Structure of Combined BCCH in GSM
It is used in the configuration of cells of low traffic density and small capacity. The Combined BCCH is only configured at timeslot 0.
Channel combination: FCCH?SCH?BCCH?CCCH+SDCCH/4+SACCH/4
SDCCH/4: Stand-alone dedicated control channel. Each TDMA multiframe with 51 frames has 4 SDCCH;
SACCH/4: Slow SDCCH/4 associated control channel;
Compared with the main BCCH channel, 4 signaling channels are added to the 51 frames. The functions of these 4 signaling channels are the same as those of the SDCCH8 channel. Therefore, this channel combination can be taken as a combination of the functions of the above two channels. This combination take effect on two aspects: first, this reduced the quantity of AGCH+PCH on CCCH and only a small-capacity system is provided; second, this combination provides a certain quantity of signaling channels in timeslot 0. Thus, it is unnecessary to assign SDCCH8 channels in a small-capacity system. This channel suitable for small-capacity systems. And it is also an example of the flexible GSM network configuration.