WCDMA Cell Search Procedure Explained

The primary objectives of the cell search procedure performed by a User Equipment (UE) in a WCDMA network are:

• Slot synchronization
• Frame synchronization
• Scrambling code group identification

Here’s a breakdown of the steps involved when a WCDMA-compliant mobile device is switched on and initially boots up:

Step 1: Frequency Band Measurement

The UE begins by taking measurements across all the frequency bands (e.g., 2010 to 2070 MHz, with a 5 MHz bandwidth). Unless specifically instructed by higher layers to focus on a particular set of frequency channels, the UE scans the spectrum to determine the location of active cells.

Step 2: Synchronization Acquisition

The UE then proceeds, starting with the strongest signal it detected, and moving to the next strongest, and so on. This continues until the UE achieves synchronization with a cell. This entire process is known as Cell Search.

Step 3: SCH Analysis (Primary and Secondary)

Each WCDMA cell transmits two crucial synchronization channels: the Primary Synchronization Channel (PSCH) and the Secondary Synchronization Channel (SSCH).

• Primary SCH (PSCH): The PSCH contains a single, identical data sequence (Cp) that is repeated in every slot. This code sequence is the same for all cells. By detecting the peak signal in each slot, the UE can accurately determine the slot boundaries.

• Secondary SCH (SSCH): The SSCH repeats 15 different code sequences over a frame. The UE attempts to correlate and identify the specific sequences in the SSCH to find the frame boundary and the repeating sequence. This repeating sequence of the SSCH indicates the primary scrambling code group used by the sector.

Therefore, by analyzing the PSCH and SSCH, the UE obtains both the frame boundary and the scrambling code group.

Scrambling Code Details

The scrambling code used in UTRA FDD is a 38400-chip segment derived from a Gold code with a length of $2^{18}-1$. A total of $2^{18}-1$ scrambling codes can be generated, numbered from 0 to 262142, although not all of them are actually used.

The downlink scrambling codes are divided into 512 sets. Each set comprises one primary scrambling code and 15 secondary scrambling codes. In total, 8192 codes are allocated. This can be verified as $512 \cdot 16 = 8192$ (16 = 1 Primary + 15 Secondary).

The 512 primary scrambling codes are further grouped into 64 scrambling code groups, with each group containing 8 primary scrambling codes. There is a one-to-one mapping between these 64 groups and the sequence of secondary synchronization codes. This means that $512 = 64 \cdot 8$.

Each cell is assigned only one primary scrambling code. The Primary Common Control Physical Channel (PCCPCH) and the Primary Common Pilot Channel (PCPICH) are always transmitted using this specific primary scrambling code.

Step 4: Common Pilot Channel Correlation

The UE then cross-correlates the Common Pilot Channel (CPICH) with each of the eight possible scrambling codes within the identified scrambling code group. This allows the UE to deduce the most probable primary scrambling code being used by the cell.

Once the primary scrambling code is identified, the UE can descramble and despread the Primary CCPCH. This channel carries the Broadcast Channel (BCH) transport channel, which contains crucial system and cell-specific broadcast information (System Information Blocks or SIBs).