5G Handling of Measurement Gaps

In 5G New Radio (NR), Measurement Gaps are critical time intervals during which the User Equipment (UE) suspends its normal data reception and processing activities to perform measurements on other radio frequencies or radio access technologies (RATs). This mechanism is essential for efficient Radio Link Management (RLM), Mobility (Handover/Cell Reselection), and Dual Connectivity (EN-DC/MR-DC) operations.

What are Measurement Gaps?

Measurement Gaps are pre-configured time periods where the gNB (5G base station) instructs the UE to pause data transmission/reception and instead focus on measuring radio conditions on specified frequencies or RATs. These gaps are essential because:

• Simultaneous Transmission/Reception: In many scenarios, especially in Dual Connectivity (EN-DC), the UE cannot simultaneously transmit/receive on the LTE and 5G NR carriers due to hardware limitations (e.g., power amplifier constraints).
• Efficient Measurements: Performing measurements while actively decoding data can interfere with the measurement accuracy. Measurement gaps ensure the UE has dedicated time for reliable measurements.

Types of Measurement Gaps

5G NR supports several types of measurement gaps, each serving different purposes:

1. Intra-frequency Measurement Gaps

Used for measuring the same frequency band but potentially different cells (e.g., neighboring cells on the same frequency).

2. Inter-frequency Measurement Gaps

Used for measuring different frequency bands (e.g., measuring an LTE band while connected to a 5G NR band).

3. Inter-RAT Measurement Gaps

Used for measuring different Radio Access Technologies (RATs), such as:

• 5G NR to LTE (for EN-DC scenarios)
• 5G NR to NB-IoT/LTE-M
• 5G NR to Wi-Fi

Measurement Gap Configuration

The gNB configures measurement gaps using the RRC (Radio Resource Control) protocol through the MeasGapConfig IE (Information Element).

Key Configuration Parameters

Measurement Gap Patterns

The measurement gaps are organized into patterns that repeat periodically. The pattern is defined by:

• Gap Length: The duration of the measurement gap (e.g., 1 ms, 2 ms, 3 ms).
• Gap Repetition Period: The interval at which the gap pattern repeats (e.g., 40 ms, 80 ms, 160 ms).

Handling of Measurement Gaps

1. Measurement Gap Detection

The UE monitors the MeasGapConfig IE received from the gNB. This configuration defines:

• Whether measurement gaps are enabled
• The pattern of the gaps
• The specific time instances when gaps occur

2. Measurement Gap Execution

When a measurement gap occurs, the UE:

1. Stops all data reception and processing activities
2. Performs the configured measurements (e.g., RSRP, RSRQ, SINR)
3. Updates the measurement results
4. Resumes normal data activities

3. Measurement Gap Sharing (EN-DC)

In EN-DC (E-UTRA-NR Dual Connectivity), the UE is connected to both an LTE eNB (Master Node) and a 5G NR gNB (Secondary Node). Measurement gap handling becomes more complex:

• Master Node (LTE): Configures measurement gaps for the UE to measure the 5G NR carrier.
• Secondary Node (5G NR): Configures measurement gaps for the UE to measure the LTE carrier.

Gap Sharing Mechanism: To avoid excessive interruption of data services, the LTE and 5G NR measurement gaps are often synchronized or shared. The UE can perform measurements for both RATs within the same gap period if the hardware allows.

Importance of Measurement Gaps

1. Accurate Mobility Decisions

Measurement gaps enable the UE to measure neighboring cells without interference, leading to:

• More accurate RSRP/RSRQ measurements
• Better handover decisions
• Reduced handover failures

2. Efficient Dual Connectivity

In EN-DC, measurement gaps allow the UE to monitor both LTE and 5G NR carriers, enabling:

• Seamless data transfer across both technologies
• Efficient load balancing between LTE and 5G NR
• Quick handover between RATs when needed

3. Reduced Power Consumption

By consolidating measurements into dedicated gap periods, the UE can power down its data processing units during these times, leading to:

• Improved battery life
• More efficient power management

Practical Considerations

1. Gap Overhead

Measurement gaps introduce overhead, as the UE cannot transmit or receive data during these periods. The network must carefully configure the gap pattern to minimize this overhead while ensuring sufficient measurement opportunities.

2. UE Capability

The UE reports its measurement capabilities to the network, including:

• Whether it supports measurement gaps
• The maximum gap length it can support
• Whether it supports gap sharing

3. Measurement Gap Repetition Patterns

5G NR supports multiple measurement gap repetition patterns, allowing the network to select the most appropriate pattern based on:

• Network conditions
• UE capabilities
• Mobility requirements

Measurement Gaps are a fundamental mechanism in 5G NR that enables efficient radio resource management, particularly for mobility and dual connectivity operations. By providing dedicated time intervals for measurements, the network can ensure accurate radio link monitoring while minimizing the impact on data throughput. The flexible configuration of measurement gaps allows operators to optimize performance based on specific network conditions and UE capabilities, contributing to the overall efficiency and reliability of 5G networks.

Further Reading

• Radio Link Monitoring (RLM) in 5G NR
• 5G NR Handover Process