Modulation

If we transmit a constant signal at a given frequency between a receiver can pick that signal up, but, if the signal never varies, it cannot convey information. Communication requires a variable signal. Paul Revere said "one if by land and two if by sea," not "one if by land and one if by sea."

To encode information, we must be able to vary the signal at the transmitting end and detect the changes at the receiving end. If we are using a binary code, we must be able to differentiate between 0 and 1.

Modulate is a technical term for "change" or "vary." Engineers have invented many modulation schemes. For example, we can differentiate 0 from 1 by changing the amplitude, frequency or phase of the carrier signal:

Amplitude modulation

0: low amplitude
1: high amplitude

Frequency modulation

0: low frequency
1: high frequency

Phase shift modulation

0: no phase shift during a clock period
1: 180 degree phase shift during a clock period

You have encountered these modulation schemes in devices you have used. For example, amplitude modulation is used in AM radio, frequency modulation is used in FM radio, and modems use phase-shift modulation.

These modulation methods use a carrier signal at a given frequency and modify it to encode information. Spread spectrum modulation schemes use a range or band of frequencies rather than a single carrier frequency. The information is diffused throughout the band by hopping from one frequency to another or by sending parallel information on several subfrequencies.

At first that might seem wasteful, since a frequency band is used rather than a single carrier frequency. However, a guard band must also be reserved around a carrier frequency to avoid interference. The carrier frequency is in the center of the band.

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