Currently (without this patch) if tch_dl_dequeue() yields nothing we're
scheduling dummy FACCH/H *regardless* if it's permitted to start at the
given TDMA FN or not. This may result in misaligned FACCH transmission,
so the MS will not able to decode anything and will report BER>0.
With this patch applied we schedule FACCH/H if it's allowed to start
at the current TDMA FN; otherwise send half-filled bursts with even
numbered bits contaning 232 encoded bits of the previous L2 frame
and 232 odd numbered bits all set to 0.
This patch does not guard against sporadic gaps in the Downlink TCH
queue. However when tch_dl_dequeue() constantly yields nothing, e.g.
when we end up with a codec mismatch, then this additional check saves
us from starting misaligned FACCH/H transmission. If we start at a wrong
TDMA offset, then all subsequent FACCH frames will be scheduled at wrong
offsets and thus none of them will be decoded by the MS.
Change-Id: I6f8af140a6ccf3d5fd7b98f6cb5c18e2c5e2f61b
Related: SYS#5919, OS#4823
Vadim Yanitskiy
11fb108221