Module 2 › Cell Search › Section 2.5
Module 2 — Cell Search

PCI —
Physical Cell Identity

With N²_ID = 1 from PSS and N¹_ID = 147 from SSS, the UE computes the Physical Cell Identity using a single formula. PCI = 442. This number seeds every reference signal, scrambling sequence, and DMRS pattern used in the cell. It is the DNA of the cell.

TS 38.211 §7.4.2.1 TS 38.211 §7.4.1

The PCI formula

The physical-layer cell identity NcellID is defined by

    NcellID = 3·N¹ID + N²ID

where N¹ID ∈ {0,1,...,335} is the physical-layer cell-identity group and N²ID ∈ {0,1,2} is the physical-layer identity within the cell-identity group.
3GPP TS 38.211, Section 7.4.2.1, Equation 7.4.2.1-1

Applying this to our example:

PCI calculation — our example TS 38.211 §7.4.2.1
Step by step
From PSS: N²_ID = 1    ← sequence index that gave correlation peak
From SSS: N¹_ID = 147  ← candidate index that gave correlation peak

PCI = 3 × N¹_ID + N²_ID
    = 3 × 147  + 1
    = 441     + 1
    = 442

// Reverse check:
N²_ID = 442 mod 3 = 1  ✓
N¹_ID = ⌊442 / 3⌋  = 147 ✓

Why 1008 PCIs?

With N¹_ID ranging over 336 values and N²_ID over 3 values, the total number of unique PCIs is 336 × 3 = 1008. This is not arbitrary — it is a careful engineering trade-off between network planning flexibility and UE processing complexity.

In dense urban deployments, nearby cells must have different PCIs to avoid PSS collision (same N²_ID would confuse UE timing), SSS collision (same N¹_ID causes interference in SSS detection), and reference signal collision (same PCI means identical scrambling seeds — DMRS interfere coherently). With 1008 PCIs and typical cell density, the reuse distance is large enough that interference is negligible.

Each N¹_ID value "owns" exactly 3 consecutive PCIs, one per N²_ID:

PCI space — 336 groups × 3 = 1008 unique PCIs TS 38.211 §7.4.2.1
■ N²_ID = 0 ■ N²_ID = 1 ■ N²_ID = 2 ● Our cell: PCI 442 (N¹_ID=147, N²_ID=1)

PCI seeds everything in the cell

Once PCI = 442 is known, the UE can generate every reference signal in the cell. The PCI is used as a seed for pseudorandom sequence generators that scramble PBCH, PDCCH, PDSCH, and all DMRS signals. Two neighbouring cells with the same PCI would have identical reference signals — they would coherently interfere with each other, making reception impossible.

What PCI = 442 seeds TS 38.211 §7.4.1, §7.3.3
Signals derived from PCI = 442
// PBCH DMRS scrambling (TS 38.211 §7.4.1.4.1)
v = PCI mod 4 = 442 mod 4 = 2  ← DMRS subcarrier offset

// PBCH scrambling seed (TS 38.211 §7.3.3.1)
c_init = PCI = 442

// PDCCH DMRS (TS 38.211 §7.4.1.3.1)
n_ID = PCI = 442  (unless overridden by higher layer)

// SSS sequence parameters (already used)
m₀ = 15×⌊147/112⌋ + 5×1 = 20
m₁ = 147 mod 112          = 35

// CSI-RS, SRS, PUSCH DMRS — all seeded by PCI when not overridden

Interactive PCI calculator

PCI ↔ N¹_ID / N²_ID calculator TS 38.211 §7.4.2.1
From N²_ID + N¹_ID → PCI
PCI = 442
From PCI → N²_ID + N¹_ID
N²_ID = 1
N¹_ID = 147
Module 2 complete — full cell search result
GSCN       → 7845 (f_SSB = 3498.24 MHz)
N²_ID      → 1 (from PSS, t_peak = 4523 samples)
N¹_ID      → 147 (from SSS, corr = 72.4)
PCI        → 442 (= 3×147 + 1)
SCS        → 30 kHz
Next step  → PBCH decoding → MIB → CORESET#0 → SIB1
← Previous SSS Detection