Thanks all for this clarification.
BCCH must be on the lower freq in case of fixed allocation.
BCCH ARFCN carrier known as the Beacon of sector (for access, signaling & control function).
FOR channel dimensioning we can use different method but as standard (BCCH & SDCCH on Ts0 of carrier (combined mode) for cell with low signaling load. OR BCCH on Ts0 & SDCCH on Ts1, for significant high load on signaling.
But we can allocate more Ts for SDCCH for further high signaling load.
Further more for half rate TCH the SDCCH will be more than Full rate, Because the call set up, SMS, Fax, HO,LA update registration & other signaling will increase by increasing of calls & so you allocate more SDDCH for this load by fixed or auto allocation according to the vendor system.