Work of Synchronization Channels P-SCH & SSCH in 3G WCDMA

Synchronization Channel mainly Used for cell search its having Two sub channels: P-SCH and S-SCH, SCH is transmitted at the first 256 chips of every time slot. Primary synchronization code is transmitted repeatedly in each time slot. Secondary synchronization code specifies the scrambling code groups of the cell.

When a UE is turned on, the first thing it does is to scan the UMTS spectrum and find a UMTS cell. After that, it has to find the primary scrambling code used by that cell in order to be able to decode the BCCH (for system information). This is done with the help of the Synchronization Channel.

Each cell of a NodeB has its own SCH timing, so that there is no overlapping.

The SCH is a pure downlink physical channel broadcasted over the entire cell. It is transmitted unscrambled during the first 256 chips of each time slot, in time multiplex with the P-CCPCH. It is the only channel that is not spread over the entire radio frame. The SCH provides the primary scrambling code group (one out of 64 groups), as well as the radio frame and time slot synchronization.

The SCH consists of two sub-channels, the primary and secondary SCH. These sub channels are sent in parallel using code division during the first 256 chips of each time slot. P-SCH always transmits primary synchronization code. S-SCH transmits secondary synchronization codes.

The primary synchronization code is repeated at the beginning of each time slot. The same code is used by all the cells and enables the mobiles to detect the existence of the UMTS cell and to synchronize it on the time slot boundaries. This is normally done with a single matched filter or any similar device. The slot timing of the cell is obtained by detecting peaks in the matched filter output.

This is the first step of the cell search procedure. The second step is done using the secondary synchronization channel.

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