5G NR & RAN
12 min read
Published: Oct 2026
Zero Telemetry • Client-Side
5G NR Flexible Numerology: Subcarrier Spacing, Slot Durations & Latency
Master 3GPP 5G NR flexible numerology (mu = 0 to 4). Calculate subcarrier spacing (15 to 240 kHz), OFDM symbol timing, cyclic prefix, and slot durations.
1. The Limitation of LTE's Fixed 15 kHz Subcarrier Spacing
In 4G LTE, the Orthogonal Frequency Division Multiplexing (OFDM) physical layer was built upon a single, rigid numerology: a fixed 15 kHz subcarrier spacing (SCS) with a constant 1.0 ms subframe duration containing exactly 14 OFDM symbols. While this fixed design was optimal for sub-3 GHz mobile broadband, it became a fatal bottleneck for 5G.
At millimeter-wave (mmWave) frequencies (28 GHz and above), severe oscillator phase noise and high Doppler shifts from vehicular movement shatter the orthogonality of closely spaced 15 kHz subcarriers. Furthermore, industrial automation and autonomous vehicles require user-plane air interface latencies well below 1 millisecond—an impossible feat when waiting for a 1.0 ms LTE slot to finish transmission.
To overcome this limitation, the 3rd Generation Partnership Project (3GPP) introduced Flexible Numerology in Release 15 (TS 38.211). By defining a scaling parameter μ (mu) that ranges from 0 to 4, 5G NR exponentially scales subcarrier spacing by powers of two:
Because the useful symbol duration Tu in OFDM is the exact reciprocal of subcarrier spacing (Tu = 1 / Δf), doubling the subcarrier spacing cuts the symbol duration and slot duration exactly in half:
Numerology (μ)
Subcarrier Spacing (Δf)
OFDM Symbol Time
Slots per Subframe (1 ms)
Slot Duration
Frequency Range Spectrum
μ = 0
15 kHz
66.67 μs
1 slot
1.000 ms
FR1 (< 3 GHz, Legacy FDD)
μ = 1
30 kHz
33.33 μs
2 slots
0.500 ms
FR1 Mid-Band (C-Band, 3.5 GHz)
μ = 2
60 kHz
16.67 μs
4 slots
0.250 ms
FR1 / FR2 (Optional / SUL)
μ = 3
120 kHz
8.33 μs
8 slots
0.125 ms (125 μs)
FR2 mmWave (24 to 40 GHz)
μ = 4
240 kHz
4.17 μs
16 slots
0.0625 ms (62.5 μs)
FR2 Synchronization / High Bands
3. The Impact on URLLC Latency and Cyclic Prefix Protection
Every 5G NR slot always contains exactly 14 OFDM symbols (under standard Normal Cyclic Prefix). By deploying μ = 3 (120 kHz SCS) in mmWave bands, the entire transmission of a 14-symbol transport block completes in just 125 microseconds, enabling true Ultra-Reliable Low-Latency Communications (URLLC).
However, there is an engineering trade-off: as symbol duration shrinks, the absolute time length of the Cyclic Prefix (CP) shrinks proportionally (from 4.69 μs at 15 kHz down to just 0.59 μs at 120 kHz). Because the cyclic prefix must be longer than the maximum multipath delay spread of the physical environment to prevent Inter-Symbol Interference (ISI), higher numerologies are restricted to compact microcells with tight line-of-sight propagation paths.
Interactive Engineering Calculators
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5G TDD Slot Duration Calculator
Calculate frame, subframe, slot, and symbol durations across 3GPP numerologies.