coding: implement encoding/decoding API for TCH/F2.4

See 3GPP TS 45.003 section 3.6.  This channel mode is a bit special,
because unlike the other CSD specific channel modes it's interleaved
over 8 (not 22!) consecutive bursts, just like TCH/FS.

Change-Id: I4685376c8deb04db670684c9ebf685ad6fc989fa
Related: OS#1572

include/osmocom/coding/gsm0503_coding.h

@@ -101,6 +101,10 @@ int gsm0503_tch_hr48_encode(ubit_t *bursts, const ubit_t *data);
 int gsm0503_tch_hr48_decode(ubit_t *data, const sbit_t *bursts,
 			    int *n_errors, int *n_bits_total);
 
+int gsm0503_tch_fr24_encode(ubit_t *bursts, const ubit_t *data);
+int gsm0503_tch_fr24_decode(ubit_t *data, const sbit_t *bursts,
+			    int *n_errors, int *n_bits_total);
+
 int gsm0503_tch_hr24_encode(ubit_t *bursts, const ubit_t *data);
 int gsm0503_tch_hr24_decode(ubit_t *data, const sbit_t *bursts,
 			    int *n_errors, int *n_bits_total);

src/coding/gsm0503_coding.c

@@ -3472,6 +3472,54 @@ int gsm0503_tch_hr48_decode(ubit_t *data, const sbit_t *bursts,
 	return gsm0503_tch_fr96_decode(data, bursts, n_errors, n_bits_total);
 }
 
+/*! Perform channel encoding of a TCH/F2.4 channel as per section 3.6.
+ *  \param[out] bursts Caller-allocated buffer for symbols of 8 bursts,
+ *                     8 * 2 * 58 == 928 bits total.
+ *  \param[in] data Data to be encoded (72 unpacked bits).
+ *  \returns 0 in case of success; negative on error */
+int gsm0503_tch_fr24_encode(ubit_t *bursts, const ubit_t *data)
+{
+	ubit_t iB[8 * 114], cB[4 * 114];
+	const ubit_t h = 0;
+
+	/* 3.6.{1-3} Block code and Convolutional encoder */
+	osmo_conv_encode(&gsm0503_tch_f24, &data[0], &cB[0]);
+
+	/* 3.6.4 Interleaving: as specified for the TCH/FS in subclause 3.1.3 */
+	gsm0503_tch_fr_interleave(&cB[0], &iB[0]);
+
+	/* 3.6.5 Mapping on a burst: as specified for TCH/FS in subclause 3.1.4 */
+	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 TCH/F2.4 channel as per section 3.6.
+ *  \param[out] data Caller-allocated buffer for decoded data (72 unpacked bits).
+ *  \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 unpacked bits used in the output buffer; negative on error. */
+int gsm0503_tch_fr24_decode(ubit_t *data, const sbit_t *bursts,
+			    int *n_errors, int *n_bits_total)
+{
+	sbit_t iB[8 * 114], cB[4 * 114];
+
+	/* 3.6.5 Mapping on a burst: as specified for TCH/FS in subclause 3.1.4 */
+	for (unsigned int i = 0; i < 8; i++)
+		gsm0503_tch_burst_unmap(&iB[i * 114], &bursts[i * 116], NULL, i >> 2);
+
+	/* 3.6.4 Interleaving: as specified for the TCH/FS in subclause 3.1.3 */
+	gsm0503_tch_fr_deinterleave(&cB[0], &iB[0]);
+
+	/* 3.6.{1-3} Block code and Convolutional encoder */
+	osmo_conv_decode_ber(&gsm0503_tch_f24, &cB[0], &data[0], n_errors, n_bits_total);
+
+	return 72;
+}
+
 /*! Perform channel encoding of a TCH/H2.4 channel as per section 3.7.
  *  \param[out] bursts Caller-allocated buffer for symbols of 22 bursts,
  *                     22 * 2 * 58 == 2552 bits total.

src/coding/libosmocoding.map

@@ -135,6 +135,8 @@ gsm0503_tch_fr48_encode;
 gsm0503_tch_fr48_decode;
 gsm0503_tch_hr48_encode;
 gsm0503_tch_hr48_decode;
+gsm0503_tch_fr24_encode;
+gsm0503_tch_fr24_decode;
 gsm0503_tch_hr24_encode;
 gsm0503_tch_hr24_decode;
 gsm0503_tch_fr144_encode;

tests/coding/coding_test.c

@@ -563,9 +563,15 @@ static const struct csd_test_case csd_tests[] = {
 		.dec_fn = &gsm0503_tch_hr48_decode,
 		.half_rate = true,
 	},
+	{
+		.name = "TCH/F2.4",
+		.num_bits = 2 * 36,
+		.enc_fn = &gsm0503_tch_fr24_encode,
+		.dec_fn = &gsm0503_tch_fr24_decode,
+	},
 	{
 		.name = "TCH/H2.4",
-		.num_bits = 2 * 72,
+		.num_bits = 4 * 36,
 		.enc_fn = &gsm0503_tch_hr24_encode,
 		.dec_fn = &gsm0503_tch_hr24_decode,
 		.half_rate = true,

tests/coding/coding_test.ok

@@ -445,6 +445,16 @@ test_csd(TCH/H4.8): block #2 (pattern 0xff): n_errors=0 / n_bits_total=456
 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111
 11111111 11111111 11111111 11111111 11111111 11111111
 
+test_csd(TCH/F2.4): block #0 (pattern 0x00): n_errors=0 / n_bits_total=456
+00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
+00000000
+test_csd(TCH/F2.4): block #1 (pattern 0xaa): n_errors=0 / n_bits_total=456
+01010101 01010101 01010101 01010101 01010101 01010101 01010101 01010101
+01010101
+test_csd(TCH/F2.4): block #2 (pattern 0xff): n_errors=0 / n_bits_total=456
+11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111
+11111111
+
 test_csd(TCH/H2.4): block #0 (pattern 0x00): n_errors=0 / n_bits_total=456
 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
@@ -527,6 +537,18 @@ test_csd(TCH/H4.8): block #2 (pattern 0xff): n_errors=3 / n_bits_total=456
 test_csd(TCH/H4.8): FACCH/H (pattern 0x2b): n_errors=0 / n_bits_total=456
 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
 
+test_csd(TCH/F2.4): block #0 (pattern 0x00): n_errors=0 / n_bits_total=456
+00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
+00000000
+test_csd(TCH/F2.4): block #1 (pattern 0xaa): n_errors=138 / n_bits_total=456
+11000010 10010010 10010010 10010010 10010010 10010010 10010101 00011000
+01001010
+test_csd(TCH/F2.4): block #2 (pattern 0xff): n_errors=0 / n_bits_total=456
+11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111
+11111111
+test_csd(TCH/F2.4): FACCH/F (pattern 0x2b): n_errors=0 / n_bits_total=456
+2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+
 test_csd(TCH/H2.4): block #0 (pattern 0x00): n_errors=38 / n_bits_total=456
 00000000 00000000 00000101 00000000 00000000 00000000 00000000 00000000
 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000