Search Space
A set of PDCCH candidate locations within a CORESET where the UE monitors for DCI. Can be UE-specific or common across all UEs in the cell.
A search space is the schedule and seating chart for PDCCH monitoring. It tells the UE, for a given CORESET, how often to check for DCI (the monitoring periodicity and the symbols within a slot) and how many candidate locations to blind-decode at each aggregation level. Without it the UE wouldn't know when or where to look.
Two types exist, and the distinction is about who the control is for. Common Search Spaces carry DCI relevant to many or all UEs — system information scheduling, paging, random access responses, and the fallback DCI formats. The Type0 CSS, paired with CORESET#0, is the very first one a UE uses to find SIB1. UE-Specific Search Spaces carry DCI addressed to one device, scrambled with its C-RNTI, for its own data scheduling. A single CORESET can host several search spaces of both kinds, and the network tunes their periodicity to trade scheduling responsiveness against the UE's blind-decode budget — too many candidates and the UE can't keep up.
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