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Modulation

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

Carrier signals are usually high frequency electromagnetic waves.

Analog modulation techniques

Angular modulation
- Phase modulation (PM)
- Frequency modulation (FM)
Amplitude modulation (AM)
- Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)
- Vestigial-sideband modulation (VSB, or VSB-AM)
Sigma-delta modulation (∑Δ)

Digital modulation techniques

Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data.

In the case of PSK, a finite number of phases are used.
In the case of FSK, a finite number of frequencies are used.
In the case of ASK, a finite number of amplitudes are used. This is very similar to pulse code modulation

Each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase.

These are the general steps used by the modulator to transmit data:

Accept incoming digital data;
Group the data into symbols;
Use these symbols to set or change the phase, frequency or amplitude of the reference signal appropriately;
Pass the modulated signal on for further processing, such as filtering and channel-coding, before transmission.

At the receiver, the demodulator

Is passed the de-filtered and de-channel-coded signal;
Determines its phase, frequency or amplitude;
Maps the phase, frequency or amplitude to its corresponding symbol;
Translates the symbol into its individual bits;
Passes the resultant bit stream on for further processing such as removal of any error-correcting codes.

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

MSK and GMSK are particular cases of continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as continuous phase-frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signalling).

Often incorrectly referred to as a modulation scheme, orthogonal frequency division multiplexing (OFDM) usually takes advantage of one of the digital techniques. It is also known as discrete multitone (DMT). When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division multiplexing (COFDM). OFDM is strictly a channel access method and not a modulation scheme.

Pulse modulation

These are hybrid digital and analogue techniques.

Pulse-code modulation (PCM)
Pulse-width modulation (PWM)
Pulse-amplitude modulation (PAM)
Pulse-position modulation (PPM)
Pulse-density modulation (PDM)

Miscellaneous techniques

The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
Adaptive modulation
Wavelet modulation

Last modified at : Thursday, December 11st 2008 14:03:15.

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