DCI
Downlink Control Information: control messages carried on PDCCH that convey resource allocation, MCS, HARQ process information, and other scheduling parameters.
DCI is the payload PDCCH delivers — the scheduling instructions that orchestrate every transfer. A downlink-grant DCI tells the UE where its PDSCH is, the MCS, the HARQ process and redundancy version, and the PUCCH resource for its ACK. An uplink-grant DCI does the same for PUSCH. Beyond grants, DCI also carries power-control commands, slot format indications, and pre-emption notices.
The reason DCI comes in many formats is a balance between flexibility and overhead. Formats 0_0 and 1_0 are the compact "fallback" versions — fewer fields, used during access and in fallback situations. Formats 0_1 and 1_1 are the full-featured ones for a configured UE, with all the fields needed for advanced features like multi-layer MIMO and carrier aggregation. There are also group-common formats (2_x) that address many UEs at once for things like slot format and pre-emption. A DCI's CRC is scrambled with an RNTI, which is how the UE knows whether a given message is addressed to it, to a group, or to the cell.
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