Secondary Synchronisation Channel

Description

The Secondary Synchronisation Channel (SSCH) is a critical component of the UMTS (Universal Mobile Telecommunications System) WCDMA (Wideband Code Division Multiple Access) air interface, defined in the 3GPP specifications. It operates in conjunction with the Primary Synchronisation Channel (P-SCH) to enable the User Equipment (UE) to perform cell search and achieve synchronisation with a Node B (base station). The SSCH is a downlink-only physical channel that is transmitted simultaneously with the P-SCH during specific time slots.

Technically, the SSCH transmits a sequence known as a Secondary Synchronisation Code (SSC). Unlike the single, fixed code used on the P-SCH, there are 16 possible SSCs. These 16 codes are not used individually but are arranged into 64 unique sequences of 15 SSCs, each sequence corresponding to one of the 64 scrambling code groups used in the system. The sequence is repeated every 10 ms radio frame. The UE, after first detecting the P-SCH to achieve slot synchronisation, correlates against the possible SSC sequences on the SSCH. By identifying the specific 15-SSC sequence received, the UE determines the scrambling code group (narrowing the search from 512 possible primary scrambling codes to just 8) and, crucially, achieves frame synchronisation by identifying the start of the 10 ms frame.

Once the scrambling code group and frame timing are known, the UE proceeds to the final stage of cell search, where it performs a correlation against the 8 possible primary scrambling codes within the identified group on the Common Pilot Channel (CPICH) to find the exact primary scrambling code of the cell. The SSCH's design, using a carefully selected set of orthogonal sequences, ensures reliable detection even in challenging radio conditions. Its operation is a foundational, low-level procedure that underpins all higher-layer network access, mobility, and connectivity in UMTS networks, enabling the UE to camp on a cell and initiate communication.

Purpose & Motivation

The SSCH was created to solve the critical problem of initial cell acquisition and synchronisation in asynchronous CDMA networks like UMTS. In such networks, base stations are not time-synchronised with each other, unlike in synchronous systems like GSM. Therefore, a UE powering on has no prior knowledge of timing or the specific codes used by nearby cells. The purpose of the two-stage SCH (P-SCH and S-SCH) design is to efficiently guide the UE through this search.

The P-SCH provides a simple, universally known signal for coarse slot timing. The SSCH builds upon this by conveying two key pieces of information: the scrambling code group and the frame boundary. Conveying the scrambling code group drastically reduces the complexity of the final code search. Identifying the frame timing is essential because all higher-layer system information is broadcast in structured frames; without knowing where a frame starts, the UE cannot decode this information. The design of the SSCH sequences also provides a degree of frequency error robustness. Prior to UMTS, simpler synchronisation schemes existed, but the asynchronous nature and wider bandwidth of WCDMA necessitated this more sophisticated, hierarchical approach to ensure fast and reliable cell search, which is vital for reducing call setup times and enabling seamless mobility.

Key Features

• Transmits a sequence of 15 Secondary Synchronisation Codes (SSCs) per 10 ms radio frame
• Identifies one of 64 possible scrambling code groups, narrowing the primary scrambling code search
• Provides frame synchronisation by indicating the start of the radio frame
• Uses orthogonal variable spreading factor (OVSF) codes for the SSCs
• Always transmitted simultaneously with the Primary Synchronisation Channel (P-SCH)
• Fundamental for the second step of the 3-step UMTS cell search procedure

Evolution Across Releases

Defining Specifications