5G NR Radio Frames, Slots and Mini-Slots: How Time Is Divided
The 10 ms frame, the numerology-dependent slot, the 14-symbol structure, the extended-CP exception, and why NR added mini-slots.
The mental model: a ruler with adjustable fine ticks
NR's time axis is a ruler. The big marks (10 ms frames, 1 ms subframes) are engraved permanently — they are the same in every numerology and the same as LTE, so the two systems can share a clock. The fine ticks — slots — are adjustable: you choose how many fit into each 1 ms subframe when you choose the numerology. And unlike LTE, NR lets you start writing between ticks: that is a mini-slot.
The fixed part
- Radio frame: 10 ms, numbered by the System Frame Number (SFN, 0–1023, so the SFN cycle is 10.24 s).
- Half-frame: 5 ms. Matters because the SS burst set lives in a half-frame.
- Subframe: 1 ms, 10 per frame.
These are independent of μ. The subframe is purely a timing reference in NR — it is no longer the scheduling unit it was in LTE.
The adjustable part: slots
A slot is the unit the scheduler normally works with. Its length depends on μ:
| μ | SCS | Slot length | Slots per subframe (2^μ) | Slots per frame (10·2^μ) |
|---|---|---|---|---|
| 0 | 15 kHz | 1 ms | 1 | 10 |
| 1 | 30 kHz | 500 µs | 2 | 20 |
| 2 | 60 kHz | 250 µs | 4 | 40 |
| 3 | 120 kHz | 125 µs | 8 | 80 |
| 4 | 240 kHz | 62.5 µs | 16 | 160 |
Inside every slot: 14 OFDM symbols with the normal cyclic prefix, or 12 symbols with the extended cyclic prefix — which exists only at 60 kHz. So "a slot" always means 14 symbols unless you are in the one extended-CP corner case.
Because slots per subframe is 2^μ and symbols per slot is fixed, a 1 ms subframe always contains 14 × 2^μ symbols: 14 at 15 kHz, 28 at 30 kHz, 56 at 60 kHz, and so on. The total symbol "budget" per millisecond scales with SCS exactly as the symbol shrinks.
The clock underneath: T_c and κ
NR expresses every duration as a multiple of a basic time unit
T_c = 1 / (480 000 × 4096) s ≈ 0.509 ns
i.e. the sample period of a 4096-point FFT at the widest defined SCS (480 kHz). LTE's basic unit was T_s = 1/(15 000 × 2048) ≈ 32.55 ns, and the ratio
κ = T_s / T_c = 64
appears in the cyclic prefix formulas. You do not need T_c for everyday work, but it explains why CP lengths look like "144κ·2^−μ·T_c": they are integer sample counts at the fastest rate, guaranteeing that every numerology's symbols are integer multiples of the same tick.
Slot formats: which symbols are DL, UL or flexible
In TDD each symbol of a slot is downlink (D), uplink (U) or flexible (X). The pattern comes from three layers, each able to override the previous:
- Cell-common TDD pattern (
tdd-UL-DL-ConfigurationCommonin SIB1): e.g. "over 5 ms: 7 DL slots, 1 mixed slot with 6 DL + 4 UL symbols, 2 UL slots" — the familiar DDDSU-type patterns. - UE-dedicated pattern (
tdd-UL-DL-ConfigurationDedicated): can turn flexible symbols into D or U for one UE. - Dynamic SFI via DCI format 2_0: selects one of 56 slot formats (TS 38.213 Table 11.1.1-1) per slot for a group of UEs.
The flexible symbols are what let a cell shift capacity between directions dynamically; they are also where the guard period for DL→UL switching lives.
Mini-slots: writing between the ticks
In LTE, a transmission always started at a subframe boundary and lasted the full 1 ms. NR allows a PDSCH or PUSCH to start at any symbol and last 2, 4 or 7 symbols (Rel-15; Rel-16 allows any length 2–13). The spec never says "mini-slot"; it calls this mapping type B, as opposed to slot-based mapping type A.
| Mapping type A ("slot-based") | Mapping type B ("mini-slot") | |
|---|---|---|
| Start symbol S | 0–3 | 0–12 |
| Length L | 3–14 | 2, 4, 7 (Rel-15) |
| First DM-RS | fixed at slot symbol 2 or 3 | first symbol of the allocation |
| Typical use | full-slot eMBB | URLLC, mmWave beam bursts, unlicensed (start the instant the channel is free), SSB-adjacent gaps |
Why it matters: a 1 ms latency budget cannot afford to wait up to a full slot for the next boundary. A 2-symbol mini-slot at 30 kHz is 71 µs of air time and can begin within one symbol of the data arriving. Mini-slots are also the tool for pre-emption: a URLLC mini-slot is punched into an ongoing eMBB slot.
Slot aggregation: the opposite direction
Just as a transmission can be shorter than a slot, it can be longer: pdsch-/pusch-AggregationFactor repeats the same transport block over 2, 4 or 8 consecutive slots for coverage. So the slot is the default unit, not a hard boundary in either direction.
A worked example
30 kHz, TDD pattern DDDSU (2.5 ms period, 5 slots): slots 0–2 are DL, slot 3 is special (say 10 DL symbols, 2 flexible as guard, 2 UL), slot 4 is UL. In one 10 ms frame: 20 slots, 12 full DL, 4 special, 4 UL. A URLLC packet arriving during slot 1 symbol 5 can be sent as a 4-symbol mini-slot at symbols 6–9 of the same slot, roughly 0.2 ms after arrival, instead of waiting for slot 2.
Common confusions
- "A slot is 1 ms." Only at 15 kHz. It is 1/2^μ ms.
- "Mini-slot is a separate numerology or a separate channel." It is a scheduling/mapping option (type B) on the normal slot grid.
- "More slots per subframe means more symbols per second per subcarrier." Symbols per second per Hz is constant; you get more symbols only because each is shorter.
- "Extended CP gives 14 symbols." It gives 12, and only at 60 kHz.
- "The subframe is the scheduling unit." That was LTE. In NR the slot (or mini-slot) is.
Spec pointers
- TS 38.211 §4.1 — T_c, κ; §4.3 — frames, subframes, slots (Tables 4.3.2-1 and 4.3.2-2)
- TS 38.211 §7.4.1.1.2 — PDSCH mapping types A/B and DM-RS positions
- TS 38.213 §11.1 — slot formats and DCI 2_0
- TS 38.214 §5.1.2.1 / §6.1.2.1 — time-domain allocation, S and L limits, aggregation factor
- TS 38.331 — TDD-UL-DL-ConfigCommon / Dedicated
Recap
Frame 10 ms and subframe 1 ms are fixed; the slot is 1/2^μ ms with 14 symbols (12 for extended CP at 60 kHz). Everything is an integer multiple of T_c ≈ 0.5 ns with κ = 64 tying NR to LTE timing. Slot formats set D/U/X per symbol in three layers. Mapping type B (mini-slots) lets transmissions start at any symbol and run 2–7 symbols; aggregation lets them run across several slots. The slot is the default, not the law.
Interview questions
The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.
18 questions
- The frame is 10 ms and the subframe 1 ms, always. Neither changes with numerology.
- The slot length and therefore the number of slots per subframe, which is 2^μ.
- Fourteen with normal cyclic prefix, twelve with extended cyclic prefix.
- T_c is the basic NR time unit, roughly 0.509 ns, and κ is the ratio between it and the LTE time unit T_s, equal to 64. They give NR a fine enough time granularity to express all numerologies exactly with integer arithmetic.
- A transmission shorter than a full slot, occupying 2, 4 or 7 symbols. It exists so a transmission need not wait for the next slot boundary, which is how NR achieves low latency for URLLC.
- Type B mapping. Type A mapping is slot-based and starts near the beginning of the slot; type B can start at any symbol.
- At 30 kHz a slot is 500 µs, so an arrival just after a boundary waits nearly that long. A mini-slot can begin at the next symbol, cutting the wait to tens of microseconds.
- Transmitting one transport block across several consecutive slots. It is used at cell edge or for coverage-limited links, where repeating the transmission buys energy rather than throughput.
- A per-slot description of which symbols are downlink, uplink or flexible. It is what lets a TDD carrier change direction inside a slot.
- Semi-statically through RRC configuration, and dynamically through the Slot Format Indicator carried in DCI format 2_0 on a group-common PDCCH.
- It has no fixed direction from the semi-static configuration and can be assigned dynamically, or left unused as a guard when switching between downlink and uplink.
- The radio cannot transmit and receive at once, and propagation delay means the uplink arrives late. Guard symbols absorb the switching time and the round-trip delay.
- Twenty. Two slots per subframe at μ = 1, times ten subframes.
- Yes. The symbol count per slot is fixed but slots get shorter, so symbols arrive proportionally faster — which is where both the latency gain and the higher processing load come from.
- The subframe is a fixed 1 ms reference interval inherited from LTE; the slot is the actual scheduling unit and its length depends on numerology. At 15 kHz they coincide, which is a common source of confusion.
- No. A transmission is contained within a slot, which is why aggregation exists for the case where more symbols are needed than remain in the current slot.
- It preserves alignment with LTE timing, which matters for coexistence, dual connectivity and refarming, and it gives a stable reference for system frame numbering.
- A 10 ms frame holds ten 1 ms subframes; each subframe holds 2^μ slots; each slot holds fourteen symbols; and a mini-slot is a short transmission that ignores the slot boundary.
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