GNU Radio C++ API Reference: cc_encoder.h Source File

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cc_encoder.h
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1/* -*- c++ -*- */
2/*
3 * Copyright 2013-2014 Free Software Foundation, Inc.
4 *
5 * This file is part of GNU Radio
6 *
7 * SPDX-License-Identifier: GPL-3.0-or-later
8 *
9 */
10
11#ifndef INCLUDED_FEC_CC_ENCODER_H
12#define INCLUDED_FEC_CC_ENCODER_H
13
14#include <gnuradio/fec/api.h>
15#include <gnuradio/fec/cc_common.h>
16#include <gnuradio/fec/encoder.h>
17#include <map>
18#include <string>
19
20namespace gr {
21namespace fec {
22namespace code {
23
24/*!
25 * \brief Convolutional Code Encoding class.
26 * \ingroup error_coding_blk
27 *
28 * \details
29 * This class performs convolutional encoding for unpacked bits
30 * for frames of a constant length. This class is general in its
31 * application of the convolutional encoding and allows us to
32 * specify the constraint length, the coding rate, and the
33 * polynomials used in the coding process.
34 *
35 * The parameter \p k sets the constraint length directly. We
36 * set the coding rate by setting \p rate to R given a desired
37 * rate of 1/R. That is, for a rate 1/2 coder, we would set \p
38 * rate to 2. And the polynomial is specified as a vector of
39 * integers, where each integer represents the coding polynomial
40 * for a different arm of the code. The number of polynomials
41 * given must be the same as the value \p rate.
42 *
43 * The encoding object holds a shift register that takes in each
44 * bit from the input stream and then ANDs the shift register
45 * with each polynomial, and places the parity of the result
46 * into the output stream. The output stream is therefore also
47 * unpacked bits.
48 *
49 * The encoder is set up with a number of bits per frame in the
50 * constructor. When not being used in a tagged stream mode,
51 * this encoder will only process frames of the length provided
52 * here. If used in a tagged stream block, this setting becomes
53 * the maximum allowable frame size that the block may process.
54 *
55 * The \p mode is a cc_mode_t that specifies how the convolutional
56 * encoder will behave and under what conditions.
57 *
58 * \li 'CC_STREAMING': mode expects an uninterrupted flow of
59 * samples into the encoder, and the output stream is
60 * continually encoded.
61 *
62 * \li 'CC_TERMINATED': is a mode designed for packet-based systems. This mode
63 * flushes the encoder with K-1 bits which adds rate*(K-1) bits to the output.
64 * This improves the protection of the last bits of a block and helps the
65 * decoder.
66 *
67 * \li 'CC_TAILBITING': is another packet-based method. Instead of adding bits
68 * onto the end of a packet (as with 'CC_TERMINATED'), this mode will
69 * pre-initialize the state of the encoder with a packet’s last (k-1) bits.
70 *
71 * \li 'CC_TRUNCATED': a truncated code always resets the registers
72 * to the \p start_state between frames.
73 *
74 * A common convolutional encoder uses K=7, Rate=1/2, and the polynomials
75 * \li 1 + x^2 + x^3 + x^5 + x^6
76 * \li 1 + x + x^2 + x^3 + x^6
77 * This is the Voyager code from NASA.
78 *
79 * Another encoder class is provided with gr-fec called the
80 * gr::fec::code::ccsds_encoder, which implements the above code
81 * that is more highly optimized for just those specific
82 * settings.
83 */
84 class FEC_API cc_encoder : virtual public generic_encoder
85{
86public:
87 /*!
88 * Build a convolutional code encoding FEC API object.
89 *
90 * \param frame_size Number of bits per frame; must be > 1. If using in the
91 * tagged stream style, this is the maximum allowable number of bits
92 * per frame.
93 * \param k Constraint length (K) of the encoder; must be in the range [2, 31].
94 * K = 1 implies a code without memory which does not make sense;
95 * upper limit is due the way the polynomials of the code are passed
96 * in \p polys.
97 * \param rate Inverse of the coder's rate; must be > 1.
98 * (rate=2 means 2 output bits per 1 input).
99 * \param polys Vector of polynomials as integers. The least significant bit
100 * (LSB) denotes the coefficient of exponent zero of the coding
101 * polynomial. The position of the most significant set bit
102 * (zero based counting) is \p K-1. Note: this representation
103 * is reversed compared to the common representation as found
104 * in most books and references. The common representation puts
105 * the coefficient of the highest exponent into the LSB and the
106 * coefficient of exponent zero is the highest set bit.
107 * Example: The common binary representation of the Voyager
108 * code polynomials (see above) is 1011011 and 1111001; the
109 * octal representation is 133 and 171. For this block, the
110 * binary representation must be reversed: 1101101 and 1001111;
111 * octal this is 155 and 117; decimal this is 109 and 79. Some
112 * standards (e.g. CCSDS 131.0-B-3) require the inversion of
113 * some outputs. This is supported by providing the negative
114 * value of the polynomial, e.g. -109.
115 * \param start_state Initialization state of the shift register; must be in
116 * range [0, 2^(K-1)-1] where K is the constraint length.
117 * The bits in \p start_state are also used to flush the
118 * encoder in mode 'CC_TERMINATED'.
119 * Note: Most books and references use a shift register
120 * shifting from left to right. This implementation,
121 * however, shifts from right to left. This means that
122 * the start state must be reversed. (The different shift
123 * direction is also the reason why the polynomials must
124 * be reversed as described above.)
125 * \param mode cc_mode_t mode of the encoding.
126 * \param padded true if the encoded frame should be padded
127 * to the nearest byte.
128 */
129 static generic_encoder::sptr make(int frame_size,
130 int k,
131 int rate,
132 std::vector<int> polys,
133 int start_state = 0,
134 cc_mode_t mode = CC_STREAMING,
135 bool padded = false);
136
137 /*!
138 * Sets the uncoded frame size to \p frame_size. If \p
139 * frame_size is greater than the value given to the
140 * constructor, the frame size will be capped by that initial
141 * value and this function will return false. Otherwise, it
142 * returns true.
143 */
144 bool set_frame_size(unsigned int frame_size) override = 0;
145
146 /*!
147 * Returns the coding rate of this encoder.
148 */
149 double rate() override = 0;
150};
151
152} /* namespace code */
153} /* namespace fec */
154} /* namespace gr */
155
156#endif /* INCLUDED_FEC_CC_ENCODER_H */
cc_common.h
cc_mode_t
enum _cc_mode_t cc_mode_t
CC_STREAMING
@ CC_STREAMING
Definition cc_common.h:17
gr::fec::code::cc_encoder
Convolutional Code Encoding class.
Definition cc_encoder.h:85
gr::fec::code::cc_encoder::rate
double rate() override=0
gr::fec::code::cc_encoder::set_frame_size
bool set_frame_size(unsigned int frame_size) override=0
gr::fec::code::cc_encoder::make
static generic_encoder::sptr make(int frame_size, int k, int rate, std::vector< int > polys, int start_state=0, cc_mode_t mode=CC_STREAMING, bool padded=false)
gr::fec::generic_encoder::sptr
std::shared_ptr< generic_encoder > sptr
Definition generic_encoder.h:37
gr::fec::generic_encoder::generic_encoder
generic_encoder(void)
Definition generic_encoder.h:119
api.h
FEC_API
#define FEC_API
Definition gr-fec/include/gnuradio/fec/api.h:18
encoder.h
gr::fec::code
Definition cc_decoder.h:22
gr::fec
Definition async_decoder.h:20
gr
GNU Radio logging wrapper.
Definition basic_block.h:29