MAC- CE and DCI

5G NR
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1

Hello!

I read somewhere DCI with MAC-CE A-CSI Report Trigger would do the following: DCI with MAC-CE is sent to UE to measure and report L1-CSI RSRP and starts CSI-RS resources transmission.

Does that mean that the MAC-CE would trigger the CSI-RS resources and the DCI further triggers subset of them? which of the MAC-CE and DCI are used to indicate reporting and which is used to configure the resources?

Thank you

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Hello there! Nice question indeed but let shed some light first to clarify whole procedure that is usually related to beam management.

  1. Through RRC:
    -gNB sends TCI states (up to 64 for PDCCH, 128 PDSCH)
    -It also configures an aperiodic (AP) CSI-RS (nzp/CSI-IM) with “reportquantity” configured to L1-RSRP (for instance).

  2. a “CSI Request” field is flagged in DCI to trigger AP CSI-RS.

  3. After that, the UE reports L1-RSRP based on aperiodic offset list (for instance), if there is only CSI on PUSCH it will follow another rule.

  4. the gNB sends a MAC-CE command to activate a specific TCI state or a down-select (8 out of 128 relation for PDSCH, for instance) from the the list of TCI state sent previously (step 1).

  5. A TCI field (3bits) could select one out of 8 TCI states sent by MAC-CE (step 4). This step requires that “tci-PrensentInDCI” to be sent previously (step1).

obs.: TCI state might be related to CSI-RS or SSB.

So MAC-CE activates(changes) CSI-RS/SSB, but do not trigger CSI-RS reporting for L1-RSRP.
DCI can trigger CSI-RS for L1-RSRP reporting. The resources available for L1-RSRP are configured through RRC and how they should be reported as well.

obs.: this procedure could also be periodic without need of DCI triggering.