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Diffstat (limited to 'gnuradio-core/src/lib/viterbi/metrics.c')
| -rw-r--r-- | gnuradio-core/src/lib/viterbi/metrics.c | 126 |
1 files changed, 126 insertions, 0 deletions
diff --git a/gnuradio-core/src/lib/viterbi/metrics.c b/gnuradio-core/src/lib/viterbi/metrics.c new file mode 100644 index 000000000..0d91c301f --- /dev/null +++ b/gnuradio-core/src/lib/viterbi/metrics.c @@ -0,0 +1,126 @@ +/* + * Copyright 1995 Phil Karn, KA9Q + * Copyright 2008 Free Software Foundation, Inc. + * + * This file is part of GNU Radio + * + * GNU Radio is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 3, or (at your option) + * any later version. + * + * GNU Radio is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with GNU Radio; see the file COPYING. If not, write to + * the Free Software Foundation, Inc., 51 Franklin Street, + * Boston, MA 02110-1301, USA. + */ + +/* + * Generate metric tables for a soft-decision convolutional decoder + * assuming gaussian noise on a PSK channel. + * + * Works from "first principles" by evaluating the normal probability + * function and then computing the log-likelihood function + * for every possible received symbol value + * + */ + +#ifdef HAVE_CONFIG_H +#include <config.h> +#endif + +/* Symbols are offset-binary, with 128 corresponding to an erased (no + * information) symbol + */ +#define OFFSET 128 + +#include <stdlib.h> +#include <math.h> + +//declare erf in case it was missing in math.h and provided for by the build system +extern double erf(double x); + +/* Normal function integrated from -Inf to x. Range: 0-1 */ +#define normal(x) (0.5 + 0.5*erf((x)/M_SQRT2)) + +/* Logarithm base 2 */ +#define gr_log2(x) (log(x)*M_LOG2E) + +/* Generate log-likelihood metrics for 8-bit soft quantized channel + * assuming AWGN and BPSK + */ +void +gen_met(int mettab[2][256], /* Metric table, [sent sym][rx symbol] */ + int amp, /* Signal amplitude, units */ + double esn0, /* Es/N0 ratio in dB */ + double bias, /* Metric bias; 0 for viterbi, rate for sequential */ + int scale) /* Scale factor */ +{ + double noise; + int s,bit; + double metrics[2][256]; + double p0,p1; + + /* Es/N0 as power ratio */ + esn0 = pow(10.,esn0/10); + + noise = 0.5/esn0; /* only half the noise for BPSK */ + noise = sqrt(noise); /* noise/signal Voltage ratio */ + + /* Zero is a special value, since this sample includes all + * lower samples that were clipped to this value, i.e., it + * takes the whole lower tail of the curve + */ + p1 = normal(((0-OFFSET+0.5)/amp - 1)/noise); /* P(s|1) */ + + /* Prob of this value occurring for a 0-bit */ /* P(s|0) */ + p0 = normal(((0-OFFSET+0.5)/amp + 1)/noise); + metrics[0][0] = gr_log2(2*p0/(p1+p0)) - bias; + metrics[1][0] = gr_log2(2*p1/(p1+p0)) - bias; + + for(s=1;s<255;s++){ + /* P(s|1), prob of receiving s given 1 transmitted */ + p1 = normal(((s-OFFSET+0.5)/amp - 1)/noise) - + normal(((s-OFFSET-0.5)/amp - 1)/noise); + + /* P(s|0), prob of receiving s given 0 transmitted */ + p0 = normal(((s-OFFSET+0.5)/amp + 1)/noise) - + normal(((s-OFFSET-0.5)/amp + 1)/noise); + +#ifdef notdef + printf("P(%d|1) = %lg, P(%d|0) = %lg\n",s,p1,s,p0); +#endif + metrics[0][s] = gr_log2(2*p0/(p1+p0)) - bias; + metrics[1][s] = gr_log2(2*p1/(p1+p0)) - bias; + } + /* 255 is also a special value */ + /* P(s|1) */ + p1 = 1 - normal(((255-OFFSET-0.5)/amp - 1)/noise); + /* P(s|0) */ + p0 = 1 - normal(((255-OFFSET-0.5)/amp + 1)/noise); + + metrics[0][255] = gr_log2(2*p0/(p1+p0)) - bias; + metrics[1][255] = gr_log2(2*p1/(p1+p0)) - bias; +#ifdef notdef + /* The probability of a raw symbol error is the probability + * that a 1-bit would be received as a sample with value + * 0-128. This is the offset normal curve integrated from -Inf to 0. + */ + printf("symbol Pe = %lg\n",normal(-1/noise)); +#endif + for(bit=0;bit<2;bit++){ + for(s=0;s<256;s++){ + /* Scale and round to nearest integer */ + mettab[bit][s] = floor(metrics[bit][s] * scale + 0.5); +#ifdef notdef + printf("metrics[%d][%d] = %lg, mettab = %d\n", + bit,s,metrics[bit][s],mettab[bit][s]); +#endif + } + } +} |