Continuous-Phase Frequency-Shift Keying Modem

Keywords: continuous phase frequency shift keying FSK modem Gauss minimum shift keying CPFSK FSK GMSK MSK

Theory of Operation

Shown in the figure below is a compaarison of the transmitted spectrum for various combinations of parameters.


Figure [fig-cpfskmodem-psd]. Power spectral density of different CPFSK modem types.


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

// 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
    return 0;