Member Avatar for shazzy99

hi,

One of my freind is asking for bpsk modulator and demodulator. It's out of my domain knowledge and i'm not sure what is it about.

Can you refer to some C language example codes of the above.

will appreciate your quick reply
thx

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Short, practical note that builds on the thread: is right that a production-grade BPSK modem (carrier recovery, timing recovery, channel equalization, framing, error control) gets complex. For learning or a lab demo there is a very small, workable C example that shows the core idea: map bits to +1/−1, multiply a carrier (phase 0 or π), and at the receiver multiply by the same carrier and integrate over the symbol interval (coherent demodulation). The snippet below is intentionally minimal — it assumes a perfectly synchronized local carrier and ideal channel so it’s suitable for experimentation and teaching, not RF hardware use.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(void){
    const double PI=3.141592653589793;
    const int Fs=8000, fc=1000, Ns=80, Nbits=32;
    double phase_inc=2.0*PI*fc/Fs, phase=0.0;
    double *sig = malloc(sizeof(double)*Ns*Nbits);
    int bits[Nbits], dec[Nbits];
    for(int i=0;i<Nbits;i++) bits[i]=rand()&1;
    for(int i=0;i<Nbits;i++){
        double sym = bits[i]?1.0:-1.0;
        for(int n=0;n<Ns;n++){
            sig[i*Ns+n]=sym*cos(phase);
            phase+=phase_inc; if(phase>2*PI) phase-=2*PI;
        }
    }
    double local=0.0;
    for(int i=0;i<Nbits;i++){
        double acc=0;
        for(int n=0;n<Ns;n++){ acc += sig[i*Ns+n]*cos(local); local+=phase_inc; if(local>2*PI)local-=2*PI; }
        dec[i] = acc>=0;
    }
    for(int i=0;i<8;i++) printf("%d -> %d\n", bits[i], dec[i]);
    free(sig);
    return 0;
}

Notes and quick troubleshooting:

  • This demo needs a coherent carrier reference. If the local carrier phase is unknown or can flip by π, use differential BPSK (DBPSK) or implement a Costas loop for carrier recovery.
  • Pick Fs and fc so Fs/fc is convenient (integer cycles/sample) to avoid slow phase drift in tests.
  • Improve performance with a matched filter (windowed carrier), low-pass filtering, and adding AWGN tests to measure BER.
  • For real I/O, write the signal to a WAV file or feed through ADC/DAC and add timing recovery and error-control layers before considering it a usable modem.

This provides a compact starting point for while acknowledging ’s point about real-system complexity.

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Member Avatar for shazzy99

did you try this?

Yes, but i couldn't find much as i really have no idea what this is about. I just need a simple c code; if there is any freely available for the above.

thx

Member Avatar for jephthah

hmm... you're serious aren't you?

okay, no: there is no "simple c code" for developing a binary phase shift key modem.

if there were, then people wouldn't get paid salary to develop solutions to relatively complex engineering projects.

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