DSP (Digital Signal Processing)

Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. There are several reasons to modulate a signal before transmission in a medium. These include the ability of different users sharing a medium (multiple access), and making the signal properties physically compatible with the propagation medium. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).

In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.

In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.

Modulation is generally performed to overcome signal transmission issues such as to allow:
· Easy (low loss, low dispersion) propagation as electromagnetic waves
· Multiplexing - the transmission of multiple data signals in one frequency band, on different carrier frequencies.
· Smaller, more directional antennas

As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.

The principal classes of modulation are:
· Phase-shift keying (PSK)
· Frequency-shift keying (FSK) and audio frequency-shift keying (AFSK)
· Minimum-shift keying (MSK)
· Gaussian minimum-shift keying (GMSK)
· Very minimum-shift keying (VMSK)
· Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
· Quadrature amplitude modulation (QAM) a combination of PSK and ASK
· Continuous phase modulation (CPM)
· Trellis coded modulation (TCM) also known as trellis modulation
· Polar modulation like QAM a combination of PSK and ASK, but using a different circuit architecture

http://en.wikipedia.org/wiki/Modulation

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