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coding: prevent marking FACCH frames as AMR's special DTX frames 

Both gsm0503_tch_a[fh]s_decode_dtx() functions accept an optional
'dtx' pointer, which is used to indicate type of a received AMR
block to the caller in DTX mode of operation.  If not NULL, it's
expected to be updated by gsm0503_detect_a[fh]s_dtx_frame() every
time one of the mentioned functions is called.

However, in case of FACCH both functions return early, so the value
of dtx remains unchanged and thus FACCH frames may be misinterpreted
as AMR's special DTX frames.  This is rather critical during the DTX
silence periods, when all special DTX frames (e.g. SID_UPDATE) are
being treated as SUB frames.  Each unsuccessful FACCH decoding
attempt will 'poison' SUB measurements, causing unexpected RxQual-
SUB values in the Uplink measurement reports.

Fix this by resetting *dtx to AMR_OTHER in the FACCH specific path.

Change-Id: I2e6f4b748c6445725211e264ab5f3f5a2712087a
Related: SYS#5853
This commit is contained in:
Vadim Yanitskiy 2022-05-16 19:40:45 +03:00
parent 71e8091c9d
commit a2bee8bc88
1 changed files with 13 additions and 0 deletions
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13
src/coding/gsm0503_coding.c
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@ -2171,6 +2171,12 @@ int gsm0503_tch_afs_decode_dtx(uint8_t *tch_data, const sbit_t *bursts,
gsm0503_tch_fr_deinterleave(cB, iB);
if (steal > 0) {
/* If not NULL, dtx indicates type of previously decoded TCH/AFS frame.
* It's normally updated by gsm0503_detect_afs_dtx_frame(), which is not
* reached in case of FACCH. Reset it here to avoid FACCH/F frames being
* misinterpreted as AMR's special DTX frames. */
if (dtx != NULL)
*dtx = AMR_OTHER;
rv = _xcch_decode_cB(tch_data, cB, n_errors, n_bits_total);
if (rv) {
/* Error decoding FACCH frame */
@ -2633,6 +2639,13 @@ int gsm0503_tch_ahs_decode_dtx(uint8_t *tch_data, const sbit_t *bursts, int odd,
/* if we found a stole FACCH, but only at correct alignment */
if (steal > 0) {
/* If not NULL, dtx indicates type of previously decoded TCH/AHS frame.
* It's normally updated by gsm0503_detect_ahs_dtx_frame(), which is not
* reached in case of FACCH. Reset it here to avoid FACCH/H frames being
* misinterpreted as AMR's special DTX frames. */
if (dtx != NULL)
*dtx = AMR_OTHER;
for (i = 0; i < 6; i++) {
gsm0503_tch_burst_unmap(&iB[i * 114],
&bursts[i * 116], NULL, i >> 2);