Continuous phase modulation: Difference between revisions

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== Phase trajectory ==

[[Minimum-shift keying]] (MSK) is another name for CPM with an excess bandwidth of 1/2 and a linear ''phase trajectory''. Although this linear phase trajectory is continuous, it is not ''[[smoothness|smooth]]'' since the derivative of the phase is not continuous. The spectral efficiency of CPM can be further improved by using a smooth phase trajectory. This is typically accomplished by filtering the phase trajectory prior to modulation, commonly using a Raised[[raised Cosinecosine]] or a [[Gaussian filter]]. The raised cosine filter has zero crossings offset by exactly one symbol time, and so it can yield a ''full-response'' CPM waveform that prevents [[intersymbol interference]] (ISI).

or a [[Gaussian filter]]. The raised cosine filter has zero crossings offset by exactly one symbol time, and so it can yield a ''full-response'' CPM waveform that prevents [[Intersymbol interference|Intersymbol Interference (ISI)]].

== Partial response CPM ==

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== Continuous-phase frequency-shift keying ==

'''Continuous-phase frequency-shift keying''' (CPFSK) is a commonly used variation of [[frequency-shift keying]] (FSK), which is itself a special case of analog [[frequency modulation]]. FSK is a method of modulating [[digital data]] onto a [[sinusoidal]] [[carrier wave]], encoding the information present in the data to variations in the carrier's [[instantaneous phase#Instantaneous frequency|instantaneous frequency]] between one of two frequencies (referred to as the [[space frequency]] and [[mark frequency]]). In general, a standard FSK signal does not have [[continuous function|continuous]] phase, as the modulated waveform switchescuts instantaneously between two sinusoids with different frequencies.

As the name suggests, the phase of a CPFSK is in fact continuous; this attribute is desirable for signals that are to be transmitted over a [[bandlimited]] channel, as discontinuities in a signal introduce [[wideband]] frequency components. In addition, some classes of amplifiers exhibit nonlinear behavior when driven with nearly discontinuous signals; this could have undesired effects on the shape of the transmitted signal.

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==See also==

* [[Minimum-shift keying|MSK]] (MSK)

== References ==