The qpacketmodem object conveniently combines linear modulation with interleaving , two levels of forward error-correction coding, and a data integrity check . The object combines the packetizer object with symbol packing for modulation. Both hard-decision and soft-decision decoding are supported, assuming the forward error-correction codec supports it.

Example Interface

Listed below is a simple example demonstrating the basic interface to the qpacketmodem encoder/decoder object:

#include <liquid/liquid.h>

int main() {
    // options
    modulation_scheme ms     = LIQUID_MODEM_QPSK;
    crc_scheme   check       = LIQUID_CRC_32;
    fec_scheme   fec0        = LIQUID_FEC_GOLAY2412; // inner
    fec_scheme   fec1        = LIQUID_FEC_NONE;      // outer
    unsigned int payload_len = 400; // number of bytes in message

    // create and configure packet encoder/decoder objects
    qpacketmodem enc = qpacketmodem_create();   // encoder
    qpacketmodem dec = qpacketmodem_create();   // decoder

    // configure
    qpacketmodem_configure(enc, payload_len, check, fec0, fec1, ms);
    qpacketmodem_configure(dec, payload_len, check, fec0, fec1, ms);

    // get frame length
    unsigned int frame_len = qpacketmodem_get_frame_len(enc);

    // initialize payload
    unsigned char payload_org[payload_len]; // original message
    float complex frame_enc  [  frame_len]; // encoded symbols
    float complex frame_rec  [  frame_len]; // received symbols
    unsigned char payload_dec[payload_len]; // decoded message

    // repeat as necessary
        // ... initialize message ...

        // encode message
        qpacketmodem_encode(enc, payload_org, frame_enc);

        // ... push through channel ...

        // decode message and perform integrity check
        int crc_pass = qpacketmodem_decode(dec, frame_rec, payload_dec);

    // clean up objects

    return 0;

Performance in AWGN

A useful metric for assessing the performance of a particular waveform is to analyze its ability to decode a packet in variouis channel environments. The figure below shows the performance of the qpacketmodem encoder/decoder in additive white Gauss noise channels:


Performance of the qpacketmodem encoder/decoder in the presence of additive white Gauss noise with QPSK modulation and the Golay (24,12) codec .