PDCCH Search Space — Light + animated · 3GPP TS 38.213 §10.1

1 Two families

Common vs UE-specific

Common Search Space (CSS) carries system-wide DCI — SIB scheduling, RAR, paging, slot-format and power-control — addressed by group RNTIs, located at fixed candidates everyone can find. UE-specific Search Space (USS) carries your unicast grants (C-RNTI), with candidate locations hashed from your RNTI so UEs spread out.

Common (CSS)

Type 0/0A SI-RNTI — SIB1 / SI Type 1 RA-RNTI — Msg2/Msg4 Type 2 P-RNTI — paging Type 3 group-common — SFI, TPC, C-RNTI

UE-specific (USS)

Carries C-RNTI / CS-RNTI / MCS-C-RNTI DCI for your PDSCH & PUSCH grants. Candidate CCE positions are derived from a per-UE hash, so two UEs in the same CORESET rarely fully collide.

USS candidate location (38.213 10.1)n_CCE = L·{ (Y_p,n + ⌊m·N_CCE/(L·M)⌋) mod ⌊N_CCE/L⌋ }
Y_p,n = (A_p·Y_p,n−1) mod 65537, Y_p,−1 = n_RNTI

2 The map

Candidates across aggregation levels

Each search space set lists how many candidates to monitor per aggregation level. For USS, the CCE start of each candidate comes from the hash above — change the RNTI and watch the candidates relocate within the CCE pool.

Space

N_CCE in CORESET 24

C-RNTI 1234

3 When

Monitoring occasions

A search space set monitors PDCCH on a periodicity of k_s slots, starting at an offset, for a duration of consecutive slots. Outside those occasions the UE's PDCCH receiver can sleep — that's the basis of power saving.

Periodicity k_s 4

Offset 1

Duration 1

4 Limits

Blind-decode & CCE budget

Per slot the UE can only monitor so many candidates and estimate so many CCEs — limits set by sub-carrier spacing (TS 38.213 Tables 10.1-2/3). Pile on candidates and watch the budget; over-budget search spaces are dropped by rule.

SCS

Candidates this slot 30

Blind decodes—

CCE channel estimates—

5 Knowledge check

Test yourself

CCE-REG Mapping DCI Formats