Continuous phase modulation: Difference between revisions

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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 switches 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.

=== Theory ===

If a finitely- valued digital signal to be transmitted (the message) is ''m''(''t''), then the corresponding CPFSK signal is

:<math>s(t) = A_c \cos\left(2 \pi f_c t + D_f \int_{-\infty}^{t} m(\alpha) d \alpha\right)\,</math>

where ''A_c'' represents the amplitude of the CPFSK signal, ''f_c'' is the base [[Carrier wave|carrier frequency]], and ''D_f'' is a parameter that controls the [[frequency deviation]] of the modulated signal. The [[integral]] located inside of the [[cosine]]'s argument is what gives the CPFSK signal its continuous phase; an integral over any finitely- valued function (which ''m''(''t'') is assumed to be) will not contain any discontinuities. If the message signal is assumed to be [[causal]], then the limits on the integral change to a lower bound of zero and a higher bound of ''t''.

Note that this does not mean that ''m''(''t'') must be continuous; in fact, most ideal digital data waveforms contain discontinuities. However, even a discontinuous message signal will generate a proper CPFSK signal.