LTE-M Frame Structure Explained

This page describes the LTE-M frame structure and its constituents, covering both downlink and uplink frame structures.

Introduction to LTE-M

LTE-M (Long Term Evolution for Machine-Type Communications) adheres to 3GPP specifications, similar to LTE technology. It’s primarily designed to meet the objectives of cellular IoT devices, such as low device cost, deep coverage, extended battery life, and higher cell capacity.

There are different categories of LTE-M devices, including Cat-0, Cat-M1, and Cat-M2, as defined by various 3GPP releases. LTE-M Cat-0, Cat-M1, and Cat-M2 comply with the 3GPP TS 36 series of releases 12, 13, and 14, respectively.

Let’s delve into the components of the LTE-M frame structure.

LTE-M Frame Structure

LTE-M Frame Structure

The figure illustrates the LTE-M frame structure, which is similar to that of LTE technology. A hyperframe cycle consists of 1024 hyperframes. Each hyperframe contains 1024 frames.

One frame is composed of 10 subframes. Each subframe contains two slots, each with a duration of 0.5ms. Consequently, each subframe has a duration of 1 ms, while each frame lasts for 10ms.

Each slot occupies either 7 symbols (for normal CP) or 6 symbols (for extended CP).

Each subframe is uniquely identified by H-SFN (range 0-1024), SFN (range 0-1024), and SN (range 0-9).

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

Normal CP makes the system robust in propagation conditions with a delay spread of up to 4.7 µs, while Extended CP can handle a delay spread of up to 16.7 µs.

LTE-M PRB Pair

LTE-M PRB Pair

The smallest resource unit utilized by an LTE-M device is one PRB (Physical Resource Block) pair mapped over two slots. One PRB pair spans 12 subcarriers with a subcarrier spacing of 15 KHz. This results in a total bandwidth of 180 KHz for 12 subcarriers.

The figure illustrates 12 subcarriers over 14 symbols for normal CP operation, which are used as a PRB pair in LTE-M. Another unit, called RE (Resource Element), refers to one subcarrier in one symbol.