Quadrature amplitude modulation

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QAM redirects here; for other uses of that abbreviation, see QAM (disambiguation).

Quadrature amplitude modulation (QAM) is the encoding of information into a carrier wave by variation of the amplitude of both the carrier wave and a 'quadrature' carrier that is 90° out of phase with the main carrier in accordance with two input signals.

Alternately, this can be regarded (using complex number notation) as simple amplitude modulation of a complex-valued carrier wave by a single complex-valued signal.

What this actually means is that the amplitude and the phase of the carrier wave are simultaneously changed according to the information you want to transmit.

Phase modulation can also be regarded as a special case of quadrature amplitude modulation, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation, as this can be regarded as a special case of phase modulation.

QAM is used in NTSC and PAL television systems, where the in-phase and 90° components carry the components of chroma (colour) information. "Compatible QAM" or C-QUAM is used in AM stereo radio to carry the stereo difference information.

It is also used extensively in modems, and other forms of digital communication over analogue channels. In digital applications, the modulating signal is generally quantised in both its in-phase and 90° components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation.

This constellation, and therefore the number of bits which can be transmitted at once, can be increased for higher bitrates and faster throughput, or decreased for more reliable transmission with fewer bit errors. The number of "dots" in the constellation is given as a number before the QAM, and is always an integer power of two -- from 21 (2-QAM) to 212 (4096-QAM).

64-QAM and 256-QAM are often used in digital cable television and cable modem applications. In the US, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable, as standardized by the SCTE in the standard ANSI/SCTE 07 2000 (http://www.scte.org/documents/pdf/ANSISCTE072000DVS031.pdf). Note that many marketing people will refer to these as QAM-64 and QAM-256.

Bit-mapping simplified overview

QAM bits per symbol symbol values
4 2 0-3
8 3 0-7
16 4 0-15
32 5 0-31
64 6 0-63
128 7 0-127
256 8 0-255

4-QAM will map every 2 data bits (0 to 3) onto 1 of 4 symbols (one symbol per constellation quadrant).

8-QAM will map every 3 data bits (0 to 7) onto 1 of 8 symbols (two symbols per constellation quadrant).

16-QAM will map every 4 data bits (0 to 15) onto 1 of 16 symbols (three symbols per constellation quadrant).

So with every QAM generation there are diminishing returns (doubling the symbols gains one bit).

4-QAM signal constellation (2 bits per symbol)

0 1
3 2

16-QAM signal constellation (4 bits per symbol)

00    01

02    03
08    09

10    11
04    05

06    07
12    13

14    15

See also

hu:Kvadratúra amplitúdómoduláció


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