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Therefore, CPM is attractive because the phase continuity yields high spectral efficiency, and the [[constant envelope]] yields excellent power efficiency. The primary drawback is the high implementation complexity required for an optimal receiver.
== Phase
Each symbol is modulated by gradually changing the phase of the carrier from the starting value to the final value, over the symbol duration. The modulation and [[demodulation]] of CPM is complicated by the fact that the initial phase of each symbol is determined by the cumulative total phase of all previous transmitted symbols, which is known as the ''phase memory''.
Therefore, the optimal receiver cannot make decisions on any isolated symbol without taking the entire sequence of transmitted symbols into account. This requires a [[Maximum Likelihood]] Sequence Estimator (MLSE), which is efficiently implemented using the [[Viterbi algorithm]].
== Phase
[[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 ''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 Cosine
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
Partial-response signaling, such as [[duo-binary signaling]], is a form of intentional ISI where
a certain number of adjacent symbols interfere with each symbol in a controlled manner.
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