High Speed Dedicated Physical Control Channel (Uplink)

Description

The High Speed Dedicated Physical Control Channel (HS-DPCCH) is a fundamental uplink physical channel in the UMTS/WCDMA radio interface, specifically introduced for High-Speed Packet Access (HSPA) operation. It is always associated with a downlink HS-DSCH (High-Speed Downlink Shared Channel) transmission. The UE uses this channel to send time-critical control information back to the serving Node B, enabling the high-speed downlink to function efficiently. The HS-DPCCH is code-multiplexed with other uplink dedicated physical channels (DPDCH and DPCCH).

The channel carries two primary types of information: HARQ Acknowledgement (ACK/NACK) and Channel Quality Indicator (CQI). The HARQ ACK/NACK is a feedback signal for each transmitted HS-DSCH transport block. After the UE attempts to decode a block, it sends an ACK if successful or a NACK if a retransmission is required. This enables the Node B's Hybrid ARQ (HARQ) process to rapidly manage retransmissions, providing link-level reliability. The CQI is a measured value reported by the UE that indicates the downlink channel conditions, such as the received signal-to-interference ratio. The Node B's scheduler uses the CQI reports from all UEs to decide which user to serve next and what modulation and coding scheme (M.g., QPSK, 16QAM) and transport block size to use, implementing link adaptation.

The physical structure of the HS-DPCCH is divided into two distinct parts within a 2 ms subframe (three time slots). The first slot is dedicated to carrying the HARQ ACK/NACK information. The remaining two slots carry the CQI (or PCI - Pre-coding Control Information in MIMO modes). The channel uses a specific spreading factor (typically 256) and is transmitted with a power offset relative to the uplink DPCCH, which carries pilot bits and other control information. The power of the HS-DPCCH is carefully controlled to ensure reliable reception of this critical feedback without causing excessive uplink interference.

From a network architecture perspective, the HS-DPCCH terminates at the Node B. The information it carries is processed by the Node B's MAC-hs (or MAC-ehs) entity and the scheduler. The fast feedback loop (on the order of milliseconds) created by the HS-DPCCH is what allows HSPA to achieve its high spectral efficiency and low latency compared to earlier WCDMA releases. It transforms the downlink into a channel-aware, rapidly adaptive system.

Purpose & Motivation

HS-DPCCH was introduced in 3GPP Release 5 alongside the HS-DSCH to overcome the limitations of the original WCDMA downlink for packet data. Pre-HSPA, downlink packet access used dedicated channels (DCH), which were inefficient for bursty traffic due to long setup times, fixed allocation of resources, and lack of fast channel-dependent scheduling. The vision for HSDPA was to create a shared channel with very short Transmission Time Intervals (TTI=2 ms) and fast retransmissions (HARQ) directly managed by the Node B.

For this vision to work, the Node B needed two key pieces of information from the UE with minimal delay: the success/failure of each transmission (for HARQ) and a measure of current channel quality (for scheduling and link adaptation). The existing uplink DPCCH was not designed to carry this new, frequent, and time-critical information. Therefore, a new dedicated physical control channel, the HS-DPCCH, was standardized to provide a dedicated, low-latency feedback path.

This solved the core problems of the old approach. The fast ACK/NACK (via HS-DPCCH) enabled Node B-based HARQ, which drastically reduced retransmission delay compared to RLC-level retransmissions controlled by the RNC. The frequent CQI reports allowed the scheduler to exploit multi-user diversity by transmitting to users when their channel conditions were best, and to adapt the modulation and coding instantly, maximizing throughput. The creation of HS-DPCCH was thus the essential uplink enabler that made the high-performance HSDPA downlink possible, forming the foundation for all subsequent HSPA enhancements.

Key Features

• Carries uplink control information for HSDPA: HARQ ACK/NACK and Channel Quality Indicator (CQI).
• Uses a 2 ms subframe structure aligned with the HS-DSCH Transmission Time Interval (TTI).
• Code-multiplexed in the uplink with the DPDCH and DPCCH, using a specific spreading factor (e.g., 256).
• Transmit power is set relative to the uplink DPCCH via a network-configured power offset.
• Enables fast Node B-controlled scheduling and link adaptation through frequent CQI feedback.
• Supports fast Hybrid ARQ (HARQ) with ACK/NACK feedback for each HS-DSCH transport block.

Evolution Across Releases

Defining Specifications