such discovery using software packages such as Matlab. Thus, we have the opportunity to get a new generation of students excited about this field: by covering abstractions “just in time” to shed light on engineering design, and by reinforcing concepts immediately using software experiments in addition to conventional pen-and-paper problem solving, we can remove the lag between learning and application, and ensure that the concepts stick.

This textbook represents my attempt to act upon the preceding observations and is an outgrowth of my lectures for a two-course undergraduate elective sequence on communication at UCSB, which is often also taken by some beginning graduate students. Thus, it can be used as the basis for a two-course sequence in communication systems, or a single course on digital communication, at the undergraduate or beginning graduate level. The book also provides a review or introduction to communication systems for practitioners, easing the path to study of more advanced graduate texts and the research literature. The prerequisite is a course on signals and systems, together with an introductory course on probability. The required material on random processes is included in the text.

A student who masters the material here should be well-prepared for either graduate school or the telecommunications industry. The student should leave with an understanding of baseband and passband signals and channels, modulation formats appropriate for these channels, random processes, and noise, a systematic framework for optimum demodulation based on signal space concepts, performance analysis and power-bandwidth tradeoffs for common modulation schemes, a hint of the power of information theory and channel coding, and introduction to communication techniques for dispersive channels and multiple antenna systems. Given the significant ongoing research and development activity in wireless communication, and the fact that an understanding of wireless link design provides a sound background for approaching other communication links, a material enabling hands-on discovery of key concepts for wireless system design is interspersed throughout the textbook.

I should add that I firmly believe that the utility of this material goes well beyond communications, important as that field is. Communications systems design merges concepts from signals and systems, probability and random processes, and statistical inference. Given the broad applicability of these concepts, a background in communications is of value in a large variety of areas requiring “systems thinking,” as I discuss briefly at the end of Chapter 1.

The goal of the lecture-style exposition in this book is to clearly articulate a selection of concepts that I deem fundamental to communication system design, rather than to provide comprehensive coverage. “Just in time” coverage is provided by organizing and limiting the material so that we get to core concepts and applications as quickly as possible, and by sometimes asking the reader to operate with partial information (which is, of course, standard operating procedure in the real world of engineering design). However, the topics that we do cover are covered in sufficient detail to enable the student to solve nontrivial problems and to obtain hands-on involvement via software labs. Descriptive material that can easily be looked up online is omitted.

Organization

• Chapter 1 provides a perspective on communication systems, including a discussion of the transition from analog to digital communication and how it colors the selection of material in this text.

• Chapter 2 provides a review of signals and systems (biased towards communications applications), and then discusses the complex baseband representation of passband signals and systems, emphasizing its critical role in modeling, design and implementation. A software lab on modeling and undoing phase offsets in complex baseband, while providing a sneak preview of digital modulation, is included. Chapter 2 also includes a section on wireless channel modeling in complex
baseband using ray tracing, reinforced by a software lab which applies these ideas to simulate link time variations for a lamppost based broadband wireless network.

• Chapter 3 covers analog communication techniques which are relevant even as the world goes digital, including superheterodyne reception and phase locked loops. Legacy analog modulation techniques are discussed to illustrate core concepts, as well as in recognition of the fact that suboptimal analog techniques such as envelope detection and limiter-discriminator detection may have to be resurrected as we push the limits of digital communication in terms of speed and power consumption. Software labs reinforce and extend concepts in amplitude and angle modulation.

• Chapter 4 discusses digital modulation, including linear modulation using constellations such as Pulse Amplitude Modulation (PAM), Quadrature Amplitude Modulation (QAM), and Phase Shift Keying (PSK), and orthogonal modulation and its variants. The chapter includes discussion of the number of degrees of freedom available on a bandlimited channel, the Nyquist criterion for avoidance of intersymbol interference, and typical choices of Nyquist and square root Nyquist signaling pulses. We also provide a sneak preview of power-bandwidth tradeoffs (with detailed discussion postponed until the effect of noise has been modeled in Chapters 5 and 6). A software lab providing a hands-on feel for Nyquist signaling is included in this chapter. The material in Chapters 2 through 4 requires only a background in signals and systems.

• Chapter 5 provides a review of basic probability and random variables, and then introduces random processes. This chapter provides a detailed discussion of Gaussian random variables, vectors, and processes; this is essential for modeling noise in communication systems. Examples which provide a preview of receiver operations in communication systems, and computation of performance measures such as error probability and signal-to-noise ratio (SNR), are provided. Discussion of circular symmetry of white noise and noise analysis of analog modulation techniques is placed in an appendix since this is material that is often skipped in modern courses on communication systems.

• Chapter 6 covers classical material on optimum demodulation for M-ary signaling in the presence of additive white Gaussian noise (AWGN). The background on Gaussian random variables, vectors and processes developed in Chapter 5 is applied to derive optimal receivers and to analyze their performance. After discussing error probability computation as a function of SNR, we are able to combine the materials in Chapters 4 and 6 for a detailed discussion of power-bandwidth tradeoffs. Chapter 6 concludes with an introduction to link budget analysis, which provides guidelines on the choice of physical link parameters such as transmit and receive antenna gains, and distance between transmitter and receiver, using what we know about the dependence of error probability as a function of SNR. This chapter includes a software lab which builds on the Nyquist signaling lab in Chapter 4 by investigating the effect of noise. It also includes another software lab simulating performance over a time-varying wireless channel, examining the effects of fading and diversity, and introduces the concept of differential demodulation for the avoidance of explicit channel tracking. Chapters 2 through 6 provide a systematic lecture-style exposition of what I consider core concepts in communication at an undergraduate level.

• Chapter 7 provides a glimpse of information theory and coding whose goal is to stimulate the reader to explore further using more advanced resources such as graduate courses and textbooks. It shows the critical role of channel coding, provides an initial exposure to information-theoretic performance benchmarks, and discusses belief propagation in detail, reinforcing the basic concepts through a software lab.

• Chapter 8 provides a first exposure to the more advanced topics of communication over dispersive channels, and of multiple antenna systems, often termed space-time communication or Multiple Input Multiple Output (MIMO) communication. These topics are grouped together because they use similar signal processing tools. We emphasize lab-style “discovery” in this chapter using three software labs, one on adaptive linear equalization for single carrier modulation, one on basic OFDM transceiver operations, and one on MIMO signal processing for space-time coding and spatial multiplexing. The goal is for students to acquire hands-on insight that hopefully motivates them to undertake a deeper and more systematic investigation.

• Finally, the epilogue contains speculation on future directions in communications research and
technology. The goal is to provide a high-level perspective on where mastery of the introductory material in this textbook could lead, and to argue that the innovations that this field has already seen set the stage for many exciting developments to come.

The role of software: Software problems and labs are integrated into the text, with “code fragments” implementing core functionalities provided in the text. While code can be provided online,
separate from the text (and indeed, sample code is made available online for instructors), code
fragments are integrated into the text for two reasons. First, they enable readers to immediately
see the software realization of a key concept as they read the text. Second, I feel that students
learn more by putting in the work of writing their own code, building on these code fragments
if they wish, rather than using code that is easily available online. The particular software that
we use is Matlab, because of its widespread availability, and because of its importance in design
and performance evaluation in both academia and industry. However, the code fragments can
also be viewed as “pseudocode,” and can be easily implemented using other software packages or
languages. Block-based packages such as Simulink (which builds upon Matlab) are avoided here,
because the use of software here is pedagogical rather than aimed at, say, designing a complete
system by putting together subsystems as one might do in industry.

Suggestions for using this book

I view Chapter 2 (complex baseband), Chapter 4 (digital modulation), and Chapter 6 (optimum
demodulation) as core material that must be studied to understand the concepts underlying
modern communication systems. Chapter 6 relies on the probability and random processes
material in Chapter 5, especially the material on jointly Gaussian random variables and WGN,
but the remaining material in Chapter 5 can be skipped or covered selectively, depending on
the students’ background. Chapter 3 (analog communication techniques) is designed such that
it can be completely skipped if one wishes to focus solely on digital communication. Finally,
Chapter 7 and Chapter 8 contain glimpses of advanced material that can be sampled according
to the instructor’s discretion. The qualitative discussion in the epilogue is meant to provide the
student with perspective and is not intended for formal coverage in the classroom.
In my own teaching at UCSB, this material forms the basis for a two-course sequence, with
Chapters 2-4 covered in the first course, and Chapters 5-6 covered in the second course, with
the dispersive channels portion of Chapter 8 provides the basis for the labs in the second
course. The content of these courses are constantly being revised, and it is anticipated that the
material on channel coding and MIMO may displace some of the existing material in the future.
UCSB is on a quarter system, hence the coverage is fast-paced, and many topics are omitted or
skimmed. There is ample material here for a two-semester undergraduate course sequence. For
a one-semester course, one possible organization is to cover Chapter 2 (focusing on the complex
envelope), Chapter 4, a selection of Chapter 5, Chapter 6, and if time permits, Chapter 7.
The slides accompanying the book are not intended to provide comprehensive coverage of the
material, but rather, to provide an example of selections from the material to be covered in
the classroom. I must comment in particular on Chapter 5. While much of the book follows
the format in which I lecture, Chapter 5 is structured as a reference on probability, random
variables and random processes that the instructor must pick and choose from, depending on
the background of the students in the class. The particular choices I make in my own lectures
on this material are reflected in the slides for this chapter.


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1 comment:

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