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5G NR Mini-slot: Basics and Functionality

5g 5g nr mini slot frame structure latency

This page explains the basics of 5G NR Mini-slots and their function. It also highlights the difference between a regular slot and a mini-slot in 5G NR.

Introduction

As illustrated in Figure 1, a frame in 5G NR has a duration of 10 ms. Similar to LTE, a frame is composed of 10 subframes, each lasting 1ms. Each subframe then consists of 2μ slots.

Each slot comprises either 14 (with normal Cyclic Prefix or CP) or 12 (with extended CP) OFDM symbols.

5G NR Frame Structure-slot,mini-slot

Figure 1: 5G NR Frame Structure showing slots and mini-slots

A standard slot is the typical unit for transmission, used by the scheduling mechanism. However, NR allows transmission to begin at any OFDM symbol and to continue for only the number of symbols necessary for the communication. This is what we call a “mini-slot” transmission.

This feature facilitates very low latency for critical data communication and also minimizes interference with other RF links. In essence, mini-slots contribute significantly to achieving lower latency in the 5G NR architecture.

5G NR Frame Structure: Fixed Slots

The table below outlines typical fixed slots used in a 5G NR frame structure.

μ (subcarrier spacing)Slots/slotSlots/subframeSlots/frameSlot duration
0 (15 KHz)141101 ms
1 (30 KHz)14220500 µs
2 (60 KHz), normal CP14440250 µs
2 (60 KHz), Extended CP12440250 µs
3 (120 KHz)14880125 µs
4 (240 KHz)141616062.5 µs

Unlike standard slots, mini-slots aren’t tied to the frame structure. They allow for puncturing the existing frame without needing to wait for a scheduled slot.

Difference Between Slot and Mini-slot in 5G NR

Here’s a breakdown of the key differences:

  • Slot:

    • A normal slot occupies either 14 (normal CP) or 12 (Extended CP) OFDM symbols.

    • Enables slot-based scheduling. One slot is the basic scheduling unit, and slot aggregation is also supported.

    • Slot length scales with subcarrier spacing, as shown by the formula:

      Slot length=1 ms2μSlot\ length = \frac{1\ ms}{2^\mu}

  • Mini-slot:

    • A mini-slot occupies 2, 4, or 7 OFDM symbols.
    • Enables non-slot based scheduling.
    • It represents the minimum scheduling unit used in 5G NR.
    • Mini-slots can be as short as 2 OFDM symbols and have variable lengths.
    • They can be positioned asynchronously with respect to the beginning of a standard slot, providing greater flexibility in scheduling.