2
Modulation
The aim of modulation is to transfer a source data over a channel in a way most suitable for this channel. That is, the original data should be translated into a form that is compatible with the channel. Since the scope of this book is wireless communication only radio channel is under consideration. In this case the data modulates a radio frequency bearer in the form of a sinusoid which is called a carrier wave. This kind of modulation is called the bandpass modulation since it deals with a bandpass channel. It is possible to divide the modulation process into two stages: baseband modulation and bandpass modulation. In this way the baseband modulation consists of translating the original data (analogue or digital) into some waveforms of low frequency and bandpass modulation consists of modifying the high frequency carrier wave, or simply carrier, in accordance with waveforms obtained at the output of the baseband modulation process. Why do we need such a complicated process? Why not transmit the waveforms directly over the radio channel? One of the reasons, and possibly the main one, is the antenna size. The typical antenna size is 
, where 
is the wavelength. Assume that the waveform at the output of baseband modulation is the ...
