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Quadrature amplitude modulation (QAM) sends information by combining two amplitude-modulated carrier components that are 90 degrees out of phase. A receiver can distinguish the components, called in-phase (I) and quadrature (Q), and use their amplitudes to identify each transmitted symbol.
How QAM represents information
Imagine a graph with I on the horizontal axis and Q on the vertical axis. Each permitted pair of I and Q amplitudes marks a point on the graph; that point represents one symbol. The set of permitted points is called a constellation. The same signal can also be described by its overall amplitude and phase.
“Quadrature” names the 90-degree phase relationship between the two carrier components. Because the components are orthogonal, a receiver can coherently separate them. MathWorks describes the I/Q basis and how constellation shape varies with modulation order in its amplitude modulation documentation; Analog Devices explains the constellation in its digital pulse-shaping filter overview.
What changes between 16-QAM and 64-QAM?
The number in a power-of-two QAM order gives the number of possible constellation points. The bits represented by each symbol are log2 of that number: 16-QAM represents 4 bits per symbol, while 64-QAM represents 6. These counts are stated in the ITU-R’s 2010 broadcasting report.
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| Modulation | Possible points | Bits per symbol | Illustrative SNR for bit-error target 10−7 |
|---|---|---|---|
| 16-QAM | 16 | 4 | 21.5 dB, in Analog Devices’ example |
| 64-QAM | 64 | 6 | 27.7 dB, in Analog Devices’ example |
The SNR figures are examples from Analog Devices, not universal thresholds; required signal quality depends on the system and its assumptions. The tradeoff is geometric: at comparable transmitted power, adding points makes them closer together. A receiver therefore needs a cleaner signal to distinguish neighboring symbols at a given error rate. Higher-order QAM can represent more bits per symbol, but that alone does not guarantee higher delivered data rates, which also depend on bandwidth, symbol rate, coding, channel conditions, and system design. See Analog Devices’ discussion of QAM orders and illustrative SNR requirements.
How a receiver decides which symbol was sent
A receiver estimates the incoming signal’s I and Q values and locates the corresponding point in the constellation. Noise or distortion can shift that estimate; if it crosses a decision boundary, the receiver may choose a neighboring point and decode the wrong symbol. Gray coding is one possible mapping in which neighboring points have similar bit patterns, so a nearest-neighbor symbol error may change fewer bits. Not every QAM constellation is square, and the shape and bit mapping depend on the modulation order and system choices.
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Imbalance between the I and Q paths can also affect the signal: Analog Devices notes that amplitude and phase mismatch can appear as an effective phase error. QAM describes a modulation method, not one mandatory transmitter or receiver design. The ITU-R report, for example, discusses optical QAM using an IQ modulator driven by multilevel signals, a context-specific implementation rather than a universal requirement.
Where QAM is used
QAM appears in data modems and in PAL/NTSC color television transmission, among other applications identified by Analog Devices. The particular implementation varies by system; the name refers to encoding information through the combined I/Q components, not to a single piece of hardware or one fixed constellation.
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