126 lines
5.9 KiB
HTML
126 lines
5.9 KiB
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<TITLE> Notes on Modifying the LDPC Programs </TITLE>
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</HEAD><BODY>
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<H1> Notes on Modifying the LDPC Programs </H1>
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<P>Here are a few notes on how to modify the programs to add new types of
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channel, new decoding procedures, etc. You should also look at the <A
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HREF="modules.html">module documentation</A>.
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<H2> Adding a new type of channel </H2>
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<P>Channels are involved in two programs:
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<A HREF="channel.html#transmit"><B>transmit</B></A> and
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<A HREF="decoding.html#decode"><B>decode</B></A>.
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Adding another type of memoryless channel should be straightforward.
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Adding a channel with memory may involve more work. Here are the
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steps needed if no major reorganizations are required:
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<OL>
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<LI> Decide on a syntax for specifying the new channel type and its
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parameters, and on an internal name for the channel type. Add
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the internal name as a possibility in the enumerated <TT>channel_type</TT>
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declared in <A HREF="channel.h"><TT>channel.h</TT></A>. You
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may also need to declare new global variables to store parameters of
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the channel in <A HREF="channel.h"><TT>channel.h</TT></A> and
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<A HREF="channel.c"><TT>channel.c</TT></A>.
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<LI> Modify the <TT>channel_parse</TT> and
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<TT>channel_usage</TT> procedures in
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<A HREF="channel.c"><TT>channel.c</TT></A> to
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parse the specification of the new channel and display an appropriate
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usage message.
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<LI> Decide on how the new channel's output is represented in a file
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(eg, for an erasure channel, what the symbol for an erasure is), and
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update <A HREF="transmit.c"><TT>transmit.c</TT></A> to write the
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new channel's output for each transmitted bit, after randomly generating
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any noise (see
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the <A HREF="rand.html">documentation on random number generation</A>).
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<LI> Modify <A HREF="decode.c"><TT>decode.c</TT></A> in three places to
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accommodate the new channel. The three sections of code to modify
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allocate space for data from the channel, read data from the channel,
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and set likelihood ratios based on the data read. The setting of
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likelihood ratios is based on the assumption that the channel is
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memoryless (ie, data received for different bits is independent).
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Adding a channel with memory would require changing this assumption,
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which would involve modifications to the decoding procedures too.
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<LI> Document the new channel type in <A HREF="channel.html">channel.html</A>
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and <A HREF="decoding.html">decoding.html</A>.
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</OL>
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<H2> Adding a new decoding procedure </H2>
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A new decoding method can be implemented as follows:
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<OL>
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<LI> Decide on a syntax for specifying the method and its parameters,
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using the trailing arguments to the
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<A HREF="decoding.html#decode"><TT>decode</TT></A> program. Pick an
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internal name for the method, and add it as a possibility in the
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enumerated <TT>decoding_method</TT> type in
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<A HREF="dec.h"><TT>dec.h</TT></A>. You may also need to declare
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new variables for the method's parameters in
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<A HREF="dec.h"><TT>dec.h</TT></A> and <A HREF="dec.c"><TT>dec.c</TT></A>.
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<LI> Modify the argument parsing code in
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<A HREF="decode.c"><TT>decode.c</TT></A>
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to handle specifications of the new method, and change the <TT>usage</TT>
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procedure to display the syntax for specifying the new method.
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<LI> Write a setup procedure for your decoding method, putting it in
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<A HREF="dec.c"><TT>dec.c</TT></A>, with a declaration in
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<A HREF="dec.h"><TT>dec.h</TT></A>. At a minimum, this procedure
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should print headers for the table of detailed decoding information
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when the <B>-T</B> option was specified.
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<LI> Write a decode procedure implementing your method, putting it in
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<A HREF="dec.c"><TT>dec.c</TT></A>, with a declaration in
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<A HREF="dec.h"><TT>dec.h</TT></A>. This procedure should output
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detailed trace information when the <B>-T</B> option was specified.
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<LI> Modify <A HREF="decode.c"><TT>decode.c</TT></A> in the appropriate
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places to call the setup procedure and the decode procedure you wrote.
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<LI> Document the new decoding method in
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<A HREF="decoding.html">decoding.html</A> and
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<A HREF="decode-detail.html">decode-detail.html</A>.
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</OL>
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<H2> Adding a new method of making a low-density parity-check matrix </H2>
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<P>The <A HREF="pchk.html#make-ldpc"><B>make-ldpc</B></A> program can be
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changed to add a new method for generating a LDPC code by modifying
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<A HREF="make-ldpc.c"><TT>make-ldpc.c</TT></A>. A radically different
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method might better be implemented by writing a new program of similar
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structure.
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<H2> Adding a new encoding method </H2>
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<P>A new heuristic for finding a sparse LU decomposition can be
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implemented by changing <A HREF="make-gen.c">make-gen.c</A> to allow
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the new heuristic to be specified on the command line, changing the <A
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HREF="mod2sparse.html#decomp"><B>mod2sparse_decomp</B></A> procedure
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in <A HREF="mod2sparse.c"><TT>mod2sparse.c</TT></A> to implement the
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heuristic, and documenting the new heuristic in <A
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HREF="encoding.html">encoding.html</A>, <A
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HREF="sparse-LU.html">sparse-LU.html</A>, and <A
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HREF="mod2sparse.html">mod2sparse.html</A>.
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<P>To implement a completely new encoding method, you will first need
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to define a new file format for a generator matrix, modify <A
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HREF="make-gen.c">make-gen.c</A> appropriately to write out this new
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format, and modify the <TT>read_gen</TT> procedure in <A
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HREF="rcode.c"><TT>rcode.c</TT></A> to read this format. You will
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need to implement the new method in a procedure in <A
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HREF="enc.c">enc.c</A>, and modify <A HREF="encode.c">encode.c</A> so
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that it will call this new procedure when the new method is used. The
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<TT>enum_decode</TT> procedure in <A HREF="dec.c">dec.c</A> will also
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need to be modified so it can call the new encoding method. Finally,
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you should document the new method in <A
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HREF="encoding.html">encoding.html</A>
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<P>
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<HR>
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<A HREF="index.html">Back to index for LDPC software</A>
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</BODY></HTML>
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