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Continuous phase modulation
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{{Modulation techniques}} '''Continuous phase modulation''' ('''CPM''') is a method for [[frequency modulation|modulation]] of data commonly used in [[wireless modem]]s. In contrast to other coherent digital [[phase modulation]] techniques where the [[carrier wave|carrier]] phase abruptly resets to zero at the start of every symbol (e.g. M-[[phase-shift keying|PSK]]), with CPM the carrier phase is modulated in a continuous manner. For instance, with [[QPSK]] the carrier instantaneously jumps from a sine to a cosine (i.e. a 90 degree [[phase shift]]) whenever one of the two message [[bit]]s of the current symbol differs from the two message bits of the previous symbol. This discontinuity requires a relatively large percentage of the power to occur outside of the intended band (e.g., high fractional [[out-of-band power]]), leading to poor [[spectral efficiency]]. Furthermore, CPM is typically implemented as a constant-envelope [[waveform]], i.e., the transmitted carrier power is constant. 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.
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