Continuous phase modulation: Difference between revisions

Revision as of 11:03, 15 August 2013

Revision as of 13:21, 6 March 2014

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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.