Shared Information Channel for HS-DSCH

Description

The Shared Information Channel for HS-DSCH (HS-SICH) is the uplink counterpart to the HS-SCCH in the UMTS High-Speed Downlink Packet Access (HSDPA) system, introduced in 3GPP Release 5. It is a physical layer channel transmitted from the User Equipment (UE) to the Node B. The primary role of the HS-SICH is to carry time-critical feedback information that enables the Node B's scheduler and Hybrid ARQ (HARQ) entity to operate effectively. This feedback is a fundamental part of the closed-loop control mechanisms that give HSDPA its high performance and efficiency.

The HS-SICH carries two crucial pieces of information: the Channel Quality Indicator (CQI) and the HARQ Acknowledgment (ACK or NACK). The CQI is a measurement reported by the UE that indicates the downlink channel conditions it is experiencing. This value is calculated by the UE based on the received signal-to-interference ratio of the common pilot channel (CPICH). The CQI essentially informs the Node B's scheduler about what modulation scheme and coding rate the UE can reliably support in the next transmission, forming the basis for Adaptive Modulation and Coding (AMC). The HARQ ACK/NACK indicates whether the data packet received on the HS-PDSCH was decoded successfully (ACK) or not (NACK), triggering a retransmission if necessary.

From an architectural perspective, the HS-SICH is transmitted by the UE a predefined time after receiving an HS-PDSCH sub-frame. This timing is strictly defined relative to the downlink HS-SCCH/HS-PDSCH transmission to ensure low-latency feedback. The channel uses a spreading factor of 16. Unlike a dedicated channel, the HS-SICH is a shared resource, but its transmission is triggered in response to a downlink allocation signaled on the HS-SCCH. The Node B must correctly receive and interpret the HS-SICH to decide on the parameters for the next downlink transmission (based on CQI) and to manage the HARQ process state (based on ACK/NACK).

How it works is integral to the HSDPA loop. After the UE decodes the HS-SCCH and the associated HS-PDSCH data, it attempts to decode the transport block. Based on the CRC check, it generates an ACK or NACK. Simultaneously, it measures the downlink channel quality to generate a CQI value. These two pieces of information are formatted, encoded, and transmitted on the HS-SICH back to the Node B. The Node B's scheduler uses the CQI from all active UEs to decide which UE to schedule next and with what data rate. Its HARQ process uses the ACK/NACK to either clear the buffer (on ACK) or prepare a redundancy-version retransmission (on NACK). This tight, fast feedback loop, enabled by the HS-SICH, is what allows HSDPA to achieve high spectral efficiency and robust link performance.

Purpose & Motivation

The HS-SICH was created to address a key missing link in the HSDPA architecture: fast, reliable uplink feedback. Moving scheduling and HARQ to the Node B (a core principle of HSDPA) required that the Node B have immediate knowledge of two things: the success or failure of its downlink transmission, and the current downlink channel quality at the UE. The existing Release 99 signaling channels were too slow and not designed for sub-2ms feedback cycles.

Without a dedicated, low-latency feedback channel like the HS-SICH, the advanced features of HSDPA would be impossible. Adaptive Modulation and Coding (AMC) relies on frequent and accurate CQI reports to select the optimal modulation and code rate for the current radio conditions. The fast Hybrid ARQ process requires the ACK/NACK to be received with minimal delay to keep the HARQ process round-trip time short, enabling rapid retransmissions and efficient use of the stop-and-wait protocol.

The HS-SICH solved the problem of how to get this critical information from the UE to the Node B within the stringent timing of the 2ms TTI framework. It provided a standardized, efficient physical channel with defined structure, coding, and timing. This allowed the Node B to make autonomous, informed scheduling decisions every 2ms, reacting to channel variations and packet errors in near real-time. The creation of the HS-SICH, alongside the HS-SCCH, completed the low-latency control plane necessary for the packet-optimized air interface that HSDPA represented.