Communication Systems Principles Using MATLAB

3Modulation and Demodulation

3.1 Chapter Objectives

On completion of this chapter, the reader should:

1. Be able to define many of the common types of modulation and their variants.
2. Be able to explain the spectral effects of modulation and why certain types of modulation are used in differing situations.
3. Be able to draw block diagrams of modulators and demodulators, both analog and digital, and derive the mathematical expressions for their form where appropriate.
4. Be able to explain, using mathematical notation, the operation of modulators and demodulators.

3.2 Introduction

If a signal is to be transmitted over some distance, be it an analog signal such as audio or digital data, then it must somehow be transformed or modulated so as to conform to the required characteristics of the transmission medium. This might be a wired connection, or a wireless/radio link, or even an optical transmission system. The transmission medium is, in effect, the carrier of the information, and it is necessary to modulate, or change, the characteristics of that carrier waveform in order to carry the signal information. The reverse operation, demodulation, must occur at the receiver. It is necessary to take the received signal and transform it back into the original. In practice, exact reversal may not be possible due to nonlinear operations in the system or external noise.

As might be expected, there is no one single method of modulation that suits all types of situations. The main aspect is the ...