Why BSIC Planning in GSM ? Because when tight frequency planning required or we have very limited frequency to plan then to reduce Mobile confusion about cell id BSIC used to identify cell id after BCCH. lets understand How BSIC Planning in GSM?
In addition to the assignment of frequency group to a cell, a Base Station Identity Code (BSIC) must be assigned in association with the frequency group. This will eliminate the possibility of incorrect cell identification and will allow the evolution to future cell architecture.
The BSIC is a two-digit code wherein the first digit is indicates NCC (Network Color Code) and the second digit indicates BCC (Base Station Color Code).
The NCC and BCC have values ranging from 0 to 7, where the NCC is fixed for an operator, signifying at any given point there can be maximum of 8 operators in an area. The BCC defines the cluster number which means a group of 8 clusters carry unique identity which is re-used for another group of 8 clusters and so on.
The principal for allocation of the BSIC is the same as for the RF carriers but at cluster level rather than cell level. The concept can be understood in the following example,
BSIC Planning Example:
Assume a network with 100 base stations each having three sectors. The BCCH and TCH share the same re-use plan 4 x 3. Which means we have cluster of 4 base stations, and in all we have 100/4 = 25 clusters.
Assume NCC code allocated is 6, which gives us clusters starting from number 61 to 67. Hence seven clusters form a group and hence we have 25/7 that is 3 groups of 7 clusters plus additional 4 clusters which form part of the 4th group.
The reuse of these 7 clusters group for BSIC numbered from 61 to 67 is shown in the figure below,
It should be noted that since BSIC are defined at cell (sector) level, hence there are every possible chances that the three sectors within the same site can have different BSIC. The reason being as BSIC is used for cell identification hence cells with same BCCH frequency but different BSIC can be easily discriminated by the MS.