.. _cpfskmodem:

Continuous-Phase Frequency-Shift Keying (CPFSK) Modem
=====================================================

.. author   jgaeddert Joseph D. Gaeddert <joseph@liquidsdr.org>
.. date     January 1, 1975
.. location Boston, MA
.. header   /doc/cpfskmodem/banner.png
.. keywords continuous phase, frequency shift keying, FSK, modem, Gauss, minimum shift keying, CPFSK, FSK, GMSK, MSK

Shown in :numref:`fig-cpfsk-psd` below is a compaarison of the transmitted spectrum for
various combinations of parameters.

.. qplot:: series modem/cpfskmodem_psd.c
    :kwargs: {"series" : [["f","square", {}],
                          ["f","full",   {}],
                          ["f","partial",{}],
                          ["f","gmsk",   {}]
                         ],
              "xlabel" : "Normalized Frequency [f/Fs]",
              "ylabel" : "Power Spectral Density [dB]",
              "xrange" : [-0.5, 0.5],
              "yrange" : [-70, 10],
              "legend" : ["square", "rcos (full)", "rcos (partial)", "GMSK, BT=0.35"]
             }
    :width: 75%
    :name: fig-cpfsk-psd
    :caption: Power spectral density of different CPFSK modem types with :math:`k=8` samples
              per symbol

Interface
---------

Here is a basic example of the `cpfksmod` and `cpfskdem` objects.

.. code-block:: c

    // demonstrate interface to cpfskmod and cpfskdem objects
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <liquid/liquid.h>

    int main(int argc, char*argv[])
    {
        // options
        unsigned int bps         = 1;       // number of bits/symbol
        float        h           = 0.5f;    // modulation index (h=1/2 for MSK)
        unsigned int k           = 4;       // filter samples/symbol
        unsigned int m           = 3;       // filter delay (symbols)
        float        beta        = 0.35f;   // filter bandwidth-time product
        int          filter_type = LIQUID_CPFSK_SQUARE; // filter type

        // create modem objects
        cpfskmod mod = cpfskmod_create(bps, h, k, m, beta, filter_type);
        cpfskdem dem = cpfskdem_create(bps, h, k, m, beta, filter_type);

        // arrays
        unsigned int i;
        unsigned int M = 1 << bps;  // constellation size
        float complex buf[k];       // sample buffer
        for (i=0; i<20; i++) {
            // generate random message signal
            unsigned int sym_in = rand() % M;

            // modulate signal
            cpfskmod_modulate(mod, sym_in, buf);

            // demodulate signal
            unsigned int sym_out = cpfskdem_demodulate(dem, buf);
        }

        // destroy modem objects
        cpfskmod_destroy(mod);
        cpfskdem_destroy(dem);
        return 0;
    }