PDCCH is comprised of fixed DCI formats which contain specific details for PDSCH assignment (i.e. no. of slots, RBS, modulation etc.). Different DCI formats are used for different UE states, different MIMO states and whether it is UL or DL allocation? UE scans all possible PDCCH search spaces based on its state (SI-RNTI, P-RNTI, RA-RNTI, C-RNTI etc.) and decodes specific DCI format to get its assigned PDSCH.