gnss-sdr/src/algorithms/libs/gps_sdr_signal_processing.cc

/*!
 * \file gps_sdr_signal_processing.cc
 * \brief This class implements various functions for GPS L1 CA signals
 * \author Javier Arribas, 2011. jarribas(at)cttc.es
 *
 * Detailed description of the file here if needed.
 *
 * -------------------------------------------------------------------------
 *
 * Copyright (C) 2010-2015  (see AUTHORS file for a list of contributors)
 *
 * GNSS-SDR is a software defined Global Navigation
 *          Satellite Systems receiver
 *
 * This file is part of GNSS-SDR.
 *
 * GNSS-SDR is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * GNSS-SDR is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNSS-SDR. If not, see <http://www.gnu.org/licenses/>.
 *
 * -------------------------------------------------------------------------
 */

#include "gps_sdr_signal_processing.h"

auto auxCeil = [](float x){ return static_cast<int>(static_cast<long>((x)+1)); };

void gps_l1_ca_code_gen_complex(std::complex<float>* _dest, signed int _prn, unsigned int _chip_shift)
{
    const unsigned int _code_length = 1023;
    bool G1[_code_length];
    bool G2[_code_length];
    bool G1_register[10], G2_register[10];
    bool feedback1, feedback2;
    bool aux;
    unsigned int lcv, lcv2;
    unsigned int delay;
    signed int prn_idx;

    /* G2 Delays as defined in GPS-ISD-200D */
    const signed int delays[51] = {5 /*PRN1*/, 6, 7, 8, 17, 18, 139, 140, 141, 251, 252, 254 ,255, 256, 257, 258, 469, 470, 471, 472,
            473, 474, 509, 512, 513, 514, 515, 516, 859, 860, 861, 862 /*PRN32*/,
            145 /*PRN120*/, 175, 52, 21, 237, 235, 886, 657, 634, 762,
            355, 1012, 176, 603, 130, 359, 595, 68, 386 /*PRN138*/};

    // compute delay array index for given PRN number
    if(120 <= _prn && _prn <= 138)
        {
            prn_idx = _prn - 88;    // SBAS PRNs are at array indices 31 to 50 (offset: -120+33-1 =-88)
        }
    else
        {
            prn_idx = _prn - 1;
        }

    /* A simple error check */
    if((prn_idx < 0) || (prn_idx > 51))
        return;

    for(lcv = 0; lcv < 10; lcv++)
        {
            G1_register[lcv] = 1;
            G2_register[lcv] = 1;
        }

    /* Generate G1 & G2 Register */
    for(lcv = 0; lcv < _code_length; lcv++)
        {
            G1[lcv] = G1_register[0];
            G2[lcv] = G2_register[0];

            feedback1 = G1_register[7]^G1_register[0];
            feedback2 = (G2_register[8] + G2_register[7] + G2_register[4] + G2_register[2] + G2_register[1] + G2_register[0]) & 0x1;

            for(lcv2 = 0; lcv2 < 9; lcv2++)
                {
                    G1_register[lcv2] = G1_register[lcv2 + 1];
                    G2_register[lcv2] = G2_register[lcv2 + 1];
                }

            G1_register[9] = feedback1;
            G2_register[9] = feedback2;
        }

    /* Set the delay */
    delay = _code_length - delays[prn_idx];
    delay += _chip_shift;
    delay %= _code_length;

    /* Generate PRN from G1 and G2 Registers */
    for(lcv = 0; lcv < _code_length; lcv++)
        {
            aux = G1[(lcv + _chip_shift) % _code_length]^G2[delay];
            if(aux == true)
                {
                    _dest[lcv] = std::complex<float>(1, 0);
                }
            else
                {
                    _dest[lcv] = std::complex<float>(-1, 0);
                }
            delay++;
            delay %= _code_length;
        }
}



/*
 *  Generates complex GPS L1 C/A code for the desired SV ID and sampled to specific sampling frequency
 */
void gps_l1_ca_code_gen_complex_sampled(std::complex<float>* _dest, unsigned int _prn, signed int _fs, unsigned int _chip_shift)
{
    // This function is based on the GNU software GPS for MATLAB in the Kay Borre book
    std::complex<float> _code[1023];
    signed int _samplesPerCode, _codeValueIndex;
    float _ts;
    float _tc;
    float aux;
    const signed int _codeFreqBasis = 1023000; //Hz
    const signed int _codeLength = 1023;

    //--- Find number of samples per spreading code ----------------------------
    _samplesPerCode = static_cast<signed int>(static_cast<double>(_fs) / static_cast<double>(_codeFreqBasis / _codeLength));

    //--- Find time constants --------------------------------------------------
    _ts = 1.0 / static_cast<float>(_fs);   // Sampling period in sec
    _tc = 1.0 / static_cast<float>(_codeFreqBasis);  // C/A chip period in sec
    gps_l1_ca_code_gen_complex(_code, _prn, _chip_shift); //generate C/A code 1 sample per chip

    for (signed int i = 0; i < _samplesPerCode; i++)
        {
            //=== Digitizing =======================================================

            //--- Make index array to read C/A code values -------------------------
            // The length of the index array depends on the sampling frequency -
            // number of samples per millisecond (because one C/A code period is one
            // millisecond).

            // _codeValueIndex = ceil((_ts * ((float)i + 1)) / _tc) - 1;
            aux = (_ts * (i + 1)) / _tc;   
            _codeValueIndex = auxCeil( aux ) - 1;

            //--- Make the digitized version of the C/A code -----------------------
            // The "upsampled" code is made by selecting values form the CA code
            // chip array (caCode) for the time instances of each sample.
            if (i == _samplesPerCode - 1)
                {
                    //--- Correct the last index (due to number rounding issues) -----------
                    _dest[i] = _code[_codeLength - 1];

                }
            else
                {
                    _dest[i] = _code[_codeValueIndex]; //repeat the chip -> upsample
                }
        }
}