Transport Format

Description

A Transport Format (TF) is defined in 3GPP TS 21.905 and TS 44.160 as a format offered by Layer 1 (L1, the physical layer) to the Medium Access Control (MAC) layer for the delivery of a Transport Block Set (TBS) during a Transmission Time Interval (TTI) on a Transport Channel. The Transport Format constitutes two parts: a dynamic part and a semi-static part. This definition is fundamental to the UMTS (and later GERAN) radio interface, where transport channels are the services offered by the physical layer to MAC. The Transport Format specifies how the Transport Block Set is to be transmitted, including parameters such as the transport block size, the number of transport blocks, the transmission time interval, channel coding, and modulation.

The dynamic part of a Transport Format can change from one TTI to the next. It typically includes the Transport Block Size (TBS) and the Transport Block Set Size (number of blocks). This allows the network to adapt the instantaneous data rate to varying channel conditions and traffic demands. For example, in a dedicated channel (DCH), the dynamic part might indicate a larger transport block size when the channel is good, and a smaller one when it is poor. The semi-static part changes less frequently, often on the order of seconds or minutes, and includes parameters such as the Transmission Time Interval (TTI), the type of channel coding (e.g., convolutional, turbo), the coding rate, and the static rate matching parameters. The semi-static part is configured by higher layers (RRC) and remains constant for a period.

The combination of dynamic and semi-static parts defines a Transport Format. A set of Transport Formats is called a Transport Format Set (TFS). For each transport channel, a TFS is defined, and the MAC layer selects a Transport Format from the TFS for each TTI based on the amount of data to be transmitted and the available radio resources. The selected Transport Format is then used by the physical layer to encode and transmit the Transport Block Set. The receiver must be able to determine which Transport Format was used to correctly decode the data. In UMTS, this is achieved through Transport Format Combination Indicator (TFCI) signaling, which explicitly indicates the combination of Transport Formats used across multiple transport channels in a given TTI. In later releases, blind transport format detection was also introduced to reduce signaling overhead.

The Transport Format concept is closely tied to the Transport Format Combination (TFC) and Transport Format Combination Set (TFCS). When multiple transport channels are multiplexed onto a single physical channel (coded composite transport channel, CCTrCH), the MAC layer must select a combination of Transport Formats—one for each transport channel—that fits within the total available transmit power and physical channel capacity. This is known as Transport Format Combination Selection (TFC selection). The TFCS defines all valid combinations, and the network controls which combinations are allowed. The TFCI then signals the chosen combination to the receiver. This mechanism ensures efficient multiplexing and power control while maintaining the required QoS for each transport channel.

In GERAN (GSM/EDGE Radio Access Network), the Transport Format concept is also used, particularly for the packet-switched domain (GPRS/EDGE). TS 44.160 defines the Transport Formats for GERAN, where the dynamic and semi-static parts are adapted to the GERAN physical layer. The Transport Format concept thus spans multiple radio access technologies, providing a unified way to describe the interface between MAC and L1. In LTE and NR, the concept of Transport Format is less explicit; instead, the MCS and resource allocation in the scheduling grant effectively define the transport format. However, the underlying principle of dynamic and semi-static parameters remains.

In summary, the Transport Format is a structured description of how a Transport Block Set is transmitted over a Transport Channel. Its dynamic and semi-static parts allow the network to balance flexibility and stability, adapting to changing conditions while maintaining a predictable configuration. It is a cornerstone of UMTS and GERAN radio interface design, enabling efficient multiplexing, link adaptation, and QoS management.

Purpose & Motivation

The Transport Format exists to provide a standardized, flexible interface between the MAC layer and the physical layer in UMTS and GERAN. It solves the problem of how to describe the transmission parameters for a Transport Block Set in a way that allows dynamic adaptation while keeping some parameters stable. The dynamic part enables fast link adaptation to channel conditions and traffic variations, while the semi-static part reduces signaling overhead by keeping slowly changing parameters constant.

Before the Transport Format concept, early systems used fixed transmission formats, which limited flexibility and efficiency. The introduction of Transport Formats in UMTS allowed the network to multiplex multiple transport channels with different QoS requirements onto a single physical channel, using Transport Format Combinations. This enabled efficient use of radio resources and support for diverse services (voice, data, video) with varying data rates and reliability requirements. The Transport Format concept thus addresses the need for a scalable, QoS-aware radio interface that can adapt to changing conditions while maintaining interoperability across vendors and networks.