WCDMA cell search procedure used by UE
The primary objectives of the cell search procedure by UE in WCDMA cell are slot synchronization, frame synchronization and scrambling code group identification.
When we switch on the UE or WCDMA compliant mobile and initial boot up and following happens.
Step1: UE shall make measurements on all the frequency bands (e.g. 2010 to 2070 MHz,5 MHz BW), unless instructed by higher layers to look at a specific set of frequency channels, to establish where the active cells are located.
Step2: The UE shall proceed, starting with the strongest signal, next strongest etc. until synchronization is achieved. This procedure is called as Cell Search.
Step3: Each cell transmits primary SCH and secondary SCH.
• The primary SCH contains same data sequence (Cp) and repeats the same code sequence over every slots and this is same for all cells. As PSCH is known so, by finding the peak on every slot. This helps to find slot boundary.
• The secondary SCH repeats over 15 different code sequences (over a frame). Using this SCH, UE try to correlate and find out the possible sequences of Secondary SCH and find the frame boundary and repeat sequence. This determined repetition sequence of secondary SCH indicates the primary scrambling code group of the sector. Hence we get scrambling code group and frame boundary from step3.
The scrambling code employed in UTRA FDD is a 38400 chip segment of a 218-1 length Gold code. A total of 218-1 scrambling codes, numbered 0 through to 262142 can be generated,but all are not used. The downlink scrambling codes are divided into 512 sets. Each set consists of a primary scrambling code and 15 secondary scrambling codes. In all 8192 codes are allocated. 512*16 (16 = 1 Primary + 15 Secondary) = 8192 used The 512 Primary are further divided into 64 scrambling code groups, each group consisting of 8 primary scrambling codes. The 64 groups have a one-to-one mapping to the sequence of secondary synchronisation codes. 512 = 64 * 8 Each cell is allocated only one primary scrambling code and the PCCPCH & PCPICH is always transmitted using this primary scrambling code.
Step4: It cross correlates the common pilot channel with each of the eight scrambling codes and thereby deduce which code is most probable. Now it is ready to descramble and despread the Primary CCPCH which contains BCH transport channel which contains the system and cell specific broadcast information (SIB).