Modulation

Modulation describes a range of techniques for encoding information on a carrier signal, typically a sine-wave signal.

Modulation techniques include:

Amplitude modulation (AM)
Phase modulation (PM)
Frequency modulation (FM)
Single sideband modulation (SSB)
Vestigial sideband modulation (VSB, or VSB-AM)
Quadrature amplitude modulation (QAM)
Orthogonal frequency division modulation (OFDM), also known as 'Discrete multitone modulation' (DMT)

When OFDM is used in conjunction with channel coding techniques, it is described as 'Coded orthogonal frequency division modulation' (COFDM).

Pulse modulation techniques include:

Where the signal is a simple low speed on-off indication, as in morse code or radioteletype (RTTY) transmission, modulation is often known as 'keying' as in the terms

Frequency shift keying (FSK) see Frequency modulation
Phase shift keying (FSK) see Phase modulation

RTTY can also be regarded as a simple kind of pulse code modulation.

Where Morse code is used to turn the carrier wave on and off, the term used is not 'amplitude keying', but 'continuous wave' (CW) operation.

Modulation is frequently used in conjunction with various channel access methods, such as

Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Frequency hopping spread spectrum (FHSS)
Space division multiple access (SDMA)
Carrier sensing multiple access with collision detection (CSMA/CD)

Where these methods are used for dividing forward and reverse communication channels, they are known as duplexing methods, such as:

Time division duplex (TDD)
Frequency division duplex (FDD)