In 5G NR, the UE does not know where its Downlink Control Information (DCI) is located. The gNB schedules PDCCH in a Control Resource Set (CORESET), but the UE must blindly decode multiple candidates across different Aggregation Levels (AL) until it finds a CRC that matches its RNTI (e.g., C-RNTI).

Resource Grid Structure

The grid shows Resource Blocks (RBs) on the Y-axis and OFDM Symbols on the X-axis. One slot = 14 symbols. The CORESET is a rectangular region (blue tint) where PDCCH can be transmitted. Typically the first 1–3 symbols.

CCE and REG (3GPP TS 38.211 §7.3.2)

1 REG = 1 RB × 1 OFDM symbol (12 REs). 1 CCE = 6 REGs = 72 REs. A PDCCH candidate uses L CCEs, where L ∈ {1, 2, 4, 8, 16}. Higher L = more robust (better for poor radio) but fewer candidates.

Search Space Formula (3GPP TS 38.213)

The starting CCE index for candidate m at aggregation level L is:

s_n = L ⋅ {(Y_p + ⌊m⋅N_CCE,p/(L⋅M_s,max)⌋ + n_CICCE,p/L⌋}

Y_p is a pseudo-random value derived from RNTI: Y_p = (A⋅Y_p−1) mod D, with A=39827, D=65537, and Y₋₁ = RNTI. Crucially, Y_p varies per slot: each slot updates Y_p, so CCE indices "hop" to new positions. This provides interference avoidance and load balancing. (Common Search Space uses Y_p=0.)

CCE-to-REG Mapping

Non-interleaved: CCE n maps to 6 contiguous REGs. Interleaved: A block interleaver (R=2, 3, or 6) scatters REGs across frequency for frequency diversity.

The "Blind" Nature

The UE tries every (AL, candidate) pair in its monitored set. For each, it demodulates, decodes, and checks CRC XOR RNTI. If it matches, the DCI is for this UE. Otherwise, it tries the next candidate.

Usage

C-RNTI: UE identifier (hex, e.g. 1234). Used in the hashing formula to determine which CCE candidates to monitor.

Number of candidates (table): For each aggregation level L, set how many candidates to monitor (0–10). Use 0 to exclude that AL from the search. Changing any value reinitializes the search.

CORESET Duration: 1, 2, or 3 OFDM symbols. More symbols = more CCEs. Changing reinitializes the search.

CORESET RB length: Frequency extent of the CORESET in RBs. Changing reinitializes the search.

Interleaved: Toggle block interleaver. When on, REGs of each CCE are scattered across frequency for diversity.

Slot: Slot index n_s,f (0–1024). Y_p is updated per slot, so candidate CCE indices "hop" to different positions.

Initialize Search: Resets the step to the beginning (before candidate 1). Keeps the DCI location unchanged.

Set DCI: Places DCI at a random candidate among the monitored set. Red dots mark the scheduled CCEs. START BLIND DECODING uses this placement and does not overwrite it.

Step Fwd / Step Bwd: Advance or go back one candidate at a time.

START BLIND DECODING: Animates through all candidates from the beginning. Uses the DCI location from Set DCI (or picks one if none set). When the scan reaches the DCI, the block gets a red boundary and the dots turn green.

Visual Legend

• CORESET: Blue region where PDCCH can be transmitted.
• Active Probe: Orange fill — the candidate currently being checked.
• DCI Found: Green fill with red boundary — CRC matched.
• DCI location: Red dots on scheduled CCEs; green when found.
• CCE boundary: Dashed cyan (interleaved) or solid (non-interleaved).

Key Concepts

• Blind: UE doesn't know the exact location; it must try every combination.
• 1 CCE = 6 REGs = 72 REs: Fundamental unit of PDCCH capacity.
• Frequency diversity: Interleaving spreads a CCE across RBs to combat fading.
• AL trade-off: L=8 gives fewer candidates but better reliability in poor conditions.