Enhanced Downlink Shared Channel Power Control

Description

Enhanced Downlink Shared Channel Power Control (EDSCHPC) is a mechanism defined for the UMTS Radio Access Network (UTRAN), specifically within the High-Speed Packet Access (HSPA) evolution. It operates on the High-Speed Downlink Shared Channel (HS-DSCH), which is the primary channel for downlink user data in HSPA. Architecturally, EDSCHPC involves coordination between the Node B (base station) and the User Equipment (UE). The Node B's scheduler, a key component, uses feedback from the UE to determine the optimal transmit power level for the HS-DSCH on a per-Transmission Time Interval (TTI) basis.

How EDSCHPC works is rooted in closed-loop power control principles adapted for shared channel operation. The UE continuously measures the downlink channel quality, typically based on the Common Pilot Channel (CPICH), and reports Channel Quality Indicators (CQIs) to the Node B. The Node B's scheduler interprets these CQIs along with other factors like available power, queue status, and Quality of Service (QoS) requirements. Based on this information, the scheduler dynamically allocates a portion of the total Node B transmit power to the HS-DSCH. Unlike dedicated channel power control, which targets a specific Signal-to-Interference Ratio (SIR), EDSCHPC aims to maximize throughput or fairness by allocating power where it is most effective, often favoring users with good radio conditions.

The key components are the UE's channel measurement and reporting functions, the Node B's HS-DSCH scheduler, and the power allocation algorithm. The specification 3GPP TS 25.423 details the lur interface signaling (between RNCs) that may support aspects of this coordination, though the primary control is between Node B and UE. EDSCHPC's role is critical for efficient HSPA operation; it allows the network to adapt rapidly to fading and interference, ensuring that the scarce downlink power resource is utilized to deliver the highest possible data rates and system capacity, directly impacting user experience for mobile broadband services.

Purpose & Motivation

EDSCHPC was created to address the inefficiencies of static or simple power allocation schemes for the high-speed shared channel in HSPA. Prior to its enhancement, power control for downlink channels was primarily designed for dedicated voice channels, aiming for constant quality, which is suboptimal for bursty, high-rate data traffic. The limitations included wasted power on users in poor radio conditions and an inability to fully exploit multi-user diversity.

The historical context is the introduction and evolution of HSPA in 3GPP Releases 5 and 6, which dramatically increased peak downlink data rates. With HS-DSCH, multiple users share channelization codes and power. EDSCHPC, introduced in Release 8, solved the problem of how to best distribute the available Node B transmit power among these competing users. It was motivated by the need to improve cell-edge performance and overall cell throughput simultaneously, a key challenge for data-centric networks.

Ultimately, EDSCHPC enabled a more intelligent, channel-aware scheduling strategy. By dynamically adjusting HS-DSCH power, it allows the network to serve users near the cell center with lower power (freeing up resources) and to boost power for cell-edge users when necessary, balancing fairness and capacity. This was a crucial step in optimizing UMTS/HSPA networks for the growing demand for mobile internet access.

Key Features

• Dynamic per-TTI power allocation for the HS-DSCH
• Utilizes UE-reported Channel Quality Indicator (CQI) as primary input
• Implemented within the Node B's fast scheduler for low latency
• Aims to maximize system throughput and/or enforce fairness
• Works in conjunction with Adaptive Modulation and Coding (AMC)
• Can be coordinated with other RRM functions like code allocation

Evolution Across Releases

Defining Specifications