2.1 Modulation

As a junior high-school student, I was introduced to the concept of modulation when my science teacher asked me if I could get to my grandmother's house, which was in a different town, on foot. I responded with “I would not be able to do it. It is far away and we usually take the train.” The teacher told me “Aha! The train is your carrier; you get modulated on the train, which can go faster and when you arrive to your grandmother's town, you are demodulated from the train.” As I later studied modulation in depth, this example struck me as being inaccurate. Nevertheless, even to this day, I believe that the comparison of the train to the carrier signal, and the comparison of myself to the information signal still rings true. A simple example of modulation is cell phone use. Your voice is at a frequency ranging between 300 Hz and 6 kHz and the frequency is modulated over the radio frequency signal between the cell phone and the base station. This carrier signal remains at a much higher frequency and can propagate for long distances with much less attenuation than your actual voice signal.

Before we continue, let us review some of the important aspects of digital modulation over a point-to-point link or a multiple access medium. A stream of bits is modulated at the sending node, transmitted over the medium and demodulated at the receiving end. A symbol is a sequence of bits in the stream that allows us to modulate one or more bits at a time. Modulation techniques are ...

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