Uplink Sharing from UE Perspective

Description

Uplink Sharing from UE Perspective (ULSUP) is a set of UE capabilities and procedures defined across multiple 3GPP specifications that govern how a User Equipment manages and utilizes multiple uplink carriers for transmission. It is a critical aspect of advanced radio features like Carrier Aggregation (CA) and Dual Connectivity (DC), where a UE is configured with more than one uplink carrier, either from the same or different base stations (e.g., MeNB and SeNB in EN-DC). ULSUP addresses the practical constraints of a UE, such as limited transmission power and hardware capabilities, to determine how uplink data and control information are distributed across these carriers.

Architecturally, ULSUP is governed by UE capability signaling and network configuration via RRC messages. The UE reports its ULSUP capability to the network, indicating, for example, whether it supports simultaneous transmission on all aggregated uplink carriers, or if it has limitations (like power sharing or time-division multiplexing constraints). Based on this, the network (gNB or eNB) configures the UE with specific Cell Groups, Primary Cells (PCell), Primary Secondary Cells (PSCell), and Secondary Cells (SCell), each with associated uplink resources. The Medium Access Control (MAC) layer in the UE is then responsible for implementing the ULSUP behavior according to the configuration and scheduling grants received.

How it works involves several key mechanisms. The UE receives uplink grants (via DCI formats on the PDCCH) for its configured carriers. ULSUP rules determine how the UE's available transmission power is allocated among these grants, adhering to maximum power limits and priority rules (e.g., UCI on PUCCH has highest priority, followed by PRACH, then PUSCH with SRB data, then PUSCH with DRB data). In scenarios with dual connectivity (e.g., NR-DC), ULSUP also defines how power is shared between the Master Node (MN) and Secondary Node (SN) cell groups. The UE performs logical channel prioritization (LCP) per cell group, mapping data to specific MAC PDUs for transmission on the granted resources of each carrier.

Its role is to maximize uplink performance and ensure reliable control signaling. By efficiently sharing uplink resources, ULSUP enables higher peak data rates, lower latency for critical data, and robust transmission of uplink control information (UCI) even when multiple carriers are active. It ensures that the UE operates within its RF and power constraints, preventing violations that could lead to transmission errors or non-compliance with regulatory emission limits. It is essential for realizing the full benefits of carrier aggregation and dual connectivity in both LTE and NR.

Purpose & Motivation

ULSUP was created to define standardized UE behavior for uplink transmission in multi-carrier and multi-connectivity scenarios, solving the problem of uncoordinated and potentially inefficient uplink resource usage. Early LTE releases with single carrier operation had straightforward uplink management. However, with the introduction of Carrier Aggregation in Release 10, a UE could have multiple downlink carriers but initially only one uplink carrier (associated with the PCell). Later releases added uplink CA, creating a need for rules on how a UE should handle simultaneous grants on multiple uplink carriers given its finite power amplifier and processing capabilities.

The primary problem ULSUP addresses is the optimal distribution of uplink data and control channels across available carriers while respecting the UE's hardware limitations (maximum output power, power sharing capability). Without such rules, different UE implementations would behave inconsistently, making network scheduling and performance optimization extremely difficult for operators. ULSUP provides a predictable framework, allowing the network to schedule resources effectively based on known UE capabilities.

Furthermore, with the advent of Dual Connectivity (e.g., LTE-NR EN-DC) and more advanced CA combinations, the complexity increased. ULSUP evolved to handle power sharing between different radio access technologies (e.g., LTE and NR) and between cell groups controlled by different nodes. This ensures that critical signaling (like HARQ-ACK on PUCCH) is always transmitted reliably, even if it means throttling data transmission on another carrier, thereby maintaining network stability and user experience in advanced deployment scenarios.

Key Features

• Defines UE power sharing and allocation rules across multiple uplink carriers
• Specifies prioritization of uplink channels (PUCCH, PRACH, PUSCH with SRB/DRB)
• Includes UE capability signaling for simultaneous transmission and power sharing classes
• Governs uplink behavior in both Carrier Aggregation and Dual Connectivity setups
• Manages logical channel prioritization (LCP) and buffer status reporting per cell group
• Ensures compliance with maximum power reduction (MPR) and RF exposure requirements

Evolution Across Releases

Defining Specifications