For a thesis, consider reading the 3GPP…
The TRX is indeed using 16kb/s channels on Abis to carry the voice. The signalling of all calls of a TRX is carried over the “RSL” (radio signalling link).
Basically, 1 TRX is connected to 2 ABIS timeslots for the voice (8 x 16kb/s) and 1 RSL. The RSL can be carried over 16kb/s or 32kb/s or 64kb/s, it depends on the signalling load, and the choice made by the OMCR configurator.