Uplink (UL) Gap for Tx Power Management in 5G NR

Introduced in 3GPP Release 17, the Uplink (UL) Gap for Tx Power Management is a specialized gap mechanism designed to address the challenges of managing peak transmit power and complying with regulatory limits, such as Specific Absorption Rate (SAR) constraints. Unlike traditional measurement gaps which are primarily used for cell identification and RRM, UL Gaps are purely focused on optimizing the UE's transmit duty cycle to maintain high-performance connectivity within power and thermal boundaries.

1. Context & Scope

The UL Gap framework is a Release 17 enhancement to the existing 5G NR gap mechanisms. Legally and technically, it falls under the purview of RAN2 (Layer 2/3 signaling) and RAN4 (RF requirements and power management).

Its primary scope includes:

• MR-DC Scenarios: Specifically EN-DC or NR-DC where simultaneous UL on FR1 and FR2 (or multiple FR1 bands) can exceed aggregate power limits.
• SAR Compliance: Managing the time-averaged power exposure for proximity sensors or regulatory safety limits.
• Thermal Management: Mitigating UE heat dissipation during prolonged high-throughput UL sessions (e.g., video streaming or large file uploads).

2. Problem Statement: Peak vs. Averaged Power

The SAR Constraint

SAR requirements limit the amount of RF energy absorbed by the human body. In many modern UE designs, complying with SAR while maintaining maximum spectral efficiency is a balancing act. Without UL Gaps, the network may be unaware of the UE's internal power constraints, leading to:

1. Autonomous Power Backoff: The UE reduces its own Tx power without informing the gNB, leading to link budget imbalance and potential RLF.
2. Increased Dropped Packets: If the gNB schedules UL transmissions that the UE cannot fulfill due to power limits, the CRC failure rate spikes.

The objective of the UL Gap is to provide a deterministic, network-controlled window where the UE does not transmit. This allows the gNB to schedule data more aggressively outside the gap, knowing that the UE’s averaged power and duty cycle are being managed within the gap itself.

3. Detailed Technical Description

UL Gap Configuration and Patterns

UL Gaps are configured via RRC signaling within the MeasGapConfig or a dedicated UplinkGapConfig structure. Key parameters include:

• Gap Periodicity: The interval at which the gap repeats.
• Gap Duration: The length of the window where UL is prohibited.
• Reference Cell: The timing reference for the gap pattern start.

Unlike measurement gaps, UL Gaps are transmission-prohibitive but not necessarily reception-prohibitive. The UE can potentially still receive DL data during a UL gap, provided the RF architecture allows for Tx/Rx isolation.

Multi-Gap Coordination

A Rel-17 UE may be configured with:

• Standard Measurement Gaps (for Inter-frequency/Inter-RAT).
• MUSIM Gaps (for Multi-SIM scenarios).
• UL Gaps (for Power Management).

The coordination of these patterns is handled by the MN (Master Node). If the different gap types collide, 3GPP specifications define a priority handling mechanism where measurement gaps typically take precedence to ensure mobility robustness.

4. Signaling and UE Capability

For the network to configure UL Gaps, the UE must explicitly indicate support via its capability reporting.

RRC Information Element

The configuration is typically signaled during the RRC Reconfiguration procedure.

UplinkGapConfig-r17 ::= SEQUENCE {
    gapOffset-r17        INTEGER (0..159),
    mgl-r17              ENUMERATED {ms1dot5, ms3, ms3dot5, ms4, ms5dot5, ms6},
    mgrp-r17             ENUMERATED {ms20, ms40, ms80, ms160},
    mgta-r17             ENUMERATED {ms0, ms0dot25, ms0dot5} OPTIONAL,
    ...
}

UE Capability Induction

The UE reports uplinkGap-r17 support under the Phy-Parameters or MeasParameters IEs. This reporting can also include the specific frequency bands or combinations where the gap is required (e.g., specific FR1+FR2 combinations).

UL Gaps for Tx Power Management represent a significant step toward "Network-Assisted Power Management." By allowing the gNB to be "in the loop" regarding the UE's regulatory and thermal constraints:

• Link Reliability is improved by avoiding unpredictable autonomous power drops.
• Spectral Efficiency is optimized through better scheduling awareness.
• Device Design is simplified, as OEMs can rely on network-assisted duty cycles to meet SAR requirements.

As 5G systems move toward mmWave and ubiquitous MR-DC, the management of the uplink duty cycle via mechanisms like UL Gaps will become a standard tool in the network optimizer's toolkit.

Further Reading

• Network-Controlled Small Gap (NCSG) in 5G NR
• MUSIM Gaps in 5G NR
• 5G Handling of Measurement Gaps
• 5G NR Timers and Constants