Serial concatenated convolutional codes: Difference between revisions

Revision as of 16:47, 18 April 2014

Revision as of 19:36, 18 April 2014

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During the encoding of a frame, the input data bits (D) are repeated and distributed to a set of constituent encoders. The constituent encoders are typically accumulators and each accumulator is used to generate a parity symbol. A single copy of the original data D is transmitted (S) with the parity bits (P) to make up the code symbols. The S bits from each constituent encoder are discarded.

In some cases a parity bit is encoded by a second constituent code (serial concatenation), but more typically all or most of the constituent encoding for the LDPC is done in parallel.

In an example using the DVB-S2 rate 2/3 code the encoded block size is 64800 symbols (N=64800) with 42300 data bits (K=43200) and 21600 parity bits ( M=21600). Each constituent code (check node) encodes 16 data bits except for the first party bit which encodes 8 data bits. The first 4680 data bits are repeated 13 times (used in 13 parity codes), while the remaining data bits are used in 3 parity codes (irregular LDPC code).

ToFor comparecomparison, classic turbo codes typically use two convolutional codes of moderate depth (v=38 or 416 state) configured in parallel, each of which encodes the entire input block (K) of data bits. These constituent encoders are combined with a code interleaver which interleaves one copy of the frame to generate the parity symbols.

The LDPC, in contrast, uses many low depth (v=12 state) 'convolutional codes' (accumulators) in parallel, each of which encode only a small portion of the input frame. The many constituent codes are connected via the repeat and distribute operationoperations which performsperform the function of the interleaver in the turbo code.

The ability to more precisely manage the connections of the various constituent codes and the level of redundancy for each input bit give more flexibility in the design of LDPC codes, which can lead to slightly better performance than turbo codes in some instances. Turbo Codescodes still seem to perform better than LDPCs at low code rates, or at least the design of good performing low rate codes is easier for Turbo Codes.

As a practical matter, the hardware that forms the accumulators is reused during the encoding process. That is, once a first set of party bits are generated and the parity bits stored, the same accumulator hardware is used to generate a next set of parity bits.