Module 4 — Control Channels

Search Space#0 —
When to monitor CORESET#0

CORESET#0 tells the UE where to look for PDCCH. Search Space#0 tells it when. From the 4-bit searchSpaceZero index in the MIB, the UE derives a precise monitoring schedule — which slots, which symbols, how many candidates to try. This section derives the full schedule for our example.

TS 38.213 §13 TS 38.213 §10.1

Search Space concept

The UE cannot monitor CORESET#0 every symbol — that would waste power and processing. Instead, the network defines a Search Space: a periodic schedule of monitoring occasions when the UE must check for PDCCH.

Think of it like a scheduled inspection: "Check the noticeboard every Monday at 9am." The UE shows up at exactly those times, tries to decode every candidate PDCCH it might find there, and moves on.

A search space set is defined by monitoring occasions, the number of PDCCH candidates per aggregation level, and the PDCCH DCI formats. A UE monitors PDCCH candidates in the search space set on the monitoring occasions as defined for the search space set configuration.

3GPP TS 38.213, Section 10.1

Table 13-11 lookup

Our searchSpaceZero index = 2 (from MIB bits [16:19] = 0010). We look this up in TS 38.213 Table 13-11, using CORESET#0 duration = 2 symbols (from section 4.1).

TS 38.213 Table 13-11 — searchSpaceZero index = 2, CORESET duration = 2 TS 38.213 §13, Table 13-11

Index	Period (slots)	Slot offset	AL=4 candidates	AL=8 candidates
0	20	0	4	2
1	20	0	4	2
2 ← ours	20	2	4	2
3	20	2	4	2
4	20	5	4	2
5	20	5	4	2

Index 2 → period = 20 slots, slot offset = 2, first symbol = 0, candidates: 4 at AL=4, 2 at AL=8

Converting to actual slot numbers

With 30 kHz SCS, there are 20 slots per 10 ms half-frame. A period of 20 slots = 10 ms. The slot offset = 2 means the UE monitors slot 2 in each period.

Search Space#0 monitoring schedule — SCS = 30 kHzTS 38.213 §13

// Period = 20 slots × 0.5 ms per slot = 10 ms
// Offset = 2 → monitor slot 2 in each period

Monitoring occasions (slot numbers within radio frame):
  Slot 2    →  t = SFN×10ms + 2×0.5ms  = 5651.0 ms (our SFN=565)
  Slot 22   →  t = SFN×10ms + 22×0.5ms = 5661.0 ms
  Slot 42   →  (next frame)
  ...every 10 ms...

// Within each monitoring slot:
Symbol = 0  (first symbol of slot)
CORESET#0 occupies symbols 0 and 1

// At each occasion, UE tries:
4 candidates at AL=4  →  each uses 4 CCEs
2 candidates at AL=8  →  each uses 8 CCEs (= all 8 CCEs in CORESET#0)
Total = 6 blind decoding attempts per occasion

Monitoring occasion timeline

Search Space#0 monitoring occasions — 10 ms period TS 38.213 §13, Table 13-11

What happens at each monitoring occasion

At each monitoring occasion, the UE performs blind decoding: it tries all 6 candidate PDCCH positions, decodes each one, and checks the CRC. If one passes, that is the PDCCH scheduling SIB1.

Wait for monitoring slot (slot 2)

UE waits until SFN=565, slot=2. This is the first Search Space#0 occasion after SIB1 decoding begins. t = 5651.0 ms from start of the SFN cycle.

Extract CORESET#0 symbols 0 and 1

UE extracts the 24 RBs × 2 symbols of CORESET#0 from the received signal. This is the search space — the UE processes only these frequency-time resources.

Try 4 candidates at AL=4

AL=4 means each candidate uses 4 CCEs. With 8 total CCEs, possible AL=4 positions are: CCE{0,1,2,3}, CCE{2,3,4,5}, CCE{4,5,6,7}, CCE{0,2,4,6} (interleaved). UE decodes each and checks CRC.

Try 2 candidates at AL=8

AL=8 means each candidate uses all 8 CCEs. With 8 CCEs available, there is effectively only 1 unique position: CCE{0..7}. Both candidates test this position under different hypotheses. CRC pass → PDCCH found.

Search Space#0 — complete schedule

Monitoring period → every 20 slots = 10 ms

First occasion → slot 2 (offset = 2)

Symbol position → symbol 0

Total candidates → 6 (4 at AL=4 + 2 at AL=8)

DCI format → 1_0 with SI-RNTI (see next section)

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