Editing Signal modulation (section)

===Modulator and detector principles of operation===
PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a [[complex-valued]] signal ''I''+''jQ'' (where ''j'' is the [[imaginary unit]]). The resulting so called [[equivalent lowpass signal]] or [[equivalent baseband signal]] is a complex-valued representation of the [[real-valued]] modulated physical signal (the so-called [[passband signal]] or [[RF signal]]).

These are the general steps used by the [[modulator]] to transmit data:
# Group the incoming data bits into codewords, one for each symbol that will be transmitted.
# Map the codewords to attributes, for example, amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values.
# Adapt [[pulse shaping]] or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing.
# Perform digital to analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using [[digital signal processing]], DSP).
# Generate a high-frequency sine carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine waveform with the I and Q signals, resulting in the equivalent low pass signal being frequency shifted to the modulated [[passband signal]] or RF signal. Sometimes this is achieved using DSP technology, for example [[direct digital synthesis]] using a [[waveform table]], instead of analog signal processing. In that case, the above DAC step should be done after this step.
# Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum.

At the receiver side, the [[demodulator]] typically performs:
# Bandpass filtering.
# [[Automatic gain control]], AGC (to compensate for [[attenuation]], for example [[fading]]).
# Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sine wave and cosine wave frequency (see the [[superheterodyne receiver]] principle).
# Sampling and analog-to-digital conversion (ADC) (sometimes before or instead of the above point, for example by means of [[undersampling]]).
# Equalization filtering, for example, a [[matched filter]], compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid [[intersymbol interference]] and symbol distortion.
# Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal.
# Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.
# Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).
# Parallel-to-serial conversion of the codewords into a bit stream.
# Pass the resultant bit stream on for further processing such as removal of any error-correcting codes.

As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair has prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.

Asynchronous methods do not require a receiver reference clock signal that is [[phase synchronisation|phase synchronized]] with the sender [[carrier signal]]. In this case, modulation symbols (rather than bits, characters, or data packets) are [[asynchronous communication|asynchronously]] transferred. The opposite is [[Bit-synchronous operation|synchronous modulation]].