In GSM system, each BTS is allocated with a color code, which is called BSIC. MS can identify two cells with the same BCCH by the help of BSIC, In network planning, effort should be made to make sure that BCCH of neighbor cells different from the serving cell’s BCCH to reduce the interference. Practically it is still possible that a same BCCH is re-used in the surrounding cells. For cells using the same BCCH in a relevant near distance, their BSIC must be different so that MS can identify two neighbor cells with same BCCH.
BSIC is transmitted on Synchronous Channel (SCH) of each cell. Its functions include:
1. If MS have read SCH, it is considered as being synchronous with that cell. But to correctly read the information on the downlink common signaling channel, MS must get TSC adopted by common signaling channel. According to GSM specification, TSC has eight fixed formats, which are represented by 0~7 respectively. TSC number adopted by common signaling channel of each cell is the BCC of this cell. So one of the functions of BSIC is to inform MS of the TSC adopted by the common signaling channel of this cell.
2. Since BSIC attends the coding process of information bit of random access burst, it can be used to avoid the case that BTS takes the RACH transmitted from MS in a neighbor cell as the access signal of the MS from the serving cell.
3. When MS is in dedicated mode, it must measure the BCCH level of the neighbor cell and report it to BTS according to BA2 on SACCH, including their respective BSIC. In special circumstance, when there are two or more cells using the same BCCH in the neighbor cells, BSC can rely on BSIC to distinguish these cells and avoid wrong handover or even handover failure.
4. MS must measure the BCCH signals of neighbor cells in dedicated mode, and report the result to the network. Since MS sends measurement report which contains contents of maximum 6 neighbor cells each time, it is necessary to control MS to merely report those cells that have neighbor relationships with the serving cell. The higher three digits of BSIC are used for the above purpose. Network operators can use parameter “ NCC Permitted” to control MS to report the neighbor cells with NCC permitted in the serving cell only.