Linear Dependence in Parity Check Matrices

If a code is specified by means of a M by N parity check matrix, H, in which some rows are linearly dependent - a situation that is usually avoided - it would be possible to map more than the usual K=N-M message bits into a codeword, since one or more rows of H could have been deleted without affecting which bit vectors are codewords.

However, this software does not increase the number of message bits in this case, but instead produces a generator matrix in which some rows are all zero, which will cause some bits of the codeword to always be zero, regardless of the source message. Referring to the description of generator matrix representations, this is accomplished by partially computing what would normally be A-1 (for a dense or mixed representations) or the L and U matrices (for a sparse representation), even though singularity prevents this computation from being carried out fully.

Example: The parity check matrix created below is redundant, since the 10100 row is equal to the sum of the 11000 and 01100 rows.

The generator matrix above can be used to encode message blocks containing one bit. This message bit is copied unchanged to the last bit (numbered 4) of the codeword, and the first four bits of the codeword are set by multiplying this message bit (seen as a vector of length one) by A-1B, shown above, and then storing the results in positions given by the column ordering. The result is that bit 3 of the codeword produced is also set to the message bit, and bits 0, 1, and 2 are set to zero.

Which bits are used for message bits, and which bits are fixed at zero, depends on arbitrary choices in the algorithm, which may differ from one encoding method to another. No attempt is made to make the best choice.

Note that codeword bits that are always zero can arise even when H does not have linearly dependent rows. For example, if a row of H has just one 1 in it, the codeword bit at that position must be zero in any codeword. The way the software handles parity check matrices with less than M independent rows is equivalent to adding additional rows to H in which only one bit is 1, in order to produce M independent checks.

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