The requirement that the BCCH TRX must transmit continuously in all the time slots sets strict limitations on how the frequency hopping can be realized in a cell. The current solutions are Baseband Frequency Hopping (BB FH) and Synthesized Frequency Hopping (RF FH).
In the baseband frequency hopping the TRXs operates at fixed frequencies. Frequency hopping is generated by switching consecutive bursts in each time slot through different TRXs according to the assigned hopping sequence. The number of frequencies to hop over is determined by the number of TRXs. Because the first time slot of the BCCH TRX is not allowed to hop, it must be excluded from the hopping sequence. This leads to three different hopping groups. The first group doesn’t hop and it includes only the BCCH time slot. The second group consists of the first time slots of the non-BCCH TRXs. The third group includes time slots one through seven from every TRX. This is illustrated in below Figure.
In the synthesized frequency hopping all the TRXs except the BCCH TRX change their frequency for every TDMA frame according to the hopping sequence. Thus the BCCH TRX doesn’t hop. The number of frequencies to hop over is limited to 63, which is the maximum number of frequencies in the Mobile Allocation (MA) list covered in Section 1.4. Synthesized hopping is illustrated in below Figure.
The biggest limitation in baseband hopping is that the number of the hopping frequencies is the same as the number of TRXs. In synthesised hopping the number of the hopping frequencies can be anything between the number of hopping TRXs and 63. However in synthesised hopping the BCCH TRX is left completely out of the hopping sequence.