What is TTI?
TTI (Transmission Time Interval) is the duration of a single transmission on the air interface — the minimum time unit over which a transport block is delivered and an independent HARQ process operates. In LTE, the TTI was fixed at 1 ms (one subframe). In 5G NR, the TTI is flexible, varying from 1 ms down to approximately 0.125 ms depending on the numerology (subcarrier spacing) and scheduling type (slot-based or mini-slot), enabling the low-latency performance required by uRLLC applications.
How Does TTI Work?
5G NR defines two scheduling granularities: slot-based and mini-slot (non-slot). A slot contains 14 OFDM symbols, and its duration depends on numerology: 1 ms at 15 kHz SCS, 0.5 ms at 30 kHz, 0.25 ms at 60 kHz, 0.125 ms at 120 kHz. Mini-slots can be as short as 2 OFDM symbols (approximately 0.036 ms at 30 kHz SCS), enabling extremely low-latency transmission for uRLLC. Each TTI carries one transport block and one HARQ-ACK process. Shorter TTIs reduce both the transmission delay and the round-trip time for HARQ retransmissions, directly lowering the user-plane latency. The trade-off is higher control overhead per unit of data for very short TTIs.
Use Cases
5G NR uRLLC latency reduction, dynamic scheduling for diverse QoS requirements, HARQ round-trip optimization, factory automation real-time control, and remote surgery communication latency management.
3GPP / Standards Reference
3GPP TS 38.211 (NR slot and mini-slot structure), 3GPP TS 38.214 (NR scheduling), 3GPP TS 38.300 (NR overall — TTI flexibility)
Related Terms
Numerology | Latency | uRLLC | NR | OFDM
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