NodeB triggered HS-DPCCH transmission

Description

NodeB triggered HS-DPCCH transmission is a mechanism introduced in Release 11 that enables the User Equipment (UE) operating in the CELL_FACH state to transmit High Speed Dedicated Physical Control Channel (HS-DPCCH) feedback using a common Enhanced Dedicated Channel (E-DCH) resource. The transmission is triggered by the reception of an HS-SCCH (High Speed Shared Control Channel) order from the Node B. This mechanism is defined in 3GPP TS 25.321 (MAC protocol specification) and TS 25.331 (RRC protocol specification).

In UMTS, the CELL_FACH state is a low-activity state where the UE has no dedicated physical channel allocated but can still transmit and receive small amounts of data over common channels. Traditionally, HSDPA operation was limited to the CELL_DCH state where dedicated resources were available. Release 8 introduced Enhanced Uplink in CELL_FACH, allowing the UE to use common E-DCH resources for uplink transmission. However, providing HS-DPCCH feedback (which carries Channel Quality Indicator (CQI) and HARQ ACK/NACK information) in CELL_FACH was challenging because the UE lacked a dedicated uplink control channel.

The NodeB triggered HS-DPCCH transmission solves this problem by allowing the Node B to explicitly trigger the UE to send HS-DPCCH feedback on a common E-DCH resource. The trigger is an HS-SCCH order, which is a special signalling message sent on the HS-SCCH with a specific bit pattern. When the UE receives this order, it initiates transmission of the HS-DPCCH on the allocated common E-DCH resource. This feedback is essential for HSDPA link adaptation and HARQ operation, as it provides the Node B with information about the downlink channel quality and the success or failure of previous downlink transmissions.

The mechanism operates as follows: the Node B, when it has downlink data to send to a UE in CELL_FACH state, sends an HS-SCCH order to trigger HS-DPCCH transmission. The UE, upon receiving the order, transmits the HS-DPCCH on the common E-DCH resource. The HS-DPCCH carries the CQI, which the Node B uses to select the appropriate modulation and coding scheme for the HS-DSCH, and HARQ ACK/NACK feedback for previously received HS-DSCH transmissions. This enables efficient HSDPA operation in CELL_FACH, improving throughput and reducing latency for UEs in this state.

This feature is particularly important for machine-type communication (MTC) and other applications where UEs frequently transition between states and may remain in CELL_FACH for extended periods. By enabling HSDPA in CELL_FACH, it reduces the need for state transitions to CELL_DCH, saving signalling overhead and improving battery life. The mechanism is part of the broader HSPA enhancements in Release 11 that aimed to improve performance for low-activity and bursty traffic scenarios.

Purpose & Motivation

The NodeB triggered HS-DPCCH transmission was created to address the limitations of HSDPA operation in the CELL_FACH state. Prior to Release 11, UEs in CELL_FACH could not provide HS-DPCCH feedback because they lacked dedicated uplink resources, which prevented efficient HSDPA operation in this state. This meant that UEs in CELL_FACH could only receive downlink data at lower rates or had to transition to CELL_DCH, incurring signalling overhead and latency.

The mechanism solves this by allowing the Node B to trigger HS-DPCCH transmission on a common E-DCH resource, enabling link adaptation and HARQ in CELL_FACH. This improves throughput and reduces latency for UEs with bursty or low-volume traffic, such as MTC devices, without requiring state transitions. It also reduces signalling load on the network and improves battery life for UEs that would otherwise need to transition to CELL_DCH.