Secondary Common Pilot Channel

Description

The Secondary Common Pilot Channel (S-CPICH) is a downlink physical channel in the UMTS/WCDMA radio interface. Like the Primary CPICH (P-CPICH), it continuously broadcasts a predefined bit sequence (a pilot) that is scrambled with the cell's primary scrambling code or a secondary scrambling code. Its primary function is to provide a coherent phase reference for User Equipment (UE) to demodulate other downlink physical channels. However, unlike the P-CPICH, which is broadcast over the entire cell and is mandatory, the S-CPICH is an optional channel that can be transmitted with a different channelization code and may cover only a part of the cell, such as a specific sector or beam.

From a technical perspective, the S-CPICH is characterized by its channelization code and, optionally, its scrambling code. It is always transmitted with a fixed spreading factor (SF) of 256. A cell can have zero, one, or several S-CPICHs. Each S-CPICH can be associated with a different antenna beam or a specific transmission scheme. For instance, an S-CPICH can be used as the phase reference for a set of Secondary Common Control Physical Channels (S-CCPCHs) carrying transport channels like the FACH and PCH, or for the Downlink Dedicated Physical Channel (DPCH) assigned to specific UEs. This allows the network to steer beams or apply transmit diversity techniques (like Space Time Transmit Diversity - STTD) to specific areas or users, using the S-CPICH as the dedicated pilot for those transmissions.

The UE uses measurements from the S-CPICH, specifically the Received Signal Code Power (RSCP) and Ec/No, for cell selection, reselection, and handover decisions, similar to the P-CPICH. The network can inform the UE via higher-layer signaling (RRC messages) about which S-CPICH(s) to monitor and use as a reference. This flexibility is key for advanced antenna systems and network optimization, allowing operators to improve coverage and capacity in targeted zones without affecting the whole cell's common pilot channel.

Purpose & Motivation

The S-CPICH was introduced in Release 5 of UMTS to support the deployment of advanced antenna technologies and enhance network capacity and coverage flexibility. The Primary CPICH (P-CPICH) is a cell-wide resource that provides the basic phase reference for all UEs. However, this single, omnipresent pilot becomes a limitation when implementing features like adaptive antenna systems (beamforming) or when needing to assign different phase references to different logical sectors or user groups within the same cell.

Before S-CPICH, all downlink channels were demodulated using the phase reference from the P-CPICH. This meant that any beamforming or targeted transmission had to be aligned with the cell's common pilot, which restricted optimization. The S-CPICH solved this by providing additional, dedicated pilot resources. This enabled several key advancements: it allowed the implementation of Site Selection Diversity Transmission (SSDT) in soft handover, supported the use of secondary scrambling codes to overcome channelization code limitations in high-capacity cells, and became fundamental for the later introduction of High-Speed Downlink Packet Access (HSDPA) where it could be used as a phase reference for the HS-PDSCH. Essentially, S-CPICH provided the necessary granularity in pilot signaling to unlock more sophisticated physical layer techniques, moving beyond the 'one-size-fits-all' approach of the primary pilot.

Key Features

• Provides an additional, optional phase reference for downlink channel demodulation in UMTS
• Transmitted with a fixed spreading factor of 256
• Can be associated with a specific antenna beam, sector, or transmission scheme
• Used as a reference for S-CCPCH, DPCH, and HSDPA channels
• Enables advanced features like beamforming and transmit diversity (STTD)
• UE measurements on S-CPICH (RSCP/Ec/No) aid in mobility procedures

Evolution Across Releases

Defining Specifications