coding: gsm0503_tch_{afs,ahs}_encode(): add ability to emit BFI

Similar to Change ID Iade3310e16b906efb6892d28f474a0d15204e861, add
BFI support to TCH AFS and TCH AHS encoder.

Every BTS needs to have some graceful handling for the scenario
where it is time to send out a speech frame on TCH DL, but there is
no frame to be sent. One possible solution is to transmit dummy
FACCH, but this option is unattractive for TCH/AHS where FACCH
displaces two speech frames rather than one. A more elegant solution
is to emit a speech frame with inverted CRC6, causing the MS receiver
to declare a BFI condition to trigger substitution and muting procedure.
Setting all u(k) bits to 0 is one way to produce such an inverted-CRC
speech frame (normal TCH FR/HR CRC6 for an all-zeros frame would be
111111). This patch adds the abiliy to gsm0503_tch_{afs,ahs}_encode()
functions, indicated by payload length of 0.

See §6.2 of 3GPP TS 26.191 for substitution and muting procedure.

Related: OS#6049
Change-Id: I82ce2adf995a4b42d1f378c5819f88d773b9104a

1 files changed, 148 insertions, 62 deletions

diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index 8effef54..365ae6e9 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -2439,7 +2439,7 @@ out:
 /*! Perform channel encoding on a TCH/AFS channel according to TS 05.03
  *  \param[out] bursts caller-allocated output buffer for bursts bits
  *  \param[in] tch_data Codec input data in RTP payload format
- *  \param[in] len Length of \a tch_data in bytes
+ *  \param[in] len Length of \a tch_data in bytes or 0 to generate a bad frame
  *  \param[in] codec_mode_req Use CMR (1) or FT (0)
  *  \param[in] codec Array of codecs (active codec set)
  *  \param[in] codecs Number of entries in \a codec
@@ -2473,12 +2473,19 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 	switch (codec[ft]) {
 	case 7: /* TCH/AFS12.2 */
-		if (len != 31)
-			goto invalid_length;
-
-		tch_amr_disassemble(d, tch_data, 244);
-
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 81, p);
+		if (!len) {
+			/* No data, induce BFI in the receiver by inverted CRC bits.
+			 * The data bit are all 0, so the correct parity bits would be 111111.  */
+			memset(d, 0, 244);
+			memset(p, 0, 6);
+		} else {
+			if (len != 31)
+				goto invalid_length;
+
+			tch_amr_disassemble(d, tch_data, 244);
+
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 81, p);
+		}
 
 		tch_amr_merge(conv, d, p, 244, 81);
 
@@ -2486,12 +2493,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 6: /* TCH/AFS10.2 */
-		if (len != 26)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 204);
+			memset(p, 0, 6);
+		} else {
+			if (len != 26)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 204);
+			tch_amr_disassemble(d, tch_data, 204);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 65, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 65, p);
+		}
 
 		tch_amr_merge(conv, d, p, 204, 65);
 
@@ -2499,12 +2512,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 5: /* TCH/AFS7.95 */
-		if (len != 20)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 159);
+			memset(p, 0, 6);
+		} else {
+			if (len != 20)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 159);
+			tch_amr_disassemble(d, tch_data, 159);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 75, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 75, p);
+		}
 
 		tch_amr_merge(conv, d, p, 159, 75);
 
@@ -2512,12 +2531,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 4: /* TCH/AFS7.4 */
-		if (len != 19)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 148);
+			memset(p, 0, 6);
+		} else {
+			if (len != 19)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 148);
+			tch_amr_disassemble(d, tch_data, 148);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 61, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 61, p);
+		}
 
 		tch_amr_merge(conv, d, p, 148, 61);
 
@@ -2525,12 +2550,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 3: /* TCH/AFS6.7 */
-		if (len != 17)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 134);
+			memset(p, 0, 6);
+		} else {
+			if (len != 17)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 134);
+			tch_amr_disassemble(d, tch_data, 134);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+		}
 
 		tch_amr_merge(conv, d, p, 134, 55);
 
@@ -2538,12 +2569,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 2: /* TCH/AFS5.9 */
-		if (len != 15)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 118);
+			memset(p, 0, 6);
+		} else {
+			if (len != 15)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 118);
+			tch_amr_disassemble(d, tch_data, 118);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+		}
 
 		tch_amr_merge(conv, d, p, 118, 55);
 
@@ -2551,12 +2588,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 1: /* TCH/AFS5.15 */
-		if (len != 13)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 103);
+			memset(p, 0, 6);
+		} else {
+			if (len != 13)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 103);
+			tch_amr_disassemble(d, tch_data, 103);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 49, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 49, p);
+		}
 
 		tch_amr_merge(conv, d, p, 103, 49);
 
@@ -2564,12 +2607,18 @@ int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 0: /* TCH/AFS4.75 */
-		if (len != 12)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 95);
+			memset(p, 0, 6);
+		} else {
+			if (len != 12)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 95);
+			tch_amr_disassemble(d, tch_data, 95);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 39, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 39, p);
+		}
 
 		tch_amr_merge(conv, d, p, 95, 39);
 
@@ -2927,7 +2976,7 @@ out:
 /*! Perform channel encoding on a TCH/AHS channel according to TS 05.03
  *  \param[out] bursts caller-allocated output buffer for bursts bits
  *  \param[in] tch_data Codec input data in RTP payload format
- *  \param[in] len Length of \a tch_data in bytes
+ *  \param[in] len Length of \a tch_data in bytes or 0 to generate a bad frame
  *  \param[in] codec_mode_req Use CMR (1) or FT (0)
  *  \param[in] codec Array of codecs (active codec set)
  *  \param[in] codecs Number of entries in \a codec
@@ -2970,12 +3019,19 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 	switch (codec[ft]) {
 	case 5: /* TCH/AHS7.95 */
-		if (len != 20)
-			goto invalid_length;
-
-		tch_amr_disassemble(d, tch_data, 159);
-
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 67, p);
+		if (!len) {
+			/* No data, induce BFI in the receiver by inverted CRC bits.
+			 * The data bit are all 0, so the correct parity bits would be 111111.  */
+			memset(d, 0, 159);
+			memset(p, 0, 6);
+		} else {
+			if (len != 20)
+				goto invalid_length;
+
+			tch_amr_disassemble(d, tch_data, 159);
+
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 67, p);
+		}
 
 		tch_amr_merge(conv, d, p, 123, 67);
 
@@ -2985,12 +3041,18 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 4: /* TCH/AHS7.4 */
-		if (len != 19)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 148);
+			memset(p, 0, 6);
+		} else {
+			if (len != 19)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 148);
+			tch_amr_disassemble(d, tch_data, 148);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 61, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 61, p);
+		}
 
 		tch_amr_merge(conv, d, p, 120, 61);
 
@@ -3000,12 +3062,18 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 3: /* TCH/AHS6.7 */
-		if (len != 17)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 134);
+			memset(p, 0, 6);
+		} else {
+			if (len != 17)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 134);
+			tch_amr_disassemble(d, tch_data, 134);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+		}
 
 		tch_amr_merge(conv, d, p, 110, 55);
 
@@ -3015,12 +3083,18 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 2: /* TCH/AHS5.9 */
-		if (len != 15)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 118);
+			memset(p, 0, 6);
+		} else {
+			if (len != 15)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 118);
+			tch_amr_disassemble(d, tch_data, 118);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 55, p);
+		}
 
 		tch_amr_merge(conv, d, p, 102, 55);
 
@@ -3030,12 +3104,18 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 1: /* TCH/AHS5.15 */
-		if (len != 13)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 103);
+			memset(p, 0, 6);
+		} else {
+			if (len != 13)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 103);
+			tch_amr_disassemble(d, tch_data, 103);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 49, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 49, p);
+		}
 
 		tch_amr_merge(conv, d, p, 91, 49);
 
@@ -3045,12 +3125,18 @@ int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 
 		break;
 	case 0: /* TCH/AHS4.75 */
-		if (len != 12)
-			goto invalid_length;
+		if (!len) {
+			/* See comment above. */
+			memset(d, 0, 95);
+			memset(p, 0, 6);
+		} else {
+			if (len != 12)
+				goto invalid_length;
 
-		tch_amr_disassemble(d, tch_data, 95);
+			tch_amr_disassemble(d, tch_data, 95);
 
-		osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 39, p);
+			osmo_crc8gen_set_bits(&gsm0503_amr_crc6, d, 39, p);
+		}
 
 		tch_amr_merge(conv, d, p, 83, 39);