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Quadrature amplitude modulation (QAM) sends information by combining two amplitude-modulated carrier components whose phases are 90 degrees apart. In digital QAM, those components are called in-phase (I) and quadrature (Q); their amplitudes together specify each transmitted symbol.
What do “quadrature,” I and Q mean?
“Quadrature” describes the 90-degree phase difference between the two carrier components. That separation makes the components orthogonal, allowing a receiver to distinguish them when it coherently detects the signal. The I and Q amplitudes can be viewed as coordinates for a symbol; together they also determine the resultant signal’s amplitude and phase. Analog Devices’ QAM glossary and MathWorks’ modulation explanation describe this arrangement.
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How does a QAM constellation diagram work?
A constellation diagram plots I horizontally and Q vertically. Each allowed coordinate pair is one possible symbol, and the full set of allowed points is the constellation. A receiver estimates the received point and decides which allowed symbol it most likely represents. The constellation’s shape depends on the modulation order and the selected design; not every QAM constellation is square. Analog Devices’ pulse-shaping overview also illustrates QAM constellations.
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The number names the possible constellation points. For a power-of-two modulation order, the bits represented by each symbol equal log2 of the number of points:
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| Modulation | Possible symbols | Bits per symbol |
|---|---|---|
| 16-QAM | 16 | 4 |
| 64-QAM | 64 | 6 |
These counts are given in the ITU-R’s 2010 broadcasting report; they describe the modulation symbols, not a guaranteed user data rate. ITU-R Report BT.2140-2 discusses QAM in broadcasting contexts.
Why does higher-order QAM need a cleaner signal?
At comparable transmitted power, adding more constellation points packs them closer together. The receiver therefore has less room to distinguish a symbol from its neighbors when noise or distortion shifts the received point. Higher-order QAM can represent more bits per symbol, but maintaining a given error rate generally requires better signal quality.
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For a bit-error target of 10−7, an Analog Devices comparison lists illustrative SNR requirements of 21.5 dB for 16-QAM and 27.7 dB for 64-QAM. These are example values under the source’s stated conditions, not universal thresholds; actual requirements depend on system assumptions and implementation.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesSome systems use Gray coding, assigning similar bit patterns to nearby constellation points. In that mapping, a decision error to a nearest neighbor may change fewer bits than it otherwise would. The exact mapping is a design choice, not a property guaranteed by the QAM name.
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Where is QAM used?
QAM is used in data modems and has also been used for PAL and NTSC color television transmission, as described in Analog Devices’ glossary. The ITU-R report also describes optical QAM implementations, including an IQ modulator driven by multilevel signals; that is one context-specific implementation, not the only way to generate QAM.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What affects QAM performance in practice?
The ideal I/Q picture assumes the two paths behave as intended. Amplitude or phase mismatch between them can distort the transmitted or received constellation; Analog Devices notes that mismatch can appear as an effective phase error. Its digital-communications discussion covers this implementation issue.
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Bits per symbol alone do not determine delivered throughput. Symbol rate, bandwidth, coding, channel quality, and system design also matter. Thus, 64-QAM’s six bits per symbol do not by themselves establish that a system will deliver more real-world data than one using 16-QAM.
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