EDCH HARQ Acknowledgement Indicator Channel

Description

The EDCH HARQ Acknowledgement Indicator Channel (E-HICH) is a dedicated downlink physical control channel defined within the 3GPP UMTS and High-Speed Packet Access (HSPA) specifications, specifically for the Enhanced Uplink (EUL), also known as HSUPA. It operates in the physical layer (Layer 1) of the radio interface. The E-HICH is transmitted from the Node B (base station) to a specific User Equipment (UE) to provide feedback for the Hybrid Automatic Repeat Request (HARQ) process used on the uplink Enhanced Dedicated Channel (E-DCH). Its sole function is to carry the HARQ Acknowledgement (ACK) or Negative Acknowledgement (NACK) indicator for a previously received E-DCH transport block. Technically, the channel uses a sequence of QPSK symbols to represent the ACK/NACK signal. A specific signature sequence, derived from parameters like the E-DCH Radio Network Temporary Identifier (E-RNTI) and HARQ process identifier, is used to differentiate E-HICH transmissions intended for different UEs or different HARQ processes. The UE continuously monitors its assigned E-HICH during the appropriate timing interval after it has transmitted an E-DCH packet. If an ACK is detected, the UE knows the packet was successfully decoded by the Node B and can proceed to send new data. If a NACK is detected (or if nothing is detected, implying a DTX scenario), the UE will schedule a retransmission of the same data packet, using incremental redundancy as part of the HARQ process. This rapid feedback loop, occurring over a 2ms Transmission Time Interval (TTI) in many configurations, is what enables the low-latency and high-reliability characteristics of HSUPA. The E-HICH is always paired with an E-RGCH (Relative Grant Channel) on the same downlink channelization code, and their signals are code-multiplexed.

Purpose & Motivation

The E-HICH was created specifically to support the Enhanced Uplink (E-DCH) feature introduced in 3GPP Release 6 (HSUPA). Prior to HSUPA, uplink packet data on the Dedicated Channel (DCH) relied on RLC layer retransmissions controlled by the RNC, which introduced significant latency. The purpose of the E-HICH is to enable fast physical layer Hybrid ARQ (HARQ) retransmissions controlled directly by the Node B. This solves the problem of slow uplink error recovery and inefficient spectrum usage. By providing ACK/NACK feedback within milliseconds directly from the receiving entity (Node B), it allows for very rapid retransmissions of erroneous data packets, dramatically improving uplink throughput, reducing latency, and increasing overall spectral efficiency. The motivation was to make the UMTS uplink competitive with the downlink enhancements provided by HSDPA, creating a symmetric high-speed packet access experience. The E-HICH is a fundamental enabler of the Node B-controlled scheduling and fast HARQ that define HSUPA's performance gains over previous UMTS releases.