coding: implement dedicated codec API for FACCH/[FH]

Currently FACCH/[FH] encoding and decoding is implemented as part of
the gsm0503_tch_[fh]r_{en,de}code and gsm0503_tch_a[fh]s_{en,de}code
API.  This works fine for speech because one FACCH frame completely
replaces one or two speech frames, but this is not the case for CSD.

According to 3GPP TS 45.002, sections 4.2.5 and 4.3.5, for TCH data
channels FACCH does not replace data frames but disturbs some amount
of bits from them.  Therefore we need to be able to perform FACCH
encoding and decoding independently from CSD specific coding API.

Change-Id: I0c7a9c180dcafe64e6aebe53518d3d11e1f29886
Related: OS#1572

2 files changed, 139 insertions, 0 deletions

diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index b761cd07..5badbd09 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -3589,4 +3589,138 @@ int gsm0503_tch_fr144_decode(ubit_t *data, const sbit_t *bursts,
 	return 290;
 }
 
+/*
+ * FACCH/[FH] transcoding
+ */
+
+/*! Perform channel encoding of a FACCH/F data as per section 4.2.
+ *  \param[out] bursts Caller-allocated buffer for symbols of 8 bursts,
+ *                     8 * 2 * 58 == 928 bits total.
+ *  \param[in] data FACCH MAC block to be encoded (GSM_MACBLOCK_LEN).
+ *  \returns 0 in case of success; negative on error */
+int gsm0503_tch_fr_facch_encode(ubit_t *bursts, const uint8_t *data)
+{
+	ubit_t iB[8 * 114], cB[4 * 114];
+	const ubit_t h = 1;
+
+	/* 4.2.1-3 as specified for the SACCH in 4.1.1-3 */
+	_xcch_encode_cB(&cB[0], &data[0]);
+
+	/* 4.2.4 Interleaving: as specified for the TCH/FS in subclause 3.1.3 */
+	gsm0503_tch_fr_interleave(&cB[0], &iB[0]);
+
+	/* 4.2.5 Mapping on a Burst:
+	 * - hu(B)=1 the even numbered bits in the first 4 bursts and
+	 * - hl(B)=1 the odd numbered bits of the last 4 bursts are stolen. */
+	for (unsigned int i = 0; i < 8; i++)
+		gsm0503_tch_burst_map(&iB[i * 114], &bursts[i * 116], &h, i >> 2);
+
+	return 0;
+}
+
+/*! Perform channel decoding of a FACCH/F data as per section 4.2.
+ *  \param[out] data Caller-allocated buffer for decoded FACCH (GSM_MACBLOCK_LEN).
+ *  \param[in] bursts Buffer containing the symbols of 8 bursts,
+ *                     8 * 2 * 58 == 928 bits total.
+ *  \param[out] n_errors Number of detected bit errors.
+ *  \param[out] n_bits_total Total number of bits.
+ *  \returns Number of bytes used in the output buffer; negative on error. */
+int gsm0503_tch_fr_facch_decode(uint8_t *data, const sbit_t *bursts,
+				int *n_errors, int *n_bits_total)
+{
+	sbit_t iB[8 * 114], cB[4 * 114];
+	int steal = 0;
+
+	/* FACCH decision: sum of 4 first hu(B) and 4 last hl(B) soft-bits */
+	for (unsigned int i = 0; i < 4; i++)
+		steal -= bursts[i * 116 + 58]; /* hu(B) */
+	for (unsigned int i = 4; i < 8; i++)
+		steal -= bursts[i * 116 + 57]; /* hl(B) */
+	if (steal <= 0)
+		return -1;
+
+	/* 4.2.5 Mapping on a Burst:
+	 * - hu(B)=1 the even numbered bits in the first 4 bursts and
+	 * - hl(B)=1 the odd numbered bits of the last 4 bursts are stolen. */
+	for (unsigned int i = 0; i < 8; i++)
+		gsm0503_tch_burst_unmap(&iB[i * 114], &bursts[i * 116], NULL, i >> 2);
+
+	/* 4.2.4 Interleaving: as specified for the TCH/FS in subclause 3.1.3 */
+	gsm0503_tch_fr_deinterleave(&cB[0], &iB[0]);
+
+	/* 4.2.1-3 as specified for the SACCH in 4.1.1-3 */
+	if (_xcch_decode_cB(&data[0], &cB[0], n_errors, n_bits_total) != 0)
+		return -1;
+
+	return GSM_MACBLOCK_LEN;
+}
+
+/*! Perform channel encoding of a FACCH/H data as per section 4.3.
+ *  \param[out] bursts Caller-allocated buffer for symbols of 6 bursts,
+ *                     6 * 2 * 58 == 696 bits total.
+ *  \param[in] data FACCH MAC block to be encoded (GSM_MACBLOCK_LEN).
+ *  \returns 0 in case of success; negative on error */
+int gsm0503_tch_hr_facch_encode(ubit_t *bursts, const uint8_t *data)
+{
+	ubit_t iB[8 * 114], cB[4 * 114];
+	const ubit_t h = 1;
+
+	/* 4.3.1-3 as specified for the SACCH in 4.1.1-3 */
+	_xcch_encode_cB(&cB[0], &data[0]);
+
+	/* 4.3.4 Interleaving */
+	gsm0503_tch_fr_interleave(&cB[0], &iB[0]);
+
+	/* 4.3.5 Mapping on a Burst:
+	 * - hu(B)=1 the even numbered bits of the first 2 bursts,
+	 * - hu(B)=1 & hl(B)=1 all bits of the middle 2 bursts and
+	 * - hl(B)=1 the odd numbered bits of the last 2 bursts are stolen. */
+	for (unsigned int i = 0; i < 6; i++)
+		gsm0503_tch_burst_map(&iB[i * 114], &bursts[i * 116], &h, i >> 2);
+	for (unsigned int i = 2; i < 4; i++)
+		gsm0503_tch_burst_map(&iB[i * 114 + 456], &bursts[i * 116], &h, 1);
+
+	return 0;
+}
+
+/*! Perform channel decoding of a FACCH/H data as per section 4.3.
+ *  \param[out] data Caller-allocated buffer for decoded FACCH (GSM_MACBLOCK_LEN).
+ *  \param[in] bursts Buffer containing the symbols of 6 bursts,
+ *                     6 * 2 * 58 == 696 bits total.
+ *  \param[out] n_errors Number of detected bit errors.
+ *  \param[out] n_bits_total Total number of bits.
+ *  \returns Number of bytes used in the output buffer; negative on error. */
+int gsm0503_tch_hr_facch_decode(uint8_t *data, const sbit_t *bursts,
+				int *n_errors, int *n_bits_total)
+{
+	sbit_t iB[8 * 114], cB[4 * 114];
+	int steal = 0;
+
+	/* FACCH decision: sum of 4 first hu(B) and 4 last hl(B) soft-bits */
+	for (unsigned int i = 0; i < 4; i++)
+		steal -= bursts[i * 116 + 58]; /* hu(B) */
+	for (unsigned int i = 2; i < 6; i++)
+		steal -= bursts[i * 116 + 57]; /* hl(B) */
+	if (steal <= 0)
+		return -1;
+
+	/* 4.3.5 Mapping on a Burst:
+	 * - hu(B)=1 the even numbered bits of the first 2 bursts,
+	 * - hu(B)=1 & hl(B)=1 all bits of the middle 2 bursts and
+	 * - hl(B)=1 the odd numbered bits of the last 2 bursts are stolen. */
+	for (unsigned int i = 0; i < 6; i++)
+		gsm0503_tch_burst_unmap(&iB[i * 114], &bursts[i * 116], NULL, i >> 2);
+	for (unsigned int i = 2; i < 4; i++)
+		gsm0503_tch_burst_unmap(&iB[i * 114 + 456], &bursts[i * 116], NULL, 1);
+
+	/* 4.3.4 Interleaving */
+	gsm0503_tch_fr_deinterleave(&cB[0], &iB[0]);
+
+	/* 4.3.1-3 as specified for the SACCH in 4.1.1-3 */
+	if (_xcch_decode_cB(&data[0], &cB[0], n_errors, n_bits_total) != 0)
+		return -1;
+
+	return GSM_MACBLOCK_LEN;
+}
+
 /*! @} */
diff --git a/src/coding/libosmocoding.map b/src/coding/libosmocoding.map
index a645a58f..b1087e32 100644
--- a/src/coding/libosmocoding.map
+++ b/src/coding/libosmocoding.map
@@ -140,5 +140,10 @@ gsm0503_tch_hr24_decode;
 gsm0503_tch_fr144_encode;
 gsm0503_tch_fr144_decode;
 
+gsm0503_tch_fr_facch_encode;
+gsm0503_tch_fr_facch_decode;
+gsm0503_tch_hr_facch_encode;
+gsm0503_tch_hr_facch_decode;
+
 local: *;
 };