Modulation refers to the way in which one or more properties of a carrier signal may be varied in order to embed an information signal into the carrier signal and send this over a physical transmission channel, e.g. over air or cable. One generally distinguishes between analog modulation, where an analog information signal is modulated, and digital modulation, where a sequence of data bits is modulated onto a carrier signal.
This chapter provides a brief overview of analog modulation techniques and describes in detail the digital modulation techniques that are most relevant to today's telecommunication systems.
10.2 General
Let us assume that we want to transmit an audio signal such as the simple sine wave depicted in Figure 10.1, having a frequency within the audible range of, say, 10 kHz. Clearly, if we transmit this as it is, for instance just sending this 10 kHz signal over a wireless channel, this would be rather inefficient because
propagation conditions at these frequencies are very bad, and
it would only be possible to transmit one such audio signal in a certain area, as multiple transmissions would otherwise be interfering with each other.
Figure 10.1 Example of a signal or message to be modulated (sine wave).
For this reason, information signals such as audio signals are typically ...
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