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https://github.com/SatDump/SatDump
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152 lines
6.7 KiB
C++
152 lines
6.7 KiB
C++
/* -*- c++ -*- */
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/*
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* Copyright 2013-2014 Free Software Foundation, Inc.
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*
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* This file is part of GNU Radio
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*
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* SPDX-License-Identifier: GPL-3.0-or-later
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*
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*/
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#pragma once
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#include "cc_common.h"
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#include "generic_encoder.h"
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#include <map>
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#include <string>
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namespace fec
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{
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namespace code
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{
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/*!
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* \brief Convolutional Code Encoding class.
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* \ingroup error_coding_blk
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*
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* \details
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* This class performs convolutional encoding for unpacked bits
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* for frames of a constant length. This class is general in its
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* application of the convolutional encoding and allows us to
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* specify the constraint length, the coding rate, and the
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* polynomials used in the coding process.
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*
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* The parameter \p k sets the constraint length directly. We
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* set the coding rate by setting \p rate to R given a desired
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* rate of 1/R. That is, for a rate 1/2 coder, we would set \p
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* rate to 2. And the polynomial is specified as a vector of
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* integers, where each integer represents the coding polynomial
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* for a different arm of the code. The number of polynomials
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* given must be the same as the value \p rate.
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*
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* The encoding object holds a shift register that takes in each
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* bit from the input stream and then ANDs the shift register
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* with each polynomial, and places the parity of the result
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* into the output stream. The output stream is therefore also
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* unpacked bits.
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*
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* The encoder is set up with a number of bits per frame in the
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* constructor. When not being used in a tagged stream mode,
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* this encoder will only process frames of the length provided
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* here. If used in a tagged stream block, this setting becomes
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* the maximum allowable frame size that the block may process.
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*
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* The \p mode is a cc_mode_t that specifies how the convolutional
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* encoder will behave and under what conditions.
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*
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* \li 'CC_STREAMING': mode expects an uninterrupted flow of
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* samples into the encoder, and the output stream is
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* continually encoded.
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*
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* \li 'CC_TERMINATED': is a mode designed for packet-based systems. This mode
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* flushes the encoder with K-1 bits which adds rate*(K-1) bits to the output.
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* This improves the protection of the last bits of a block and helps the
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* decoder.
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*
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* \li 'CC_TAILBITING': is another packet-based method. Instead of adding bits
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* onto the end of a packet (as with 'CC_TERMINATED'), this mode will
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* pre-initialize the state of the encoder with a packet’s last (k-1) bits.
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*
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* \li 'CC_TRUNCATED': a truncated code always resets the registers
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* to the \p start_state between frames.
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*
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* A common convolutional encoder uses K=7, Rate=1/2, and the polynomials
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* \li 1 + x^2 + x^3 + x^5 + x^6
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* \li 1 + x + x^2 + x^3 + x^6
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* This is the Voyager code from NASA.
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*
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* Another encoder class is provided with gr-fec called the
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* gr::fec::code::ccsds_encoder, which implements the above code
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* that is more highly optimized for just those specific
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* settings.
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*/
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class cc_encoder : virtual public generic_encoder
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{
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public:
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/*!
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* Build a convolutional code encoding FEC API object.
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*
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* \param frame_size Number of bits per frame; must be > 1. If using in the
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* tagged stream style, this is the maximum allowable number of bits
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* per frame.
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* \param k Constraint length (K) of the encoder; must be in the range [2, 31].
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* K = 1 implies a code without memory which does not make sense;
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* upper limit is due the way the polynomials of the code are passed
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* in \p polys.
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* \param rate Inverse of the coder's rate; must be > 1.
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* (rate=2 means 2 output bits per 1 input).
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* \param polys Vector of polynomials as integers. The least significant bit
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* (LSB) denotes the coefficient of exponent zero of the coding
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* polynomial. The position of the most significant set bit
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* (zero based counting) is \p K-1. Note: this representation
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* is reversed compared to the common representation as found
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* in most books and references. The common representation puts
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* the coefficient of the highest exponent into the LSB and the
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* coefficient of exponent zero is the highest set bit.
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* Example: The common binary representation of the Voyager
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* code polynomials (see above) is 1011011 and 1111001; the
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* octal representation is 133 and 171. For this block, the
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* binary representation must be reversed: 1101101 and 1001111;
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* octal this is 155 and 117; decimal this is 109 and 79. Some
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* standards (e.g. CCSDS 131.0-B-3) require the inversion of
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* some outputs. This is supported by providing the negative
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* value of the polynomial, e.g. -109.
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* \param start_state Initialization state of the shift register; must be in
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* range [0, 2^(K-1)-1] where K is the constraint length.
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* The bits in \p start_state are also used to flush the
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* encoder in mode 'CC_TERMINATED'.
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* Note: Most books and references use a shift register
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* shifting from left to right. This implementation,
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* however, shifts from right to left. This means that
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* the start state must be reversed. (The different shift
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* direction is also the reason why the polynomials must
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* be reversed as described above.)
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* \param mode cc_mode_t mode of the encoding.
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* \param padded true if the encoded frame should be padded
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* to the nearest byte.
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*/
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static generic_encoder::sptr make(int frame_size,
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int k,
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int rate,
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std::vector<int> polys,
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int start_state = 0,
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cc_mode_t mode = CC_STREAMING,
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bool padded = false);
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/*!
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* Sets the uncoded frame size to \p frame_size. If \p
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* frame_size is greater than the value given to the
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* constructor, the frame size will be capped by that initial
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* value and this function will return false. Otherwise, it
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* returns true.
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*/
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bool set_frame_size(unsigned int frame_size) override = 0;
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/*!
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* Returns the coding rate of this encoder.
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*/
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double rate() override = 0;
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};
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} /* namespace code */
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} /* namespace fec */
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