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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. Purpose and UsageThis 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.
ConfigurationThis 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.
Fallback for C-RNTIIn 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.
Interaction With Other Search SpacesUnlike 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.
Reference[1] Type0-PDCCH common search space
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