The Type-3 PDCCH Common Search Space is a critical element in 5G NR for broadcasting system information and ensuring network accessibility. Its configuration and interaction with other search spaces provide a balanced approach to resource utilization while catering to both common and dedicated signaling requirements. This setup ensures a robust and scalable mechanism for control channel operations in diverse network conditions.

NOTE : For formal description of this type of search space, refer to 38.213-10.1

• Purpose and Usage
• Configuration
• Interaction With Other Search Spaces

Purpose and Usage

The Type-3 Common Search Space (CSS) is a foundational component in 5G NR, specifically designed to facilitate the efficient transmission of system information. It ensures that all UEs within a cell, regardless of their state or location, can access critical broadcast messages such as SystemInformationBlockType1 (SIB1) and other essential system information. With its wide coverage area, the Type-3 CSS is optimized for large geographical reach, enabling consistent information delivery to UEs across diverse network environments. Furthermore, its support for beamformed systems enhances coverage and signal quality, even in scenarios requiring advanced spatial transmission techniques. Additionally, the Type-3 CSS serves as a reliable fallback mechanism, particularly for UEs in idle mode or those undergoing initial connection procedures, ensuring seamless network access and robust communication.

• System Information Transmission:
• The Type-3 CSS is primarily used for broadcasting system information messages. This includes critical information such as SystemInformationBlockType1 (SIB1) and other broadcast messages that UEs need for network access.
• Wide Coverage Area:
• Type-3 CSS is configured to cover large geographical areas and ensure all UEs within the cell can decode the required information.
• Support for Beamformed Systems:
• It plays a role in scenarios involving beamforming. Since the Type-3 CSS is broadcast-oriented, the beam configuration ensures maximum coverage for UEs.
• Fallback Mechanism:
• Provides a fallback mechanism for UEs that have limited configuration information or are in early stages of communication, such as idle mode or random access procedure.

Configuration

The configuration of the Type-3 Common Search Space (CSS) revolves around CSS set identifiers, which precisely define the allocation of control channel resources within this search space. Central to its operation is the association with specific Control Resource Sets (CORESETs), which specify the physical resources in both frequency and time for transmitting PDCCHs. To ensure timely access to control information, UEs monitor the Type-3 CSS at predefined monitoring occasions, configured through higher-layer signaling. Additionally, the Type-3 CSS supports multiple aggregation levels for Control Channel Elements (CCEs), allowing for flexible resource allocation that aligns with varying coverage and reliability requirements across the network.

• CSS Set Identifiers:
• Type-3 CSS is configured using CSS set identifiers, which determine the exact location of control channel resources allocated to this search space.
• Control Resource Set (CORESET):
• Type-3 CSS is associated with specific CORESET(s). These define the physical resources (frequency and time) available for transmitting PDCCHs in the CSS.
• Monitoring Occasions:
• UEs monitor the Type-3 CSS at predefined monitoring occasions, which are configured via higher layer signaling.
• Aggregation Levels:
• The Type-3 CSS supports multiple aggregation levels for control channel elements (CCEs). These levels enable flexible allocation based on the coverage and reliability requirements.

Interaction With Other Search Spaces

The Type-3 Common Search Space (CSS) distinguishes itself from Type-1 and Type-2 CSS by focusing on common signaling intended for all UEs within a cell, rather than the UE-specific or group-specific functions of the latter. While there may be overlap in the use of CORESET resources, the scheduling priority and purpose of Type-3 CSS emphasize its role in broadcasting critical system information. In contrast, UE-specific search spaces (USS) handle dedicated signaling, such as DCI for individual UEs, requiring UEs to differentiate between these search spaces to accurately decode control information. Type-3 CSS also takes precedence in control channel prioritization for essential broadcast messages, with resource allocation carefully managed to prevent conflicts with USS. Additionally, Type-3 CSS plays a vital role in the Random Access Channel (RACH) procedures, supporting initial system access and the transmission of broadcast signaling essential for UEs to complete the random access process.

• Type-1 and Type-2 CSS:
• Unlike Type-1 and Type-2 CSS, which are primarily UE-specific or group-specific, Type-3 CSS focuses on common signaling for all UEs in the cell.
• Type-3 CSS may overlap with Type-1 and Type-2 CSS in terms of CORESET usage, but its scheduling priority and purpose differentiate it.
• UE-Specific Search Spaces:
• While Type-3 CSS is for common signaling, UE-specific search spaces (USS) are used for dedicated signaling (e.g., DCI for individual UEs). UEs must differentiate between these search spaces to decode the appropriate control information.
• Control Channel Prioritization:
• Type-3 CSS is given higher priority for broadcasting essential system information. However, resource allocation is managed to avoid conflicts with USS.
• Random Access Procedure:
• Type-3 CSS interacts with the Random Access Channel (RACH) procedures by enabling initial system access and transmission of broadcast signaling required for UEs to complete the random access process.

Reference