5G-NR Radio Frame Structure

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Introduction to 5G Frame Structure

5G New Radio (5G NR) represents a significant leap in wireless communication technology, designed to support a diverse range of use cases, from enhanced Mobile Broadband (eMBB) to Ultra-Reliable Low-Latency Communication (URLLC) and massive Machine-Type Communication (mMTC).

A key feature of 5G is its flexible and scalable frame structure, which allows the network to optimize the transmission of data across different services and applications.

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Overview of 5G Frame Structure

At the heart of 5G NR is its flexible frame structure, which is based on a hierarchical organization of time-domain units:

• Radio Frame: A 5G radio frame is 10 ms long, the same as in LTE.
• Subframe: A 5G subframe is 1 ms in length, also similar to LTE.
• Slot: A subframe is divided into several slots, depending on the numerology used.
• Symbols: Each slot contains 14 OFDM symbols when using a normal cyclic prefix (CP).

While the basic structure remains like LTE, 5G introduces key innovations, such as multiple numerologies and variable subcarrier spacing, which significantly enhance its flexibility.

Key Differences from LTE

Although the overall structure of frames, subframes, and slots remains like LTE, there are key distinctions in 5G:

• Multiple Numerologies: Unlike LTE, which uses a fixed subcarrier spacing of 15 kHz, 5G supports multiple subcarrier spacings. This flexibility allows 5G to cater to a wider range of use cases, from low-latency applications to high-throughput services.
• Variable Slot Length: In 5G, the number of slots within a subframe depends on the subcarrier spacing (numerology). As the subcarrier spacing increases, the duration of each slot decreases.
• Mini-Slot Scheduling: 5G introduces mini-slot scheduling, which allows data transmission within a portion of a slot. This is critical for low-latency applications like URLLC.

Unique Features of 5G Subframe: Multiple Numerology

One of the most significant innovations in 5G is the introduction of multiple numerologies. This concept allows for different subcarrier spacings, enabling 5G to accommodate a wide variety of use cases across different frequency ranges and deployment scenarios.

In LTE, a fixed subcarrier spacing of 15 kHz is used, but 5G introduces multiple options for subcarrier spacing, as shown below:

Numerology : https://www.techedgewireless.com/post/concept-numerology-in-5g-nr

From above table and diagram, we can understand:

• μ (Numerology): Represents different numerologies (μ = 0, 1, 2, 3, 4).
• Nslot_symb: This column indicates that each slot consists of 14 OFDM symbols (fixed for all numerologies).
• Nframe_slot: The number of slots in a 10 ms radio frame increases as numerology increases. For example:

·??????? μ = 0: 10 slots per radio frame (for 15 kHz subcarrier spacing).

·??????? μ = 1: 20 slots per radio frame (for 30 kHz subcarrier spacing).

·??????? μ = 2: 40 slots per radio frame (for 60 kHz subcarrier spacing), and so on.

• Nsubframe_slot: This column shows how many slots fit within a 1 ms subframe. For example:

·??????? μ = 0: 1 slot per subframe (for 15 kHz).

·??????? μ = 1: 2 slots per subframe (for 30 kHz).

·??????? μ = 2: 4 slots per subframe (for 60 kHz), and so on.

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Subcarrier Spacing and Slot Configuration:

The lines branching from the table explain the subcarrier spacing and slot details for each numerology:

• 15 kHz (μ = 0):

1. Subcarrier bandwidth = 12 subcarriers * 15 kHz = 180 kHz.
2. 1 ms per slot, with 10 slots in the radio frame.

• 30 kHz (μ = 1):

1. Subcarrier bandwidth = 12 subcarriers * 30 kHz = 360 kHz.
2. 0.5 ms per slot, with 20 slots in the radio frame.

• 60 kHz (μ = 2):

1. Subcarrier bandwidth = 12 subcarriers * 60 kHz = 720 kHz.
2. 0.25 ms per slot, with 40 slots in the radio frame.

• 120 kHz (μ = 3):

1. Subcarrier bandwidth = 12 subcarriers * 120 kHz = 1440 kHz.
2. 0.125 ms per slot, with 80 slots in the radio frame.

• 240 kHz (μ = 4):

1. Subcarrier bandwidth = 12 subcarriers * 240 kHz = 2880 kHz.
2. 0.0625 ms per slot, with 160 slots in the radio frame

Points to remember:

• As the numerology (μ) increases, subcarrier spacing also increases (from 15 kHz up to 240 kHz).
• The larger the subcarrier spacing, the smaller the slot duration becomes. For example, with 15 kHz subcarrier spacing, each slot lasts 1 ms, while with 240 kHz subcarrier spacing, each slot lasts only 0.0625 ms.
• The number of slots within a subframe and the entire radio frame increases as the subcarrier spacing grows, enabling 5G NR to handle both low-latency services and high-throughput applications efficiently.

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Impact of Multiple Numerologies

The introduction of multiple numerologies in 5G allows for significant flexibility in optimizing network performance. This flexibility is particularly beneficial in a variety of scenarios:

• High-bandwidth scenarios: Applications requiring high throughput, such as 8K video streaming, can benefit from larger subcarrier spacing (e.g., 120 kHz or 240 kHz) for faster data transmission.
• Low-latency scenarios: For applications such as autonomous driving, where latency is critical, higher numerologies (e.g., 120 kHz or 240 kHz) can be used to reduce symbol duration and achieve the required low-latency performance.
• Energy-efficient communications: Lower numerologies, such as 15 kHz or 30 kHz, can be used in scenarios like mMTC, where power consumption is a concern, allowing for energy-efficient communication between devices.

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Mini-Slot Scheduling

Mini-slot scheduling is another key feature of 5G NR. Unlike traditional slot-based scheduling, which requires waiting for an entire slot to elapse, mini-slot scheduling allows data to be transmitted within a portion of a slot, using only 2, 4, or 7 OFDM symbols.

This is particularly useful for low-latency applications, where critical data needs to be transmitted without waiting for a full slot duration. By enabling faster data transmission, mini-slot scheduling reduces latency and enhances the performance of services like real-time gaming and industrial automation.

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https://www.techedgewireless.com/post/5g-nr-radio-frame-structure

references : 3GPP TS 38.211 V17.2.0 (2022-03) "5G; NR; Physical channels and modulation (Release 17)." Available from: 3GPP Specification

Rohde & Schwarz (2020) "5G New Radio Numerology and Frame Structure." Available from: Rohde & Schwarz Whitepaper

Dahlman, E., Parkvall, S., and Sk?ld, J. (2018) "5G NR: The Next Generation Wireless Access Technology." Academic Press.

Keysight Technologies (2021) "Understanding 5G Numerology." Available from: Keysight Whitepaper

Qualcomm Technologies, Inc. (2020) "5G NR Physical Layer Overview." Available from: Qualcomm 5G NR Overview

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