i didn’t understand your question. What are you asking exactly ?
(ps: ha ha, i never needed to graduate : i work there ! 🙂 )
trx1 provides better rxqual due to better frequency planning
trx 2, 3… provide better rx lev due to frequency diversity (if FH is activated), but this is not always true, and hard to quantify anyway.
if bbh is activated, trx1 can also get frequency hopping (except ts0)
so why SDCCH and BCCH on TRX1 : because rxqual is better, therefore less interference. (that’s a measurable improvement, whereas freq diversity is hard to measure)