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#include <complex.h>
#include <stdio.h>
#include <math.h>
#include <string.h>
#include <osmocom/core/bits.h>
#include <osmocom/core/conv.h>
#include <osmocom/core/utils.h>
#include <osmocom/core/crcgen.h>
#include "sch.h"
/* GSM 04.08, 9.1.30 Synchronization channel information */
struct sch_packed_info {
ubit_t t1_hi[2];
ubit_t bsic[6];
ubit_t t1_md[8];
ubit_t t3p_hi[2];
ubit_t t2[5];
ubit_t t1_lo[1];
ubit_t t3p_lo[1];
} __attribute__((packed));
struct sch_burst {
sbit_t tail0[3];
sbit_t data0[39];
sbit_t etsc[64];
sbit_t data1[39];
sbit_t tail1[3];
sbit_t guard[8];
} __attribute__((packed));
static const uint8_t sch_next_output[][2] = {
{ 0, 3 }, { 1, 2 }, { 0, 3 }, { 1, 2 },
{ 3, 0 }, { 2, 1 }, { 3, 0 }, { 2, 1 },
{ 3, 0 }, { 2, 1 }, { 3, 0 }, { 2, 1 },
{ 0, 3 }, { 1, 2 }, { 0, 3 }, { 1, 2 },
};
static const uint8_t sch_next_state[][2] = {
{ 0, 1 }, { 2, 3 }, { 4, 5 }, { 6, 7 },
{ 8, 9 }, { 10, 11 }, { 12, 13 }, { 14, 15 },
{ 0, 1 }, { 2, 3 }, { 4, 5 }, { 6, 7 },
{ 8, 9 }, { 10, 11 }, { 12, 13 }, { 14, 15 },
};
static const struct osmo_conv_code gsm_conv_sch = {
.N = 2,
.K = 5,
.len = GSM_SCH_UNCODED_LEN,
.next_output = sch_next_output,
.next_state = sch_next_state,
};
const struct osmo_crc16gen_code gsm0503_sch_crc10 = {
.bits = 10,
.poly = 0x175,
.init = 0x000,
.remainder = 0x3ff,
};
#define GSM_MAX_BURST_LEN 157
#define GSM_SYM_RATE (1625e3 / 6)
/* Pre-generated FCCH measurement tone */
static complex float fcch_ref[GSM_MAX_BURST_LEN];
int float_to_sbit(const float *in, sbit_t *out, float scale, int len)
{
int i;
for (i = 0; i < len; i++) {
out[i] = (in[i] - 0.5f) * scale;
}
return 0;
}
/* Check if FN contains a SCH burst */
int gsm_sch_check_fn(int fn)
{
int fn51 = fn % 51;
switch (fn51) {
case 1:
case 11:
case 21:
case 31:
case 41:
return 1;
}
return 0;
}
/* SCH (T1, T2, T3p) to full FN value */
int gsm_sch_to_fn(struct sch_info *sch)
{
int t1 = sch->t1;
int t2 = sch->t2;
int t3p = sch->t3p;
if ((t1 < 0) || (t2 < 0) || (t3p < 0))
return -1;
int tt;
int t3 = t3p * 10 + 1;
if (t3 < t2)
tt = (t3 + 26) - t2;
else
tt = (t3 - t2) % 26;
return t1 * 51 * 26 + tt * 51 + t3;
}
/* Parse encoded SCH message */
int gsm_sch_parse(const uint8_t *info, struct sch_info *desc)
{
struct sch_packed_info *p = (struct sch_packed_info *) info;
desc->bsic = (p->bsic[0] << 0) | (p->bsic[1] << 1) |
(p->bsic[2] << 2) | (p->bsic[3] << 3) |
(p->bsic[4] << 4);
desc->t1 = (p->t1_lo[0] << 0) | (p->t1_md[0] << 1) |
(p->t1_md[1] << 2) | (p->t1_md[2] << 3) |
(p->t1_md[3] << 4) | (p->t1_md[4] << 5) |
(p->t1_md[5] << 6) | (p->t1_md[6] << 7) |
(p->t1_md[7] << 8) | (p->t1_hi[0] << 9) |
(p->t1_hi[1] << 10);
desc->t2 = (p->t2[0] << 0) | (p->t2[1] << 1) |
(p->t2[2] << 2) | (p->t2[3] << 3) |
(p->t2[4] << 4);
desc->t3p = (p->t3p_lo[0] << 0) | (p->t3p_hi[0] << 1) |
(p->t3p_hi[1] << 2);
return 0;
}
/* From osmo-bts */
int gsm_sch_decode(uint8_t *info, sbit_t *data)
{
int rc;
ubit_t uncoded[GSM_SCH_UNCODED_LEN];
osmo_conv_decode(&gsm_conv_sch, data, uncoded);
rc = osmo_crc16gen_check_bits(&gsm0503_sch_crc10,
uncoded, GSM_SCH_INFO_LEN,
uncoded + GSM_SCH_INFO_LEN);
if (rc)
return -1;
memcpy(info, uncoded, GSM_SCH_INFO_LEN * sizeof(ubit_t));
return 0;
}
#define FCCH_TAIL_BITS_LEN 3
#define FCCH_DATA_LEN 142
/* Compute FCCH frequency offset */
double gsm_fcch_offset(float *burst, int len)
{
int i, start, end;
float a, b, c, d, ang, avg = 0.0f;
double freq;
if (len > GSM_MAX_BURST_LEN)
len = GSM_MAX_BURST_LEN;
for (i = 0; i < len; i++) {
a = burst[2 * i + 0];
b = burst[2 * i + 1];
c = crealf(fcch_ref[i]);
d = cimagf(fcch_ref[i]);
burst[2 * i + 0] = a * c - b * d;
burst[2 * i + 1] = a * d + b * c;
}
start = FCCH_TAIL_BITS_LEN;
end = start + FCCH_DATA_LEN;
for (i = start; i < end; i++) {
a = cargf(burst[2 * (i - 1) + 0] +
burst[2 * (i - 1) + 1] * I);
b = cargf(burst[2 * i + 0] +
burst[2 * i + 1] * I);
ang = b - a;
if (ang > M_PI)
ang -= 2 * M_PI;
else if (ang < -M_PI)
ang += 2 * M_PI;
avg += ang;
}
avg /= (float) (end - start);
freq = avg / (2 * M_PI) * GSM_SYM_RATE;
return freq;
}
/* Generate FCCH measurement tone */
static __attribute__((constructor)) void init()
{
int i;
double freq = 0.25;
for (i = 0; i < GSM_MAX_BURST_LEN; i++) {
fcch_ref[i] = sin(2 * M_PI * freq * (double) i) +
cos(2 * M_PI * freq * (double) i) * I;
}
}
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