Transport Block Set Size

Description

Transport Block Set Size (TBSS) is defined as the number of bits in a Transport Block Set. It is a fundamental parameter in 3GPP radio access networks, particularly in UMTS, where it is used to specify the size of the data payload delivered from the MAC layer to the physical layer in a given transmission time interval (TTI). The TBSS is calculated as the product of the number of transport blocks in the set and the size of each transport block. It directly influences the channel coding, modulation, and resource allocation processes in the physical layer.

In UMTS, the Transport Block Set Size is one of the parameters that define a Transport Format (TF). Each transport channel is configured with a set of transport formats, each specifying a particular TBSS along with other parameters such as the transmission time interval and the coding scheme. The MAC layer selects a Transport Format Combination (TFC) based on the amount of data to be transmitted and the available radio resources. The selected TFC determines the TBSS for each transport channel, which is then used by the physical layer to perform rate matching and mapping onto physical channels. The TBSS is also used in the calculation of the gain factors for uplink transmission, ensuring that the transmitted power is sufficient to meet the required quality of service.

In LTE and NR, the concept of a Transport Block Set Size is less commonly used 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. In these systems, the equivalent parameter is the Transport Block Size (TBS), which specifies the size of a single transport block. The TBS is used in link adaptation and scheduling, where the base station selects an appropriate MCS and resource allocation to achieve a target block error rate. The TBS is signalled to the user equipment via the control channel (e.g., PDCCH) to enable decoding.

The Transport Block Set Size is closely related to the Transport Block and Transport Block Set concepts. It provides a quantitative measure of the amount of data being transmitted in a given TTI, which is essential for resource management and performance evaluation. In system-level simulations and network planning, the TBSS is used to model the traffic load and to dimension the radio interface. It also plays a role in the transport format combination selection algorithm, which aims to maximize the use of available resources while satisfying the constraints on transmit power and interference.

In summary, the Transport Block Set Size is a key parameter in 3GPP systems, particularly in UMTS, where it defines the payload size for a set of transport blocks. Although its importance has diminished with the evolution to LTE and NR, it remains an important concept for understanding the historical development of 3GPP radio access networks and for maintaining backward compatibility.

Purpose & Motivation

The Transport Block Set Size was introduced to provide a precise measure of the data payload in a Transport Block Set, enabling efficient resource allocation and transport format selection in UMTS. It solved the problem of how to quantify the amount of data being transmitted in a TTI, which is essential for link adaptation, power control, and scheduling. Without a defined size, the system could not accurately determine the required coding, modulation, and transmit power, leading to inefficient use of radio resources.

Historically, the TBSS emerged in Release 4 as part of the UMTS specifications, where it was used in the transport format combination selection process. It allowed the MAC layer to choose a transport format that matched the offered load to the available radio resources, ensuring that the quality of service requirements were met while minimizing interference. This was particularly important for variable-rate services such as packet data.

With the advent of HSDPA and HSUPA, the concept of a Transport Block Set Size became less relevant because these systems introduced fast scheduling with a single transport block per TTI. In LTE and NR, the equivalent parameter is the Transport Block Size, which serves a similar purpose but for a single transport block. The TBSS thus represents an intermediate step in the evolution of 3GPP radio access, bridging the gap between circuit-switched and packet-switched data transmission.