Transmit invalid AMR speech blocks instead of dummy FACCH

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 that is bad, causing the MS receiver to
declare a BFI condition to trigger substitution and muting procedure.
A bad frame is generated by gsm0503_tch_{afs,ahs}_encode() by setting
the payload length to 0.

Depends: libosmocore.git I82ce2adf995a4b42d1f378c5819f88d773b9104a
Related: OS#6049
Change-Id: I056f379715c91ad968f198e112d363a9009dc1c3

2 files changed, 36 insertions, 8 deletions

diff --git a/src/osmo-bts-trx/sched_lchan_tchf.c b/src/osmo-bts-trx/sched_lchan_tchf.c
index 0138a2a2..b621b234 100644
--- a/src/osmo-bts-trx/sched_lchan_tchf.c
+++ b/src/osmo-bts-trx/sched_lchan_tchf.c
@@ -538,15 +538,14 @@ int tx_tchf_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 		/* - If the channel mode is TCH/FS or TCH/EFS, transmit a dummy
 		 *   speech block with inverted CRC3, designed to induce a BFI
 		 *   condition in the MS receiver.
+		 * - If the channel mode is TCH/AFS, transmit a dummy speech
+		 *   block with inverted CRC6, designed to induce a BFI
+		 *   condition in the MS receiver.
 		 * - If the channel mode is one of the CSD modes, transmit an
 		 *   idle frame as described in 3GPP TS 44.021, sections 8.1.6
 		 *   and 10.2.3 (all data, status and E-bits set to binary '1').
 		 * - In all other channel modes, transmit dummy FACCH
 		 *   like we always did before.
-		 *
-		 * FIXME: someone who knows AMR needs to look at this problem
-		 * and decide what is the correct BTS Tx behavior for frame
-		 * gaps in TCH/AFS.  See OS#6049.
 		 */
 		switch (tch_mode) {
 		case GSM48_CMODE_DATA_12k0:
@@ -562,6 +561,21 @@ int tx_tchf_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 			/* fall-through */
 		case GSM48_CMODE_SIGN:
 		default:
+			if (tch_mode == GSM48_CMODE_SPEECH_AMR) {
+				/* the first FN 4,13,21 defines that CMI is included in frame,
+				 * the first FN 0,8,17 defines that CMR is included in frame.
+				 */
+				rc = gsm0503_tch_afs_encode(BUFPOS(bursts_p, 0),
+							    NULL, 0,
+							    !sched_tchf_dl_amr_cmi_map[br->fn % 26],
+							    chan_state->codec,
+							    chan_state->codecs,
+							    chan_state->dl_ft,
+							    chan_state->dl_cmr);
+				if (rc == 0)
+					goto send_burst;
+			}
+
 			/* TODO: use randomized padding */
 			msg_facch = tch_dummy_msgb(GSM_MACBLOCK_LEN, GSM_MACBLOCK_PADDING);
 			/* dummy LAPDm func=UI frame */
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index e0d7aca6..05e1af4b 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -453,15 +453,14 @@ int tx_tchh_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 		/* - If the channel mode is TCH/HS, transmit a dummy speech block
 		 *   with inverted CRC3, designed to induce a BFI condition in
 		 *   the MS receiver.
+		 * - If the channel mode is TCH/AHS, transmit a dummy speech
+		 *   block with inverted CRC6, designed to induce a BFI
+		 *   condition in the MS receiver.
 		 * - If the channel mode is one of the CSD modes, transmit an
 		 *   idle frame as described in 3GPP TS 44.021, sections 8.1.6
 		 *   and 10.2.3 (all data, status and E-bits set to binary '1').
 		 * - In all other channel modes, transmit dummy FACCH
 		 *   like we always did before.
-		 *
-		 * FIXME: someone who knows AMR needs to look at this problem
-		 * and decide what is the correct BTS Tx behavior for frame
-		 * gaps in TCH/AHS.  See OS#6049.
 		 */
 		switch (tch_mode) {
 		case GSM48_CMODE_DATA_6k0:
@@ -474,6 +473,21 @@ int tx_tchh_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 			/* fall-through */
 		case GSM48_CMODE_SIGN:
 		default:
+			if (tch_mode == GSM48_CMODE_SPEECH_AMR) {
+				/* the first FN 4,13,21 or 5,14,22 defines that CMI is included
+				 * in frame, the first FN 0,8,17 or 1,9,18 defines that CMR is
+				 * included in frame. */
+				rc = gsm0503_tch_ahs_encode(BUFPOS(bursts_p, 0),
+							    NULL, 0,
+							    !sched_tchh_dl_amr_cmi_map[br->fn % 26],
+							    chan_state->codec,
+							    chan_state->codecs,
+							    chan_state->dl_ft,
+							    chan_state->dl_cmr);
+				if (rc == 0)
+					goto send_burst;
+			}
+
 			/* FACCH/H can only be scheduled at specific TDMA offset */
 			if (!sched_tchh_dl_facch_map[br->fn % 26]) {
 				/* FACCH/H is not allowed, send half-filled bursts with even numbered