Five Layers of Time
1) Frame: 10 ms reference cycle (legacy from LTE). 2) Half-Frame: 5 ms — SSB and paging windows. 3) Subframe: 1 ms — invariant across all numerologies. 4) Slot: 1 ms / 2^μ — depends on numerology. 5) OFDM Symbol: 14 per slot (Normal CP) or 12 (Extended CP).
SFN · System Frame Number
SFN is a 10-bit counter (0..1023) that wraps every 10.24 seconds. The MIB carries the upper 6 bits; the lower 4 bits derive from SSB position. SFN aligns DRX, paging, HARQ, and measurement gaps.
TDD Slot Format · D / U / F per Symbol
Each OFDM symbol in a TDD slot is classified as Downlink (D), Uplink (U), or Flexible (F). D and U follow gNB→UE and UE→gNB transmission directions. F is dynamically resolved by DCI 2_0 (group-common scheduler) or determined by UE behavior.
Semi-Static TDD Configuration
tdd-UL-DL-ConfigurationCommon (RRC IE) defines the periodic D/U pattern. Pattern 1: short cycle (e.g., 5 ms). Pattern 2: long cycle (e.g., 10 ms). Combined patterns enable flexible DL/UL ratios.
Dynamic Override · DCI 2_0
DCI Format 2_0 sends a Slot Format Indicator (SFI) to all UEs in a CORESET — overriding the semi-static F symbols with explicit D or U for the next K slots. Lets the gNB react to traffic bursts.
56 Predefined TDD Formats
TS 38.213 Table 11.1.1-1 lists 56 standard slot formats (e.g., Format 28: DDDDDDDDDDDDFU = 12 DL + 1 F + 1 U). Operators pick formats matching their traffic mix.
Mini-Slot Scheduling
Mini-slots use SLIV (Start and Length Indicator) to allocate 2/4/7-symbol scheduling within a single slot. Used for URLLC: a packet arriving at symbol 5 can be transmitted on symbols 5-6 instead of waiting for the next slot boundary.
Frame Stays 10 ms Forever
No matter the numerology, the 10 ms frame is invariant. This keeps NR backward-compatible with LTE's neighbor-list timing and lets DRX, SIB1, paging, and HARQ all reference a common cycle.
Frequently Asked Questions
How long is a 5G NR radio frame?
10 ms. This is invariant across all numerologies and identical to LTE — preserved for backward neighbor-list compatibility.
How many slots are in a 1 ms subframe?
2^μ slots, where μ is the numerology index. μ=0 → 1 slot per subframe (1 ms). μ=1 → 2 slots (500 μs each). μ=3 → 8 slots (125 μs each). μ=6 → 64 slots (15.625 μs each).
What is a mini-slot in 5G NR?
A mini-slot is a sub-slot scheduling unit (2, 4, or 7 OFDM symbols) used for low-latency transmissions like URLLC. The Start and Length Indicator (SLIV) in DCI specifies the symbol range within the slot.
How does dynamic TDD configuration work?
DCI Format 2_0 (Slot Format Indicator) is sent to all UEs in a CORESET and overrides the semi-static D/U/F pattern with explicit D or U for the next K slots. This lets the gNB react to traffic bursts and asymmetric loads.