As was demonstrated in [1], there is a TCH/AHS specific problem in
libosmocoding causing unexpected BER ~50% in decoded AHS_SID_UPDATE
frames. The reason is that A[H]S_SID_UPDATE employs quite tricky
interleaving algorithm, which is different from the algorithm used
by normal TCH/AHS speech frames or A[F]S_SID_UPDATE frames.
An AHS_SID_UPDATE frame consists of two halves (228 bits each):
+---------+--------------------|---------+--------------------+
| in-band | SID marker | in-band | coded data |
+---------+--------------------|---------+--------------------+
| 16 bits | 212 bits | 16 bits | 212 bits |
The first half contains coded in-band signalling data (16 bits) and
the identification marker (212 bits), which allows to detect that
it's an AHS_SID_UPDATE. This half is carried by even bits of the
first two bursts and odd bits of the last two bursts.
The other half also contains the in-band data (16 bits), while the
remaining 212 bits contain encoded SID_UPDATE (212 bits). This
half is carried by even bits of the last two bursts and odd bits
of the first two bursts.
Current implementation does not use odd bits of the first two
bursts at all, so buffer cB[] in gsm0503_tch_ahs_decode_dtx()
contains only 114 out of 228 bits.
This patch changes the logic, so that gsm0503_tch_ahs_decode_dtx()
would not split AHS_SID_UPDATE onto two frames anymore like its
TCH/AFS equivalent does, but attempt to deinterleave the second
half and attempt to decode the payload immediately.
Change-Id: I8686d895e96fa0e606c1898b6574cc80a8f46983
Related: [1] I434157e2091a306c039123cea08d84bd8533c937
Related: SYS#5853
This patch adds a new test confirming that [1] actually fixes the bug.
Change-Id: I3d295a15d4446b3e440fbf4c90a1688d6c7275ae
Related: [1] I2e6f4b748c6445725211e264ab5f3f5a2712087a
Related: SYS#5853
This patch extends the existing unit test coverage for AMR's special
DTX frames. The new tests confirm that the problem with unexpected
BER in decoded AFS_SID_UPDATE frames has been actually fixed [1].
Additionally this patch demonstrates another TCH/AHS specific problem,
which negatively affects RxQual-SUB measurements in osmo-bts-trx: the
actual content of AHS_SID_UPDATE_CN is decoded with ~50% BER, because
the burst buffer contains only half of the burst bits.
Change-Id: I434157e2091a306c039123cea08d84bd8533c937
Related: [1] I813081a4c0865958eee2496fe251ae17235ac842
Related: SYS#5853
The value string array that explain the type of the AMR DTX / SID
frames is incomplete, lets add the missing strings.
Change-Id: If9e80b4bd8bdc31323c7c276155b2538e20a99be
Related: OS#2978
gsm0503_coding contains AMR decoder functions for HR and FR. Those can
only decode AMR payload frames but not amr DTX frames. Lets add
functionality to detect DTX frames. Also lets add decoding for SID_UPDATE
frames as well as error checking for the SID frame recognition patterns.
Related: OS#2978
Change-Id: I2bbdb39ea20461ca08b2e6f1a33532cb55cd5195
Vadim Yanitskiy
Philipp Maier