LTE-M Channel Types: Downlink and Uplink Channels Explained

This article dives into the different LTE-M channel types and their respective functions, covering both downlink and uplink channels.

Introduction to LTE-M

LTE-M, short for Long Term Evolution for Machine-Type Communications, adheres to 3GPP specifications, similar to standard LTE technology. It was primarily developed to address the specific needs of cellular IoT devices, focusing on:

• Low device cost
• Deep coverage
• Extended battery life
• Increased cell capacity

LTE-M devices are categorized into Cat-0, Cat-M1, and Cat-M2 based on different 3GPP releases (Rel. 12, Rel. 13, and Rel. 14, respectively, following the TS 36 series).

Let’s explore the functions of different LTE-M channel types.

LTE-M Channel Types

LTE-M channel types are broadly divided into downlink channels and uplink channels. Signals are also utilized for synchronization and channel estimation/equalization.

LTE-M Downlink Channels and Signals

LTE-M uses a specific set of downlink and uplink channels and signals, as visualized above. The mapping between logical, transport, and physical channels is crucial to understand how data flows. PSS, SSS, and RS serve as downlink signals, while PDSCH, MPDCCH, and PBCH are downlink channels.

Location of Downlink Channels and Signals:

• PSS, SSS, and PBCH: Transmitted periodically in the center of the LTE carrier.
• MPDCCH and PDSCH: Transmitted in narrowband.
• DL Reference Signals (RS): Transmitted in all PRBs (Physical Resource Blocks).

Synchronization Signals

These signals are essential for the UE (User Equipment) to synchronize with the network.

Downlink Reference Signals

These signals are used for channel estimation and demodulation.

PBCH (Physical Broadcast Channel)

The PBCH delivers the MIB (Master Information Block), which provides essential information for the device to operate within the network.

MPDCCH (Narrowband Physical Downlink Control Channel)

MPDCCH carries Downlink Control Information (DCI).

PDSCH (Physical Downlink Shared Channel)

PDSCH is primarily used to transmit unicast data.

LTE-M Uplink Channels and Signals

As shown above, PRACH, PUSCH, and PUCCH are used as uplink channels. RS and DMRS serve as reference signals.

PRACH (Physical Random Access Channel)

The PRACH is used by the device to initiate a connection and allows the serving base station to estimate the time of arrival of the UL transmission.

Uplink Reference Signals (SRS, DMRS)

RS signals are used by the eNB (base station) to perform channel estimation, UL quality measurements, and issue timing advance commands.

PUSCH (Physical Uplink Shared Channel)

PUSCH is used to transmit unicast data.

PUCCH (Physical Uplink Control Channel)

The PUCCH is used to carry Uplink Control Information (UCI), which includes:

• UL scheduling request (SR)
• DL HARQ feedback (ACK or NACK)
• DL CSI