Communications Systems Glossary

Quadrature amplitude modulation (QAM).  Quadrature amplitude modulation (QAM) is a modulation technique in which both the phase and amplitude of a carrier is varied by the symbols of the message.

Quadrature phase-shift keying (QPSK).  Phase shift keying (PSK) between four phase states, normally 90 degrees apart.

Quantisation.  The process of converting a continuous variable (an analogue signal) into discrete samples (a digital number). Only a limited number of amplitudes are allowed in the discrete samples—for example, if three bits are used, only eight amplitudes can be represented. The difference between the sample value and the original waveform value is called quantisation error, or quantisation noise.

Quantum.  Quantum is derived from the Latin word quantus meaning ‘how many'. The difference between the discrete set of amplitudes allowed during the quantisation of an analogue waveform.

Quaternary.  The term quaternary is used to describe a system having four states.

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