5G NR Slot Duration: Why It Halves When SCS Doubles

5G NR numerology slot duration derived step by step: why a slot lasts 1/2^mu ms, exact symbol and CP lengths, FR1 vs FR2 use, and worked examples.

By BitForBytes Editorial & Hardware Research Team, Official BitForBytes Hardware Publication · 2026-10-09 · 10 min read

⚡ Quick Summary

In 5G NR, a slot is 14 OFDM symbols with normal cyclic prefix, and it lasts 1 ms divided by 2^μ, where μ is the numerology index. Subcarrier spacing is 15 × 2^μ kHz and an OFDM symbol lasts 1/Δf, so doubling the spacing halves every symbol and therefore the slot. The 1 ms subframe never changes.

1. Introduction & Core Motivation

LTE used one subcarrier spacing for normal data transmission, 15 kHz, and had one answer to "how long is a slot". That worked for carriers below 6 GHz and broadband data. 5G NR has to run at 700 MHz and at 28 GHz, serve a video stream and a factory robot, and a single spacing cannot suit all of that. At millimetre-wave frequencies, oscillator phase noise and Doppler shift would smear 15 kHz subcarriers into each other; at low frequencies, a very wide spacing would shrink the cyclic prefix below the channel's delay spread.

NR's answer is scalable numerology: a family of subcarrier spacings related by powers of two. Most explainers present the result as a table of five rows to memorise. The table hides the fact that every column follows from one relationship, and it invites errors, such as the claim found online that 15 kHz and 30 kHz correspond to "normal" and "extended" cyclic prefix. They do not.

Numerology is the first topic in any 5G physical-layer syllabus and a guaranteed interview opener for telecom roles. After reading, you will be able to derive the numerology table from the definition of an OFDM symbol, state which spacings apply to which frequency range, and compute slot counts and timings without looking them up.

2. Detailed Technical Breakdown

What Is Numerology in 5G NR?

Numerology in 5G NR is the choice of subcarrier spacing and cyclic-prefix type, identified by an index μ. 3GPP TS 38.211 defines the subcarrier spacing as

Δf = 2^μ × 15 kHz

where Δf is the spacing between adjacent OFDM subcarriers in kHz and μ is a non-negative integer. So μ = 0 gives 15 kHz (the LTE value), μ = 1 gives 30 kHz, μ = 2 gives 60 kHz, μ = 3 gives 120 kHz and μ = 4 gives 240 kHz.

Why Does Doubling the Subcarrier Spacing Halve the Symbol?

Doubling the subcarrier spacing halves the symbol because OFDM subcarriers stay orthogonal only when the useful symbol duration equals the reciprocal of the spacing:

T_u = 1 / Δf

where T_u is the useful (FFT) part of the OFDM symbol in seconds. At 15 kHz, T_u = 1/15000 = 66.67 µs. At 30 kHz, T_u = 33.33 µs. This is the same time-frequency trade as in any Fourier pair: wider in frequency means shorter in time.

The cyclic prefix scales with it. The normal cyclic prefix is about 7% of the useful symbol, 4.69 µs at 15 kHz and 2.34 µs at 30 kHz. A slot is always 14 symbols with normal cyclic prefix, so if each symbol halves, the slot halves:

T_slot = 1 ms / 2^μ

Worked example. For μ = 1: 14 symbols of 33.33 + 2.34 = 35.67 µs come to 499.5 µs. The missing 0.5 µs is a slightly longer prefix on one symbol, explained below, which brings the slot to exactly 0.5 ms.

How Is the NR Frame Built From Slots and Symbols?

The NR frame is built on fixed 10 ms frames and 1 ms subframes, and only the number of slots inside a subframe changes with numerology.

 Frame: 10 ms (always)
 +----+----+----+----+----+----+----+----+----+----+
 | SF0| SF1| SF2| SF3| SF4| SF5| SF6| SF7| SF8| SF9|   10 subframes
 +----+----+----+----+----+----+----+----+----+----+
   |
   v  one subframe = 1 ms (always)

 mu=0  15 kHz  |<---------------- 1 slot ---------------->|
 mu=1  30 kHz  |<----- slot 0 ----->|<----- slot 1 ----->|
 mu=2  60 kHz  |<- s0 ->|<- s1 ->|<- s2 ->|<- s3 ->|
 mu=3 120 kHz  |s0 |s1 |s2 |s3 |s4 |s5 |s6 |s7 |

 each slot = 14 OFDM symbols (normal CP):
 |CP|sym0|CP|sym1|CP|sym2| ...                |CP|sym13|

A subframe holds 2^μ slots and a frame holds 10 × 2^μ slots. Because slot boundaries of every numerology line up at subframe boundaries, carriers with different spacings can share a common time reference.

What Are the Exact Values for Each Numerology?

The exact values follow from NR's basic time unit T_c = 1/(480000 × 4096) s, about 0.509 ns. The useful symbol is 2048κ · 2^-μT_c and the normal cyclic prefix is 144κ · 2^-μT_c, with κ = 64.

The word "average" in the slot column is deliberate. Once every 0.5 ms, one symbol carries a cyclic prefix that is 16κT_c = 0.52 µs longer than the rest. That padding makes the arithmetic close: in half a subframe there are 7 × 2^μ symbols of (2048 + 144)κ · 2^-μT_c each, plus 16κT_c once, which totals 15360κT_c = 0.5 ms for every μ.

A consequence that tables omit: at 60 kHz, the first slot in each half-subframe is 250.26 µs and the next is 249.74 µs. Slots are equal on average, not individually.

Which Numerology Is Used in Which Frequency Range?

Lower spacings are used in Frequency Range 1 and higher spacings in Frequency Range 2, and 240 kHz is reserved for synchronisation signals only.

Extended cyclic prefix exists only for μ = 2. It lengthens the prefix to 4.17 µs, a quarter of the useful symbol, and fits 12 symbols in a slot. It is a separate option on the 60 kHz numerology, not a property of 30 kHz.

Why Not Use One Subcarrier Spacing Everywhere?

One spacing cannot work everywhere because Doppler and phase noise push the spacing up, while delay spread pushes it down. Two short calculations show both limits.

Doppler. Doppler shift is f_d = v f_c / c, where v is speed in m/s, f_c is carrier frequency in Hz and c = 3 × 10^8 m/s. At 120 km/h (33.33 m/s):

A Doppler shift of 20.7% of the spacing would cause severe inter-carrier interference, so 15 kHz is not usable at 28 GHz for a moving user. At 120 kHz the same Doppler shift is a manageable 2.6%.

Delay spread. The cyclic prefix must be longer than the channel's delay spread. A path-length difference d gives a delay d/c, so a 4.69 µs prefix absorbs echoes from paths up to about 1.4 km longer than the direct path, while a 0.59 µs prefix absorbs only about 176 m. Wide spacings therefore suit small millimetre-wave cells with short echoes, and narrow spacings suit large low-band cells.

The third effect is latency. Scheduling is slot-based, so a 0.125 ms slot at 120 kHz lets the network react eight times faster than a 1 ms slot at 15 kHz.

3. Practical Takeaways for Students

• Exam angle: Derive, do not recite. Write Δf = 15 × 2^μ kHz, then T_u = 1/Δf, then "14 symbols per slot, subframe fixed at 1 ms, so 2^μ slots per subframe". Draw the frame diagram with slots aligned at the subframe boundary. That sequence earns full marks and regenerates the table.
• Interview angle: "How many slots in a frame at 120 kHz?" 80. "Is 240 kHz used for data?" No, only for the SS/PBCH block in FR2. "Why can't FR2 use 15 kHz?" Phase noise and Doppler are too large relative to the spacing.
• Common mistake: Saying the subframe gets shorter. The frame (10 ms) and subframe (1 ms) are fixed for all numerologies; only the slot and symbol scale. The second trap is tying cyclic-prefix type to 15 or 30 kHz.
• Try it yourself: In Python, compute 1e6 / (15e3 * 2**mu) for the useful symbol in µs and multiply by 144/2048 for the prefix; you should reproduce every row of the table. Revise OFDM basics on the BitForBytes Signal Playground first if orthogonality is unclear.

4. Frequently Asked Questions

How many slots are there in one 5G NR frame?

One 5G NR frame of 10 ms contains 10 × 2^μ slots. That is 10 slots at 15 kHz, 20 at 30 kHz, 40 at 60 kHz, 80 at 120 kHz and 160 at 240 kHz. Each slot carries 14 OFDM symbols with normal cyclic prefix.

The number of symbols per subframe scales the same way: 14 at 15 kHz and 112 at 120 kHz. Slot numbering restarts at the beginning of every frame.

Is the 5G NR subframe duration different for different numerologies?

No. The subframe is 1 ms and the frame is 10 ms for every numerology. Only the slot and the OFDM symbol change with subcarrier spacing. A subframe simply contains more slots as the spacing increases: one at 15 kHz and eight at 120 kHz.

Keeping the subframe fixed gives all numerologies a common 1 ms grid, which simplifies coexistence with LTE and timing alignment between carriers that use different spacings.

What is a mini-slot in 5G NR?

A mini-slot is a transmission shorter than a full slot that can start at a symbol other than the first. NR allows data allocations of a few OFDM symbols instead of all 14, so the network can send urgent data without waiting for the next slot boundary.

Mini-slots are aimed at low-latency services and at unlicensed or millimetre-wave operation, where starting a transmission immediately after a channel becomes free matters more than filling a full slot.

Why does a higher subcarrier spacing reduce latency?

A higher subcarrier spacing reduces latency because the slot, the basic scheduling unit, gets shorter. At 15 kHz the network makes a scheduling decision once per 1 ms slot; at 120 kHz it makes one every 0.125 ms, so data waits less time before it can be sent and acknowledged.

The cost is a shorter cyclic prefix and more signalling per second. Operators therefore pick the spacing from the band and the cell size first, and take the latency that comes with it.

5. References & Further Reading

• 3GPP TS 38.211, "NR; Physical channels and modulation" (clause 4.2 numerologies, clause 4.3 frame structure, clause 5.3.1 OFDM baseband signal generation).
• 3GPP TS 38.101-1 and TS 38.101-2, "NR; User Equipment (UE) radio transmission and reception" (frequency ranges and supported subcarrier spacings).
• 3GPP TS 38.300, "NR; NR and NG-RAN Overall Description; Stage 2".
• E. Dahlman, S. Parkvall and J. Sköld, "5G NR: The Next Generation Wireless Access Technology", Academic Press.