5G NR › Cell Search › Section 2.5
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
Foundations · 5G NR This is the established 5G NR basis that 6G builds on. 6G is this site's primary focus — here is how the idea evolves: initial access & sync in 6G. How this evolves in 6G →

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
Cell search result — PCI identified
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