trx TCH DL: transmit invalid speech blocks instead of dummy FACCH

In speech TCH operation, there will always be times when a speech frame
needs to be transmitted on the downlink, but there is no frame available
to transmit (gap in the incoming RTP stream), or the logic of DTXd says
that no frame shall be transmitted at this FN, but we are not doing
physical DTXd.  Previous osmo-bts-trx code sent dummy FACCH during such
conditions, but this approach is bad for TCH/HS where we would like to
transmit good speech frames or speech frame gaps one 20 ms frame at a
time, rather than take out pairs of frames for dummy FACCH/H.  Other
BTS models (sysmoBTS PHY) send out invalid speech blocks with inverted
CRC3 under the conditions in question - do the same in osmo-bts-trx.

The present change modifies osmo-bts-trx TCH DL frame gap behavior
only for TCH/FS, TCH/HS and TCH/EFS; for all other channel modes,
including AMR speech, the previous behavior of sending dummy FACCH
is left unchanged.

Depends: Iade3310e16b906efb6892d28f474a0d15204e861 (libosmocore)
Change-Id: I78106802a0aa4af39859c75d29fe0e77037899fe

2 files changed, 36 insertions, 2 deletions

diff --git a/src/osmo-bts-trx/sched_lchan_tchf.c b/src/osmo-bts-trx/sched_lchan_tchf.c
index f0658038..0188d859 100644
--- a/src/osmo-bts-trx/sched_lchan_tchf.c
+++ b/src/osmo-bts-trx/sched_lchan_tchf.c
@@ -472,10 +472,29 @@ int tx_tchf_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
 			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
 		};
+		int rc;
 
 		LOGL1SB(DL1P, LOGL_DEBUG, l1ts, br, "No TCH or FACCH prim for transmit.\n");
-		gsm0503_tch_fr_encode(bursts_p, dummy, sizeof(dummy), 1);
-		chan_state->dl_facch_bursts = 8;
+		/* 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.  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_SPEECH_V1:
+		case GSM48_CMODE_SPEECH_EFR:
+			rc = gsm0503_tch_fr_encode(bursts_p, NULL, 0, 1);
+			if (rc == 0)
+				break;
+			/* fall-through */
+		default:
+			gsm0503_tch_fr_encode(bursts_p, dummy, sizeof(dummy), 1);
+			chan_state->dl_facch_bursts = 8;
+		}
 		goto send_burst;
 	}
 
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index 5772e9f4..2f1194ae 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -385,8 +385,23 @@ int tx_tchh_fn(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br)
 			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
 			0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b, 0x2b,
 		};
+		int rc;
 
 		LOGL1SB(DL1P, LOGL_INFO, l1ts, br, "No TCH or FACCH prim for transmit.\n");
+		/* 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.  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.
+		 */
+		if (tch_mode == GSM48_CMODE_SPEECH_V1) {
+			rc = gsm0503_tch_hr_encode(bursts_p, NULL, 0);
+			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]) {