--- /dev/null
+/*
+ * 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>
+
+/* 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
+ }
+ }
+}
projects / debian / gnuradio / blobdiff