Primary and Secondary Synchronization Signals occupy 2 blocks of symbols per 10 ms (central 72 subcarriers for all channel bandwidths).
An example is shown below for 72 subcarrier- (1.4 MHz) / single antenna case.
During the initial cell search, the mobile station searches for the strongest cell. It then determines that cell’s physical layer id and frame synchronization. The initial cell search uses the Synchronization Channels (primary and secondary). Since the radio frame timing of all common physical channels is related to its timing, it is enough to find the timing of the Synchronization Channel only.
The cell search is carried out in three steps: search for strongest cell, frame
synchronization/code group identification and scrambling code identification
Step 1 (Strongest cell): Depending on its radio capability the mobile station scans for the strongest cell.
Step 2 (Slot synchronization / L1 id identification): Based on the correlation peaks detected for the primary synchronization code, the mobile station detects the L1 id (0,1 or 2). Furthermore it finds the start of the slot (0/10)..
Step 3 (Frame synchronization / Physical Layer Cell ID): The frame
synchronization and the group identification are achieved in this step. There are 168 possibilities for the secondary synchronization code word The terminal seeks the codes that belongs to the particular group id. Each code group consists of three id’s.
Together there are 504 different physical layer cell id’s.