path: root/src/osmo-bts-trx/tch_fr.c

/*
 * tch_fr.c
 *
 * Copyright (c) 2013  Andreas Eversberg <jolly@eversberg.eu>
 */

#include <stdio.h>
#include <stdint.h>
#include <string.h>

#include <osmocom/core/bits.h>
#include <osmocom/core/conv.h>
#include <osmocom/core/crcgen.h>

#include "xcch.h"
#include "tch_fr.h"


/*
 * GSM TCH FR/EFR parity
 *
 * g(x) = x^3 + x + 1
 */

const struct osmo_crc8gen_code tch_fr_crc3 = {
	.bits = 3,
	.poly = 0x2,
	.init = 0x0,
	.remainder = 0x7,
};


/*
 * GSM TCH FR/EFR convolutional coding
 *
 * G_0 = 1 + x^3 + x^4
 * G_1 = 1 + x + x^3 + x^4
 */

static const uint8_t conv_tch_fr_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 conv_tch_fr_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 conv_tch_fr = {
	.N = 2,
	.K = 5,
	.len = 185,
	.next_output = conv_tch_fr_next_output,
	.next_state  = conv_tch_fr_next_state,
};


/*
 * GSM TCH FR/EFR interleaving and burst mapping
 *
 * Interleaving:
 *
 * Given 456 coded input bits, form 8 blocks of 114 bits,
 * where event bits of the first 4 block and off bits of the last 4 block
 * are used:
 *
 *      i(B, j) = c(n, k)       k = 0, ..., 455
 *                              n = 0, ..., N, N + 1, ...
 *                              B = B_0 + 4n + (k mod 8)
 *                              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 hl(B) and hn(B) are bits in burst B indicating flags.
 */

static void
tch_fr_deinterleave(sbit_t *cB, sbit_t *iB)
{
	int j, k, B;

	for (k=0; k<456; k++) {
		B = k & 7;
		j = 2 * ((49 * k) % 57) + ((k & 7) >> 2);
		cB[k] = iB[B * 114 + j];
	}
}

static void
tch_fr_interleave(ubit_t *cB, ubit_t *iB)
{
	int j, k, B;

	for (k=0; k<456; k++) {
		B = k & 7;
		j = 2 * ((49 * k) % 57) + ((k & 7) >> 2);
		iB[B * 114 + j] = cB[k];
	}
}

static void
tch_fr_burst_unmap(sbit_t *iB, sbit_t *eB, sbit_t *h, int odd)
{
	int i;

	/* brainfuck: only copy even or odd bits */
	for (i=odd; i<57; i+=2)
		iB[i] = eB[i];
	for (i=58-odd; i<114; i+=2)
		iB[i] = eB[i+2];

	if (h && !odd)
		*h = eB[57];

	if (h && odd)
		*h = eB[58];
}

static void
tch_fr_burst_map(ubit_t *iB, ubit_t *eB, ubit_t *h, int odd)
{
	int i;

	/* brainfuck: only copy even or odd bits */
	for (i=odd; i<57; i+=2)
		eB[i] = iB[i];
	for (i=58-odd; i<114; i+=2)
		eB[i+2] = iB[i];

	if (h && !odd)
		eB[57] = *h;
	if (h && odd)
		eB[58] = *h;
}

/* this corresponds to the bit-lengths of the individual codec
 * parameters as indicated in Table 1.1 of TS 06.10 */
static const uint8_t gsm_fr_map[] = {
	6, 6, 5, 5, 4, 4, 3, 3,
	7, 2, 2, 6, 3, 3, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 7, 2, 2, 6, 3, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 7, 2, 2, 6, 3, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 3, 7, 2, 2, 6, 3,
	3, 3, 3, 3, 3, 3, 3, 3,
	3, 3, 3, 3
};

static void
tch_fr_reassemble(uint8_t *tch_data, ubit_t *d_bits)
{
	int i, j, k, l, o;

	tch_data[0] = 0xd << 4;
	/* reassemble d-bits */
	i = 0; /* counts bits */
	j = 4; /* counts output bits */
	k = gsm_fr_map[0]-1; /* current number bit in element */
	l = 0; /* counts element bits */
	o = 0; /* offset input bits */
	while (i < 260) {
		tch_data[j>>3] |= (d_bits[k+o] << (7-(j&7)));
		if (--k < 0) {
			o += gsm_fr_map[l];
			k = gsm_fr_map[++l]-1;
		}
		i++;
		j++;
	}
}

static void
tch_fr_disassemble(ubit_t *d_bits, uint8_t *tch_data)
{
	int i, j, k, l, o;

	i = 0; /* counts bits */
	j = 4; /* counts input bits */
	k = gsm_fr_map[0]-1; /* current number bit in element */
	l = 0; /* counts element bits */
	o = 0; /* offset output bits */
	while (i < 260) {
		d_bits[k+o] = (tch_data[j>>3] >> (7-(j&7))) & 1;
		if (--k < 0) {
			o += gsm_fr_map[l];
			k = gsm_fr_map[++l]-1;
		}
		i++;
		j++;
	}
}

static void
tch_fr_unreorder(ubit_t *d, ubit_t *p, ubit_t *u)
{
	int i;

	for (i=0; i<91; i++) {
		d[i<<1] = u[i];
		d[(i<<1)+1] = u[184-i];
	}
	for (i=0; i<3; i++)
		p[i] = u[91+i];
}

static void
tch_fr_reorder(ubit_t *u, ubit_t *d, ubit_t *p)
{
	int i;

	for (i=0; i<91; i++) {
		u[i] = d[i<<1];
		u[184-i] = d[(i<<1)+1];
	}
	for (i=0; i<3; i++)
		u[91+i] = p[i];
}

int
tch_fr_decode(uint8_t *tch_data, sbit_t *bursts)
{
	sbit_t iB[912], cB[456], h;
	ubit_t conv[185], d[260], p[3];
	int i, rv, len, steal = 0;

	for (i=0; i<8; i++) {
		tch_fr_burst_unmap(&iB[i * 114], &bursts[i * 116], &h, i>>2);
		if (h < 0)
			steal++;
	}

	tch_fr_deinterleave(cB, iB);

	if (steal < 4) {
		osmo_conv_decode(&conv_tch_fr, cB, conv);

		tch_fr_unreorder(d, p, conv);

		for (i=0; i<78; i++)
			d[i+182] = (cB[i+378] < 0) ? 1:0;

		rv = osmo_crc8gen_check_bits(&tch_fr_crc3, d, 50, p);
		if (rv)
			return -1;

		tch_fr_reassemble(tch_data, d);

		len = 33;
	} else {
		rv = xcch_decode_cB(tch_data, cB);
		if (rv)
			return -1;

		len = 23;
	}

	return len;
}

int
tch_fr_encode(ubit_t *bursts, uint8_t *tch_data, int len)
{
	ubit_t iB[912], cB[456], h;
	ubit_t conv[185], d[260], p[3];
	int i;

	switch (len) {
	case 33: /* TCH FR */
		tch_fr_disassemble(d, tch_data);

		osmo_crc8gen_set_bits(&tch_fr_crc3, d, 50, p);

		tch_fr_reorder(conv, d, p);

		memcpy(cB+378, d+182, 78);

		osmo_conv_encode(&conv_tch_fr, conv, cB);

		h = 0;

		break;
	case 23: /* FACCH */
		xcch_encode_cB(cB, tch_data);

		h = 1;

		break;
	default:
		return -1;
	}

	tch_fr_interleave(cB, iB);

	for (i=0; i<8; i++)
		tch_fr_burst_map(&iB[i * 114], &bursts[i * 116], &h, i>>2);

	return 0;
}