ax100_mode6.h

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/* -*- c++ -*- */
/*
 *  gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
 *
 *  Copyright (C) 2020-2023, Libre Space Foundation <http://libre.space>
 *
 *  This program 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.
 *
 *  This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *  SPDX-License-Identifier: GNU General Public License v3.0 or later
 */
 
#ifndef INCLUDED_SATNOGS_AX100_MODE6_H
#define INCLUDED_SATNOGS_AX100_MODE6_H
 
#include <gnuradio/digital/lfsr.h>
#include <gnuradio/satnogs/api.h>
#include <gnuradio/satnogs/crc.h>
#include <gnuradio/satnogs/decoder.h>
#include <gnuradio/satnogs/whitening.h>
 
#include <deque>
 
namespace gr {
namespace satnogs {
 
/*!
 * \brief AX.100 mode 6 decoder
 *
 * Mode 6 Decoder for the AX100 modem of GomSpace.
 *
 * In this particular mode, the modem encapsulates the payload inside an
 * AX.25 frame. The payload is appended with a Castagnoli CRC32, scrambled with
 * a CCSDS scrambler and encoded using the CCSDS compliant Reed Solomon.
 *
 * This non-sense framing scheme is used mainly for getting data
 * from legacy ham stations receiving and reporting AX.25 frames.
 *
 * The implementation drops any kind of AX.25 information and does not check
 * the 16-bit CRC, allowing the Reed Solomon to correct any error bits.
 *
 * \ingroup satnogs
 *
 */
class SATNOGS_API ax100_mode6 : public decoder
{
public:
    using sptr = std::shared_ptr<ax100_mode6>;
    static sptr make(crc::type crc = crc::type::CRC32_C,
                     whitening::sptr descrambler = whitening::make_ccsds(),
                     bool ax25_descramble = true);
 
    ax100_mode6(crc::type crc, whitening::sptr descrambler, bool ax25_descramble);
 
    ~ax100_mode6();
 
    decoder_status_t decode(const void* in, int len);
 
    void reset();
 
private:
    typedef enum { NO_SYNC, IN_SYNC, DECODING } decoding_state_t;
 
    const crc::type d_crc;
    const bool d_ax25_descramble;
    const size_t d_max_frame_len;
    whitening::sptr d_descrambler;
    decoding_state_t d_state;
    uint32_t d_shift_reg;
    uint8_t d_dec_b;
    uint8_t d_prev_bit_nrzi;
    size_t d_received_bytes;
    size_t d_decoded_bits;
    digital::lfsr d_lfsr;
    uint8_t* d_frame_buffer;
    std::deque<uint8_t> d_bitstream;
    size_t d_start_idx;
    uint64_t d_frame_start;
    uint64_t d_sample_cnt;
 
    void reset_state();
    void enter_sync_state();
    void enter_decoding_state();
    bool enter_frame_end(decoder_status_t& status);
 
    bool _decode(decoder_status_t& status);
 
    inline void decode_1b(uint8_t in);
};
 
} // namespace satnogs
} // namespace gr
 
#endif /* INCLUDED_SATNOGS_AX100_MODE6_H */