diff options
| author | rear1019 <rear1019@posteo.de> | 2020-01-19 15:18:46 +0100 |
|---|---|---|
| committer | Michael Dickens <michael.dickens@ettus.com> | 2020-02-01 16:05:14 -0500 |
| commit | 9ed2fde11337edab2629195b45de2d1338a5f703 (patch) | |
| tree | ef0967ea76e3b51ef93a294ee6c5893ca22e1880 /gr-fec | |
| parent | 978b0b08a1ae55888fc0dcdb9c2fd1e98ee6f950 (diff) | |
gr-fec/cc_encoder: Fix and improve documentation
* Fix wrong Voyager polynomials. The binary and decimal representation
which must be passed as an argument to the block, however, is
correct.
* Clarify how the generator polynomials are to be represented. Add the
important note that an uncommonly used representation is required.
* Mention that start_state must be possibly reversed and that
start_state is used to flush the encoder in terminated mode.
* Mention support for inversion of the output of a generator polynomial.
* Improve description of tailbiting and terminated mode.
* Mention valid values for parameters.
Diffstat (limited to 'gr-fec')
| -rw-r--r-- | gr-fec/include/gnuradio/fec/cc_encoder.h | 64 |
1 files changed, 45 insertions, 19 deletions
diff --git a/gr-fec/include/gnuradio/fec/cc_encoder.h b/gr-fec/include/gnuradio/fec/cc_encoder.h index bf29a1c941..5d730360f5 100644 --- a/gr-fec/include/gnuradio/fec/cc_encoder.h +++ b/gr-fec/include/gnuradio/fec/cc_encoder.h @@ -59,23 +59,22 @@ namespace code { * samples into the encoder, and the output stream is * continually encoded. * - * \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_TERMINATED': is a mode designed for packet-based systems. This mode + * flushes the encoder with K-1 bits which adds rate*(K-1) bits to the output. + * This improves the protection of the last bits of a block and helps 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_TAILBITING': is another packet-based method. Instead of adding bits + * onto the end of a packet (as with 'CC_TERMINATED'), this mode will + * pre-initialize the state of the encoder with a packet’s last (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 + * A common convolutional encoder uses K=7, Rate=1/2, and the polynomials + * \li 1 + x^2 + x^3 + x^5 + x^6 + * \li 1 + x + x^2 + x^3 + x^6 + * This is the Voyager code from NASA. * * Another encoder class is provided with gr-fec called the * gr::fec::code::ccsds_encoder, which implements the above code @@ -88,14 +87,41 @@ public: /*! * Build a convolutional code encoding 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 + * \param frame_size Number of bits per frame; must be > 1. 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; must be in the range [2, 31]. + * K = 1 implies a code without memory which does not make sense; + * upper limit is due the way the polynomials of the code are passed + * in \p polys. + * \param rate Inverse of the coder's rate; must be > 1. * (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 polys Vector of polynomials as integers. The least significant bit + * (LSB) denotes the coefficient of exponent zero of the coding + * polynomial. The position of the most significant set bit + * (zero based counting) is \p K-1. Note: this representation + * is reversed compared to the common representation as found + * in most books and references. The common representation puts + * the coefficient of the highest exponent into the LSB and the + * coefficient of exponent zero is the highest set bit. + * Example: The common binary representation of the Voyager + * code polynomials (see above) is 1011011 and 1111001; the + * octal representation is 133 and 171. For this block, the + * binary representation must be reversed: 1101101 and 1001111; + * octal this is 155 and 117; decimal this is 109 and 79. Some + * standards (e.g. CCSDS 131.0-B-3) require the inversion of + * some outputs. This is supported by providing the negative + * value of the polynomial, e.g. -109. + * \param start_state Initialization state of the shift register; must be in + * range [0, 2^(K-1)-1] where K is the constraint length. + * The bits in \p start_state are also used to flush the + * encoder in mode 'CC_TERMINATED'. + * Note: Most books and references use a shift register + * shifting from left to right. This implementation, + * however, shifts from right to left. This means that + * the start state must be reversed. (The different shift + * direction is also the reason why the polynomials must + * be reversed as described above.) * \param mode cc_mode_t mode of the encoding. * \param padded true if the encoded frame should be padded * to the nearest byte. |