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经典的Fano编码源代码

Fano decoder v1.1
Copyright 1995 Phil Karn

This package includes an encoder and a soft-decision sequential
decoder for K=32, rate 1/2 convolutional codes. The decoder uses the
Fano algorithm.

Also included are support routines to generate metric tables that are
optimized for gaussian noise with a specified Eb/N0 ratio, and a test
driver that exercises the encoder/decoder routines and keeps statistics.

The files are as follows:

README		this file
Makefile	Makefile for GCC under BSDI 2.0 (edit to taste)
fano.h		header file with declarations for fano.c
fano.c		the encoder and Fano decoder routines
metrics.c	metric table generator
sim.c		transmitter/channel simulator (including gaussian noise gen)
seqtest.c	driver program for testing
tab.c		parity lookup table

The test program in seqtest.c creates a test frame, encodes it, adds
gaussian noise and decodes it. It then repeats a specified number of
times, keeping a histogram on the number of decoder cycles required
per bit. By default, the program continuously displays the statistics
using the UNIX "curses" package; this can be suppressed with the -q
(quiet) option.

The gaussian random number generator in sim.c uses the traditional
"rejection" method. This requires slightly more than one floating
point square root and log function per pair of gaussian numbers. This
makes noise generation rather slow, much slower in fact than the
actual sequential decoding process (except for very noisy
packets). The BSDI 2.0 math library routines do not make use of the
native 387 FPU instructions, and this made it even slower. So the
makefile specifies a separate library that I built locally (-lm387)
with versions of the log and sqrt functions that do use the FPU. Other
386/486 UNIX clones apparently do have updated math libraries, so this
special library shouldn't be necessary. In that case, simply eliminate
the reference to -lm387. If you do need my lm387 library, let me know
and I can package it up for release.

If you want to time the speed of the decoder, use the -t (timetest)
option.  This executes the decoder in a tight loop repeatedly decoding
the same packet, allowing you to test just the decoder and not the
noise generator or screen update routines. Use the UNIX "time" command
to get your results.

Three code polynomials are supported as described in fano.c. A #define
statement at the top of fano.c selects the polynomial.

The arguments to the encoder and decoder routines are documented in
comments in fano.c. Note that the encoded symbols created by encode()
take on the values 0 and 1, while fano() ordinarily expects 8-bit
soft-decision receive symbols. The interpretation of these symbols by
the decoder is completely determined by the metric table.

The metric table generator in gen_met() in metrics.c assumes the
channel is corrupted by gaussian noise at some specified level, that
the received symbols from the modem are offset-128 binary with some
specified amplitude, and that symbol value corresponding to a
transmitted "1" is larger than for a "0".  Given these parameters it
builds the table from by computing the log-likelihood function for
each possible received symbol value.

The performance of a sequential decoder depends critically on the
accuracy of its metric table. In some cases (e.g., the estimated noise
level is incorrect) the degradation may be relatively minor. But if
the table is way off, e.g., if the signal level is so low compared to
the expected values that a negative metric results even on the correct
path, then the decoder won't work even if the signal is otherwise
completely clean.

By generating the appropriate metric table, you could use the decoder
on some other kind of channel. For example, on a binary (hard
decision) channel there would only be four table entries corresponding
to the four channel transition probabilities. And if the channel is
symmetric (BSC), this 2x2 matrix would also be symmetric.

If you use this decoder in an actual application, you won't need sim.c
and seqtest.c. Also, instead of including metrics.c to compute the
metric tables at application run time, you could build a set of metric
tables into static tables, perhaps for several fixed Eb/N0s. This
would avoid floating point math in your runtime package.

Any real application will also probably require an interleaver, since
convolutional coders (especially sequential decoders) are highly
sensitive to burst errors.

Phil Karn

March 1995: version 1.0
Original release for BSDI 1.1

August 5, 1995: version 1.1
Updated for BSDI 2.0
 - changes in curses/termcap
Several minor bugs fixed:
 - fencepost error in cycle count returned by fano()
 - correct returned value of cumulative metric
 - clean up computation of noise level for gen_met() in seqtest.c, change
   convention to conform to viterbi 1.1 package