Book contents
- Frontmatter
- Contents
- List of illustrations
- List of tables
- Preface
- 1 Introduction
- 2 Antenna Fundamentals
- 3 Propagation
- 4 Noise
- 5 Analog Modulation
- 6 Digital Modulation
- 7 Radio Link Analysis
- 8 Two-Port Concepts
- 9 Impedance Matching
- 10 Amplifiers
- 11 Linearity, Multistage Analysis, and Dynamic Range
- 12 Antenna Integration
- 13 Analog Filters and Multiplexers
- 14 Frequency and Quadrature Conversion in the Analog Domain
- 15 Receivers
- 16 Frequency Synthesis
- 17 Transmitters
- 18 Digital Implementation of Radio Functions
- Appendix A Empirical Modeling of Mean Path Loss
- Appendix B Characteristics of Some Common Radio Systems
- References
- Index
6 - Digital Modulation
Published online by Cambridge University Press: 28 May 2018
Book contents
- Frontmatter
- Contents
- List of illustrations
- List of tables
- Preface
- 1 Introduction
- 2 Antenna Fundamentals
- 3 Propagation
- 4 Noise
- 5 Analog Modulation
- 6 Digital Modulation
- 7 Radio Link Analysis
- 8 Two-Port Concepts
- 9 Impedance Matching
- 10 Amplifiers
- 11 Linearity, Multistage Analysis, and Dynamic Range
- 12 Antenna Integration
- 13 Analog Filters and Multiplexers
- 14 Frequency and Quadrature Conversion in the Analog Domain
- 15 Receivers
- 16 Frequency Synthesis
- 17 Transmitters
- 18 Digital Implementation of Radio Functions
- Appendix A Empirical Modeling of Mean Path Loss
- Appendix B Characteristics of Some Common Radio Systems
- References
- Index
Summary
INTRODUCTION
In Section 5.1 we defined modulation as the representation of information in a form suitable for transmission. Digital modulation might be defined as the representation of information as a sequence of symbols drawn from a finite alphabet of possible symbols. This definition is not completely satisfactory, since it would seem to include a variety of techniques that many practitioners would consider to be analog modulations; these include continuous wave (CW) signaling (the use of an interruptible carrier to send Morse code). In Section 5.1 we settled on the notion that digital modulation also requires the information to be transmitted to be represented in digital (not merely discrete-time) form at some point in the modulation process, whereas this is not required in analog modulation.
Overview of a Digital Communications Link and Organization of this Chapter
With this definition in mind, Figure 6.1 shows an overview of a digital communications link. Here is the usual signal flow.
• The starting point of the link is source coding, which is the processing of data and/or digitized analog message signals – e.g., voice – in order to render the data in a form better-suited for digital modulation. A common goal in source coding is compression; that is, reduction of data rate so as to reduce the required bandwidth. Source coding is addressed in Section 6.2.
• Data are often encrypted for security. The principles of encryption are not specific to radio communications and so are not addressed further in this book.
• Channel coding is the processing of data with the goal of making them more robust to propagation channel impairments, including noise, multipath, and interference. Channel coding is addressed in Sections 6.13 (error control coding) and 6.14 (interleaving).
• At this point the data are processed by what is traditionally considered “modulation.” The result is an analog signal representing a discrete-time finite-alphabet message, and which is suitable for frequency conversion and amplification. It is this form of the signal that is ultimately transmitted to the other end of the link. Modulation is addressed in Sections 6.3, 6.4, and portions of Sections 6.10–6.12, 6.18 and 6.19.
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- Radio Systems Engineering , pp. 139 - 202Publisher: Cambridge University PressPrint publication year: 2016
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