Introduction to Digital Communications

By Ali Grami

Introduction to Digital Communications explores the basic principles in the analysis and design of digital communication systems, including design objectives, constraints and trade-offs. After portraying the big picture and laying the background material, this book lucidly progresses to a comprehensive and detailed discussion of all critical elements and key functions in digital communications.

• The first undergraduate-level textbook exclusively on digital communications, with a complete coverage of source and channel coding, modulation, and synchronization.
• Discusses major aspects of communication networks and multiuser communications
• Provides insightful descriptions and intuitive explanations of all complex concepts
• Focuses on practical applications and illustrative examples.
• A companion Web site includes solutions to end-of-chapter problems and computer exercises, lecture slides, and figures and tables from the text

• Language: English
• Publisher: Academic Press
• Release date: Feb 25, 2015
• ISBN: 9780124076587

Ali Grami

Dr. Grami received his PhD in Electrical Engineering from the University of Toronto. He has worked for Nortel Networks, where he was involved in the research, design, and development of North America’s first digital cellular wireless system.He later joined Telesat Canada, where he was the lead researcher and principal designer of Canada's Anik-F2 Ka-band system, the world’s first broadband access satellite system. Dr. Grami is currently an associate professor in the Faculty of Engineering and Applied Science at the University of Ontario Institute of Technology (UOIT), where as a founding faculty member he has led the development of various programs, including the BEng, MEng, and PhD programs in ECE.

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Introduction to Digital Communications

First Edition

Ali Grami

Table of Contents

Cover image

Title page

Copyright

Dedication

Preface

Acknowledgements

Chapter 1: Introduction

Abstract

1.1 Historical Review of Communications

1.2 Block Diagram of a Digital Communication System

1.3 Organization of the Book

Chapter 2: Fundamental Aspects of Digital Communications

Abstract

Introduction

2.1 Why Digital?

2.2 Communications Modalities

2.3 Communication Network Models

2.4 Guided-Transmission Media

2.5 Radio Transmission

2.6 Transmission Impairments

2.7 Modulation Process

2.8 Fundamental Limits in Digital Transmission

2.9 Digital Communication Design Aspects

Chapter 3: Signals, Systems, and Spectral Analysis

Abstract

Introduction

3.1 Basic Operations On Signals

3.2 Classification of Signals

3.3 Classification of Systems

3.4 Sinsuoidal Signals

3.5 Elementary Signals

3.6 Fourier Series

3.7 Fourier Transform

3.8 Time and Frequency Relations

3.9 Signal Transmission Through Systems

3.10 Communication Filters

3.11 Spectral Density and Autocorrelation Functions

3.12 Lowpass and Bandpass Signals

Problems

Chapter 4: Probability, Random Variables, and Random Processes

Abstract

Introduction

4.1 Probability

4.2 Random Variables

4.3 Random Processes

Problems

Chapter 5: Analog-to-Digital Conversion

Abstract

Introduction

5.1 Sampling Process

5.2 Quantization Process

5.3 Digital Pulse Modulation

5.4 Line Codes

Problems

Chapter 6: Baseband Digital Transmission

Abstract

Introduction

6.1 Baseband Binary PAM Transmission System Model

6.2 Intersymbol Interference

6.3 Optimum System Design for Noise Immunity

6.4 Baseband M-ary Signaling Schemes

6.5 Equalization

Problems

Chapter 7: Passband Digital Transmission

Abstract

Introduction

7.1 Optimum Receiver Principles

7.2 Binary Digital Modulation Schemes

7.3 Coherent Quaternary Signaling Schemes

7.4 M-ary Coherent Modulation Techniques

7.5 Orthogonal Frequency-Division Multiplexing

Problems

Chapter 8: Synchronization

Abstract

Introduction

8.1 Synchronization Levels

8.2 Scrambling

8.3 Phase-Locked Loop (PLL)

8.4 Carrier Recovery

8.5 Symbol Synchronization

Problems

Chapter 9: Information Theory

Abstract

Introduction

9.1 Measure of Information

9.2 Classification of Source Codes

9.3 Source Coding Theorem

9.4 Lossless Data Compression

9.5 Discrete Memoryless Channels

9.6 Channel Coding Theorem

9.7 Gaussian Channel Capacity Theorem

Problems

Chapter 10: Error-Control Coding

Abstract

Introduction

10.1 Errors

10.2 Error-Detection Methods

10.3 Automatic Repeat Request (ARQ)

10.4 Block Codes

10.5 Convolutional Codes

10.6 Compound Codes

Problems

Chapter 11: Communication Networks

Abstract

Introduction

11.1 Multiplexing

11.2 Duplexing

11.3 Multiple Access

11.4 Random Access

11.5 Controlled Access

11.6 Wired Communication Networks

11.7 Network Security and Cryptography

Problems

Chapter 12: Wireless Communications

Abstract

Introduction

12.1 Radio-Link Analysis

12.2 Frequency Reuse

12.3 Mobile-Radio Propagation Characteristics

12.4 Diversity

12.5 Diversity-Combining Methods

12.6 Emerging Wireless Communication Systems

Problems

Appendix: Analog Continuous-Wave Modulation

Introduction

A.1 Analog Continuous-Wave (CW) Modulation

A.2 Amplitude Modulation

A.3 Frequency Modulation

A.4 Amplitude Nonlinearity in Analog CW Modulation

A.5 Noise in Analog CW Modulation

A.6 Commercial Radio Broadcasting

A.7 Comparison of Analog CW Modulation Schemes

Summary and Sources

List of Acronyms and Abbreviations

Index

Copyright

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f00-01-9780124076822

Dedication

To the loves of my life: Shirin, Nickrooz, and Neeloufar

Preface

This is an undergraduate-level textbook that lucidly provides the basic principles in the analysis and design of digital communication systems. Since digital communications is an advanced topic, the treatment here is more descriptive and explanatory rather than merely rigorous. However, it is not devoid of mathematical analyses, as all relevant results are presented. Although the mathematical level of the book is intentionally constrained, the intuitive level of it is not. The premise is that complex concepts are best conveyed in the simplest possible context. The goal, at every step of the way, is to provide big picture through analysis, description, example, and application. In taking a systems approach to digital communications, the text hardly considers actual physical realizations of the devices (i.e., the systems are presented in block diagram form).

There is intentionally much more in this book than can be taught in one semester (of 36 lecture hours). The book consists of 12 chapters focused on digital communications. Chapters 1 & 2 provide an overview of the fundamentals of digital communications. Since digital communications at the physical layer involves the transmission of continuous-time signals where the channel noise is also in the form of continuous time, Chapters 3 & 4 cover deterministic and random signals in detail, and should serve as review materials. Chapters 5 to 12, inclusive, discuss the critical elements and major topics in the broad field of digital communications. In addition, there is an appendix with a summary of analog communications.

As this text is primarily written at a level that is most suitable for junior and senior students, the coverage of the book, in terms of depth and breadth as well as tone and scope, lends itself very well to distinct audiences in an electrical engineering program. In a third-year course on digital communications, the coverage may include Chapters 3 & 4 (review of select sections), Chapters 1, 2, 5, 6, 7, 9 & 10 (all sections). In a third-year course on communication systems, the coverage may include Chapters 3 & 4 (review of select sections), Appendix (all sections), Chapters 1, 2, 5 & 6 (all sections), Chapters 7, 9 & 10 (a couple of sections in each chapter). In a fourth-year course on digital communications, with a third-year course on communication systems as a prerequisite, the coverage may contain a review of Chapters 1, 2, 5, 6 and a complete coverage of Chapters 7 to 12, inclusive.

There is also a secondary target audience that can include undergraduate students in the software engineering and information technology programs. In teaching such a course, it is important not to get mired in too many theoretical details, and the focus, instead, must be almost exclusively on the descriptive aspects of systems and intuitive explanations of analytical results and design objectives. This book can also be used by professionals, such as practicing engineers, project leaders, and technical managers, who already have familiarity with the standards and systems and are seeking to advance their knowledge of the fundamentals of the field. It may also serve graduate students as a reference.

Upon request from the publisher, a Solutions Manual can be obtained only by instructors who use the book for adoption in a course.

Acknowledgements

Acknowledgements are a bore for the reader, but a source of true pleasure for the author. All writing is in some sense collaborative. This book is no exception, as it builds upon the ideas, approaches, and insights of teachers, students, and colleagues in both academia and industry. In the course of writing a textbook of this nature, one is bound to lean on the bits and pieces of materials developed by others. I would therefore like to greatly thank the many authors and researchers in the field of digital communications whose writings and findings helped me one way or the other.

I am truly grateful to Dr. J.F. Hayes, my master’s supervisor at McGill University, and Dr. S. Pasupathy, my PhD supervisor at the University of Toronto, who both taught and inspired me about communications. Their broad knowledge, valuable insights, and research excellence in the area of digital communications helped shape my foundation in the field. I owe them a great deal of gratitude for their advice and perspectives on numerous occasions.

I am heavily indebted to many people for their contributions to the development of this text as well as their reviews of some chapters, including Dr. I. Dincer, Dr. X. Fernando, Dr. M. Nassehi, Dr. A. Sepasi Zahmati, Dr. H. Shahnasser, and Dr. M. Zandi, as well as the anonymous reviewers who provided many valuable comments. I would also like to express my appreciation to all the students I have had over the years at various universities. A reflection of what I learned from them is on every page of this book. I would like to thank Lucas Huffman who commented on many parts of the book and prepared some of the figures. A special thanks is due to Ahmad Manzar who provided helpful comments on the entire manuscript and prepared the solutions to the computer exercises.

We have used trademarks in this book without providing a trademark symbol for each mention. We acknowledge the trademarks, and express that their use reflects no intention of infringing upon them. The financial support of Natural Sciences and Engineering Research Council (NSERC) of Canada was also crucial to this project. I am grateful to the staff of Elsevier for their support in various phases of this project, namely Stephen Merken, Nate McFadden, and Anusha Sambamoorthy as well as other members of the production team. No book is flawless, and this text is certainly no different from others. Your comments and suggestions for improvements are always welcomed. I would greatly appreciate it if you could send your feedback to [e-mail].

Chapter 1

Introduction

Abstract

This textbook introduces all fundamental aspects of digital communications, and examines the basic analysis methods, design principles, performance measures, and system trade-offs involved in a digital communication system. In this chapter, we briefly reflect on how some aspects of communications evolved, describe typical elements of a digital communication system, and lastly highlight the chapters that follow.

Keywords

Telecommunications history

communication system block diagram

Contents

1.1 Historical Review of Communications   1

1.2 Block Diagram of a Digital Communication System   5

1.3 Organization of the Book   8

References   10

1.1 Historical Review of Communications

Due to a host of well-conceived ideas, indispensable discoveries, crucial innovations, and important inventions over the past two centuries, information transmission has evolved immeasurably. The technological advances in communications and their corresponding societal impacts are moving at an accelerating pace. To understand today’s modern communication systems, networks, and devices, and to help obtain a sense of perspective about future breakthroughs, it may be insightful to have a glance at the historical developments in the broad field of communications.

A number of people have made significant contributions to help pave the way for a multitude of technological revolutions in the arena of communications. The array of collective, and sometimes collaborative, achievements made in communications are, in essence, due to a team effort, a team whose knowledgeable, talented, and ingenious members lived at different times and in different places; however, we can only afford to mention a select few here. The nineteenth century witnessed scientists and discoverers, such as Oersted, who showed that electric currents can create magnetic fields, Faraday, who discovered that electric current can be induced in a conductor by a changing magnetic field, as well as Maxwell who developed the theory of electromagnetic waves and Hertz who experimentally verified it. Their collective contributions led to the foundation of wireless communications, more specifically, that an electric signal is transmitted by varying an electromagnetic field to induce current change in a distant device. The twentieth century brought researchers and theoreticians, such as Nyquist and Reeves, who respectively contributed to signal sampling process and pulse code modulation, as well as others, such as North, Rice, Wiener, and Kolmogorov, who made contributions to optimal signal and noise processing. Finally, it was Shannon, with his exceptional contribution to the mathematical theory of communications, who laid the unique foundation for digital transmission and today’s information age [1]. Table 1.1 highlights some of the major events in the history of telecommunications [2–4].

Table 1.1

Some of the major events in the history of telecommunications

The telegraph, which provides communications at a distance by coded signals, is considered as the first invention of major significance to communications. The first commercial electrical telegraph was constructed by Wheatstone and Cooke, and perfected by Morse. The telegraph was the forerunner of digital transmission in that Morse code was used to represent a variable-length binary code, where short sequences of dots (short beeps) and dashes (long beeps) represent frequent letters and long ones represent infrequent letters. In the mid-nineteenth century, the first permanent transatlantic telegraph cable became operational, and at the outset of the twentieth century, Marconi and others demonstrated wireless telegraphy, and the first transatlantic radio message was sent. The telegraph is hardly present today.

The telephone, which provides two-way voice communications, was invented by Bell. Like telegraphy, the telephone was first viewed as a point-to-point communication system. But shortly after, the first telephone exchange was established. With the first transcontinental telephone service in the early twentieth century and the first transatlantic telephone cable in the mid-fifties, as well as inventions of the diode, triode, transistor, digital switch, and fiber optic cables, the telephone and the telephone network steadily evolved toward what it is today. Voice communications by telephone at its inception was analog in its entirety. However, half a century later, the transmission of speech signals over the backbone networks along with the switching of the signals became digital, yet the local loop remained analog. And now, for mobile telephones, even the signal between the mobile device and the network is in digital and hence, is an all-digital network.

Radio broadcasting, a one-way wireless transmission of audio signals over radio waves intended to reach a wide audience, grew out of the vacuum tube and early electronics. Following the earlier work on radio, the first amplitude modulation (AM) radio broadcasting was introduced, and grew rapidly across the globe. Shortly after, Armstrong invented the superheterodyne radio receiver and the first frequency modulation (FM) system, and was the first to advocate the principle of power-bandwidth trade-off. FM radio broadcasting gained popularity in the mid-twentieth century, especially with the introduction of FM stereo.

Television is a medium that is used for transmitting and receiving video and sound. TV technology was based on the evolution of electronics that began with the invention of the vacuum tube. TV was invented in the United States in 1929, BBC began the first commercial TV broadcasting in monochrome in 1936, the NTSC color TV was introduced in 1953, and the first commercial HDTV broadcasting began in 1996. Today, there is no analog TV transmission, and TV signals are now exclusively digital.

In 1945, Clarke published his famous article to propose the idea of a geostationary satellite as a relay for communications between various points on Earth. In the late fifties, the former Soviet Union launched the first satellite, Sputnik I, and then the United States launched Explorer I. In 1962, the US launched Telstar I, the first satellite to relay TV programs across the Atlantic. Today, there are several hundreds of satellites employing various frequency bands (e.g., L-band, C-band, X-band, Ku-Band, and Ka-band) in various orbits (LEO, MEO, and GEO) to provide a multitude of services, such as radio and TV broadcasting, mobile and business communications, and GPS.

Following the earlier radio systems, the first public mobile telephone service was introduced in the mid-forties. The system had an interface with the public-switched telephone network (PSTN), the landline phone system. Each system in a city used a single, high-powered transmitter and large tower in order to cover distances of over 50 km. It began as a push-to-talk system (i.e., half-duplex mode) employing FM with 120 kHz of radio-frequency (RF) bandwidth. By the sixties, it had become a full-duplex, auto-dial system, but due to advances in RF filters and low-noise amplifiers, the FM bandwidth transmission was cut to 30 kHz. This mobile radio communication system, which lasted until the early eighties, was very spectrally inefficient, as very few in a geographical area could subscribe to it. These non-cellular mobile systems could now be referred to as 0G systems. Due to a vast level of basic research, in the fifties and sixties, companies such as AT&T and Motorola, NTT in Japan, and Ericson in Europe developed and demonstrated various cellular mobile systems in the seventies. These systems were all analog, and employed FM. In North America, the analog cellular mobile system, known as the advanced mobile phone system (AMPS), which used 30-kHz channels to employ frequency-division multiple access (FDMA), was deployed in 1983. The analog cellular mobile systems are referred to as 1G systems. Due to the lack of mobile roaming capability across Europe and the severe shortage of system capacity in North America, the need for digital cellular mobile systems was born. In North America, IS-54, which was based on a three-slot time-division multiple access (TDMA) scheme using a 30-kHz channel, and IS-95, which was based on many users employing a 1.25 MHz-channel using code-division multiple access (CDMA) scheme, were introduced. In Europe, the GSM, through which eight users employ a 200-kHz TDMA channel, was developed, and became the most widely-used mobile system in the world. Digital cellular mobile systems, such as the IS-54, IS-95, GSM, and some others, introduced in the nineties, are referred to as 2G systems. After that, 3G systems, which provided data transmission at high rates in addition to voice transmission, began to emerge. Figure 1.1 presents the trends in mobile communications.

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Figure 1.1 Evolution of telecommunications with a focus on mobile communications.

The Internet, composed of thousands interconnected networks, is a classic example of an innovation that originated in many different places. To connect computers with their bursty traffic, the sixties witnessed the birth of packet-switched networks and the development of the Advanced Research Projects Agency Network (ARPANET). After Vint Cerf and Bob Kahn, known as the Internet pioneers, devised the protocols to achieve end-to-end delivery of computer data in the mid-seventies, the transmission control protocol and Internet protocol (TCP/IP) became the official protocol for the ARPANET, and ARPANET was renamed the Internet in the mid-eighties. In the early nineties, a hypermedia software interface to the Internet, named the World Wide Web (WWW), was proposed by Tim Berners-Lee. This was the turning point that resulted in the explosive growth of the Internet and yielded numerous commercial applications for the Internet. The Internet has evolved at a faster rate and become more widely-used than any other innovation, invention, or technology in the history of telecommunications. The set of cultural, educational, political, and financial impacts of the Internet on our way of lives is and will remain unparalleled for many years to come.

1.2 Block Diagram of a Digital Communication System

Figure 1.2a shows the basic functional blocks of a typical communication system. Regardless of the particular application and configuration, all information transmission systems involve three major subsystems: the transmitter, the channel, and the receiver.

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Figure 1.2 Block diagrams: (a) a communication system and (b) a digital communication system.

The information source produces its output, which is in probabilistic form, as there is no need to convey deterministic source outputs. An input transducer, such as a microphone, converts the source output into a time-varying electrical signal, referred to as the message signal. The transmitter then converts the message signal into a form suitable for transmission through a physical channel, such as a cable. The transmitter generally changes the characteristics of the message signal to match the characteristics of the channel by using a process called modulation. In addition to modulation, other functions, such as filtering and amplification, are also performed by the transmitter.

The communication channel is the physical medium between the transmitter and the receiver, where they are physically separated. No communication channel is ideal, and thus a message signal undergoes various forms of degradation. Sources of degradation may include attenuation, noise, distortion, and interference. As some or all of these degradations are present in a physical channel, a paramount goal in the design of a communication system is to overcome the effects of such impairments.

The function of the receiver is to extract the message signal from the received signal. The primary function is to perform the process of demodulation, along with a number of peripheral functions, such as amplification and filtering. The complexity of a receiver is generally more significant than that of the transmitter, as a receiver must additionally minimize the effects of the channel degradations. The output transducer, such as a loudspeaker, then converts the receiver output into a signal suitable for the information sink.

Figure 1.2b shows the basic functional elements of a digital communication system. In a simple, yet classical fashion, the transmitter or the receiver each is subdivided into three blocks. The transmitter consists of the source encoder, channel encoder, and modulator, and the receiver consists of the demodulator, channel decoder, and source decoder. At the receiver, the received signal passes through the inverse of the operations at the transmitter, while minimizing the effects of the channel impairments. The three functions of source coding, channel coding, and modulation may be designed in concert with one another to better meet the system design goals, yet accommodating the overall system design constraints.

The information may be inherently digital, such as computer data, or analog, such as voice. If the information is in analog form, then the source encoder at the transmitter must first perform analog-to-digital conversion to produce a binary stream, and the source decoder must then perform digital-to-analog conversion to recover the analog signal. The source encoder removes redundant information from the binary stream so as to make efficient use of the channel. Source coding, also known as data compression, leads to bandwidth conservation, as the spectrum is always at a premium. The important parameters associated with source coding are primarily the efficiency of the coder (i.e., the ratio of actual output data rate to the source information rate) and the encoder/decoder complexity.

The channel encoder at the transmitter introduces, in a controlled fashion, redundancy. The additional bits are used by the channel decoder at the receiver to overcome the channel-induced errors. The added redundancy serves to enhance the performance by reducing the bit error rate, which is the ultimate performance measure in a digital communication system. The important parameters associated with channel coding are primarily the efficiency of the coder (i.e., the ratio of data rate at the input of the encoder to the data rate at its output), error control capability, and the encoder/decoder complexity.

The modulator at the transmitter and the demodulator at the receiver serve as the interface to the communication channel. The modulator accepts a sequence of bits, and maps each sequence into a waveform. A sequence may consist of only one or several bits. At the receiver, the demodulator processes the received waveforms, and maps each to the corresponding bit sequence. The important parameters of modulation are the number of bits in a sequence represented by a waveform, the types of waveforms used, the duration of the waveforms, the power level and the bandwidth used, as well as the demodulation complexity.

There are, of course, other functional blocks, not shown in Figure 1.1b, that are required in a practical digital communication system. They may include synchronization, an essential requirement for all digital commination systems, as well equalization, amplification, and filtering, to name a few.

1.3 Organization of the Book

This textbook provides a comprehensive introduction to digital communications at a level that undergraduate students can grasp all important concepts and obtain a fundamental understanding of digital communication system analysis and design. This book consists of 12 chapters and an appendix, and is organized as follows:

Chapter 2 briefly offers a descriptive overview of major aspects of digital communications with a view to set the stage for what will be covered in the rest of the book. The focus is on the rationale behind digital (vis-à-vis analog), network models, transmission media and impairments, and radio transmission and spectrum. An array of inter-related, inter-dependent design objectives and a host of interacting and conflicting design constraints are identified.

Chapter 3 provides an extensive discussion of signals, systems, and spectral analysis. Humans appreciate time and its continuous and irreversible flow, and above all the impact of its changes. Signals are therefore always defined in the time domain. Physical signals are quite distinct in the time domain, so are many analytical signals. However, in the frequency domain, which is the key measure of how slowly or rapidly signals change in the time domain, a group of distinct signals may share common characteristics. This commonality in turn allows us to design communication systems to transmit, receive, and process a very wide range of signals.

Chapter 4 introduces the basic concepts of probability, random variables, and random processes, with the sole focus of their applications in digital communications, as time-varying, information-bearing signals from the receiving viewpoint are unpredictable. In addition, there are major sources of channel degradation that are random functions of time. This chapter helps pave the way for the performance assessments of digital communication systems with nondeterministic imperfections.

Chapter 5 presents a detailed discussion on how analog signals can be converted into their digital representations. Since humans produce and perceive audio and visual signals in analog form, for the transmission, processing, and storage of audio and visual signals as well as other analog information-bearing signals in digital form, analog-to-digital conversion in the transmitter and digital-to-analog conversion in the receiver are indispensable operations.

Chapter 6 details digital transmission over a baseband channel in the context of pulse amplitude modulation. We first introduce intersymbol interference and the Nyquist criterion to eliminate it, discuss pulse shaping and eye patterns, present the optimum system design to minimize the effect of noise, make a comparison of systems using binary and M-ary signaling, and expand means to combat intersymbol interference using various equalization techniques.

Chapter 7 focuses on digital continuous-wave (CW) binary and M-ary modulation schemes. Digital passband modulation is based on variation of the amplitude, phase, or frequency of the sinusoidal carrier, or some combination of these parameters. As there are a host of reasons why modulation may be required, including transmission of digital data over bandpass channels, a number of digital modulation techniques that have some common characteristics and yet distinct features are discussed.

Chapter 8 highlights synchronization, since in every digital communication system, some level of synchronization is required, without which a reliable transmission of information is not possible. Of the various levels of synchronization, the focus here is on symbol synchronization and carrier recovery, as the role of the former is to provide the receiver with an accurate estimate of the beginning and ending times of the symbols and the latter aims to replicate a sinusoidal carrier at the receiver whose phase is the same as that sent by the transmitter.

Chapter 9 expands on the fundamental relationships between the bit error rate performance and the information rate. Information theory leads to the quantification of the information content of the source, as denoted by entropy, the characterization of the information-bearing capacity of the communication channel, as related to its noise characteristics, and consequently the establishment of the relationship between the information content of the source and the capacity of the channel.

Chapter 10 is concerned with error-control coding, as coding can accomplish its purpose through the deliberate introduction of redundant bits into the message bits. The redundancy bits, which are derived from the message bits as well as the code features and parameters, can provide a level of error detection and correction; however, it cannot guarantee that an error will always be detectable or correctable. At the expense of the channel bandwidth, error-control coding can reduce the bit error rate for a fixed transmitted power and/or reduce the transmit power for a fixed bit error rate.

Chapter 11 describes the major facets of communication networks in an overview fashion, with a focus on multiuser communications, which includes many aspects of wireless communications, such as multiple access methods, and some aspects of wired communications, such as network topology, and public and local area networks. The concepts of cryptography and digital signature are also briefly discussed.

Chapter 12 gives and overview of some aspects of wireless communications, including radio link analysis, mobile radio propagation characteristics, and diversity techniques. As the field of wireless communications has been exponentially expanding and has led to the introduction of versatile mobile/portable devices with their high-data-rate capabilities and numerous mobile applications, there is also a mention of what the future holds.

The appendix summarizes analog continuous-wave modulation namely amplitude modulation, including double sideband (DSB), single sideband (SSB), and vestigial sideband (VSB) schemes, each with and without a carrier, along with narrowband and wideband frequency modulation. In addition, the bandwidth and power requirements, along with methods of their generation and demodulation, are briefly discussed. Detailed derivations are avoided, but the results of the analyses along with intuitive explanations are provided.

References

[1] Shannon CE. A mathematical theory of communication. Bell Syst. Tech. J. July, October, 1948;27:379–423 623–656.

[2] Verdu S. Fifty years of Shannon theory. IEEE Trans. Inform. Theor. October 1998;44:2057–2078.

[3] Huurdeman A. The Worldwide History of Telecommunications. Wiley-IEEE Press; July 2003.978-0-471-20505-0.

[4] http://en.wikipedia.org.

Chapter 2

Fundamental Aspects of Digital Communications

Abstract

This chapter briefly provides a descriptive overview of major aspects of digital communications with a view to set the stage for what will be covered in the rest of the book. A quantitative discussion and detailed analysis of critical elements of digital communication systems will be provided in the following chapters. To provide a fundamental understanding of digital communication system analysis and design, this chapter begins with the rationale behind digital, vis-à-vis analog. The focus then turns toward network models, transmission media and impairments, and radio transmission and spectrum. Following a brief discussion on the fundamental limits in digital transmission, an array of inter-related, inter-dependent design objectives and a host of interacting and conflicting design constraints are identified.

Keywords

Digital

communication modality

OSI model

TCP/IP model;twisted-pair

coaxial cable

fiber-optic cable

wave propagation mode

radio spectrum

frequency band

transmission impairment

modulation process

design objective

design constraint

Contents

Introduction   11

2.1 Why Digital?   12

2.2 Communications Modalities   15

2.3 Communication Network Models   18

2.4 Guided-Transmission Media   23

2.5 Radio Transmission   26

2.6 Transmission Impairments   31

2.7 Modulation Process   34

2.8 Fundamental Limits in Digital Transmission   37

2.9 Digital Communication Design Aspects   37

Summary and Sources   39

Introduction

In today’s world, communications are essential and pervasive, as the age of communications with anyone, anytime, anywhere has arrived. The theme is multimedia—the confluence of voice, data, image, music, text, graphics, and video warranting simultaneous transmission in an integrated fashion. With the push of advancing digital technology and the pull of public demand for an array of innovative applications, it is highly anticipated that every aspect of digital communications will continue to broaden so as to usher in even more achievements. The emerging trend is toward low-cost, high-speed, high-performance, utterly-secure, highly-personalized, context-aware, location-sensitive, and time-critical multimedia applications. After studying this chapter on the fundamental aspects of digital communications and understanding all relevant concepts, students should be able to do the following:

1. State the numerous merits of digital and its dominance in communications.

2. Know the few drawbacks of digital and how they can be mitigated.

3. Understand how text can be represented.

4. Expand on the audio characteristics and the impact of digitization on speech and music.

5. Explain the attributes of image and video and the impact of compression on them.

6. Identify how computers form packets to send them over communication networks.

7. Distinguish between the various characteristics associated with wired transmission media.

8. Highlight the benefits and shortcomings associated with radio communications.

9. Assess various modes of radio wave propagation.

10. Define the modulation process.

11. Identify the principal reasons signals may need to be modulated.

12. Describe signal attenuation.

13. Differentiate among different types of distortions along with their possible remedies.

14. Discuss various sources of interference and how to mitigate them.

15. Summarize various sources of noise.

16. Grasp the limiting factors of a band-limited Gaussian channel.

17. Appreciate the relationship among power, bandwidth, and capacity.

18. Outline digital communication design objectives.

19. List digital communication design constraints.

20. Connect the fundamental aspects of digital communications.

2.1 Why Digital?

The telegraph, invented in the mid-nineteenth century, was the forerunner of digital communications. However, it is now that we can emphatically say digital is the pervasive technology of the twenty-first century and beyond, as the first generation of cellular phones in the late seventies was the last major analog communication invention. During the past three decades, communication networks, systems, and devices have all moved toward digital. The primary examples are wireless networks, Internet, MP3 players, smartphones, HDTV, GPS, and satellite TV and radio. Digital communication technology will continue to bring about intelligent infrastructures and sophisticated end-user devices, through which a host of applications in entertainment (e.g., wireless video on demand), education (e.g., online interactive multimedia courses), information (e.g., 3-D video streaming), and business (e.g., mobile commerce) will be provided. The burgeoning field of digital communications will thus continue to affect almost all aspects of our contemporary life.

A basic definition of digital is the transmission of a message using binary digits (bits) or symbols from a finite alphabet during a finite time interval (bit or symbol duration). A bit or symbol occurring in each interval is mapped onto a continuous-time waveform that is then sent across the channel. Over any finite interval, the continuous-time waveform at the channel output belongs to a finite set of possible waveforms. This is in contrast to analog communications, where the output can assume any possible waveform. Digital can bring about many significant benefits, of course, at the expense of few shortcomings, for there is no free lunch in digital communications.

2.1.1 Advantages of Digital

Design efficiency: Digital is inherently more efficient than analog in exchanging power for bandwidth, the two premium resources in communications. Since an essentially unlimited range of signal conditioning and processing options are available to the designer, effective trade-offs among power, bandwidth, performance, and complexity can be more readily accommodated. For any required performance, there is a three-way trade-off among power, bandwidth, and complexity (i.e., an increase in one means the other two will be reduced).

Versatile hardware: The processing power of digital integrated circuits continues to approximately double every 18 months to 2 years. These programmable processors easily allow the implementation of improved designs or changed requirements. Digital circuits are generally less sensitive to physical effects, such as vibration, aging components, and external temperature. They also allow a greater dynamic range (the difference between the largest and the smallest signal values). Processing is now less costly than precious bandwidth and power resources. This in turn allows considerable flexibility in designing communication systems.

New and enhanced services: In today’s widely distributed way of life, Internet services, such as web browsing, e-mailing, texting, e-commerce, streaming and interactive multimedia services, have all become feasible and some even indispensable. It is also easier to integrate different services, with various modalities, into the same transmission scheme or to enhance services through transmission of some additional information, such as playing music or receiving a phone call with all relevant details.

Control of quality: A desired distortion level can be initially set and then kept nearly fixed at that value at every step (link) of a digital communication path. This reconstruction of the digital signal is done by appropriately-spaced regenerative repeaters, which do not allow accumulation of noise and interference. On the other hand, once the analog signal is distorted, the distortion cannot be removed and a repeater in an analog system (i.e., an amplifier) regenerates the distortion together with the signal. In a way, in an analog system, the noises add, whereas in a digital system, the bit error rates add. In other words, with many regenerative repeaters along the path, the impact in an analog system is a reduction of many decibels (dBs) in the signal-to-noise ratio (SNR), whereas the effect in a digital system is a reduction of only a few dBs in the SNR.

Improved security: Digital encryption, unlike analog encryption, can make the transmitted information virtually impossible to decipher. This applies especially to sensitive data, such as electronic banking and medical information transfer. Secure communications can be achieved using complex cryptographic systems.

Flexibility, compatibility, and switching: Combining various digital signals and digitized analog signals from different users and applications into streams of different speeds and sizes—along with control and signaling information—can be much easier and more efficient. Signal storage, reproduction, interface with computers, as well as access and search of information in electronic databases can also be quite easy and inexpensive. Digital techniques allow the development of communication components with various features that can easily interface with a different component produced by a different manufacturer. Digital transmission brings about the great ability to dynamically switch and route messages of various types, thus offering an array of network connectivities, including unicast, multicast, narrowcast, and broadcast.

2.1.2 Disadvantages of Digital

Signal-processing intensive: Digital communication systems require a very high degree of signal processing, where every one of the three major functions of source coding, channel coding, and modulation in the transceiver—each requiring an array of sub-functions (especially in the receiver)—warrants high computational load and thus complexity. Due to major advances in digital signal processing (DSP) technologies in the past two decades, this is no longer a major disadvantage.

Additional bandwidth: Digital communication systems generally require more bandwidth than analog systems, unless digital signal compression (source coding) and M-ary (vis-à-vis binary) signaling techniques are heavily employed. Due to major advances in compression techniques and bandwidth-efficient modulation schemes, the bit rate requirement and thus the corresponding bandwidth requirement can be considerably reduced by a couple of orders of magnitude. As such, additional bandwidth is no longer a critical issue.

Synchronization: Digital communication systems always require a significant share of resources allocated to synchronization, including carrier phase and frequency recovery, timing (bit or symbol) recovery, and frame and network synchronization. This inherent drawback of digital transmission cannot be circumvented. However, synchronization in a digital communication system can be accomplished to the extent required, but at the expense of a high degree of complexity.

Non-graceful performance degradation: Digital communication systems yield non-graceful performance degradation when the SNR drops below a certain threshold. A modest reduction in SNR can give rise to a considerable increase in bit error rate (BER), thus resulting in a significant degradation in performance.

2.2 Communications Modalities

The main sources of information are broadly categorized as follows: text (e.g., alphanumeric characters), audio (e.g., speech, music), and visual (e.g., image, video). The confluence of voice, data, image, music, text, graphics, and video has led to what is widely known as multimedia. The characteristics of all these modalities and their transmission requirements are distinct. Humans produce and perceive audio and visual signals in an analog form. To this effect, in digital transmission of audio and visual signals, both analog-to-digital and digital-to-analog conversions are required.

After converting analog sources into digital, they are compressed with a high compression ratio. Compression is achieved by exploiting redundancy to the largest extent possible and associating the shortest binary codes with the most likely outcomes. There are fundamentally two types of compression methods: i) lossless compression used in texts and sensitive data, so the original data can be reconstructed exactly (i.e., the compression is completely reversible) and ii) lossy compression used in audio and visual signals, in that permanent loss of information in a controlled manner is involved, and it is therefore not completely reversible. Lossy compression is, however, capable of achieving a compression ratio higher than that attainable with lossless compression. Lossy compression is employed only when degradation in performance to the end user is either unnoticeable or noticeable, but acceptable.

In a discrete memoryless source (DMS), the output symbols are statistically independent and the goal is to find a source code that gives the minimum average number of bits per symbol. Shannon’s source coding theorem states there can be no lossless source code for a DMS whose average codeword length can be less than its source entropy (i.e., the average information content per symbol). In short, for a DMS, the source entropy provides a bound for the best lossless data compression that can be done.

2.2.1 Text

By text, we mean a collection of alphanumeric characters representing, say, English text, software programs, information data, and mathematical formulas. In digital transmission, each character is converted to a sequence of bits. The text transmitted by a computer is usually encoded using American Standard Code for Information Interchange (ASCII). Each character in ASCII is represented by seven bits constituting a unique binary sequence made up of 1s and 0s. Therefore, there are 128 different characters to include all symbols and control functions produced by a typical keyboard or a keypad. An eighth bit, also known as a parity bit, is added for error detection and to make it a byte.

A widely-used compression technique for text is the Lempel-Ziv (LZ) algorithm, which is intrinsically adaptive and capable of encoding groups of characters that occur frequently, while not requiring any advance knowledge of the message statistics. The LZ algorithm is based on parsing the source data stream into segments that are the shortest sequences not encountered previously. The new sequence is encoded in terms of previously seen sequences that have been compiled in a code book (dictionary). An impressive compression of about 55% of ordinary English text can be achieved.

The target BER is a function of the content of the text. For ordinary English text, say a newspaper article, a BER of 10- 4 may be acceptable, but for a bank statement and fund transfer, a BER of 10- 10, or even lower, may be required.

2.2.2 Audio

Audio primarily includes speech and music. Speech is the primary method of human communication. Speech, both produced and perceived, is in analog form. Uttered speech thus needs to be digitized for digital transmission or storage, and converted back to analog to be perceived. The power spectrum (i.e., the distribution of long-term average power versus frequency) of speech approaches zero for zero frequency and reaches a peak in the neighborhood of a few hundred Hertz (Hz). Therefore, speech processing (i.e., production, transmission, storage, and perception) is very sensitive to frequency. More specifically, the power in speech concentrates in the frequencies 100–800 Hz, above which it declines quite drastically. Only about 1% of the power in speech lies above 4 kHz, and humans can hardly hear voice frequencies outside the 100–4000 Hz range. The power content in 100–800 Hz range generally allows speaker recognition and that in the 800–4000 Hz range allows speech recognition (intelligibility).

Telephone speech requires a bandwidth of about 100–3100 Hz and the analog speech quality typically requires an SNR of about 27–40 dB. Analog speech at the 40-dB quality can be converted into a 64 kilobits per second (kbps) digital signal using an 8-bit pulse-code modulation (PCM) technique. The measure of quality for the bits is BER. As long as the BER falls in the range of 10- 4 to 10- 5 or less, it is considered high-quality speech, also known as toll quality. A high-performance, low-rate speech compression technique is based on the human physiological models involved in speech generation. Using digital compression techniques, such as linear prediction coding (LPC), the bit rate requirements for digital speech can be significantly reduced. The 64-kbps rate can be reduced to a range of 1.2 kbps to 13 kbps, depending on the required speech quality. In principle, the resulting bit rate reduction is a function of the complexity of the speech compression method, which in turn is a function of the target application, such as toll-quality voice in telephony and low-quality voice in search and rescue operations. Various LPC techniques now form the basis of many modern cellular vocoders, voice over Internet protocol (VoIP), and other audio ITU-T G-series standards.

A note made by a musical instrument may last for a short time, such as pressing a key on a piano or hitting a prolonged note on a French horn. Typically, music has two structures, a melodic structure consisting of a time sequence of sounds, and a harmonic structure consisting of a set of simultaneous sounds. High-fidelity music requires a bandwidth of 20–20,000 Hz. We perceive both loudness and musical pitch more or less logarithmically, so a power ratio of 1 dB between two sounds may sound quite small to us. Our most acute hearing takes place over 800–3000 Hz, and it is here that we distinguish different musical instruments and perceive the direction of sound. The two elements that are important in high-fidelity music are the SNR value and the dynamic range.

The bit rate requirement for standard stereo CD-quality music is 1.411 Mbps using a 16-bit PCM technique, which in turn results in an SNR of about 90 dB. A waveform representing music varies relatively slowly, and as such past music samples can be used to rather strongly predict the present sample. Sophisticated digital-conversion methods, such as perceptual encoding employed in MP3 with frequency and temporal masking techniques, can take advantage of this predictability to reduce 1.411 Mbps to tens of kbps. The BER requirement for CD-quality music is about 10- 9 or less.

2.2.3 Visual

An image is a two-dimensional array of values that must be reduced to a one-dimensional waveform. This is typically accomplished by the scanning process. The widely-used raster scanning consists of successive lines across the image. Consider scanning, printing, or faxing a standard black-and-white 8.5 × 11 page with a modest resolution of 600 dots per inch, where a dot represents a bit one or zero. The number of bits in a page is then about 4.2 megabytes (MB). Well-known compression techniques, such as the Huffman coding—where the number of bits for each outcome is roughly equal in length to the amount of information conveyed by the outcome in question—can be used. Using Huffman coding, the number of bits required for transmission or storage of such a page can be greatly reduced by one to two orders of magnitude. For an 8 × 10 color print, with 400 × 400 pixels per square inch and 24 bits per pixel (8 bits for each of the three primary colors of red, green, and blue), the number of bits is then about 38.4 MB. By using image-compression techniques, such as the JPEG image-coding standard for compression of still images, it can be reduced by about two orders of magnitude.

Video is a moving picture, and from a signal-processing standpoint, video is actually a succession of still images. The North American NTSC-TV signal has a bandwidth of 4.2 MHz, which extends down to zero frequency, and requires a 6-MHz channel allocation. Use of direct sampling and quantization leads to an uncompressed digital video signal of about 250 Mbps, provided that there are 30 frames per second, 720 × 480 pixels per image frame, and 24 bits per pixel. The HDTV signal requires an uncompressed digital video stream of about 1.5 Gbps, provided that there are 30 frames per second, 1920 × 1080 pixels per frame, and 24 bits per pixel.

The power of video compression is staggering, as the key to video compression is based on human visual perception. MPEG-2 is a widely-used standard for video compression that supports diverse video-coding applications for a wide range of quality, from VCR to HDTV, depending on the transmission rate. MPEG-2 is a highly popular standard that is used in DVD, HDTV, terrestrial digital video broadcasting (DVB-T), and digital video broadcasting by satellite (DVB-S). MPEG-2 exploits both temporal (inter-frame) and spatial (intra-frame) compression techniques, as a relatively small number of pixels changes from frame to frame and there is a very strong correlation among neighboring pixels on a given frame. Using MPEG-2, the uncompressed digital video for the NTSC-TV and HDTV can be reduced to about 6 Mbps and 19.4 Mbps, respectively, and both can then utilize the allocated 6-MHz bandwidth.

In addition to MPEG standards, there are the ITU-T H-series standards, which compress video at a rate of multiples of 64 kbps in applications such as videophone and videoconferencing. For instance, a close variant of one of these standards, designed for very low bit rate coding applications, is now used in Flash video, a highly popular format for video sharing on Internet sites, such as YouTube. Another example is a versatile standard, developed jointly by ITU-T (International Telecommunications Union-Telecommunication) and MPEG (Moving Picture Experts Group), that supports compressed video applications for a wide range of video quality and bit rates, with applications such as mobile phone service (50–60 kbps), Internet standard-definition video (1–2 Mbps), and Internet high-definition video (5–8 Mbps).

2.3 Communication Network Models

Communication networks are needed to support a wide range of services, while allowing diverse devices to communicate seamlessly. A fundamental feature to all network architectures is the grouping of communications and related-connection functions into layers. The principles of layering consist of bidirectional communication (i.e., each layer can perform two opposite tasks, one in each direction). Layering simplifies design, implementation, and testing by partitioning the overall communication process into distinct,