Secondary Common Control Physical Channel

Description

The Secondary Common Control Physical Channel (SCCPCH) is a dedicated downlink physical channel in the UMTS WCDMA radio interface. It is defined in the physical layer specifications and is used to carry common transport channels that are not carried by the Primary Common Control Physical Channel (PCCPCH). The two main transport channels mapped to the SCCPCH are the Forward Access Channel (FACH) and the Paging Channel (PCH). These can be mapped to the same or separate SCCPCHs depending on network configuration.

From a physical layer perspective, the SCCPCH has a defined slot and frame structure, but unlike the PCCPCH, it does not carry the primary synchronization sequences. It uses a fixed spreading factor (typically SF=256) to ensure it can be received by all User Equipment (UE) in the cell, including those at the cell edge. The channel is continuously transmitted by the Node B, and its timing is aligned with the PCCPCH. The data on the SCCPCH is time-multiplexed with Transport Format Combination Indicator (TFCI) bits, which inform the UE about the format of the data being transmitted, and optionally with pilot bits for channel estimation.

The operational role of the SCCPCH is multifaceted. The FACH, carried on the SCCPCH, is used to transmit control information to UEs that have been identified by the network (e.g., during random access response) or to send small amounts of user data. The PCH, also carried on the SCCPCH, is used to page UEs when there is an incoming call or data session. UEs in idle or connected mode periodically monitor the PCH for their identity. The network can configure multiple SCCPCHs with different characteristics (e.g., different spreading codes) to manage load and prioritize traffic, such as having one SCCPCH for paging and another for FACH data.

Purpose & Motivation

The SCCPCH was introduced in UMTS to address the limitation of having only one Primary CCPCH. The PCCPCH is reserved for carrying the Broadcast Channel (BCH) with the most critical system information and uses a specific channelization code. The network, however, needs additional capacity to carry other common control information like paging messages and responses to random access, as well as to deliver small data packets without establishing a dedicated channel. The SCCPCH provides this necessary additional common channel resource.

Its creation was motivated by the need for efficiency and flexibility in common channel signaling. By dedicating a separate physical channel for FACH and PCH, the network can manage these functions independently of the broadcast stream. This separation allows for different transmission power levels, spreading factors, and coding schemes to be optimized for paging coverage versus data transmission on the FACH. It also prevents contention and interference between critical broadcast information and other common signaling traffic.

Furthermore, the SCCPCH enables efficient UE power saving. UEs in idle mode can sleep and only wake up to listen to their assigned Paging Occasion on the SCCPCH, rather than continuously monitoring a channel that also carries other traffic. This design, central to the UMTS common channel structure, supported essential procedures like cell reselection, random access, and packet data initiation in the early 3G era before high-speed dedicated channels were established.

Key Features

• Carries the Forward Access Channel (FACH) for control and small data packets
• Carries the Paging Channel (PCH) for network-initiated UE contact
• Uses a fixed, high spreading factor (e.g., SF=256) for wide cell coverage
• Time-multiplexes data with TFCI bits for transport format indication
• Can be configured in multiple instances for load balancing
• Continuously transmitted by the Node B with timing aligned to PCCPCH

Evolution Across Releases

Defining Specifications