Secondary High Speed Dedicated Physical Control Channel (Uplink)

Description

The Secondary High Speed Dedicated Physical Control Channel (HS-DPCCH2) is an extension of the original HS-DPCCH, introduced in 3GPP Release 17 as part of ongoing HSPA evolution, specifically for Multi-Cell HSPA (MC-HSPA) scenarios. It is a second, parallel uplink physical control channel that a UE may be configured to transmit when it is receiving data on a large number of downlink HS-DSCH cells (secondary serving cells). The primary HS-DPCCH has a limited capacity for feedback bits. When the number of configured cells grows, a single HS-DPCCH cannot carry all the necessary Channel Quality Indicator (CQI) and HARQ ACK/NACK information for every cell within the required timeframe.

The triggering condition for HS-DPCCH2 activation is based on the RRC parameter Secondary_Cell_Enabled. Specifically, when Secondary_Cell_Enabled is greater than 3 for a UE not configured in 4Tx MIMO mode, or greater than 1 for a UE configured in 4Tx MIMO mode, the UE is required to use HS-DPCCH2. The difference in thresholds accounts for the increased feedback payload when MIMO is used (which requires Pre-coding Control Indication - PCI). The HS-DPCCH2 carries supplemental feedback information, effectively splitting the total feedback load across two parallel control channels.

Physically, the HS-DPCCH2 is structured similarly to the original HS-DPCCH, operating with a 2 ms subframe. It is also code-multiplexed in the uplink. The network configures its channelization code and power offset relative to the DPCCH independently. The content carried on HS-DPCCH2 complements that on HS-DPCCH; together, they provide complete CQI and HARQ-ACK reports for all activated secondary serving cells. The exact mapping of which cell's feedback goes on which channel is defined in the specifications to ensure the Node B can correctly interpret the combined information.

From a network operation perspective, the introduction of HS-DPCCH2 allows the Node B to manage a higher level of downlink carrier aggregation (e.g., 8-carrier HSPA) without compromising the timeliness or granularity of the feedback. This maintains the effectiveness of fast scheduling, link adaptation, and HARQ processes across all aggregated carriers. It is a capacity enhancement for the uplink control plane that directly enables more aggressive downlink capacity boosts in later HSPA releases.

Purpose & Motivation

HS-DPCCH2 was created to address a capacity bottleneck in the uplink control signaling for advanced Multi-Cell HSPA deployments. As HSPA evolved beyond Release 8, features like multi-carrier HSDPA allowed the aggregation of an increasing number of downlink carriers (e.g., 4-carrier, 8-carrier) to a single UE. Furthermore, advanced antenna techniques like 4x4 MIMO were introduced, which require additional feedback bits (PCI). The original HS-DPCCH, designed in Release 5, had a fixed structure and limited bit capacity.

With many secondary cells and/or MIMO, the amount of CQI and HARQ-ACK data that needs to be reported every subframe can exceed what can be reliably carried on a single HS-DPCCH channel without significantly increasing its power (which would cause uplink interference) or reducing feedback frequency (which would degrade scheduler performance). The problem was the uplink control channel becoming a limiting factor for further downlink aggregation gains.

The solution, standardized in Release 17, was to introduce a secondary channel, HS-DPCCH2. This provides a parallel pipe for control information, effectively doubling the uplink feedback capacity when needed. It allows the network to configure very high levels of downlink aggregation (and complex MIMO) while preserving the low-latency, high-frequency feedback loop that is the hallmark of HSPA's efficiency. HS-DPCCH2 thus solves a specific scalability issue in the control plane, ensuring that HSPA can continue to leverage carrier aggregation for capacity growth in its later evolutionary stages.

Key Features

• Secondary uplink physical control channel for HSPA, used in Multi-Cell HSPA (MC-HSPA) configurations.
• Activated when the number of enabled secondary cells (Secondary_Cell_Enabled) exceeds a threshold (3 for non-4Tx MIMO, 1 for 4Tx MIMO).
• Carries supplemental CQI and HARQ-ACK information for secondary serving cells, complementing the primary HS-DPCCH.
• Uses a 2 ms subframe structure and is code-multiplexed in the uplink with independently configured power offset.
• Enables support for higher-order downlink carrier aggregation (e.g., 8-carrier HSDPA) without feedback compression or delay.
• Defined in updated physical layer specifications (25.212, 25.213, 25.214) for Release 17.

Evolution Across Releases

Defining Specifications