In order for UE to decode PDCCH(DCI), it has to figure out the exact value for location(CCE index), structure(Aggregation Level, Interleaving etc) and scrambling code(RNTI) etc. But these information is not informed to UE beforehand and in most case those values changes dynamically. The only thing known to UE is the information about a certain range that possibly carries PDCCH(DCI). UE knows about the information about this certain range by a predefined rule or signaling message. Within this range, UE has to try to decode PDCCH/DCI using many, many, many, many different types of parameters (CCE Index, Aggregation Level, RNTI) based on trial and error method. This way of decoding is called Blind Decodinig. The predfined region in which UE perform the blind decoding is called Search Space.

There are two types of search space called "UE-specific search space" and "Common Search Space".

UE Spacific Search Space is dedicated for each specific UE and informed to UE via RRC signaling message. It means UE need to complete RRC establishment to get the information about UE-specific search space.

Common Search Space is the specific search space that every UE need to search for the signals for every UE(e.g, PDCCH for SIB) or signaling message that is applied to every UE before dedicated channel is established for a specific UE(e.g, PDCCH used during RACH process). For example, UE need to detect PDCCH for SIB1 reception or various DCI