Change in osmo-bts[master]: TA Control Loop: Change toa256 switch threshold to 75% of a symbol

[pespin]

pespin has submitted this change. ( https://gerrit.osmocom.org/c/osmo-bts/+/25443 )

Change subject: TA Control Loop: Change toa256 switch threshold to 75% of a symbol
......................................................................

TA Control Loop: Change toa256 switch threshold to 75% of a symbol

As per 3GPP TS 45.010 sec 5.6.3 Delay assessment error:
"""
the delay shall be assessed in such a way that the assessment error
(due to  noise and interference) is less than 1/2 normal symbol periods
for stationary MS. For MS moving at a speed up to 500  km/h the additional
error shall be less than 1/4 normal symbol period.
"""

Hence, required TA loop calculation should take into account an error of
up to 1/2 + 1/4 = 75% of a symbol. By changing the threshold variable
to that percentage we make sure we take into account bigger deviations
and update TA accordingly.

Related: SYS#5371
Change-Id: I4c5825df4291d3851a113027abf7566de343a190
---
M src/common/ta_control.c
1 file changed, 5 insertions(+), 4 deletions(-)

Approvals:
  osmith: Looks good to me, but someone else must approve
  pespin: Looks good to me, approved
  Jenkins Builder: Verified



diff --git a/src/common/ta_control.c b/src/common/ta_control.c
index 6cef545..fd49d60 100644
--- a/src/common/ta_control.c
+++ b/src/common/ta_control.c
@@ -25,8 +25,9 @@
 #include <osmo-bts/gsm_data.h>
 #include <osmo-bts/logging.h>
 
-/* 90% of one bit duration in 1/256 symbols: 256*0.9 */
-#define TOA256_9OPERCENT	230
+/* 3GPP TS 45.010 sec 5.6.3 Delay assessment error:
+ * 75% of one bit duration in 1/256 symbols: 256*0.75 */
+#define TOA256_THRESH	192
 
 /* rqd_ta value range */
 #define TA_MIN 0
@@ -51,12 +52,12 @@
 
 	int16_t delta_ta = toa256/256;
 	if (toa256 >= 0) {
-		if ((toa256 - (256 * delta_ta)) > TOA256_9OPERCENT)
+		if ((toa256 - (256 * delta_ta)) > TOA256_THRESH)
 			delta_ta++;
 		if (delta_ta > TA_MAX_INC_STEP)
 			delta_ta = TA_MAX_INC_STEP;
 	} else {
-		if ((toa256 - (256 * delta_ta)) < -TOA256_9OPERCENT)
+		if ((toa256 - (256 * delta_ta)) < -TOA256_THRESH)
 			delta_ta--;
 		if (delta_ta < -TA_MAX_DEC_STEP)
 			delta_ta = -TA_MAX_DEC_STEP;

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Gerrit-Project: osmo-bts
Gerrit-Branch: master
Gerrit-Change-Id: I4c5825df4291d3851a113027abf7566de343a190
Gerrit-Change-Number: 25443
Gerrit-PatchSet: 3
Gerrit-Owner: pespin <pespin at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: laforge <laforge at osmocom.org>
Gerrit-Reviewer: osmith <osmith at sysmocom.de>
Gerrit-Reviewer: pespin <pespin at sysmocom.de>
Gerrit-MessageType: merged
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