LTE-M CE Mode A vs CE Mode B: Coverage Enhancement Explained

This page delves into the differences between CE (Coverage Enhancement) Mode A and CE Mode B in LTE-M (Long Term Evolution for Machines), a technology designed for IoT (Internet of Things) devices. These modes were introduced in 3GPP Release 13 specifically to enhance coverage.

Introduction to LTE-M and Coverage Enhancement

LTE-M aims to provide a cellular communication solution tailored for IoT devices. To address the unique challenges of IoT deployments, particularly regarding coverage, LTE-M employs coverage enhancement techniques. These challenges include:

Complexity Reduction: LTE-M devices reduce complexity using techniques such as single antenna reception and lower maximum transmit power. However, these simplifications can degrade performance and lead to coverage loss compared to standard LTE.
Challenging Coverage Conditions: LTE-M devices often need to operate in environments with poor signal strength and require better coverage than regular LTE.

CE Mode A and CE Mode B are designed to overcome these challenges, offering different levels of coverage enhancement.

LTE-M CE Mode A

CE Mode A is designed to provide moderate coverage enhancement, offering a balance between performance and power consumption. Key features include:

Repetitions: Supports 32 subframe repetitions of the data channel (PDSCH and PUSCH). This repetition increases the probability of successful data transmission in poor signal conditions.
Bandwidth: PDSCH and PUSCH use a bandwidth of 1 to 6 PRBs (Physical Resource Blocks) in CE mode A.
Coverage Enhancement: Offers sufficient CE to compensate for simplifications implemented by low-cost LTE-M devices. It also provides some additional CE beyond normal LTE coverage.
Optimization: Optimized for moderate CE, achieved through a relatively small number of repetitions.
SRS Transmission: Supports both periodic and aperiodic SRS (Sounding Reference Signal) transmission. SRS helps the base station estimate the uplink channel quality.

LTE-M CE Mode B

CE Mode B provides a deeper level of coverage enhancement compared to Mode A, suitable for more challenging environments. Key features include:

Repetitions: Supports 2048 subframe repetitions of the data channel (PDSCH and PUSCH). The significantly higher number of repetitions offers improved robustness in extremely poor signal conditions.
Bandwidth: PDSCH uses a bandwidth of 4 to 6 PRBs in CE mode B. PUSCH uses a bandwidth of 1 or 2 PRBs.
Coverage Enhancement: Provides deep coverage compared to CE mode A, needed in more challenging coverage conditions.
Optimization: Optimized for substantial CE, achieved through a large amount of repetition.
Mode Support: If a device supports CE mode B, it also supports CE mode A.
SRS Transmission: Does not support SRS transmission, but will use shortened formats for PUSCH / PUCCH (Physical Uplink Control Channel).

MCS and TBS for PDSCH and PUSCH in LTE-M

Here are the tables illustrating Modulation and Coding Schemes (MCS) and Transport Block Size (TBS) values for PDSCH and PUSCH channels in LTE-M CE Modes A and B:

MCS-and-TBS-for-PDSCH-CE-Mode-A-and-Mode-B-LTE-M

Table-1: PDSCH Modulation and Coding Schemes and TBS in LTE-M

MCS-and-TBS-for-PDSCH-CE-Mode-A-and-Mode-B-LTE-M

Table-2: PUSCH Modulation and Coding Schemes and TBS in LTE-M

These tables show the different modulation and coding schemes and corresponding transport block sizes used for PDSCH and PUSCH in CE Mode A and Mode B, impacting the data rates and robustness of the connection.

Key Considerations for CE Mode Selection

All low-cost LTE-M devices (with categories Cat-M1 and others) must support CE mode A. Support for CE mode B is optional.
Devices operate in CE mode only when required to maintain connectivity, i.e., when they are outside the normal LTE coverage range.
When devices are within normal LTE coverage, they use normal LTE operation instead of CE mode, leveraging the high performance (low latency, high data rates) of standard LTE.