added files for cch decoding, trying to make them compile

9 files changed, 1080 insertions, 2 deletions

diff --git a/src/lib/decoder/Makefile.am b/src/lib/decoder/Makefile.am
index 32b55e0..3a3e3f5 100644
--- a/src/lib/decoder/Makefile.am
+++ b/src/lib/decoder/Makefile.am
@@ -26,9 +26,18 @@ AM_CPPFLAGS = $(STD_DEFINES_AND_INCLUDES) -I$(MAIN_INCLUDEDIR)
 noinst_LTLIBRARIES = libdecoder.la
 
 libdecoder_la_SOURCES = \
-	sch.c 
+	sch.c		\
+	cch.c 		\
+	fire_crc.c	\
+	gsmstack.c	\
+	interleave.c
         
 noinst_HEADERS = \
 	sch.h \
-	burst_types.h \
+	cch.h 		\
+	fire_crc.h	\
+	gsmstack.h	\
+	interleave.h	\
 	system.h	
+	
+#	burst_types.h 
diff --git a/src/lib/decoder/cch.c b/src/lib/decoder/cch.c
new file mode 100644
index 0000000..468c1f3
--- /dev/null
+++ b/src/lib/decoder/cch.c
@@ -0,0 +1,481 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+#include "system.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <ctype.h>
+
+//#include <exception>
+//#include <stdexcept>
+#include <math.h>
+//#include "burst_types.h"
+#include "cch.h"
+#include "fire_crc.h"
+
+/*
+ * GSM SACCH -- Slow Associated Control Channel
+ *
+ * These messages are encoded exactly the same as on the BCCH.
+ * (Broadcast Control Channel.)
+ *
+ * 	Input: 184 bits
+ * 	
+ * 	1. Add parity and flushing bits. (Output 184 + 40 + 4 = 228 bit)
+ * 	2. Convolutional encode. (Output 228 * 2 = 456 bit)
+ * 	3. Interleave. (Output 456 bit)
+ * 	4. Map on bursts. (4 x 156 bit bursts with each 2x57 bit content data)
+ */
+
+
+/*
+ * Parity (FIRE) for the GSM SACCH channel.
+ *
+ * 	g(x) = (x^23 + 1)(x^17 + x^3 + 1)
+ * 	     = x^40 + x^26 + x^23 + x^17 + x^3 + 1
+ */
+
+static const unsigned char parity_polynomial[PARITY_SIZE + 1] = {
+   1, 0, 0, 0, 0, 0, 0, 0,
+   0, 0, 0, 0, 0, 0, 1, 0,
+   0, 1, 0, 0, 0, 0, 0, 1,
+   0, 0, 0, 0, 0, 0, 0, 0,
+   0, 0, 0, 0, 0, 1, 0, 0,
+   1
+};
+
+// remainder after dividing data polynomial by g(x)
+static const unsigned char parity_remainder[PARITY_SIZE] = {
+   1, 1, 1, 1, 1, 1, 1, 1,
+   1, 1, 1, 1, 1, 1, 1, 1,
+   1, 1, 1, 1, 1, 1, 1, 1,
+   1, 1, 1, 1, 1, 1, 1, 1,
+   1, 1, 1, 1, 1, 1, 1, 1
+};
+
+
+/*
+static void parity_encode(unsigned char *d, unsigned char *p) {
+
+	int i;
+	unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q;
+
+	memcpy(buf, d, DATA_BLOCK_SIZE);
+	memset(buf + DATA_BLOCK_SIZE, 0, PARITY_SIZE);
+
+	for(q = buf; q < buf + DATA_BLOCK_SIZE; q++)
+		if(*q)
+			for(i = 0; i < PARITY_SIZE + 1; i++)
+				q[i] ^= parity_polynomial[i];
+	for(i = 0; i < PARITY_SIZE; i++)
+		p[i] = !buf[DATA_BLOCK_SIZE + i];
+}
+ */
+
+
+static int parity_check(unsigned char *d) {
+
+	unsigned int i;
+	unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q;
+
+	memcpy(buf, d, DATA_BLOCK_SIZE + PARITY_SIZE);
+
+	for(q = buf; q < buf + DATA_BLOCK_SIZE; q++)
+		if(*q)
+			for(i = 0; i < PARITY_SIZE + 1; i++)
+				q[i] ^= parity_polynomial[i];
+	return memcmp(buf + DATA_BLOCK_SIZE, parity_remainder, PARITY_SIZE);
+}
+
+
+/*
+ * Convolutional encoding and Viterbi decoding for the GSM SACCH channel.
+ */
+
+/*
+ * Convolutional encoding:
+ *
+ *	G_0 = 1 + x^3 + x^4
+ *	G_1 = 1 + x + x^3 + x^4
+ *
+ * i.e.,
+ *
+ *	c_{2k} = u_k + u_{k - 3} + u_{k - 4}
+ *	c_{2k + 1} = u_k + u_{k - 1} + u_{k - 3} + u_{k - 4}
+ */
+#define K		5
+#define MAX_ERROR	(2 * CONV_INPUT_SIZE + 1)
+
+
+/*
+ * Given the current state and input bit, what are the output bits?
+ *
+ * 	encode[current_state][input_bit]
+ */
+static const unsigned int encode[1 << (K - 1)][2] = {
+	{0, 3}, {3, 0}, {3, 0}, {0, 3},
+	{0, 3}, {3, 0}, {3, 0}, {0, 3},
+	{1, 2}, {2, 1}, {2, 1}, {1, 2},
+	{1, 2}, {2, 1}, {2, 1}, {1, 2}
+};
+
+
+/*
+ * Given the current state and input bit, what is the next state?
+ * 
+ * 	next_state[current_state][input_bit]
+ */
+static const unsigned int next_state[1 << (K - 1)][2] = {
+	{0, 8}, {0, 8}, {1, 9}, {1, 9},
+	{2, 10}, {2, 10}, {3, 11}, {3, 11},
+	{4, 12}, {4, 12}, {5, 13}, {5, 13},
+	{6, 14}, {6, 14}, {7, 15}, {7, 15}
+};
+
+
+/*
+ * Given the previous state and the current state, what input bit caused
+ * the transition?  If it is impossible to transition between the two
+ * states, the value is 2.
+ *
+ * 	prev_next_state[previous_state][current_state]
+ */
+static const unsigned int prev_next_state[1 << (K - 1)][1 << (K - 1)] = {
+        { 0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2,  2,  2},
+        { 0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2,  2,  2},
+        { 2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2,  2},
+        { 2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2,  2},
+        { 2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2},
+        { 2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2,  2},
+        { 2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2},
+        { 2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2,  2},
+        { 2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2},
+        { 2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2,  2},
+        { 2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2},
+        { 2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2,  2},
+        { 2,  2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2},
+        { 2,  2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1,  2},
+        { 2,  2,  2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1},
+        { 2,  2,  2,  2,  2,  2,  2,  0,  2,  2,  2,  2,  2,  2,  2,  1}
+};
+
+
+static inline unsigned int hamming_distance2(unsigned int w) {
+
+	return (w & 1) + !!(w & 2);
+}
+
+
+/*
+static void conv_encode(unsigned char *data, unsigned char *output) {
+
+	unsigned int i, state = 0, o;
+
+	// encode data
+	for(i = 0; i < CONV_INPUT_SIZE; i++) {
+		o = encode[state][data[i]];
+		state = next_state[state][data[i]];
+		*output++ = !!(o & 2);
+		*output++ = o & 1;
+	}
+}
+ */
+
+
+static int conv_decode(unsigned char *output, unsigned char *data) {
+
+	int i, t;
+	unsigned int rdata, state, nstate, b, o, distance, accumulated_error,
+	   min_state, min_error, cur_state;
+
+	unsigned int ae[1 << (K - 1)];
+	unsigned int nae[1 << (K - 1)]; // next accumulated error
+	unsigned int state_history[1 << (K - 1)][CONV_INPUT_SIZE + 1];
+
+	// initialize accumulated error, assume starting state is 0
+	for(i = 0; i < (1 << (K - 1)); i++)
+		ae[i] = nae[i] = MAX_ERROR;
+	ae[0] = 0;
+
+	// build trellis
+	for(t = 0; t < CONV_INPUT_SIZE; t++) {
+
+		// get received data symbol
+		rdata = (data[2 * t] << 1) | data[2 * t + 1];
+
+		// for each state
+		for(state = 0; state < (1 << (K - 1)); state++) {
+
+			// make sure this state is possible
+			if(ae[state] >= MAX_ERROR)
+				continue;
+
+			// find all states we lead to
+			for(b = 0; b < 2; b++) {
+
+				// get next state given input bit b
+				nstate = next_state[state][b];
+
+				// find output for this transition
+				o = encode[state][b];
+
+				// calculate distance from received data
+				distance = hamming_distance2(rdata ^ o);
+
+				// choose surviving path
+				accumulated_error = ae[state] + distance;
+				if(accumulated_error < nae[nstate]) {
+
+					// save error for surviving state
+					nae[nstate] = accumulated_error;
+
+					// update state history
+					state_history[nstate][t + 1] = state;
+				}
+			}
+		}
+		
+		// get accumulated error ready for next time slice
+		for(i = 0; i < (1 << (K - 1)); i++) {
+			ae[i] = nae[i];
+			nae[i] = MAX_ERROR;
+		}
+	}
+
+	// the final state is the state with the fewest errors
+	min_state = (unsigned int)-1;
+	min_error = MAX_ERROR;
+	for(i = 0; i < (1 << (K - 1)); i++) {
+		if(ae[i] < min_error) {
+			min_state = i;
+			min_error = ae[i];
+		}
+	}
+
+	// trace the path
+	cur_state = min_state;
+	for(t = CONV_INPUT_SIZE; t >= 1; t--) {
+		min_state = cur_state;
+		cur_state = state_history[cur_state][t]; // get previous
+		output[t - 1] = prev_next_state[cur_state][min_state];
+	}
+
+	// return the number of errors detected (hard-decision)
+	return min_error;
+}
+
+
+/*
+ * GSM SACCH interleaving and burst mapping
+ *
+ * Interleaving:
+ *
+ * Given 456 coded input bits, form 4 blocks of 114 bits:
+ *
+ * 	i(B, j) = c(n, k)	k = 0, ..., 455
+ * 				n = 0, ..., N, N + 1, ...
+ * 				B = B_0 + 4n + (k mod 4)
+ * 				j = 2(49k mod 57) + ((k mod 8) div 4)
+ *
+ * Mapping on Burst:
+ *
+ * 	e(B, j) = i(B, j)
+ * 	e(B, 59 + j) = i(B, 57 + j)	j = 0, ..., 56
+ * 	e(B, 57) = h_l(B)
+ * 	e(B, 58) = h_n(B)
+ *
+ * Where h_l(B) and h_n(B) are bits in burst B indicating flags.
+ */
+
+/*
+static void interleave(unsigned char *data, unsigned char *iBLOCK) {
+
+	int j, k, B;
+
+	// for each bit in input data
+	for(k = 0; k < CONV_SIZE; k++) {
+		B = k % 4;
+		j = 2 * ((49 * k) % 57) + ((k % 8) / 4);
+		iBLOCK[B * iBLOCK_SIZE + j] = data[k];
+	}
+}
+ */
+
+
+#if 0
+static void decode_interleave(unsigned char *data, unsigned char *iBLOCK) {
+
+	int j, k, B;
+
+	for(k = 0; k < CONV_SIZE; k++) {
+		B = k % 4;
+		j = 2 * ((49 * k) % 57) + ((k % 8) / 4);
+		data[k] = iBLOCK[B * iBLOCK_SIZE + j];
+	}
+}
+
+#endif
+
+/*
+static void burstmap(unsigned char *iBLOCK, unsigned char *eBLOCK,
+   unsigned char hl, unsigned char hn) {
+
+	int j;
+
+	for(j = 0; j < 57; j++) {
+		eBLOCK[j] = iBLOCK[j];
+		eBLOCK[j + 59] = iBLOCK[j + 57];
+	}
+	eBLOCK[57] = hl;
+	eBLOCK[58] = hn;
+}
+ */
+
+
+static void decode_burstmap(unsigned char *iBLOCK, unsigned char *eBLOCK,
+   unsigned char *hl, unsigned char *hn) {
+
+	int j;
+
+	for(j = 0; j < 57; j++) {
+		iBLOCK[j] = eBLOCK[j];
+		iBLOCK[j + 57] = eBLOCK[j + 59];
+	}
+	*hl = eBLOCK[57];
+	*hn = eBLOCK[58];
+}
+
+
+/*
+ * Transmitted bits are sent least-significant first.
+ */
+static int compress_bits(unsigned char *dbuf, unsigned int dbuf_len,
+   unsigned char *sbuf, unsigned int sbuf_len) {
+
+	unsigned int i, j, c, pos = 0;
+
+	if(dbuf_len < ((sbuf_len + 7) >> 3))
+		return -1;
+
+	for(i = 0; i < sbuf_len; i += 8) {
+		for(j = 0, c = 0; (j < 8) && (i + j < sbuf_len); j++)
+			c |= (!!sbuf[i + j]) << j;
+		dbuf[pos++] = c & 0xff;
+	}
+	return pos;
+}
+
+
+#if 0
+int get_ns_l3_len(unsigned char *data, unsigned int datalen) {
+
+	if((data[0] & 3) != 1) {
+		fprintf(stderr, "error: get_ns_l3_len: pseudo-length reserved "
+		   "bits bad (%2.2x)\n", data[0] & 3);
+		return -1;
+	}
+	return (data[0] >> 2);
+}
+
+#endif
+
+
+static unsigned char *decode_sacch(GS_CTX *ctx, unsigned char *burst, unsigned int *datalen) {
+
+	int errors, len, data_size;
+	unsigned char conv_data[CONV_SIZE], iBLOCK[BLOCKS][iBLOCK_SIZE],
+	   hl, hn, decoded_data[PARITY_OUTPUT_SIZE];
+	FC_CTX fc_ctx;
+
+	data_size = sizeof ctx->msg;
+	if(datalen)
+		*datalen = 0;
+
+	// unmap the bursts
+	decode_burstmap(iBLOCK[0], burst, &hl, &hn); // XXX ignore stealing bits
+	decode_burstmap(iBLOCK[1], burst + 116, &hl, &hn);
+	decode_burstmap(iBLOCK[2], burst + 116 * 2, &hl, &hn);
+	decode_burstmap(iBLOCK[3], burst + 116 * 3, &hl, &hn);
+
+	// remove interleave
+	interleave_decode(&ctx->interleave_ctx, conv_data, (unsigned char *)iBLOCK);
+	//decode_interleave(conv_data, (unsigned char *)iBLOCK);
+
+	// Viterbi decode
+	errors = conv_decode(decoded_data, conv_data);
+	//DEBUGF("conv_decode: %d\n", errors);
+	if (errors)
+		return NULL;
+
+	// check parity
+	// If parity check error detected try to fix it.
+	if (parity_check(decoded_data))
+	{
+		FC_init(&fc_ctx, 40, 184);
+		unsigned char crc_result[224];
+		if (FC_check_crc(&fc_ctx, decoded_data, crc_result) == 0)
+		{
+			errors = -1;
+			DEBUGF("error: sacch: parity error (%d)\n", errors);
+			return NULL;
+		} else {
+			DEBUGF("Successfully corrected parity bits!\n");
+			memcpy(decoded_data, crc_result, sizeof crc_result);
+			errors = 0;
+		}
+	}
+
+	if((len = compress_bits(ctx->msg, data_size, decoded_data,
+	   DATA_BLOCK_SIZE)) < 0) {
+		fprintf(stderr, "error: compress_bits\n");
+		return NULL;
+	}
+	if(len < data_size) {
+		fprintf(stderr, "error: buf too small (%d < %d)\n",
+		   sizeof(ctx->msg), len);
+		return NULL;
+	}
+
+	if(datalen)
+		*datalen = (unsigned int)len;
+	return ctx->msg;
+}
+
+
+/*
+ * decode_cch
+ *
+ * 	Decode a "common" control channel.  Most control channels use
+ * 	the same burst, interleave, Viterbi and parity configuration.
+ * 	The documentation for the control channels defines SACCH first
+ * 	and then just keeps referring to that.
+ *
+ * 	The current (investigated) list is as follows:
+ *
+ * 		BCCH Norm
+ * 		BCCH Ext
+ * 		PCH
+ * 		AGCH
+ * 		CBCH (SDCCH/4)
+ * 		CBCH (SDCCH/8)
+ * 		SDCCH/4
+ * 		SACCH/C4
+ * 		SDCCH/8
+ * 		SACCH/C8
+ *
+ * 	We provide two functions, one for where all four bursts are
+ * 	contiguous, and one where they aren't.
+ */
+unsigned char *decode_cch(GS_CTX *ctx, unsigned char *burst, unsigned int *datalen) {
+
+	return decode_sacch(ctx, burst, datalen);
+}
+
+
+#if 0
+unsigned char *decode_cch(GS_CTX *ctx, unsigned char *e, unsigned int *datalen) {
+
+	return decode_sacch(ctx, e, e + eBLOCK_SIZE, e + 2 * eBLOCK_SIZE,
+	   e + 3 * eBLOCK_SIZE, datalen);
+}
+#endif
diff --git a/src/lib/decoder/cch.h b/src/lib/decoder/cch.h
new file mode 100644
index 0000000..a642721
--- /dev/null
+++ b/src/lib/decoder/cch.h
@@ -0,0 +1,56 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+#ifndef __GSMSTACK_CCH_H__
+#define __GSMSTACK_CCH_H__ 1
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "gsmstack.h"
+
+/*
+ * decode_cch
+ *
+ * 	Decode a "common" control channel.  Most control channels use
+ * 	the same burst, interleave, Viterbi and parity configuration.
+ * 	The documentation for the control channels defines SACCH first
+ * 	and then just keeps referring to that.
+ *
+ * 	The current (investigated) list is as follows:
+ *
+ * 		BCCH Norm
+ * 		BCCH Ext
+ * 		PCH
+ * 		AGCH
+ * 		CBCH (SDCCH/4)
+ * 		CBCH (SDCCH/8)
+ * 		SDCCH/4
+ * 		SACCH/C4
+ * 		SDCCH/8
+ * 		SACCH/C8
+ *
+ * 	We provide two functions, one for where all four bursts are
+ * 	contiguous, and one where they aren't.
+ */
+
+#define DATA_BLOCK_SIZE		184
+#define PARITY_SIZE		40
+#define FLUSH_BITS_SIZE		4
+#define PARITY_OUTPUT_SIZE (DATA_BLOCK_SIZE + PARITY_SIZE + FLUSH_BITS_SIZE)
+
+#define CONV_INPUT_SIZE		PARITY_OUTPUT_SIZE
+#define CONV_SIZE		(2 * CONV_INPUT_SIZE)
+
+#define BLOCKS			4
+#define iBLOCK_SIZE		(CONV_SIZE / BLOCKS)
+#define eBLOCK_SIZE		(iBLOCK_SIZE + 2)
+
+unsigned char *decode_cch(GS_CTX *ctx, unsigned char *burst, unsigned int *len);
+//unsigned char *decode_cch(GS_CTX *ctx, unsigned char *, unsigned char *, unsigned char *, unsigned char *, unsigned int *len);
+//unsigned char *decode_cch(GS_CTX *ctx, unsigned char *, unsigned int *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/lib/decoder/fire_crc.c b/src/lib/decoder/fire_crc.c
new file mode 100644
index 0000000..7cdfc0b
--- /dev/null
+++ b/src/lib/decoder/fire_crc.c
@@ -0,0 +1,179 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+/* -*- c++ -*- */
+/*
+ * Copyright 2005 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 2, 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., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include "fire_crc.h"
+#include <stdio.h>
+#include <string.h>
+
+#define REM(x, y)	(x) % (y)
+
+static int FC_syndrome_shift(FC_CTX *ctx, unsigned int bit);
+
+static int
+outit(int *data, int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++)
+		printf("%d ", data[i]);
+	printf("\n");
+}
+
+int
+FC_init(FC_CTX *ctx, unsigned int crc_size, unsigned int data_size)
+{
+	ctx->crc_size = crc_size;
+	ctx->data_size = data_size;
+	ctx->syn_start = 0;
+
+	return 0;
+}
+
+int
+FC_check_crc(FC_CTX *ctx, unsigned char *input_bits, unsigned char *control_data)
+{ 
+	int j,error_count = 0, error_index = 0, success_flag = 0, syn_index = 0;
+	unsigned int i;
+
+	ctx->syn_start = 0;
+	// reset the syndrome register
+	memset(ctx->syndrome_reg, 0, sizeof ctx->syndrome_reg);
+
+	// shift in the data bits
+    for (i=0; i < ctx->data_size; i++) {
+        error_count = FC_syndrome_shift(ctx, input_bits[i]);
+        control_data[i] = input_bits[i];
+    }
+
+    // shift in the crc bits
+    for (i=0; i < ctx->crc_size; i++) {
+        error_count = FC_syndrome_shift(ctx, 1-input_bits[i+ctx->data_size]);
+    }
+
+    // Find position of error burst
+    if (error_count == 0) {
+        error_index = 0;
+    }
+    else {
+        error_index = 1;
+        error_count = FC_syndrome_shift(ctx, 0);
+        error_index += 1;
+        while (error_index < (ctx->data_size + ctx->crc_size) ) {
+            error_count = FC_syndrome_shift(ctx, 0);
+            error_index += 1;
+            if ( error_count == 0 ) break;
+        }
+    }
+
+    // Test for correctable errors
+    //printf("error_index %d\n",error_index);
+    if (error_index == 224) success_flag = 0;
+    else {
+
+        // correct index depending on the position of the error
+        if (error_index == 0) syn_index = error_index;
+        else syn_index = error_index - 1;
+        
+        // error burst lies within data bits
+        if (error_index < 184) {
+            //printf("error < bit 184,%d\n",error_index);
+            j = error_index;
+            while ( j < (error_index+12) ) {
+                if (j < 184) {
+                    control_data[j] = control_data[j] ^ 
+                       ctx->syndrome_reg[REM(ctx->syn_start+39-j+syn_index,40)];
+                }               
+                else break;
+                j = j + 1;
+            }
+        }
+        else if ( error_index > 212 ) {
+            //printf("error > bit 212,%d\n",error_index);
+            j = 0;
+            while ( j < (error_index - 212) ) {
+                control_data[j] = control_data[j] ^ 
+                      ctx->syndrome_reg[REM(ctx->syn_start+39-j-224+syn_index,40)];
+                j = j + 1;
+            }
+        }
+        // for 183 < error_index < 213 error in parity alone so ignore
+        success_flag = 1;
+    }
+    return success_flag;
+}    
+
+static int 
+FC_syndrome_shift(FC_CTX *ctx, unsigned int bit)
+{
+	int error_count = 0;
+	unsigned int i;
+
+	if (ctx->syn_start == 0)
+		ctx->syn_start = 39;
+	else ctx->syn_start -= 1;
+
+	int temp_syndrome_reg[sizeof ctx->syndrome_reg];
+
+	memcpy(temp_syndrome_reg, ctx->syndrome_reg, sizeof temp_syndrome_reg);
+
+    temp_syndrome_reg[REM(ctx->syn_start+3,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+3,40)] ^
+                       ctx->syndrome_reg[ctx->syn_start];
+    temp_syndrome_reg[REM(ctx->syn_start+17,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+17,40)] ^
+                       ctx->syndrome_reg[ctx->syn_start];
+    temp_syndrome_reg[REM(ctx->syn_start+23,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+23,40)] ^
+                       ctx->syndrome_reg[ctx->syn_start];
+    temp_syndrome_reg[REM(ctx->syn_start+26,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+26,40)] ^
+                       ctx->syndrome_reg[ctx->syn_start];
+
+    temp_syndrome_reg[REM(ctx->syn_start+4,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+4,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+6,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+6,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+10,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+10,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+16,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+16,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+27,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+27,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+29,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+29,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+33,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+33,40)] ^ bit;
+    temp_syndrome_reg[REM(ctx->syn_start+39,40)] = 
+                       ctx->syndrome_reg[REM(ctx->syn_start+39,40)] ^ bit;
+
+    temp_syndrome_reg[ctx->syn_start] = ctx->syndrome_reg[ctx->syn_start] ^ bit;
+
+	memcpy(ctx->syndrome_reg, temp_syndrome_reg, sizeof ctx->syndrome_reg);
+
+    for (i = 0; i < 28; i++) {
+       error_count = error_count + ctx->syndrome_reg[REM(ctx->syn_start+i,40)];
+    }
+    return error_count;
+}
+ 
+
diff --git a/src/lib/decoder/fire_crc.h b/src/lib/decoder/fire_crc.h
new file mode 100644
index 0000000..aa6319c
--- /dev/null
+++ b/src/lib/decoder/fire_crc.h
@@ -0,0 +1,47 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+/* -*- c++ -*- */
+/*
+ * Copyright 2005 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 2, 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., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+
+#ifndef INCLUDED_FIRE_CRC_H
+#define INCLUDED_FIRE_CRC_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct
+{
+	unsigned int crc_size;
+	unsigned int data_size;
+	unsigned int syn_start;
+	int syndrome_reg[40];
+} FC_CTX;
+
+int FC_init(FC_CTX *ctx, unsigned int crc_size, unsigned int data_size);
+int FC_check_crc(FC_CTX *ctx, unsigned char *input_bits, unsigned char *control_data);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/lib/decoder/gsmstack.c b/src/lib/decoder/gsmstack.c
new file mode 100644
index 0000000..0f2aca6
--- /dev/null
+++ b/src/lib/decoder/gsmstack.c
@@ -0,0 +1,197 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+/*
+ * Invoke gsmstack() with any kind of burst. Automaticly decode and retrieve
+ * information.
+ */
+#include "system.h"
+#include <stdlib.h>
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include "gsmstack.h"
+#include "gsm_constants.h"
+#include "gsm_receiver_config.h"
+#include "interleave.h"
+#include "sch.h"
+#include "cch.h"
+
+//#include "out_pcap.h"
+
+static void out_gsmdecode(char type, int arfcn, int ts, int fn, char *data, int len);
+
+/* encode a decoded burst (1 bit per byte) into 8-bit-per-byte */
+static void burst_octify(unsigned char *dest, 
+			 const unsigned char *data, int length)
+{
+	int bitpos = 0;
+
+	while (bitpos < USEFUL_BITS) {
+		unsigned char tbyte;
+		int i; 
+
+		tbyte = 0;
+		for (i = 0; (i < 8) && (bitpos < length); i++) {
+			tbyte <<= 1;
+			tbyte |= data[bitpos++];
+		}
+		if (i < 8)
+			tbyte <<= 8 - i;
+		*dest++ = tbyte;
+	}	
+}
+
+
+#if 0
+static void
+diff_decode(char *dst, char *src, int len)
+{
+	const char *end = src + len;
+	unsigned char last;
+
+	src += 3;
+	last = 0;
+	memset(dst, 0, 3);
+	dst += 3;
+
+	while (src < end)
+	{
+		*dst = !*src ^ last;
+		last = *dst;
+		src++;
+		dst++;
+	}
+}
+#endif
+
+/*
+ * Initialize a new GSMSTACK context.
+ */
+int
+GS_new(GS_CTX *ctx)
+{
+	memset(ctx, 0, sizeof *ctx);
+	interleave_init(&ctx->interleave_ctx, 456, 114);
+	ctx->fn = -1;
+	ctx->bsic = -1;
+
+	ctx->tun_fd = mktun("gsm", ctx->ether_addr);
+	if (ctx->tun_fd < 0)
+		fprintf(stderr, "cannot open 'gsm' tun device, did you create it?\n");
+
+	ctx->pcap_fd = open_pcap_file("tvoid.pcap");
+	if (ctx->pcap_fd < 0)
+		fprintf(stderr, "cannot open PCAP file: %s\n", strerror(errno));
+
+	ctx->burst_pcap_fd = open_pcap_file("tvoid-burst.pcap");
+	if (ctx->burst_pcap_fd < 0)
+		fprintf(stderr, "cannot open burst PCAP file: %s\n", strerror(errno));
+
+	return 0;
+}
+
+#define BURST_BYTES	((USEFUL_BITS/8)+1)
+/*
+ * 142 bit
+ */
+int
+GS_process(GS_CTX *ctx, int ts, int type, const unsigned char *src)
+{
+	int fn;
+	int bsic;
+	int ret;
+	unsigned char *data;
+	int len;
+	struct gs_ts_ctx *ts_ctx = &ctx->ts_ctx[ts];
+	unsigned char octified[BURST_BYTES];
+
+	memset(ctx->msg, 0, sizeof(ctx->msg));
+
+	/* write burst to burst PCAP file */
+	burst_octify(octified, src, USEFUL_BITS);
+	write_pcap_packet(ctx->burst_pcap_fd, 0 /* arfcn */, ts, ctx->fn,
+			  1, type, octified, BURST_BYTES);
+	
+#if 0
+	if (ts != 0) {
+		/* non-0 timeslots should end up in PCAP */
+		data = decode_cch(ctx, ctx->burst, &len);
+		if (data == NULL)
+			return -1;
+		write_pcap_packet(ctx->pcap_fd, 0 /* arfcn */, ts, ctx->fn, data, len);
+		return;
+	}
+#endif
+
+	if (ts == 0) {
+		if (type == sch_burst) {
+			ret = decode_sch(src, &fn, &bsic);
+			if (ret != 0)
+				return 0;
+			if ((ctx->bsic > 0) && (bsic != ctx->bsic))
+				fprintf(stderr, "WARN: BSIC changed.\n");
+			//DEBUGF("FN %d, BSIC %d\n", fn, bsic);
+			ctx->fn = fn;
+			ctx->bsic = bsic;
+			/* Reset message concatenator */
+			ts_ctx->burst_count = 0;
+			return 0;
+		}
+
+		/* If we did not get Frame Number yet then return */
+		if (ctx->fn < 0)
+			return 0;
+
+		ctx->fn++;
+	}
+
+	if (type == normal_burst) {
+		/* Interested in these frame numbers (cch)
+ 		 * 2-5, 12-15, 22-25, 23-35, 42-45
+ 		 * 6-9, 16-19, 26-29, 36-39, 46-49
+ 		 */
+		/* Copy content data into new array */
+		//DEBUGF("burst count %d\n", ctx->burst_count);
+		memcpy(ts_ctx->burst + (116 * ts_ctx->burst_count), src, 58);
+		memcpy(ts_ctx->burst + (116 * ts_ctx->burst_count) + 58, src + 58 + 26, 58);
+		ts_ctx->burst_count++;
+		/* Return if not enough bursts for a full gsm message */
+		if (ts_ctx->burst_count < 4)
+			return 0;
+
+		ts_ctx->burst_count = 0;
+		data = decode_cch(ctx, ts_ctx->burst, &len);
+		if (data == NULL) {
+			DEBUGF("cannot decode fnr=0x%08x ts=%d\n", ctx->fn, ts);
+			return -1;
+		}
+		//DEBUGF("OK TS %d, len %d\n", ts, len);
+
+		out_gsmdecode(0, 0, ts, ctx->fn - 4, data, len);
+		write_interface(ctx->tun_fd, data+1, len-1, ctx->ether_addr);
+		write_pcap_packet(ctx->pcap_fd, 0 /* arfcn */, ts, ctx->fn,
+				  0, normal_burst, data, len);
+#if 0
+		if (ctx->fn % 51 != 0) && ( (((ctx->fn % 51 + 5) % 10 == 0) || (((ctx->fn % 51) + 1) % 10 ==0) ) )
+			ready = 1;
+#endif
+		
+		return 0;
+	}
+}
+
+
+/*
+ * Output data so that it can be parsed from gsmdeocde.
+ */
+static void
+out_gsmdecode(char type, int arfcn, int ts, int fn, char *data, int len)
+{
+	char *end = data + len;
+
+	/* FIXME: speed this up by first printing into an array */
+	while (data < end)
+		printf(" %02.2x", (unsigned char)*data++);
+	printf("\n");
+	fflush(stdout);
+}
+
diff --git a/src/lib/decoder/gsmstack.h b/src/lib/decoder/gsmstack.h
new file mode 100644
index 0000000..9c5730e
--- /dev/null
+++ b/src/lib/decoder/gsmstack.h
@@ -0,0 +1,43 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+#ifndef __GSMSTACK_H__
+#define __GSMSTACK_H__ 1
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <linux/if_ether.h>
+#include "interleave.h"
+
+struct gs_ts_ctx {
+	/* FIXME: later do this per each ts per each arfcn */
+	unsigned char burst[4 * 58 * 2];
+	int burst_count;
+};
+
+typedef struct
+{
+	int flags;
+	int fn;
+	int bsic;
+	char msg[23];	/* last decoded message */
+
+	INTERLEAVE_CTX interleave_ctx;
+
+	struct gs_ts_ctx ts_ctx[8];
+
+	int tun_fd;
+	unsigned char ether_addr[ETH_ALEN];
+
+	int pcap_fd;
+	int burst_pcap_fd;
+} GS_CTX;
+
+int GS_new(GS_CTX *ctx);
+int GS_process(GS_CTX *ctx, int ts, int type, const unsigned char *src);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/lib/decoder/interleave.c b/src/lib/decoder/interleave.c
new file mode 100644
index 0000000..7b45927
--- /dev/null
+++ b/src/lib/decoder/interleave.c
@@ -0,0 +1,47 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+#include <stdlib.h>
+#include <stdio.h>
+#include "interleave.h"
+
+int
+interleave_init(INTERLEAVE_CTX *ictx, int size, int block_size)
+{
+	ictx->trans_size = size;
+	ictx->trans = (unsigned short *)malloc(size * sizeof *ictx->trans);
+
+//	DEBUGF("size: %d\n", size);
+//	DEBUGF("Block size: %d\n", block_size);
+	int j, k, B;
+	for (k = 0; k < size; k++)
+	{
+		B = k % 4;
+		j = 2 * ((49 * k) % 57) + ((k % 8) / 4);
+		ictx->trans[k] = B * block_size + j;
+		/* Mapping: pos1 goes to pos2: pos1 -> pos2 */
+//		DEBUGF("%d -> %d\n", ictx->trans[k], k);
+	}
+//	exit(0);
+	return 0;
+}
+
+int
+interleave_deinit(INTERLEAVE_CTX *ictx)
+{
+	if (ictx->trans != NULL)
+	{
+		free(ictx->trans);
+		ictx->trans = NULL;
+	}
+
+	return 0;
+}
+
+void
+interleave_decode(INTERLEAVE_CTX *ictx, unsigned char *dst, unsigned char *src)
+{
+
+	int k;
+	for (k = 0; k < ictx->trans_size; k++)
+		dst[k] = src[ictx->trans[k]];
+}
+
diff --git a/src/lib/decoder/interleave.h b/src/lib/decoder/interleave.h
new file mode 100644
index 0000000..fa1912f
--- /dev/null
+++ b/src/lib/decoder/interleave.h
@@ -0,0 +1,19 @@
+//TODO: this file shouldn't be part of the GSM Receiver
+/*
+ * $Id:$
+ */
+
+#ifndef __GSMSP_INTERLEAVE_H__
+#define __GSMSP_INTERLEAVE_H__ 1
+
+typedef struct _interleave_ctx
+{
+	unsigned short *trans;
+	int trans_size;
+} INTERLEAVE_CTX;
+
+int interleave_init(INTERLEAVE_CTX *ictx, int size, int block_size);
+int interleave_deinit(INTERLEAVE_CTX *ictx);
+void interleave_decode(INTERLEAVE_CTX *ictx, unsigned char *dst, unsigned char *src);
+
+#endif