KA9Q Viterbi decoder V1.0
Copyright 1995 Phil Karn, KA9Q

This package implements an encoder and a Viterbi decoder for the NASA
standard rate 1/2 constraint length 7 convolutional code.

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 following files are included:

Makefile	for BSDI 1.1
README		this file
genbut.c	utility program for generating inline macro calls
metrics.c	generate soft-decision metrics for AWGN channel
sim.c		transmitter/channel simulator (including gaussian noise gen)
tab.c		lookup table for parity generation
viterbi.c	the encoder and Viterbi decoder routines
vittest.c	driver program for testing

The test program in vittest.c creates a test frame, encodes it, adds
gaussian noise and decodes it. It then repeats a specified number of
times, keeping count of bit errors and errored frames (frames with at
least one error).

The files sim.c and metrics.c are also in my Fano decoder package. See
the readme for that package for further comments on these two files,
including math library performance issues for the gaussian random
number generator in sim.c.

Unlike a sequential decoder, a Viterbi decoder runs at a constant
speed regardless of Eb/N0 ratio. It is also more tolerant of metric
table inaccuracies.

This decoder runs at about 45.5 kilobits/sec in 32-bit (protected)
mode on a 66 MHz 486DX2 using gcc 1.42 and -O under BSDI
1.1. Interestingly, it runs more slowly (about 40.8 kb/s) under GCC
2.5.8, even with full optimization (-O4). Although the code should be
portable, it is very heavily optimized for 32-bit machines and will
probably perform poorly on machines with smaller words.

It's possible to adapt this code for other rates, polynomials and
constraint lengths. It would be somewhat tedious, though, since the
parameters are hardwired in many places.

Phil Karn
March 18, 1995