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The multiple separate information (modulation) signals that are sent over an FDM system, such as the video signals of the television channels that are sent over a cable TV system, are called baseband signals.
At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency that serves to "carry" information. The carrier is much higher in frequency than the baseband signal.
The carrier signal and the baseband signal are combined in a modulator circuit. The modulator alters some aspect of the carrier signal, such as its amplitude, frequency, or phase, with the baseband signal, "piggybacking" the data onto the carrier.
Unlike FDM, where the total bandwidth available in a communication medium available, frequency division duplex (FDD) is a technique where separate frequency bands are used at the transmitter and receiver side.
Because the FDD technique uses different frequency bands for send and receive operations, the sending and receiving data signals don't interfere with each other.
This makes FDD a better choice than Time Division Duplex (TDD) for symmetric traffic such as voice applications in broadband wireless networks.
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