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Unformatted text preview: Chapter 3 Analog Communication Techniques From Introduction to Communication Systems Copyright by Upamanyu Madhow, 2008-2011 Modulation is the process of encoding information into a signal that can be transmitted (or recorded) over a channel of interest. In analog modulation, a baseband message signal, such as speech, audio or video, is directly transformed into a signal that can be transmitted over a designated channel, typically a passband radio frequency (RF) channel. Digital modulation differs from this only in the following additional step: bits are encoded into baseband message signals, which are then transformed into passband signals to be transmitted. Thus, despite the relentless transition from digital to analog modulation, many of the techniques developed for analog communication systems remain important for the digital communication systems designer, and our goal in this chapter is to study an important subset of these techniques, using legacy analog communication systems as examples to reinforce concepts. From Chapter 2, we know that passband signals carry information in their complex envelope, and that the complex envelope can be represented either in terms of I and Q components, or in terms of envelope and phase. We study two broad classes of techniques: amplitude modula- tion, in which the analog message signal appears directly in the I and/or Q components; and angle modulation, in which the analog message signal appears directly in the phase or in the instantaneous frequency (i.e., in the derivative of the phase), of the transmitted signal. Examples of analog communication in space include AM radio, FM radio, and broadcast television, as well as a variety of specialized radios. Examples of analog communication in time (i.e., for storage) include audiocassettes and VHS videotapes. The analog-centric techniques covered in this chapter include envelope detection, superhetero- dyne reception, limiter discriminators, and phase locked loops. At a high level, these techniques tell us how to go from baseband message signals to passband transmitted signals, and back from passband received signals to baseband message signals. For analog communication, this is enough, since we consider continuous time message signals which are directly transformed to passband through amplitude or angle modulation. For digital communication, we need to also figure out how to decode the encoded bits from the received passband signal, typically after down- conversion to baseband; this is a subject discussed in later chapters. However, between encoding at the transmitter and decoding at the receiver, a number of analog communication techniques are relevant: for example, we need to decide between direct and superheterodyne architectures for upconversion and downconversion, and tailor our frequency planning appropriately; we may use a PLL to synthesize the local oscillator frequencies at the transmitter and receiver; and 1 the basic techniques for mapping baseband signals to passband remain the same (amplitude...
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