Transport Block Set

Description

The Transport Block Set (TBS) is a 3GPP concept defined as a set of Transport Blocks that are exchanged between Layer 1 (physical layer) and the MAC sublayer at the same time instance using the same transport channel. It is equivalent to a MAC PDU Set. This concept is particularly relevant in UMTS and early LTE systems where multiple transport blocks could be transmitted simultaneously on a single transport channel within a given transmission time interval (TTI). The Transport Block Set allows the MAC layer to deliver multiple transport blocks to the physical layer in one go, which are then jointly processed and transmitted.

Architecturally, the Transport Block Set is associated with a specific transport channel, such as a Dedicated Transport Channel (DCH) or a Shared Transport Channel (e.g., HS-DSCH). Each transport channel is configured with a set of transport formats, each specifying parameters like the transport block size, the number of transport blocks in the set, and the transmission time interval. The Transport Block Set Size (TBSS) is the total number of bits in the set, calculated as the number of transport blocks multiplied by the size of each transport block. The physical layer then applies channel coding, interleaving, and modulation to the entire set, treating it as a single unit for transmission.

In UMTS, the Transport Block Set is a key element of the transport format combination (TFC) selection process. The MAC layer selects a TFC based on the available data and the allowed transport formats, which determines the number of transport blocks and their sizes. This selection is subject to constraints such as the maximum allowed transmit power and the available processing gain. The Transport Block Set is then passed to the physical layer for transmission over the air interface. At the receiver, the physical layer decodes the received signal and recovers the Transport Block Set, which is then delivered to the MAC layer for further processing.

With the evolution to LTE and NR, the concept of a Transport Block Set has become less prominent because these systems typically transmit a single transport block per TTI per transport channel. However, the term is still defined in 3GPP specifications for backward compatibility and for use in certain scenarios, such as multi-carrier aggregation or when multiple transport blocks are transmitted on different layers. In NR, the concept of a transport block set is not explicitly used; instead, multiple transport blocks can be transmitted on different layers or carriers, but they are not grouped into a set at the MAC-PHY interface.

The Transport Block Set is closely related to the Transport Block and Transport Block Size. It provides a way to handle multiple transport blocks as a single unit, which can simplify scheduling and reduce overhead in systems where multiple blocks are transmitted together. However, it also introduces complexity in terms of transport format selection and resource allocation. In modern systems, the trend is toward more flexible and granular transmission, making the Transport Block Set less relevant, but it remains an important concept for understanding the evolution of 3GPP radio access networks.

Purpose & Motivation

The Transport Block Set was introduced to enable the simultaneous transmission of multiple transport blocks over a single transport channel, which was particularly useful in early UMTS systems where data rates were relatively low and multiple small blocks could be efficiently bundled. It solved the problem of transmitting multiple MAC PDUs in a single TTI without incurring the overhead of separate transmissions. By grouping transport blocks into a set, the system could apply joint coding and interleaving, improving spectral efficiency and reducing latency.

Historically, the Transport Block Set concept emerged in Release 4 as part of the UMTS specifications, where it was used for dedicated channels. It allowed the MAC layer to select a transport format combination that specified the number of transport blocks and their sizes, providing flexibility in matching the offered load to the available radio resources. This was particularly important for services with variable data rates, such as packet-switched data.

With the advent of HSDPA and HSUPA, the need for Transport Block Sets diminished because these systems introduced fast scheduling and link adaptation with a single transport block per TTI. In LTE and NR, the concept is largely obsolete, but it remains in the specifications for legacy support. The Transport Block Set thus represents an intermediate step in the evolution of 3GPP radio access, bridging the gap between circuit-switched and packet-switched data transmission.