Communication Systems

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.
We define communication as the process of information transfer across space or time. Communication across space is something we have an intuitive understanding of: for example, radio waves carry our phone conversation between our cell phone and the nearest base station, and coaxial cables (or optical fiber, or radio waves from a satellite) deliver television from a remote location to our home. However, a moment’s thought shows that that communication across time, or storage of information, is also an everyday experience, given our use of storage media such as compact discs (CDs), digital video discs (DVDs), hard drives and memory sticks. In all of these instances, the key steps in the operation of a communication link are as follows: 

(a) insertion of information into a signal, termed the transmitted signal, compatible with the physical medium of interest.
(b) propagation of the signal through the physical medium (termed the channel) in space or
time;
(c) extraction of information from the signal (termed the received signal) obtained after propagation through the medium. 
Communications systems can not only link people or systems at great distances via audio, visual, computer, or other messages, but may link the various parts within systems, and even within single semiconductor chips. They may communicate information in two directions, or only one way, and they may involve one node broadcasting to many, one node receiving from many, or a finite set of nodes communicating among themselves in a network. Even active measurement and remote sensing systems can be regarded as communications systems. In this case the transmitted signals are designed to be maxim ally sensitive to the channel characteristics, rather than insensitive, and the receiver’s task is to extract these channel characteristics knowing what was transmitted.  
A two-node, one-way communication system consists of the channel that conveys the waves, together with a modulator and a demodulator. All communications systems can be regarded as aggregates of these basic two-node units. The modulator transforms the signal or symbol to be transmitted into the signal that is propagated across the channel; the channel may add noise and distortion. The task of the demodulator is to analyze the channel output and to make the best possible estimate of the exact symbol transmitted, accounting for the known channel characteristics and any user concerns about the relative importance of different sorts of errors. A sequence of symbols constitutes a message. A complete communications system is formed by combining many two-node, one-way systems in the desired configuration.

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