Scrambling Code Planning

Each transmitter in WCDMA has its unique code named Scrambling Code (SC). In the DL direction, SC is used to distinguish each cell and in the UL direction SC is used to distinguish each use (UE). UE detects DL SC using cell search procedure after slot and frame synchronization process. 3GPP (TS 25.213) has specified 512 Primary Scrambling Code (PSC) that can be used in the network (Actually, total of 218-1 = 262,143 scrambling codes, numbered 0…262,142 can be generated. However not all the scrambling codes are used).512 PSC are divided into 64 SC Group (SC Group 0…63) and each cell is allocated one and only one PSC.

SC in DL is assigned based on Planning and SC in UL is assigned automatically by network at call setup phase. DL PSC must be considered in the planning scope to avoid Co-SC problem that make UE can’t distinguish cell correctly due to identical scrambling codes are detected.

Scrambling code planning is not unique and can be performed in many ways. The following rules are commonly used in PSC planning:

1. Cell and its neighbors can not use the PSC from the same group

2. We should provide spare PSC to accommodate new node expansion for the future

3. Different PSC group allocation for each cell type (Macro, Micro and In Building)

Example of SC planning based on the rules above can be seen in following figure