synthesized hopping :
all TRX except BCCH TRX will hop over a list of frequencies (= frequency hopping sequence). You can put as many frequencies as you want in the list (up to more than 30 usually).
Only the BCCH TRX requires a fixed frequency (bcch frequency that shouldn’t be included in the FHS !).
baseband hopping :
All TRX can hop, but the list of frequencies is limited to the number of TRX in the cell. For instance, if you have 4 TRX, the amount of frequencies will be 4. Then, like a round robin mechanism, each TRX will use, for each TDMA frame, one of those four frequencies.
Only the 1st timeslot of the BCCH TRX cannot hop.
Each other timeslot can hop.
Everytime a timeslot uses the BCCH frequency, it cannot use dowlink power control.
Hope it helps,