trau_frame: Introduce support for Downlink Time Alignment

In the uplink direction, we provide osmo_trau_frame_dl_ta_us()
to determine the time alignment as requested by the CCU in the BTS.

In the downlink direction, the TRAU frame encoder will perform
time alignment as requested by the user in osmo_trau_frame.dl_ta_usec.

Change-Id: I3981becafc56c9a50119b9ba6bf67aa0391cc76e

2 files changed, 134 insertions, 40 deletions

diff --git a/include/osmocom/trau/trau_frame.h b/include/osmocom/trau/trau_frame.h
index 6784d1f..5f1d708 100644
--- a/include/osmocom/trau/trau_frame.h
+++ b/include/osmocom/trau/trau_frame.h
@@ -110,6 +110,9 @@ struct osmo_trau_frame {
 	enum osmo_trau_frame_type type;
 	enum osmo_trau_frame_direction dir;
 
+	/* timing alignment: 0 = no change; negative: less bits; positive: more bits */
+	int dl_ta_usec;
+
 	ubit_t c_bits[MAX_C_BITS];
 	ubit_t d_bits[MAX_D_BITS];
 	ubit_t t_bits[MAX_T_BITS];
@@ -142,5 +145,7 @@ int osmo_trau_frame_decode_16k(struct osmo_trau_frame *fr, const ubit_t *bits,
  *  \return number of bits encoded */
 int osmo_trau_frame_encode(ubit_t *bits, size_t n_bits, const struct osmo_trau_frame *fr);
 
+/*! Determine the time alignment in us requested by CCU in a UL frame */
+int osmo_trau_frame_dl_ta_us(const struct osmo_trau_frame *fr);
 
 /* }@ */
diff --git a/src/trau/trau_frame.c b/src/trau/trau_frame.c
index a57d642..ea63938 100644
--- a/src/trau/trau_frame.c
+++ b/src/trau/trau_frame.c
@@ -74,6 +74,14 @@ const struct value_string osmo_trau_frame_type_names[] = {
  * New API; introduced in 2020 for use by osmo-mgw
  *********************************************************************************/
 
+/* Bits C21..C22 (16k) or C4..C5 (8k) */
+enum ts48060_amr_frame_classification {
+	TS48060_AMR_FC_SPEECH_GOOD	= 0x3,
+	TS48060_AMR_FC_SPEECH_DEGRADED	= 0x2,
+	TS48060_AMR_FC_SPEECH_BAD	= 0x1,
+	TS48060_AMR_FC_NO_SPEECH	= 0x0,
+};
+
 /* 16k sub-slots */
 static const ubit_t ft_fr_up_bits[5] =		{ 0, 0, 0, 1, 0 };
 static const ubit_t ft_fr_down_bits[5] =	{ 1, 1, 1, 0, 0 };
@@ -110,19 +118,125 @@ static void encode_sync16(ubit_t *trau_bits)
 #define T16_500us	8	/* 500 us = 8 bits */
 #define T16_250us	4	/* 250 us = 4 bits */
 
+#define T8_250us	2	/* 250 us = 2 bits */
+#define T8_125us	1	/* 125 us = 1 bits */
+
 /* How many 16k bits to delay / advance (TS 48.060 Section 5.5.1.1.1) */
-static int fr_efr_alignment_bitcount(uint8_t c6_11)
+static int trau_frame_16_ta_us(uint8_t c6_11)
 {
 	if (c6_11 <= 0x27)
-		return c6_11 * T16_500us;	/* delay frame N x 500us */
+		return c6_11 * 500;	/* delay frame N x 500us */
 	else if (c6_11 == 0x3e)
-		return T16_250us;		/* delay frame 250us */
+		return 250;		/* delay frame 250us */
 	else if (c6_11 == 0x3f)
-		return -T16_250us;		/* advance frame 250us */
+		return -250;	/* advance frame 250us */
 	else
 		return 0;
 }
 
+/* How many 8k bits to delay / advance (TS 48.061 Table 6.1) */
+static int trau_frame_8_ta_us(uint8_t c6_8)
+{
+	/* TA2..TA1..TA0 == C6..C7..C8 */
+	switch (c6_8) {
+	case 0x7:
+		return 0;
+	case 0x6:
+		return -250;
+	case 0x5:
+		return 250;
+	case 0x3:
+		return 500;
+	case 0x4:
+		return 1000;
+	case 0x2:
+		return 2000;
+	case 0x1:
+		return 6000;
+	case 0:
+		return 9000;
+	default:
+		return 0;
+	}
+}
+
+/*! Determine the time alignment in us requested by CCU in a UL frame */
+int osmo_trau_frame_dl_ta_us(const struct osmo_trau_frame *fr)
+{
+	uint8_t c6_11, c6_8, fc;
+
+	/* timing alignment is only communicated from CCU to TRAU in UL */
+	if (fr->dir == OSMO_TRAU_DIR_DL)
+		return 0;
+
+	switch (fr->type) {
+	case OSMO_TRAU16_FT_FR:
+	case OSMO_TRAU16_FT_EFR:
+	case OSMO_TRAU16_FT_HR:
+	case OSMO_TRAU16_FT_AMR:
+		c6_11 = (fr->c_bits[5] << 5) | (fr->c_bits[6] << 4) | (fr->c_bits[7] << 3) |
+			(fr->c_bits[8] << 2) | (fr->c_bits[9] << 1) | (fr->c_bits[10] << 0);
+		return trau_frame_16_ta_us(c6_11);
+		break;
+
+	case OSMO_TRAU8_SPEECH:
+		c6_8 = (fr->c_bits[5] << 2) | (fr->c_bits[6] << 1) | (fr->c_bits[7] << 0);
+		return trau_frame_8_ta_us(c6_8);
+		break;
+
+	case OSMO_TRAU8_AMR_LOW:
+		fc = (fr->c_bits[3] << 1) | (fr->c_bits[4] << 0);
+		if (fc == TS48060_AMR_FC_NO_SPEECH) {
+			c6_11 = (fr->c_bits[5] << 5) | (fr->c_bits[6] << 4) | (fr->c_bits[7] << 3) |
+				(fr->c_bits[8] << 2) | (fr->c_bits[9] << 1) | (fr->c_bits[10] << 0);
+			/* For AMR speech on 8 kBit/s submultiplexing the same procedures as for AMR
+			 * speech on 16 kBit/s submultiplexing shall be applied, see 3GPP TS 48.060 */
+			return trau_frame_16_ta_us(c6_11);
+		} else
+			return 0;
+		break;
+
+	default:
+		return 0;
+	}
+}
+
+static int encode16_handle_ta(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
+{
+	if (fr->dir == OSMO_TRAU_DIR_DL) {
+		int num_bits = fr->dl_ta_usec * T16_250us / 250;
+		if (num_bits > 0) {
+			if (num_bits > 39 * 8)
+				num_bits = 39;
+			memset(trau_bits + 40 * 8, 1, num_bits);
+			return 40 * 8 + num_bits;
+		} else if (num_bits < 0) {
+			if (num_bits < -1 * T16_250us)
+				num_bits = -1 * T16_250us;
+			return 40 * 8 + num_bits;
+		}
+	}
+	return 40 * 8;
+}
+
+static int encode8_handle_ta(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
+{
+	if (fr->dir == OSMO_TRAU_DIR_DL) {
+		int num_bits = fr->dl_ta_usec * T8_250us / 250;
+		if (num_bits > 0) {
+			if (num_bits > 20 * 8 - 2)
+				num_bits = 20 * 8 - 2;
+			memset(trau_bits + 20 * 8, 1, num_bits);
+			return 20 * 8 + num_bits;
+		} else if (num_bits < 0) {
+			if (num_bits < -1 * T8_250us)
+				num_bits = -2;
+			return 20 * 8 + num_bits;
+		}
+	}
+	return 20 * 8;
+}
+
 /* TS 08.60 Section 3.1.1 */
 static int encode16_fr(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 {
@@ -172,16 +286,7 @@ static int encode16_fr(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	memcpy(trau_bits + 316, fr->t_bits+0, 4);
 
 	/* handle timing adjustment */
-	if (fr->dir == OSMO_TRAU_DIR_DL) {
-		uint8_t cbits6_11 = get_bits(fr->c_bits, 5, 6);
-		int ta_bits = fr_efr_alignment_bitcount(cbits6_11);
-		if (ta_bits > 0) {
-			memset(trau_bits+320, 1, ta_bits);
-			return 320 + ta_bits;
-		} else if (ta_bits < 0)
-			return 320 - ta_bits;
-	}
-	return 40 * 8;
+	return encode16_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.60 Section 3.1.1 */
@@ -236,18 +341,9 @@ static int encode16_amr(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	/* T1 .. T4 */
 	memcpy(trau_bits + 316, fr->t_bits + 0, 4);
 
-	/* handle timing adjustment */
-	if (fr->dir == OSMO_TRAU_DIR_DL) {
-		uint8_t cbits6_11 = get_bits(fr->c_bits, 5, 6);
-		int ta_bits = fr_efr_alignment_bitcount(cbits6_11);
-		if (ta_bits > 0) {
-			memset(trau_bits+320, 1, ta_bits);
-			return 320 + ta_bits;
-		} else if (ta_bits < 0)
-			return 320 - ta_bits;
-	}
+	return encode16_handle_ta(trau_bits, fr);
+
 	/* FIXME: handle TAE (Timing Alignment Extension) */
-	return 40 * 8;
 }
 
 /* TS 08.60 Section 3.1.2 */
@@ -378,16 +474,7 @@ static int encode16_hr(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	memcpy(trau_bits + 39 * 8 + 4, fr->t_bits, 4);
 
 	/* handle timing adjustment */
-	if (fr->dir == OSMO_TRAU_DIR_DL) {
-		uint8_t cbits6_11 = get_bits(fr->c_bits, 5, 6);
-		int ta_bits = fr_efr_alignment_bitcount(cbits6_11);
-		if (ta_bits > 0) {
-			memset(trau_bits+320, 1, ta_bits);
-			return 320 + ta_bits;
-		} else if (ta_bits < 0)
-			return 320 - ta_bits;
-	}
-	return 40 * 8;
+	return encode16_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.61 Section 5.1.1.1 */
@@ -797,12 +884,12 @@ static int encode8_hr(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	/* C6 .. C9 */
 	memcpy(trau_bits + 19 * 8 + 2, fr->c_bits + 5, 4);
 
-	/* FIXME: handle timing adjustment */
 	/* T1 .. T2 */
 	trau_bits[19 * 8 + 6] = fr->t_bits[0];
 	trau_bits[19 * 8 + 7] = fr->t_bits[1];
 
-	return 20 * 8;
+	/* handle timing adjustment */
+	return encode8_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.61 Section 5.2.1.2.1 */
@@ -876,7 +963,7 @@ static int encode8_amr_low(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	/* T1 */
 	trau_bits[19 * 8 + 7] = fr->t_bits[0];
 
-	return 20 * 8;
+	return encode8_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.61 Section 5.2.1.2.2 */
@@ -947,7 +1034,7 @@ static int encode8_amr_67(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 		d_idx += 15;
 	}
 
-	return 20 * 8;
+	return encode8_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.61 Section 5.1.2.3 */
@@ -1006,7 +1093,7 @@ static int encode8_amr_74(ubit_t *trau_bits, const struct osmo_trau_frame *fr)
 	/* D24 .. D151 */
 	memcpy(trau_bits + 4 * 8, fr->d_bits + d_idx, 16 * 8);
 
-	return 20 * 8;
+	return encode8_handle_ta(trau_bits, fr);
 }
 
 /* TS 08.61 Section 5.2.2 */
@@ -1185,6 +1272,7 @@ int osmo_trau_frame_decode_16k(struct osmo_trau_frame *fr, const ubit_t *bits,
 
 	fr->type = OSMO_TRAU_FT_NONE;
 	fr->dir = dir;
+	fr->dl_ta_usec = 0;
 
 	switch (cbits5) {
 	case TRAU_FT_FR_UP:
@@ -1326,6 +1414,7 @@ int osmo_trau_frame_decode_8k(struct osmo_trau_frame *fr, const ubit_t *bits,
 {
 	fr->type = OSMO_TRAU_FT_NONE;
 	fr->dir = dir;
+	fr->dl_ta_usec = 0;
 
 	if (is_hr(bits)) {
 		/* normal sync pattern */