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gnss-sdr/src/algorithms/libs/glonass_l2_signal_processing.cc

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/*!
* \file glonass_l2_signal_processing.cc
* \brief This class implements various functions for GLONASS L2 CA signals
* \author Damian Miralles, 2018, dmiralles2009(at)gmail.com
*
* Detailed description of the file here if needed.
*
* -------------------------------------------------------------------------
*
* Copyright (C) 2010-2019 (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 <https://www.gnu.org/licenses/>.
*
* -------------------------------------------------------------------------
*/
#include "glonass_l2_signal_processing.h"
#include <array>
#include <bitset>
auto auxCeil = [](float x) { return static_cast<int32_t>(static_cast<int64_t>((x) + 1)); };
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void glonass_l2_ca_code_gen_complex(gsl::span<std::complex<float>> _dest, uint32_t _chip_shift)
{
const uint32_t _code_length = 511;
std::bitset<_code_length> G1{};
auto G1_register = std::bitset<9>{}.set(); // All true
bool feedback1;
bool aux;
uint32_t delay;
uint32_t lcv;
uint32_t lcv2;
/* Generate G1 Register */
for (lcv = 0; lcv < _code_length; lcv++)
{
G1[lcv] = G1_register[2];
feedback1 = G1_register[4] xor G1_register[0];
for (lcv2 = 0; lcv2 < 8; lcv2++)
{
G1_register[lcv2] = G1_register[lcv2 + 1];
}
G1_register[8] = feedback1;
}
/* Generate PRN from G1 Register */
for (lcv = 0; lcv < _code_length; lcv++)
{
aux = G1[lcv];
if (aux == true)
{
_dest[lcv] = std::complex<float>(1, 0);
}
else
{
_dest[lcv] = std::complex<float>(-1, 0);
}
}
/* Set the delay */
delay = _code_length;
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];
if (aux == true)
{
_dest[lcv] = std::complex<float>(1, 0);
}
else
{
_dest[lcv] = std::complex<float>(-1, 0);
}
delay++;
delay %= _code_length;
}
}
/*
* Generates complex GLONASS L2 C/A code for the desired SV ID and sampled to specific sampling frequency
*/
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void glonass_l2_ca_code_gen_complex_sampled(gsl::span<std::complex<float>> _dest, int32_t _fs, uint32_t _chip_shift)
{
// This function is based on the GNU software GPS for MATLAB in the Kay Borre book
std::array<std::complex<float>, 511> _code{};
int32_t _samplesPerCode;
int32_t _codeValueIndex;
float _ts;
float _tc;
float aux;
const int32_t _codeFreqBasis = 511000; // Hz
const int32_t _codeLength = 511;
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// --- Find number of samples per spreading code ---------------------------
_samplesPerCode = static_cast<int32_t>(static_cast<double>(_fs) / static_cast<double>(_codeFreqBasis / _codeLength));
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// --- 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
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glonass_l2_ca_code_gen_complex(_code, _chip_shift); // generate C/A code 1 sample per chip
for (int32_t i = 0; i < _samplesPerCode; i++)
{
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// === Digitizing ==================================================
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// --- 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).
aux = (_ts * (i + 1)) / _tc;
_codeValueIndex = auxCeil(aux) - 1;
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// --- 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)
{
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// --- Correct the last index (due to number rounding issues) -----------
_dest[i] = _code[_codeLength - 1];
}
else
{
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_dest[i] = _code[_codeValueIndex]; // repeat the chip -> upsample
}
}
}