cc_decoder.h

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/* -*- c++ -*- */
/*
 * Copyright 2013-2014 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
#ifndef INCLUDED_FEC_CC_DECODER_H
#define INCLUDED_FEC_CC_DECODER_H
 
#include <gnuradio/fec/api.h>
#include <gnuradio/fec/cc_common.h>
#include <gnuradio/fec/generic_decoder.h>
#include <map>
#include <string>
 
namespace gr {
namespace fec {
namespace code {
 
typedef void (*conv_kernel)(unsigned char* Y,
                            unsigned char* X,
                            unsigned char* syms,
                            unsigned char* dec,
                            unsigned int framebits,
                            unsigned int excess,
                            unsigned char* Branchtab);
 
/*!
 * \brief Convolutional Code Decoding class.
 * \ingroup error_coding_blk
 *
 * \details
 * This class performs convolutional decoding via the Viterbi
 * algorithm. While it is set up to take variable values for K,
 * rate, and the polynomials, currently, the block is only
 * capable of handling the following settings:
 *
 * \li K = 7
 * \li rate = 1/2 (given as 2 to the constructor)
 * \li polynomials = [109, 79]
 *
 * This is the well-known convolutional part of the Voyager code
 * implemented in the CCSDS encoder.
 *
 * The intent of having this FECAPI code classes fully
 * parameterizable is to eventually allow it to take on generic
 * settings, much like the cc_encoder class where the CCSDS
 * settings would be a highly-optimized version of this.
 *
 * The decoder is set up with a number of bits per frame in the
 * constructor. When not being used in a tagged stream mode,
 * this encoder will only process frames of the length provided
 * here. If used in a tagged stream block, this setting becomes
 * the maximum allowable frame size that the block may process.
 *
 * The \p mode is a cc_mode_t that specifies how the convolutional
 * encoder will behave and under what conditions.
 *
 * \li 'CC_STREAMING': mode expects an uninterrupted flow of
 * samples into the encoder, and the output stream is
 * continually encoded. This mode is the only mode for this
 * decoder that has a history requirement because it requires
 * rate*(K-1) bits more to finish the decoding properly. This
 * mode does not work with any deployments that do not allow
 * history.
 *
 * \li 'CC_TERMINATED': is a mode designed for packet-based
 * systems. This mode adds rate*(k-1) bits to the output as a
 * way to help flush the decoder.
 *
 * \li 'CC_TAILBITING': is another packet-based method. Instead of
 * adding bits onto the end of the packet, this mode will
 * continue the code between the payloads of packets by
 * pre-initializing the state of the new packet based on the
 * state of the last packet for (k-1) bits.
 *
 * \li 'CC_TRUNCATED': a truncated code always resets the registers
 * to the \p start_state between frames.
 *
 * A common convolutional encoder uses K=7, Rate=1/2,
 * Polynomials=[109, 79]. This is the Voyager code from NASA:
 * \li   109: b(1101101) --> 1 + x   + x^3 + x^4 + x^6
 * \li   79:  b(1001111) --> 1 + x^3 + x^4 + x^5 + x^6
 */
class FEC_API cc_decoder : virtual public generic_decoder
{
public:
    /*!
     * Build a convolutional code decoding FEC API object.
     *
     * \param frame_size Number of bits per frame. If using in the
     *        tagged stream style, this is the maximum allowable
     *        number of bits per frame.
     * \param k Constraint length (K) of the encoder.
     * \param rate Inverse of the coder's rate
     *             (rate=2 means 2 output bits per 1 input).
     * \param polys Vector of polynomials as integers.
     * \param start_state Initialization state of the shift register.
     * \param end_state Ending state of the shift register.
     * \param mode cc_mode_t mode of the encoding.
     * \param padded true if the encoded frame is padded
     *               to the nearest byte.
     */
    static generic_decoder::sptr make(int frame_size,
                                      int k,
                                      int rate,
                                      std::vector<int> polys,
                                      int start_state = 0,
                                      int end_state = -1,
                                      cc_mode_t mode = CC_STREAMING,
                                      bool padded = false);
 
    /*!
     * Sets the uncoded frame size to \p frame_size. If \p
     * frame_size is greater than the value given to the
     * constructor, the frame size will be capped by that initial
     * value and this function will return false. Otherwise, it
     * returns true.
     */
    bool set_frame_size(unsigned int frame_size) override = 0;
 
    /*!
     * Returns the coding rate of this encoder.
     */
    double rate() override = 0;
};
 
} /* namespace code */
} /* namespace fec */
} /* namespace gr */
 
#endif /* INCLUDED_FEC_CC_DECODER_H */