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osmo-bts-trx: rx_tchh_fn(): use proper meas averaging mode

Compared to TCH/F, TCH/H is a bit special in a way that:

* speech frames are interleaved over 4 consecutive bursts,
* while FACCH frames are interleaved over 6 consecutive bursts.

This is why in rx_tchh_fn() we allocate a buffer large enough to
store up to 6 bursts.  Let's say we have that buffer filled up
completely with all 6 bursts (from 'a' to 'f').  Now attempting
to decode them may yield either a speech frame or a FACCH frame:

  +---+---+---+---+---+---+
  | a | b | c | d | e | f |  Burst 'a' received first, 'f' last
  +---+---+---+---+---+---+
   ^^^^^^^^^^^^^^^           Speech frame (bursts 'a' .. 'd')
   ^^^^^^^^^^^^^^^^^^^^^^^   FACCH frame  (bursts 'a' .. 'f')

For FACCH we use measurement averaging mode SCHED_MEAS_AVG_M_S6N6,
so that 6 last samples are averaged - so far so good.  For speech
we use SCHED_MEAS_AVG_M_S4N4, so that 4 last samples corresponding
to bursts 'c', 'd', 'e', 'f' are averaged - this is wrong.

We actually need to average the *first* 4 samples corresponding to
bursts 'a', 'b', 'c', 'd' in the case of speech.  Let's add and use
a new averaging mode SCHED_MEAS_AVG_M_S6N4 for that.

Change-Id: Iea6f4e5471550f4c2b57aaebeac83c80e879489d
---
M include/osmo-bts/scheduler.h
M src/osmo-bts-trx/sched_lchan_tchh.c
M src/osmo-bts-trx/scheduler_trx.c
3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/osmo-bts/scheduler.h b/include/osmo-bts/scheduler.h
index 8643edd..1e1a2d7 100644
--- a/include/osmo-bts/scheduler.h
+++ b/include/osmo-bts/scheduler.h
@@ -306,10 +306,12 @@
 
 /* Averaging mode for trx_sched_meas_avg() */
 enum sched_meas_avg_mode {
-	/* last 4 bursts (default for xCCH, TCH/H, PTCCH and PDTCH) */
+	/* last 4 bursts (default for xCCH, PTCCH and PDTCH) */
 	SCHED_MEAS_AVG_M_S4N4,
 	/* last 8 bursts (default for TCH/F and FACCH/F) */
 	SCHED_MEAS_AVG_M_S8N8,
+	/* first 4 of last 6 bursts (default for TCH/H) */
+	SCHED_MEAS_AVG_M_S6N4,
 	/* last 6 bursts (default for FACCH/H) */
 	SCHED_MEAS_AVG_M_S6N6,
 	/* first 4 of last 8 bursts */
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index 8264163..edbb566 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -64,7 +64,7 @@
 	 * Even FN ending at: 10,11,19,20,2,3
 	 */
 	int fn_is_odd = (((bi->fn + 26 - 10) % 26) >> 2) & 1;
-	enum sched_meas_avg_mode meas_avg_mode = SCHED_MEAS_AVG_M_S4N4;
+	enum sched_meas_avg_mode meas_avg_mode = SCHED_MEAS_AVG_M_S6N4;
 	struct l1sched_meas_set meas_avg;
 	unsigned int fn_begin;
 	unsigned int fn_tch_end;
diff --git a/src/osmo-bts-trx/scheduler_trx.c b/src/osmo-bts-trx/scheduler_trx.c
index 78c7b4a..6ce5a9a 100644
--- a/src/osmo-bts-trx/scheduler_trx.c
+++ b/src/osmo-bts-trx/scheduler_trx.c
@@ -635,6 +635,7 @@
 static const uint8_t trx_sched_meas_modeset[][2] = {
 	[SCHED_MEAS_AVG_M_S4N4] = { 4, 4 },
 	[SCHED_MEAS_AVG_M_S8N8] = { 8, 8 },
+	[SCHED_MEAS_AVG_M_S6N4] = { 6, 4 },
 	[SCHED_MEAS_AVG_M_S6N6] = { 6, 6 },
 	[SCHED_MEAS_AVG_M_S8N4] = { 8, 4 },
 	[SCHED_MEAS_AVG_M_S6N2] = { 6, 2 },

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