Quadrature amplitude modulation
Quadrature amplitude modulation (QAM) is a digital modulation technique in which both the amplitude and phase of a carrier are modulated to convey digital information. The name comes from the modulation technique, which takes two signals with the same frequency that are 90 degrees out of phase (i.e., in quadrature) and modulating their amplitude to convey information. Each signal carries part of the bit stream. The two signals are combined for transmission purposes. At the receiver they are separated, the bits are read and then recombined to create the original bit stream.
If each of the base signals has four defined amplitude levels, each signal can carry four bits per amplitude shift. Combining the two signals produces 16 unique four-bit combinations, an encoding scheme known as 16-QAM. As with phase shift keying a constellation diagram , like the one to the right, can be used to depict the encoding scheme. The constellation is labeled a bit differently, however, with the horizontal axis representing the In-phase Plane (I-Plane), and the vertical access representing the Quadrature-Phase Plane (Q-Plane), the names given to the two base signals.
A quick look at the constellation and a comparison with the phase shift keying (PSK) constellation should be enough to underscore a major advantage of QAM: it spreads the constellation points more evenly across the graph. PSK places all of the constellation points on a circle around the midpoint. The more points there are, the closer they are to one another. The closer they are to one another, the more similar they look at the receiver and the harder it is for the receiver to distinguish one from the other. But the graph has enormous untapped real estate, which QAM leverages.
A 16-bit PSK scheme would be ineffective due to the increased error rate at the receiver. 16-QAM is, however, quite reasonable. Indeed, it is the slower of the QAM options. Theoretically there are also 64-QAM, 128-QAM, and 256-QAM codes. As with PSK, however, each of these places the constellation points closer to one another. In other words, as you increase the constellation, you trade bit rate for bit reliability. If the underlying transmission system is well engineered and highly noise immune, 256-QAM is achievable.
The primary use of QAM signalling in the the HFC cable plant deployed by the cable companies.
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