E-DCH Uplink Control Channel

Description

The E-DCH Uplink Control Channel (E-UCCH) is a physical layer channel defined for the Time Division Duplex (TDD) modes of UMTS, specifically for the 3.84 Mcps and 7.68 Mcps chip rate options (often referred to as TD-SCDMA). It serves an analogous purpose to the E-DPCCH used in the Frequency Division Duplex (FDD) mode of HSUPA. The E-UCCH is transmitted by the User Equipment (UE) in the uplink timeslot(s) allocated for E-DCH transmission. Its primary role is to carry the essential in-band control information that the Node B requires to successfully decode the accompanying E-DCH data on the E-UDCH (E-DCH Uplink Data Channel). The key information fields carried on the E-UCCH are the E-DCH Transport Format Combination Indicator (E-TFCI) and the Retransmission Sequence Number (RSN). The E-TFCI indicates the transport block size and format of the simultaneous E-UDCH data transmission. The RSN is crucial for the Hybrid ARQ (HARQ) process, informing the Node B whether the current transmission is a new packet or a retransmission, and if a retransmission, which redundancy version is being used. The channel is designed with robust coding to ensure reliable reception even at the cell edge, as control information loss would cause data decoding failure. Its structure and timing are tightly integrated with the TDD frame structure, where uplink and downlink transmissions occur in different timeslots.

Purpose & Motivation

The E-UCCH was introduced to enable High-Speed Uplink Packet Access (HSUPA) functionality in UMTS TDD (TD-SCDMA) systems. While FDD HSUPA (Release 6) used the E-DPCCH, the TDD variant required a channel adapted to its time-slotted architecture. The purpose of the E-UCCH is to provide the same low-latency, reliable control signaling for TDD that E-DPCCH provides for FDD. It solves the problem of how to convey dynamic transport format and HARQ information within the constraints of a TDD frame without relying on slower higher-layer signaling. This control channel is fundamental to realizing the benefits of HSUPA—fast scheduling, adaptive modulation and coding, and incremental redundancy HARQ—in the TDD spectrum. It allows the TDD Node B to perform fast decoding of the uplink data packet immediately upon reception, enabling quick HARQ acknowledgements (sent on the E-HICH in the downlink) and efficient scheduling decisions for subsequent TTIs. Without the E-UCCH, TDD HSUPA would not be able to achieve its targeted high uplink throughput and low latency, maintaining parity with FDD HSUPA capabilities.

Key Features

• Carries uplink control information for E-DCH in 3.84/7.68 Mcps TDD (TD-SCDMA).
• Transports the E-TFCI (Transport Format Combination Indicator).
• Transports the RSN (Retransmission Sequence Number) for HARQ processes.
• Enables fast packet decoding and HARQ operation at the TDD Node B.
• Designed with robust channel coding for reliable reception.
• Timing aligned with the TDD frame and timeslot structure.

Evolution Across Releases

Defining Specifications