Continuous phase modulation

<p>Continuous phase modulation (CPM) is a method for <a href="/facts/Frequency_modulation/qIExDdtv">modulation</a> of data commonly used in <a href="/facts/Wireless_modem/91kNTutm">wireless modems</a>. In contrast to other coherent digital <a href="/facts/Phase_modulation/ws1kz7Y9">phase modulation</a> techniques where the <a href="/facts/Carrier_wave/ae98pBlY">carrier</a> phase
abruptly resets to zero at the start of every symbol (e.g. M-<a href="/facts/Phase-shift_keying/TFnpYe64">PSK</a>), with CPM the carrier phase is modulated in a continuous manner.  For instance, with <a href="/facts/QPSK/TFnpYe64">QPSK</a> the carrier instantaneously jumps from a sine to a cosine (i.e. a 90 degree <a href="/facts/Phase_shift/cpGtuVll">phase shift</a>) whenever one of the two message <a href="/facts/Bit/S972NSaD">bits</a> 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 <a href="/facts/Spectral_efficiency/GaePAvv9">spectral efficiency</a>. Furthermore, CPM is typically implemented as a constant-envelope <a href="/facts/Waveform/yAskMREw">waveform</a>, i.e., the transmitted carrier power is constant.
Therefore, CPM is attractive because the phase continuity yields high spectral efficiency, and the <a href="/facts/Constant_envelope/jRz6sxl0">constant envelope</a> yields excellent power efficiency. The primary drawback is the high implementation complexity required for an optimal receiver.  
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Continuous phase modulation open-in-new

Continuous phase modulation