Module 4 — Control Channels

Blind Decoding —
Finding PDCCH without being told where it is

The UE knows when to look (Search Space#0) and where to look (CORESET#0). But within CORESET#0, the gNB has placed PDCCH at one of several possible CCE positions. The UE must try all of them — this is blind decoding. A CRC match reveals which candidate carries the real PDCCH.

TS 38.213 §10.1 TS 38.212 §7.3.1 TS 38.321 §7.1

Why blind decoding?

The gNB schedules different UEs in different CCE positions within each monitoring occasion. A UE has no advance knowledge of which CCE positions carry its PDCCH. Instead, it tries all candidate positions and checks each decoded result against a CRC masked with its own identity. Only the correct candidate passes.

For SIB1 specifically, the UE uses the SI-RNTI = 0xFFFF (a fixed value known by all UEs). Any UE in the cell can receive SIB1 — there is no UE-specific masking.

For a PDCCH candidate, if the CRC check of the decoded DCI is correct after de-masking with the RNTI associated with the search space set, the UE shall consider the decoded DCI as valid and process the corresponding DCI format.

3GPP TS 38.213, Section 10.1

RNTI — Radio Network Temporary Identifier

The RNTI is a 16-bit value used to mask the CRC of a PDCCH. Different RNTIs serve different purposes:

RNTI types and their uses TS 38.321 §7.1, TS 38.331

RNTI	Value	Used for	Who monitors
SI-RNTI	0xFFFF	SIB1 and other SI scheduling	All UEs
P-RNTI	0xFFFE	Paging	UEs in IDLE/INACTIVE
RA-RNTI	0x0001–0xFFCF	Random Access Response	UE performing RACH
TC-RNTI	assigned	Msg3/4 during RACH	UE completing RACH
C-RNTI	assigned	UE-specific scheduling	Connected UE only

DCI Format 1_0 — the SIB1 scheduling message

When the UE finds a PDCCH with SI-RNTI, it decodes the DCI (Downlink Control Information) to find where SIB1 is on the PDSCH. The format used for SIB1 scheduling is DCI Format 1_0 — the fallback downlink assignment format.

For DCI format 1_0 with CRC scrambled by SI-RNTI: the "System information indicator" field is 1 bit where 0 indicates scheduling of SIB1 and 1 indicates scheduling of SI messages in an SI window.

3GPP TS 38.212, Section 7.3.1.2.1

DCI Format 1_0 fields — SI-RNTI scrambled (SIB1 scheduling) TS 38.212 §7.3.1.2.1, Table 7.3.1.2.1-1

Field	Bits	Our decoded value	Meaning
Identifier for DCI formats	1	1	Downlink DCI
Freq. domain resource assign.	variable	0b000111111	RBs 24–38 → SIB1 on PDSCH
Time domain resource assign.	4	0b0010 = 2	Row 2 of Table 5.1.2.1-1 → symbols 2–12
VRB-to-PRB mapping	1	0	Non-interleaved
Modulation and coding scheme	5	5	QPSK, code rate 0.38
Redundancy version	2	00	First transmission (RV=0)
System information indicator	1	0	This schedules SIB1 ← key field

Full blind decoding walkthrough — slot 2

Let's trace through exactly what the UE does at slot 2, symbols 0–1, in CORESET#0 (3497.10–3505.74 MHz).

Extract CORESET#0 from received signal

UE extracts 24 RBs × 2 symbols = 48 REGs from the received signal at slot 2. Channel estimation using PDCCH DMRS (seeded by PCI=442). Equalization applied to all data REs.

Candidate 0 — AL=4, CCEs {0,1,2,3}

Demodulate CCEs 0–3 (= RBs 0–11 × 2 symbols = 144 REs → 288 bits after QPSK). Polar decode. Demask CRC with SI-RNTI=0xFFFF. CRC check → FAIL.

CRC check — candidate 0

Decoded bits + SI-RNTI demask → CRC = 0xA3F2 ≠ 0  ← fail

Candidates 1, 2, 3 — AL=4, other CCE positions

Try CCEs {2,3,4,5}, {4,5,6,7}, {0,2,4,6}. Each decoded and CRC checked with SI-RNTI. All fail.

Candidate 4 — AL=8, CCEs {0..7} — CRC PASS

Demodulate all 8 CCEs (= all 48 REGs). Polar decode 576 bits. Demask with SI-RNTI=0xFFFF. CRC = 0x000000 → PASS. PDCCH found.

CRC check — candidate 4 (PASS)

Decoded bits + SI-RNTI demask → CRC = 0x000000  ← pass ✓
DCI 1_0 content:
  Freq assignment = RBs 24–38   ← SIB1 on PDSCH
  Time assignment = row 2        ← slot 2, symbols 2–12
  MCS            = 5             ← QPSK, rate 0.38
  SI indicator   = 0             ← this is SIB1

SIB1 on PDSCH — where and when

With the decoded DCI, the UE now knows exactly where to find SIB1. The time domain assignment row 2 from TS 38.214 Table 5.1.2.1-1 means: slot offset K0=0, symbols 2–12 within the same slot.

Slot 2 — CORESET#0 then SIB1 PDSCH TS 38.214 §5.1.2.1

Blind decoding complete — SIB1 location found

PDCCH found at → Candidate 4, AL=8, CCEs {0..7}

DCI format → 1_0, SI-RNTI=0xFFFF

SIB1 frequency → RBs 24–38 (within initial DL BWP)

SIB1 time → slot 2, symbols 2–12

MCS → 5 (QPSK, code rate 0.38)

Next step → Decode SIB1 on PDSCH → Module 5

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