GNU Radio Manual and C++ API Reference 3.8.5.0
The Free & Open Software Radio Ecosystem
 
Loading...
Searching...
No Matches
cc_encoder.h
Go to the documentation of this file.
1/* -*- c++ -*- */
2/*
3 * Copyright 2013-2014 Free Software Foundation, Inc.
4 *
5 * This file is part of GNU Radio
6 *
7 * GNU Radio is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License as published by
9 * the Free Software Foundation; either version 3, or (at your option)
10 * any later version.
11 *
12 * GNU Radio is distributed in the hope that it will be useful,
13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 * GNU General Public License for more details.
16 *
17 * You should have received a copy of the GNU General Public License
18 * along with GNU Radio; see the file COPYING. If not, write to
19 * the Free Software Foundation, Inc., 51 Franklin Street,
20 * Boston, MA 02110-1301, USA.
21 */
22
23#ifndef INCLUDED_FEC_CC_ENCODER_H
24#define INCLUDED_FEC_CC_ENCODER_H
25
26#include <gnuradio/fec/api.h>
29#include <map>
30#include <string>
31
32namespace gr {
33namespace fec {
34namespace code {
35
36/*!
37 * \brief Convolutional Code Encoding class.
38 * \ingroup error_coding_blk
39 *
40 * \details
41 * This class performs convolutional encoding for unpacked bits
42 * for frames of a constant length. This class is general in its
43 * application of the convolutional encoding and allows us to
44 * specify the constraint length, the coding rate, and the
45 * polynomials used in the coding process.
46 *
47 * The parameter \p k sets the constraint length directly. We
48 * set the coding rate by setting \p rate to R given a desired
49 * rate of 1/R. That is, for a rate 1/2 coder, we would set \p
50 * rate to 2. And the polynomial is specified as a vector of
51 * integers, where each integer represents the coding polynomial
52 * for a different arm of the code. The number of polynomials
53 * given must be the same as the value \p rate.
54 *
55 * The encoding object holds a shift register that takes in each
56 * bit from the input stream and then ANDs the shift register
57 * with each polynomial, and places the parity of the result
58 * into the output stream. The output stream is therefore also
59 * unpacked bits.
60 *
61 * The encoder is set up with a number of bits per frame in the
62 * constructor. When not being used in a tagged stream mode,
63 * this encoder will only process frames of the length provided
64 * here. If used in a tagged stream block, this setting becomes
65 * the maximum allowable frame size that the block may process.
66 *
67 * The \p mode is a cc_mode_t that specifies how the convolutional
68 * encoder will behave and under what conditions.
69 *
70 * \li 'CC_STREAMING': mode expects an uninterrupted flow of
71 * samples into the encoder, and the output stream is
72 * continually encoded.
73 *
74 * \li 'CC_TERMINATED': is a mode designed for packet-based systems. This mode
75 * flushes the encoder with K-1 bits which adds rate*(K-1) bits to the output.
76 * This improves the protection of the last bits of a block and helps the
77 * decoder.
78 *
79 * \li 'CC_TAILBITING': is another packet-based method. Instead of adding bits
80 * onto the end of a packet (as with 'CC_TERMINATED'), this mode will
81 * pre-initialize the state of the encoder with a packet’s last (k-1) bits.
82 *
83 * \li 'CC_TRUNCATED': a truncated code always resets the registers
84 * to the \p start_state between frames.
85 *
86 * A common convolutional encoder uses K=7, Rate=1/2, and the polynomials
87 * \li 1 + x^2 + x^3 + x^5 + x^6
88 * \li 1 + x + x^2 + x^3 + x^6
89 * This is the Voyager code from NASA.
90 *
91 * Another encoder class is provided with gr-fec called the
92 * gr::fec::code::ccsds_encoder, which implements the above code
93 * that is more highly optimized for just those specific
94 * settings.
95 */
96class FEC_API cc_encoder : virtual public generic_encoder
97{
98public:
99 /*!
100 * Build a convolutional code encoding FEC API object.
101 *
102 * \param frame_size Number of bits per frame; must be > 1. If using in the
103 * tagged stream style, this is the maximum allowable number of bits
104 * per frame.
105 * \param k Constraint length (K) of the encoder; must be in the range [2, 31].
106 * K = 1 implies a code without memory which does not make sense;
107 * upper limit is due the way the polynomials of the code are passed
108 * in \p polys.
109 * \param rate Inverse of the coder's rate; must be > 1.
110 * (rate=2 means 2 output bits per 1 input).
111 * \param polys Vector of polynomials as integers. The least significant bit
112 * (LSB) denotes the coefficient of exponent zero of the coding
113 * polynomial. The position of the most significant set bit
114 * (zero based counting) is \p K-1. Note: this representation
115 * is reversed compared to the common representation as found
116 * in most books and references. The common representation puts
117 * the coefficient of the highest exponent into the LSB and the
118 * coefficient of exponent zero is the highest set bit.
119 * Example: The common binary representation of the Voyager
120 * code polynomials (see above) is 1011011 and 1111001; the
121 * octal representation is 133 and 171. For this block, the
122 * binary representation must be reversed: 1101101 and 1001111;
123 * octal this is 155 and 117; decimal this is 109 and 79. Some
124 * standards (e.g. CCSDS 131.0-B-3) require the inversion of
125 * some outputs. This is supported by providing the negative
126 * value of the polynomial, e.g. -109.
127 * \param start_state Initialization state of the shift register; must be in
128 * range [0, 2^(K-1)-1] where K is the constraint length.
129 * The bits in \p start_state are also used to flush the
130 * encoder in mode 'CC_TERMINATED'.
131 * Note: Most books and references use a shift register
132 * shifting from left to right. This implementation,
133 * however, shifts from right to left. This means that
134 * the start state must be reversed. (The different shift
135 * direction is also the reason why the polynomials must
136 * be reversed as described above.)
137 * \param mode cc_mode_t mode of the encoding.
138 * \param padded true if the encoded frame should be padded
139 * to the nearest byte.
140 */
141 static generic_encoder::sptr make(int frame_size,
142 int k,
143 int rate,
144 std::vector<int> polys,
145 int start_state = 0,
146 cc_mode_t mode = CC_STREAMING,
147 bool padded = false);
148
149 /*!
150 * Sets the uncoded frame size to \p frame_size. If \p
151 * frame_size is greater than the value given to the
152 * constructor, the frame size will be capped by that initial
153 * value and this function will return false. Otherwise, it
154 * returns true.
155 */
156 virtual bool set_frame_size(unsigned int frame_size) = 0;
157
158 /*!
159 * Returns the coding rate of this encoder.
160 */
161 virtual double rate() = 0;
162};
163
164} /* namespace code */
165} /* namespace fec */
166} /* namespace gr */
167
168#endif /* INCLUDED_FEC_CC_ENCODER_H */
enum _cc_mode_t cc_mode_t
@ CC_STREAMING
Definition cc_common.h:27
Convolutional Code Encoding class.
Definition cc_encoder.h:97
virtual bool set_frame_size(unsigned int frame_size)=0
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)
virtual double rate()=0
Definition generic_encoder.h:35
boost::shared_ptr< generic_encoder > sptr
Definition generic_encoder.h:49
#define FEC_API
Definition gr-fec/include/gnuradio/fec/api.h:30
GNU Radio logging wrapper for log4cpp library (C++ port of log4j)
Definition basic_block.h:46