The TCI State Indication for UE-specific PDCCH MAC CE is a fixed-size Control Element that spans 16 bits and is comprised of three specific fields, each with a designated length and purpose. This MAC CE is crucial for the UE as it dictates the current transmission configuration(TCI) for decoding the PDCCH, which is essential for the UE to understand the downlink scheduling decisions made by the gNodeB.

<  38.321-Figure 6.1.3.15-1: TCI State Indication for UE-specific PDCCH MAC CE >

Serving Cell ID: This field indicates the identity of the Serving Cell for which the MAC CE applies. The length of the field is 5 bits. If the indicated Serving Cell is configured as part of a simultaneousTCI-UpdateList1 or simultaneousTCI-UpdateList2 as specified in 38.331, this MAC CE applies to all theServing Cells in the set simultaneousTCI-UpdateList1 or simultaneousTCI-UpdateList2, respectively;

CORESET ID: This field indicates a Control Resource Set identified with ControlResourceSetId as specified in 38.331, for which the TCI State is being indicated. In case the value of the field is 0, the field refers to the Control Resource Set configured by controlResourceSetZero as specified in TS 38.331. The length of the field is 4 bits;

TCI State ID: This field indicates the TCI state identified by TCI-StateId as specified in 38.331 applicable to the Control Resource Set identified by CORESET ID field. If the field of CORESET ID is set to 0, this field indicates a TCI-StateId for a TCI state of the first 64 TCI-states configured by tci-StatesToAddModList and tci-StatesToReleaseList in the PDSCH-Config in the active BWP. If the field of CORESET ID is set to the other value than 0, this field indicates a TCI-StateId configured by tci-StatesPDCCH-ToAddList and tci-StatesPDCCH-ToReleaseList in the controlResourceSet identified by the indicated CORESET ID. The length of the field is 7 bits.

Reference

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