NB-IoT Frame Structure: LTE-NB Overview

This page provides an overview of the NB-IoT Frame Structure as defined by the LTE-NB standard. We’ll explore the frame structures used with both 15 KHz and 3.75 KHz subcarrier spacings.

Introduction

• Figure 1 illustrates the overall time frame structure employed in the NB-IoT Access Stratum (AS).
• In both downlink (DL) and uplink (UL) directions, NB-IoT supports a 15 KHz subcarrier spacing (SCS) frame structure. This structure consists of 20 slots, as depicted in Figure 1.
• In the uplink (UL) direction, an additional frame structure with a 3.75 KHz SCS is supported. In this structure, each frame is divided into 5 slots, with each slot having a duration of 2ms, as shown in Figure 3.

NB-IoT Frame Structure for 15 KHz SCS

NB-IoT Frame Structure for 15 KHz SCS

Figure-1:

• One hyperframe Cycle = 1024 hyperframes
• One hyperframe = 1024 frames
• One frame = 10 subframes
• One subframe = 2 slots
• One slot duration = 0.5 ms

Hyperframes and Frames are designated as H-SFN and SFN, respectively.

• H-SFN stands for Hyper System Frame Number
• SFN stands for System Frame Number
• SN stands for Subframe Number

H-SFN, SFN, and SN range from 0-1023, 0-1023, and 0-9, respectively.

PRB Pair in NB-IoT Downlink

PRB Pair in NB-IoT Downlink

Figure-2:

In the downlink, the concept of Physical Resource Blocks (PRBs) is used to specify the mapping of physical channels and signals onto Resource Elements (REs). As shown, the RE is the smallest physical channel unit.

An RE is identified by its index, which includes the subcarrier index ($k$) and the symbol index ($l$) within the PRB. A PRB spans 12 subcarriers over 7 OFDM symbols, resulting in a total of 12 x 7 = 84 REs. One PRB pair, occupying 2 consecutive slots, serves as the smallest schedulable unit in most downlink scenarios.

NB-IoT Frame Structure for 3.75 KHz SCS

NB-IoT Frame Structure for 3.75 KHz SCS

Figure-3:

In the uplink, Resource Units (RUs) are used to specify the mapping of UL channels onto REs. The definition of an RU depends on the configured SCS and the number of subcarriers allocated for UL transmission.

For a 15 KHz SCS and 12 subcarriers, the RU corresponds to a PRB pair. In sub-PRB scheduling scenarios with assignments of 6, 3, or 1 subcarrier, the RU is expanded in time to compensate for the reduced frequency allocation. In the single subcarrier allocation case, also known as single-tone allocation, the RU supports an additional subcarrier spacing of 3.75 KHz. The following RU formats are supported in various UL transmit configurations.

NB-IoT NPUSCH RU

NB-IoT NPUSCH RU

The table summarizes the definitions of RUs for various NPUSCH configurations.