Notes on Modifying the LDPC Programs
Here are a few notes on how to modify the programs to add new types of
channel, new decoding procedures, etc. You should also look at the module documentation.
Adding a new type of channel
Channels are involved in two programs:
Adding another type of memoryless channel should be straightforward.
Adding a channel with memory may involve more work. Here are the
steps needed if no major reorganizations are required:
- Decide on a syntax for specifying the new channel type and its
parameters, and on an internal name for the channel type. Add
the internal name as a possibility in the enumerated channel_type
declared in channel.h. You
may also need to declare new global variables to store parameters of
the channel in channel.h and
- Modify the channel_parse and
channel_usage procedures in
parse the specification of the new channel and display an appropriate
- Decide on how the new channel's output is represented in a file
(eg, for an erasure channel, what the symbol for an erasure is), and
update transmit.c to write the
new channel's output for each transmitted bit, after randomly generating
any noise (see
the documentation on random number generation).
- Modify decode.c in three places to
accommodate the new channel. The three sections of code to modify
allocate space for data from the channel, read data from the channel,
and set likelihood ratios based on the data read. The setting of
likelihood ratios is based on the assumption that the channel is
memoryless (ie, data received for different bits is independent).
Adding a channel with memory would require changing this assumption,
which would involve modifications to the decoding procedures too.
- Document the new channel type in channel.html
Adding a new decoding procedure
A new decoding method can be implemented as follows:
- Decide on a syntax for specifying the method and its parameters,
using the trailing arguments to the
decode program. Pick an
internal name for the method, and add it as a possibility in the
enumerated decoding_method type in
dec.h. You may also need to declare
new variables for the method's parameters in
dec.h and dec.c.
- Modify the argument parsing code in
to handle specifications of the new method, and change the usage
procedure to display the syntax for specifying the new method.
- Write a setup procedure for your decoding method, putting it in
dec.c, with a declaration in
dec.h. At a minimum, this procedure
should print headers for the table of detailed decoding information
when the -T option was specified.
- Write a decode procedure implementing your method, putting it in
dec.c, with a declaration in
dec.h. This procedure should output
detailed trace information when the -T option was specified.
- Modify decode.c in the appropriate
places to call the setup procedure and the decode procedure you wrote.
- Document the new decoding method in
Adding a new method of making a low-density parity-check matrix
The make-ldpc program can be
changed to add a new method for generating a LDPC code by modifying
make-ldpc.c. A radically different
method might better be implemented by writing a new program of similar
Adding a new encoding method
A new heuristic for finding a sparse LU decomposition can be
implemented by changing make-gen.c to allow
the new heuristic to be specified on the command line, changing the mod2sparse_decomp procedure
in mod2sparse.c to implement the
heuristic, and documenting the new heuristic in encoding.html, sparse-LU.html, and mod2sparse.html.
To implement a completely new encoding method, you will first need
to define a new file format for a generator matrix, modify make-gen.c appropriately to write out this new
format, and modify the read_gen procedure in rcode.c to read this format. You will
need to implement the new method in a procedure in enc.c, and modify encode.c so
that it will call this new procedure when the new method is used. The
enum_decode procedure in dec.c will also
need to be modified so it can call the new encoding method. Finally,
you should document the new method in encoding.html
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