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

/*!
 * \file gps_sdr_signal_replica.cc
 * \brief This file implements functions for GPS L1 C/A signal replica
 * generation
 * \author Javier Arribas, 2011. jarribas(at)cttc.es
 *
 *
 * -----------------------------------------------------------------------------
 *
 * GNSS-SDR is a Global Navigation Satellite System software-defined receiver.
 * This file is part of GNSS-SDR.
 *
 * Copyright (C) 2010-2020  (see AUTHORS file for a list of contributors)
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 * -----------------------------------------------------------------------------
 */

#include "gps_sdr_signal_replica.h"
#include <array>
#include <bitset>

const auto AUX_CEIL = [](float x) { return static_cast<int32_t>(static_cast<int64_t>((x) + 1)); };

void gps_l1_ca_code_gen_int(own::span<int32_t> dest, int32_t prn, uint32_t chip_shift)
{
    constexpr uint32_t code_length = 1023;
    std::bitset<code_length> G1{};
    std::bitset<code_length> G2{};
    auto G1_register = std::bitset<10>{}.set();  // All true
    auto G2_register = std::bitset<10>{}.set();  // All true
    uint32_t lcv;
    uint32_t lcv2;
    uint32_t delay;
    int32_t prn_idx;
    bool feedback1;
    bool feedback2;
    bool aux;

    // G2 Delays as defined in GPS-ISD-200D
    const std::array<int32_t, 51> delays = {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;
        }

    // 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] xor G1_register[0];
            feedback2 = G2_register[8] xor G2_register[7] xor G2_register[4] xor G2_register[2] xor G2_register[1] xor G2_register[0];

            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] xor G2[delay];
            if (aux == true)
                {
                    dest[lcv] = 1;
                }
            else
                {
                    dest[lcv] = -1;
                }
            delay++;
            delay %= code_length;
        }
}


void gps_l1_ca_code_gen_float(own::span<float> dest, int32_t prn, uint32_t chip_shift)
{
    constexpr uint32_t code_length = 1023;
    std::array<int32_t, code_length> ca_code_int{};

    gps_l1_ca_code_gen_int(ca_code_int, prn, chip_shift);

    for (uint32_t ii = 0; ii < code_length; ++ii)
        {
            dest[ii] = static_cast<float>(ca_code_int[ii]);
        }
}


void gps_l1_ca_code_gen_complex(own::span<std::complex<float>> dest, int32_t prn, uint32_t chip_shift)
{
    constexpr uint32_t code_length = 1023;
    std::array<int32_t, code_length> ca_code_int{};

    gps_l1_ca_code_gen_int(ca_code_int, prn, chip_shift);

    for (uint32_t ii = 0; ii < code_length; ++ii)
        {
            dest[ii] = std::complex<float>(0.0F, static_cast<float>(ca_code_int[ii]));
        }
}


/*
 *  Generates complex GPS L1 C/A code for the desired SV ID and sampled to specific sampling frequency
 *  NOTICE: the number of samples is rounded towards zero (integer truncation)
 */
void gps_l1_ca_code_gen_complex_sampled(own::span<std::complex<float>> dest, uint32_t prn, int32_t sampling_freq, uint32_t chip_shift)
{
    // This function is based on the GNU software GPS for MATLAB in the Kay Borre book
    constexpr int32_t codeFreqBasis = 1023000;  // chips per second
    constexpr int32_t codeLength = 1023;
    constexpr float tc = 1.0F / static_cast<float>(codeFreqBasis);  // C/A chip period in sec

    const auto samplesPerCode = static_cast<int32_t>(static_cast<double>(sampling_freq) / (static_cast<double>(codeFreqBasis) / static_cast<double>(codeLength)));
    const float ts = 1.0F / static_cast<float>(sampling_freq);  // Sampling period in sec
    std::array<std::complex<float>, 1023> code_aux{};
    int32_t codeValueIndex;
    float aux;

    gps_l1_ca_code_gen_complex(code_aux, prn, chip_shift);  // generate C/A code 1 sample per chip

    for (int32_t 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).

            aux = (ts * (static_cast<float>(i) + 1)) / tc;
            codeValueIndex = AUX_CEIL(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_aux[codeLength - 1];
                }
            else
                {
                    dest[i] = code_aux[codeValueIndex];  // repeat the chip -> upsample
                }
        }
}