Quadrature modulation is the general technique of modulating two carriers.
Constellation diagrams are used to examine the modulation in the 2-D signal space.
Sending a signal by amplitude modulation consists of sending the function
where is the signal to encode and is the carrier wave, is the carrier frequency – one is changing the amplitude of a carrier wave to encode the signal, hence amplitude modulation.
In general one could also change the phase of the carrier to encode the signal, as in:
this 90° (the angle of a rectangle, or a 1/4 turn) is why it is called "quadrature" modulation, and the symbols and indicate the "in-phase" signal and "quadrature" signal.
In terms of Euler's formula, amplitude modulation encodes a 1-dimensional real signal, while quadrature modulation encodes a 2-dimensional complex signal. This viewpoint, that a wave of a given frequency can encode 2 dimensions of data, is elaborated in Fourier analysis, and is the principle that quadrature modulation exploits.
The added channel capacity is not costless, however.
An amplitude-modulated signal is self-clocking – it has zero-crossings at a regular frequency as a clock pulse. A quadrature-modulated signal, by contrast, has no such pulse, and thus sender and receiver must share a clock or otherwise send a clock signal – if the clocks drift by phase φ, which corresponds to rotation by φ in the plane, then the I and Q signal bleed into each other, yielding crosstalk. In this context, the clock signal is called a "phase reference" – in NTSC, which uses quadrature amplitude modulation, this is conveyed by the color burst, a synchronization signal.
By contrast, in polar modulation, clock drift simply degrades the phase-modulated signal.
Quadrature modulates two signals by changing the in-phase and quadrature phase components, corresponding to Cartesian coordinates. By contrast, one can instead consider this to be changing the amplitude and phase of a wave, which corresponds to polar coordinates. The corresponding modulation is called polar modulation, and was developed earlier, in the 1874 quadruplex telegraph by Thomas Edison.
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