channel decoding (gsmstack)

11 files changed, 1286 insertions, 1 deletions

diff --git a/gsm-tvoid/src/lib/burst_types.h b/gsm-tvoid/src/lib/burst_types.h
new file mode 100644
index 0000000..90d35ad
--- /dev/null
+++ b/gsm-tvoid/src/lib/burst_types.h
@@ -0,0 +1,213 @@
+// $Id: burst_types.h,v 1.5 2007/03/14 05:44:53 jl Exp $
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "gsm_constants.h"
+
+static const int TB_LEN	= 3;
+static const int TB_OS1	= 0;
+static const int TB_OS2	= 145;
+static const unsigned char tail_bits[] = {0, 0, 0};
+
+/*
+ * The normal burst is used to carry information on traffic and control
+ * channels.
+ */
+static const int N_TSC_NUM	= 8;	// number of training sequence codes
+static const int N_TSC_CODE_LEN	= 26;	// length of tsc
+static const int N_TSC_OS	= 61;	// tsc offset
+static const int N_EDATA_LEN_1	= 58;	// length of first data section
+static const int N_EDATA_OS_1	= 3;	// offset of first data section
+static const int N_EDATA_LEN_2	= 58;	// length of second data section
+static const int N_EDATA_OS_2	= 87;	// offset of second data section
+#if 0
+static const unsigned char n_tsc[][N_TSC_CODE_LEN] = {
+/* 0 */	{
+		0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 0,
+		0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1
+	},
+/* 1 */	{
+		0, 0, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1,
+		1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1 
+	},
+/* 2 */	{
+		0, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 1,
+		0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 1, 0
+	},
+/* 3 */	{
+		0, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 0,
+		1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 1, 1, 0
+	},
+/* 4 */	{
+		0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0,
+		1, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1
+	},
+/* 5 */	{
+		0, 1, 0, 0, 1, 1, 1, 0, 1, 0, 1, 1, 0,
+		0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1, 0
+	},
+/* 6 */	{
+		1, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 1, 1,
+		0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 1
+	},
+/* 7 */	{
+		1, 1, 1, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0,
+		0, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 0
+	}
+};
+
+#endif
+
+/*
+ * The frequency correction burst is used for frequency synchronization
+ * of the mobile.  This is broadcast in TS0 together with the SCH and
+ * BCCH.
+ *
+ * Modulating the bits below causes a spike at 62.5kHz above (below for
+ * COMPACT) the center frequency.  One can use this spike with a narrow
+ * band filter to accurately determine the center of the channel.
+ */
+static const int FC_CODE_LEN	= 142;
+static const int FC_OS		= 3;
+static const unsigned char fc_fb[] = {
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+};
+
+static const unsigned char fc_compact_fb[] = {
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
+	1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0
+};
+
+
+/*
+ * The synchronization burst is used for time synchronization of the
+ * mobile.  The bits given below were chosen for their correlation
+ * properties.  The synchronization channel (SCH) contains a long
+ * training sequence (given below) and carries the TDMA frame number and
+ * base station identity code.  It is broadcast in TS0 in the frame
+ * following the frequency correction burst.
+ */
+static const int SB_CODE_LEN	= 64;
+static const int SB_ETS_OS	= 42;
+static const int SB_EDATA_LEN_1	= 39;
+static const int SB_EDATA_OS_1	= 3;
+static const int SB_EDATA_LEN_2	= 39;
+static const int SB_EDATA_OS_2	= 106;
+static const unsigned char sb_etsc[] = {
+	1, 0, 1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 1, 0,
+	0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1,
+	0, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1,
+	0, 1, 1, 1, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 1
+};
+
+static const unsigned char sb_cts_etsc[] = {
+	1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1,
+	0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0,
+	1, 1, 1, 1, 0, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 0,
+	1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 1, 0, 1, 0, 1
+};
+
+static const unsigned char sb_compact_etsc[] = {
+	1, 1, 1, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1,
+	0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0,
+	0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0,
+	0, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 1, 1, 1, 0
+};
+
+
+/*
+ * A base tranceiver station must transmit a burst in every timeslot of
+ * every TDMA frame in channel C0.  The dummy burst will be transmitted
+ * on all timeslots of all TDMA frames for which no other channel
+ * requires a burst to be transmitted.
+ */
+static const int D_CODE_LEN	= 142;
+static const int D_MB_OS	= 3;
+static const unsigned char d_mb[] = {
+	1, 1, 1, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 0,
+	0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 1, 0,
+	0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0,
+	0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0,
+	0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 1, 1, 0, 0,
+	0, 1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0,
+	0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1,
+	1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1,
+	0, 0, 1, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0
+};
+
+
+/*
+ * The access burst is used for random access from a mobile.
+ */
+static const int AB_ETB_CODE_LEN	= 8;
+static const int AB_ETB_OS		= 0;
+static const unsigned char ab_etb[] = {
+	0, 0, 1, 1, 1, 0, 1, 0
+};
+
+static const int AB_SSB_CODE_LEN	= 41;
+static const int AB_SSB_OS		= 8;
+static const unsigned char ab_ssb[] = {
+	0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1,
+	1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0,
+	0, 0, 1, 1, 1, 1, 0, 0, 0
+};
+
+static const unsigned char ab_ts1_ssb[] = {
+	0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0,
+	1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 0, 1, 1, 1, 1,
+	0, 0, 1, 0, 0, 1, 1, 0, 1
+};
+
+static const unsigned char ab_ts2_ssb[] = {
+	1, 1, 1, 0, 1, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1,
+	0, 1, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 1,
+	1, 0, 1, 1, 1, 0, 1, 1, 1
+};
+
+
+typedef enum {
+	burst_n_0,
+	burst_n_1,
+	burst_n_2,
+	burst_n_3,
+	burst_n_4,
+	burst_n_5,
+	burst_n_6,
+	burst_n_7,
+	burst_fc,
+	burst_fc_c,
+	burst_s,
+	burst_s_cts,
+	burst_s_c,
+	burst_d,
+	burst_a,
+	burst_a_ts1,
+	burst_a_ts2,
+	burst_not_a_burst
+} burst_t;
+
+static const int N_BURST_TYPES	= ((int)(burst_not_a_burst) + 1);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/gsm-tvoid/src/lib/conv.c b/gsm-tvoid/src/lib/conv.c
new file mode 100644
index 0000000..eb97e30
--- /dev/null
+++ b/gsm-tvoid/src/lib/conv.c
@@ -0,0 +1,487 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <ctype.h>
+
+#include <math.h>
+
+//#include "burst_types.h"
+#include "conv.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];
+}
+ */
+
+
+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;
+	}
+}
+ */
+
+
+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.
+ */
+int
+compress_bits(unsigned char *dbuf, int dbuf_len,
+   unsigned char *sbuf, 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
+
+#if 0
+static unsigned char *decode_sacch(unsigned char *e0, unsigned char *e1,
+   unsigned char *e2, unsigned char *e3, unsigned int *datalen) {
+
+	int errors, len, data_size = (DATA_BLOCK_SIZE + 7) >> 3;
+	unsigned char conv_data[CONV_SIZE], iBLOCK[BLOCKS][iBLOCK_SIZE],
+	   hl, hn, decoded_data[PARITY_OUTPUT_SIZE], *data;
+
+	if(!(data = (unsigned char *)malloc(data_size))) {
+		throw std::runtime_error("error: decode_cch: malloc");
+	}
+
+	if(datalen)
+		*datalen = 0;
+
+	// unmap the bursts
+	decode_burstmap(iBLOCK[0], e0, &hl, &hn); // XXX ignore stealing bits
+	decode_burstmap(iBLOCK[1], e1, &hl, &hn);
+	decode_burstmap(iBLOCK[2], e2, &hl, &hn);
+	decode_burstmap(iBLOCK[3], e3, &hl, &hn);
+
+	// remove interleave
+	interleave_decode(&opt.ictx, 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))
+	{
+		fire_crc crc(40, 184);
+		// fprintf(stderr, "error: sacch: parity error (%d)\n", errors);
+		unsigned char crc_result[224];
+		if (crc.check_crc(decoded_data, crc_result) == 0)
+		{
+			errors = -1;
+			DEBUGF("error: sacch: parity error (%d)\n", errors);
+			return NULL;
+		} else {
+			memcpy(decoded_data, crc_result, sizeof crc_result);
+			errors = 0;
+		}
+	}
+
+	if((len = compress_bits(data, data_size, decoded_data,
+	   DATA_BLOCK_SIZE)) < 0) {
+		fprintf(stderr, "error: compress_bits\n");
+		return 0;
+	}
+	if(len < data_size) {
+		fprintf(stderr, "error: buf too small (%d < %d)\n",
+		   sizeof(data), len);
+		return 0;
+	}
+
+	if(datalen)
+		*datalen = (unsigned int)len;
+	return data;
+}
+
+#endif
+
+/*
+ * 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.
+ */
+#if 0
+unsigned char *decode_cch(unsigned char *e0, unsigned char *e1,
+   unsigned char *e2, unsigned char *e3, unsigned int *datalen) {
+
+	return decode_sacch(e0, e1, e2, e3, datalen);
+}
+
+
+unsigned char *decode_cch(unsigned char *e, unsigned int *datalen) {
+
+	return decode_sacch(e, e + eBLOCK_SIZE, e + 2 * eBLOCK_SIZE,
+	   e + 3 * eBLOCK_SIZE, datalen);
+}
+#endif
diff --git a/gsm-tvoid/src/lib/conv.h b/gsm-tvoid/src/lib/conv.h
new file mode 100644
index 0000000..0d7505d
--- /dev/null
+++ b/gsm-tvoid/src/lib/conv.h
@@ -0,0 +1,42 @@
+
+/*
+ * 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)
+
+int conv_decode(unsigned char *output, unsigned char *data);
+int parity_check(unsigned char *data);
+int compress_bits(unsigned char *dbuf, int dlen, unsigned char *src, int len);
+
diff --git a/gsm-tvoid/src/lib/gsm_burst.cc b/gsm-tvoid/src/lib/gsm_burst.cc
index f367de2..5415b2f 100755
--- a/gsm-tvoid/src/lib/gsm_burst.cc
+++ b/gsm-tvoid/src/lib/gsm_burst.cc
@@ -9,6 +9,8 @@
 #include <memory.h>
 #include <gsm_burst.h>
 #include <assert.h>
+#include "system.h"
+#include "gsmstack.h"
 
 gsm_burst::gsm_burst (void) :
 	  	d_bbuf_pos(0),
@@ -67,6 +69,8 @@ gsm_burst::gsm_burst (void) :
 		fprintf(stdout,"\n");
 	}
 		
+	/* Initialize GSM Stack */
+	GS_new(&d_gs_ctx);
 }
 
 gsm_burst::~gsm_burst ()
@@ -99,6 +103,16 @@ void gsm_burst::print_bits(const float *data,int length)
 		data[i] < 0 ? fprintf(stdout,"+") : fprintf(stdout,".");
 		
 }
+void gsm_burst::soft2hardbit(char *dst, const float *data, int len)
+{
+	for (int i=0; i < len; i++)
+	{
+		if (data[i] < 0)
+			dst[i] = 0;
+		else
+			dst[i] = 1;
+	}
+}
 
 void gsm_burst::print_burst(void)
 {
@@ -125,13 +139,32 @@ void gsm_burst::print_burst(void)
 
 	if ( print && (d_print_options & PRINT_BITS) ) {
 		if (d_print_options & PRINT_ALL_BITS)
+		{
 			print_bits(d_burst_buffer,BBUF_SIZE);
-		else
+		} else {
+			/* 142 useful bits: 2*58 + 26 training */
 			print_bits(d_burst_buffer + d_burst_start,USEFUL_BITS);
+		}
 		
 		fprintf(stdout," ");
 	}
 	
+	/*
+	 * Pass information to GSM stack. GSM stack will try to extract
+	 * information (fn, layer 2 messages, ...)
+	 */
+
+	char buf[156];
+	/* In hardbits include the 3 trial bits */
+	/* FIXME: access burst has 8 trail bits? what is d_burst_start
+ 	 * set to? make sure we start at the right position here.
+ 	 */
+	soft2hardbit(buf, d_burst_buffer + d_burst_start - 3, 156);
+	/* GS_process will differentially decode the data and then
+ 	 * extract SCH infos (and later bcch infos).
+ 	 */
+	GS_process(&d_gs_ctx, d_ts, d_burst_type, buf);
+
 	if (print) {
 
 		fprintf(stdout,"%d/%d/%+d/%lu/%lu ",
diff --git a/gsm-tvoid/src/lib/gsmstack.c b/gsm-tvoid/src/lib/gsmstack.c
new file mode 100644
index 0000000..9fc4868
--- /dev/null
+++ b/gsm-tvoid/src/lib/gsmstack.c
@@ -0,0 +1,74 @@
+/*
+ * Invoke gsmstack() with any kind of burst. Automaticly decode and retrieve
+ * information.
+ */
+#include "system.h"
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include "gsmstack.h"
+#include "gsm_constants.h"
+#include "interleave.h"
+#include "sch.h"
+
+INTERLEAVE_CTX ictx;
+
+static void
+diff_decode(char *dst, char *src, int len)
+{
+	const unsigned 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++;
+	}
+}
+
+/*
+ * Initialize a new GSMSTACK context.
+ */
+int
+GS_new(GS_CTX *ctx)
+{
+	memset(ctx, 0, sizeof *ctx);
+	interleave_init(&ictx, 456, 114);
+
+	return 0;
+}
+
+/*
+ * 156 bit
+ */
+int
+GS_process(GS_CTX *ctx, int ts, int type, char *data)
+{
+	int fn;
+	int bsic;
+	int ret;
+	char buf[156];
+
+	diff_decode(buf, data, 156);
+
+	switch (type)
+	{
+	case SCH:
+		ret = decode_sch(buf, &fn, &bsic);
+		if (ret != 0)
+			break;
+		DEBUGF("FN %d, BSIC %d\n", fn, bsic);
+		break;
+	case NORMAL:
+		break;
+	}
+}
+
+
diff --git a/gsm-tvoid/src/lib/gsmstack.h b/gsm-tvoid/src/lib/gsmstack.h
new file mode 100644
index 0000000..9f0d9d8
--- /dev/null
+++ b/gsm-tvoid/src/lib/gsmstack.h
@@ -0,0 +1,21 @@
+
+#ifndef __GSMSTACK_H__
+#define __GSMSTACK_H__ 1
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct
+{
+	int flags;
+} GS_CTX;
+
+int GS_new(GS_CTX *ctx);
+int GS_process(GS_CTX *ctx, int ts, int type, char *data);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/gsm-tvoid/src/lib/interleave.c b/gsm-tvoid/src/lib/interleave.c
new file mode 100644
index 0000000..437ed92
--- /dev/null
+++ b/gsm-tvoid/src/lib/interleave.c
@@ -0,0 +1,47 @@
+
+#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/gsm-tvoid/src/lib/interleave.h b/gsm-tvoid/src/lib/interleave.h
new file mode 100644
index 0000000..b1abf81
--- /dev/null
+++ b/gsm-tvoid/src/lib/interleave.h
@@ -0,0 +1,18 @@
+/*
+ * $Id:$
+ */
+
+#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
diff --git a/gsm-tvoid/src/lib/sch.c b/gsm-tvoid/src/lib/sch.c
new file mode 100644
index 0000000..c01d7bb
--- /dev/null
+++ b/gsm-tvoid/src/lib/sch.c
@@ -0,0 +1,323 @@
+#include "system.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include "burst_types.h"
+
+/*
+ * Synchronization channel.
+ *
+ * Timeslot	Repeat length		Frame Number (mod repeat length)
+ * 0		51			1, 11, 21, 31, 41
+ */
+
+/*
+ * Parity (FIRE) for the GSM SCH.
+ *
+ * 	g(x) = x^10 + x^8 + x^6 + x^5 + x^4 + x^2 + 1
+ */
+#define DATA_BLOCK_SIZE		25
+#define PARITY_SIZE		10
+#define TAIL_BITS_SIZE		4
+#define PARITY_OUTPUT_SIZE (DATA_BLOCK_SIZE + PARITY_SIZE + TAIL_BITS_SIZE)
+
+static const unsigned char parity_polynomial[PARITY_SIZE + 1] = {
+	1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1
+};
+
+static const unsigned char parity_remainder[PARITY_SIZE] = {
+	1, 1, 1, 1, 1, 1, 1, 1, 1, 1
+};
+
+
+static void parity_encode(unsigned char *d, unsigned char *p) {
+
+	unsigned 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 SCH.
+ * (Equivalent to the GSM SACCH.)
+ *
+ * 	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 CONV_INPUT_SIZE		PARITY_OUTPUT_SIZE
+#define CONV_SIZE		(2 * CONV_INPUT_SIZE)
+#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 *data, unsigned char *output) {
+
+	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;
+}
+
+
+int decode_sch(const unsigned char *buf, int *fn_o, int *bsic_o) {
+
+	int errors, bsic, t1, t2, t3p, t3, fn, tt;
+	unsigned char data[CONV_SIZE], decoded_data[PARITY_OUTPUT_SIZE];
+
+	// extract encoded data from synchronization burst
+	/* buf + 3, 39 bit */
+	/* buf + 3 + 39 + 64 = 106, 39 */
+	memcpy(data, buf + SB_EDATA_OS_1, SB_EDATA_LEN_1);
+	memcpy(data + SB_EDATA_LEN_1, buf + SB_EDATA_OS_2, SB_EDATA_LEN_2);
+
+	// Viterbi decode
+	if(errors = conv_decode(data, decoded_data)) {
+		// fprintf(stderr, "error: sch: conv_decode (%d)\n", errors);
+		DEBUGF("ERR: conv_decode %d\n", errors);
+		return errors;
+	}
+
+	// check parity
+	if(parity_check(decoded_data)) {
+		// fprintf(stderr, "error: sch: parity failed\n");
+		DEBUGF("ERR: parity_check failed\n");
+		return 1;
+	}
+
+	// Synchronization channel information, 44.018 page 171. (V7.2.0)
+	bsic =
+	   (decoded_data[ 7] << 5)  |
+	   (decoded_data[ 6] << 4)  |
+	   (decoded_data[ 5] << 3)  |
+	   (decoded_data[ 4] << 2)  |
+	   (decoded_data[ 3] << 1)  |
+	   (decoded_data[ 2] << 0);
+	t1 =
+	   (decoded_data[ 1] << 10) |
+	   (decoded_data[ 0] << 9)  |
+	   (decoded_data[15] << 8)  |
+	   (decoded_data[14] << 7)  |
+	   (decoded_data[13] << 6)  |
+	   (decoded_data[12] << 5)  |
+	   (decoded_data[11] << 4)  |
+	   (decoded_data[10] << 3)  |
+	   (decoded_data[ 9] << 2)  |
+	   (decoded_data[ 8] << 1)  |
+	   (decoded_data[23] << 0);
+	t2 = 
+	   (decoded_data[22] << 4)  |
+	   (decoded_data[21] << 3)  |
+	   (decoded_data[20] << 2)  |
+	   (decoded_data[19] << 1)  |
+	   (decoded_data[18] << 0);
+	t3p =
+	   (decoded_data[17] << 2)  |
+	   (decoded_data[16] << 1)  |
+	   (decoded_data[24] << 0);
+
+	t3 = 10 * t3p + 1;
+
+	// modulo arithmetic
+	if(t3 < t2)
+		tt = (t3 + 26) - t2;
+	else
+		tt = (t3 - t2) % 26;
+	fn = (51 * 26 * t1) + (51 * tt) + t3;
+
+	/*
+	 * BSIC: Base Station Identification Code
+	 * 	BCC: Base station Color Code
+	 * 	NCC: Network Color Code
+	 *
+	 * FN: Frame Number
+	 */
+        /*
+	printf("bsic: %x (bcc: %u; ncc: %u)\tFN: %u\n", bsic, bsic & 7,
+	   (bsic >> 3) & 7, fn);
+         */
+
+	if(fn_o)
+		*fn_o = fn;
+	if(bsic_o)
+		*bsic_o = bsic;
+
+	return 0;
+}
diff --git a/gsm-tvoid/src/lib/sch.h b/gsm-tvoid/src/lib/sch.h
new file mode 100644
index 0000000..6fd31b8
--- /dev/null
+++ b/gsm-tvoid/src/lib/sch.h
@@ -0,0 +1,16 @@
+
+#ifndef __GSMSTACK_H__
+#define __GSMSTACK_H__ 1
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int decode_sch(const unsigned char *src, int *fn_o, int *bsic_o);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+
diff --git a/gsm-tvoid/src/lib/system.h b/gsm-tvoid/src/lib/system.h
new file mode 100644
index 0000000..414730a
--- /dev/null
+++ b/gsm-tvoid/src/lib/system.h
@@ -0,0 +1,11 @@
+
+#ifndef __GSMTVOID_SYSTEM_H__
+#define __GSMTVOID_SYSTEM_H__ 1
+
+#define DEBUGF(a...)	{ \
+	fprintf(stderr, "%s:%d ", __FILE__, __LINE__); \
+	fprintf(stderr, a); \
+} while (0)
+
+#endif
+