E-DCH Transport Format Combination

Description

The E-DCH Transport Format Combination (E-TFC) is a central concept in the uplink of High-Speed Packet Access (HSPA), specifically for the Enhanced Dedicated Channel (E-DCH). It defines the exact set of parameters for transmitting data from the User Equipment (UE) to the NodeB on the E-DPDCH (E-DCH Dedicated Physical Data Channel). An E-TFC specifies a combination of transport formats for the different logical channels (MAC-d flows) multiplexed together for a given Transmission Time Interval (TTI). Each transport format within the combination defines attributes for its associated MAC-d flow, such as the Transport Block Size (TBS), the type of channel coding (Turbo or Convolutional), and the coding rate. The UE maintains an E-TFC Set, which is a collection of all allowable E-TFCs configured by the network via Radio Resource Control (RRC) signaling. The process works as follows: The UE's MAC-e/es entity receives data from higher layers in logical channel buffers. It must decide how much data to send in the next TTI. This decision is the E-TFC selection process. The UE considers two primary constraints: the Serving Grant (SG) from the NodeB, which indicates the maximum allowed power offset for the E-DPDCH relative to the DPCCH, and its own available transmission power (the Power Headroom). Based on these, the UE selects an E-TFC from its allowed set that maximizes data transmission without exceeding the granted power resource. It then multiplexes the appropriate amount of data from each logical channel according to the selected E-TFC's transport formats, builds the transport block(s), and transmits them. The NodeB, aware of the UE's E-TFC set, can decode the transmission based on the signaled E-TFC indicator (E-TFCI) on the E-DPCCH. This mechanism allows for fast, TTI-by-TTI (2ms or 10ms) adaptation of the uplink data rate, enabling efficient packet scheduling and interference control in the cell.

Purpose & Motivation

E-TFC was created to enable fast and efficient uplink packet scheduling in UMTS, which was a major enhancement over the original Release 99 DCH. The pre-HSPA uplink used Radio Network Controller (RNC)-controlled scheduling with slow power control, which was inefficient for bursty packet data traffic, leading to high latency and poor utilization of uplink capacity. The purpose of the E-TFC framework within the E-DCH (HSUPA) architecture was to move scheduling decisions closer to the air interface—to the NodeB—and allow the UE to autonomously select its data rate on a per-TTI basis within network-defined limits. This solved the problem of slow reaction times to changing channel conditions and buffer status. The E-TFC mechanism, coupled with NodeB grants (Absolute and Relative Grants), allows for very fast link adaptation, reducing latency and increasing peak data rates and overall cell throughput. It was motivated by the need to make UMTS competitive for symmetric data applications like video upload, online gaming, and large file transfers, addressing a key weakness compared to other evolving technologies.

Key Features

• Defines the transport block size and coding parameters for E-DCH transmission
• UE autonomously selects E-TFC based on NodeB grants and available power
• Enables fast (2ms TTI) link adaptation for uplink packet data
• Supports multiplexing of multiple logical channels (MAC-d flows) within one TTI
• Governed by an E-TFC Set configured by RRC signaling
• Uses E-TFCI on the E-DPCCH for in-band signaling of the selected combination

Evolution Across Releases

Defining Specifications