5G/NR - Type 1 PDCCH Common Search Space  

 

 

 

Type 1 PDCCH Common Search Space

Type 1 PDCCH Common Search Space is a subset of NR PDCCH Search Space that is dedicated to transmit the PDCCH with RA RNTI, TC-RNTI,C-RNTI on a primary cell. It means UE would monitor this search space during the whole period of RACH process. It means that UE searches this space in order to find DCIs for Msg2(PDSCH), Msg4(PDSCH) in the RACH process

Type1-PDCCH CSS is the common search space the UE relies on for receiving downlink control information associated with the random-access procedure on its primary cell. Without it being configured (via ra-SearchSpace), the UE has no way to detect the RA Response (RAR), MsgB, or TC-RNTI-based DCI in that cell.

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

NOTE : For the concept of all search space and comparision among different types of search space, refer to this note.

Purpose and Usage

This search space is primarily used for random-access–related DCIs on the UE’s primary cell. Specifically, it carries downlink control information with CRCs scrambled by the RA-RNTI (Random-Access RNTI), the MsgB-RNTI (used for certain MsgB transmissions in multi-step random access), and the TC-RNTI (Temporary C-RNTI assigned during random access). Hence, it is crucial for receiving Random Access Response (RAR) messages and other initial-access or temporary signaling.

  • It is primarily used for random-access–related DCIs on the UE’s primary cell. Specifically, it carries downlink control information with CRCs scrambled by:
    • RA-RNTI (Random-Access RNTI)
    • MsgB-RNTI (used for certain MsgB transmissions in multi-step random access)
    • TC-RNTI (Temporary C-RNTI assigned during random access)
  • Hence, this search space is crucial for receiving Random Access Response (RAR) messages and other initial-access or temporary signaling.

Configuration

This search space can be explicitely configured by ra-ControlResourceSet in PDCCH-ConfigCommon. The RRC Message or IE that can carry PDCCH-ConfigCommon are SIB1, BWP-DownlinkCommon, BWP-DownlinkDedicated. If the configuration for this search space is not explicitly configured in RRC message(ra-ControlResourceSet and ra-SearchSpace), UE will search the Type 1 PDCCH in Type 0 PDCCH Common Search Space.

  • The network signals the presence and parameters of the Type1-PDCCH CSS through the ra-SearchSpace field in PDCCH-ConfigCommon.
  • This signaling tells the UE where and when to monitor for DCI scrambled by RA-RNTI, MsgB-RNTI, or TC-RNTI on the primary cell.
  • If the UE does not receive any ra-SearchSpace configuration (for instance, if it is omitted or set to null), it does not attempt to decode Type1-PDCCH CSS.

Fallback for C-RNTI

In certain fallback scenarios, when a UE has been assigned a C-RNTI but lacks other dedicated or more advanced search space configurations (e.g, namely the Type3-PDCCH CSS set, Type1A-PDCCH CSS set, or any UE-specific search space) it can leverage the Type1-PDCCH CSS set for unicast transmissions using the C-RNTI. Specifically, the UE may monitor this search space for DCI formats 0_0 and 1_0 that are CRC-scrambled by the C-RNTI, ensuring the continuation of unicast communication even in the absence of other configured search spaces.

  • If the UE has not been provided:
    • a Type3-PDCCH CSS set,
    • or a Type1A-PDCCH CSS set,
    • or any UE-specific search space (USS),
  • and the UE has been assigned a C-RNTI and does have a Type1-PDCCH CSS set, the UE may also use this Type1-PDCCH CSS set to monitor DCI format 0_0 and 1_0 with CRC scrambled by the C-RNTI.

Interaction With Other Search Spaces

Unlike Type0-PDCCH CSS (for System Information), Type2-PDCCH CSS (for Paging), or Type3-PDCCH CSS (for various common RNTIs, including TPC RNTIs, INT-RNTI, etc.), the Type1-PDCCH CSS is specifically linked to the random access process or, under certain fallback conditions, to unicast if no UE-specific or other common search spaces are available.

  • Primarily focuses on random-access or fallback: Type1-PDCCH CSS is oriented toward RA-RNTI, MsgB-RNTI, and TC-RNTI, covering core random access needs or, in certain fallback conditions, unicast via C-RNTI.
  • Contrast with Type0-PDCCH CSS: Type0 is allocated to system information (e.g., for SIB broadcasting).
  • Contrast with Type2-PDCCH CSS: Type2 is dedicated to paging activities.
  • Contrast with Type3-PDCCH CSS: Type3 is used for various “common” or specialized RNTIs (e.g., TPC, INT, SFI).
  • Ensures essential RA or unicast: This design allows a UE to continue its random access procedures or unicast reception, even if other common or UE-specific search spaces are unavailable.

Reference