Transport Block

Description

The Transport Block (TB) is a fundamental concept in 3GPP radio access networks, defined as the basic data unit exchanged between Layer 1 (the physical layer) and the Medium Access Control (MAC) sublayer. It is equivalent to a MAC PDU (Protocol Data Unit). The transport block represents the payload that the physical layer encodes, modulates, and transmits over the air interface during a specific transmission time interval (TTI). The size of a transport block—measured in bits—is determined by the modulation and coding scheme (MCS), the number of allocated resource blocks, and the number of MIMO layers. This size is signalled to the receiver via control channels (e.g., PDCCH in LTE and NR) to enable correct decoding.

Architecturally, the transport block sits at the boundary between the MAC layer and the physical layer. The MAC layer multiplexes logical channel data into transport blocks and passes them to the physical layer for transmission. The physical layer then performs a series of operations: CRC attachment, code block segmentation, channel coding (e.g., turbo coding in LTE, LDPC in NR), rate matching, scrambling, modulation, and layer mapping. At the receiver, the reverse operations are performed to recover the transport block, which is then delivered to the MAC layer. The transport block size (TBS) is a critical parameter that directly affects throughput and link adaptation. The scheduler in the base station (eNB or gNB) dynamically selects the TBS based on channel conditions, available resources, and QoS requirements.

In LTE, the transport block is transmitted over a 1 ms subframe, and the TBS is determined by the MCS and the number of allocated physical resource blocks (PRBs). In 5G NR, the concept is similar but more flexible: the transmission duration can be as short as a few OFDM symbols (mini-slot), and the TBS calculation depends on the MCS, resource allocation, and the number of layers. NR also introduces the concept of a transport block spanning multiple slots for certain use cases. The transport block is protected by a 24-bit CRC, and for large transport blocks, code block segmentation is applied, with each code block having its own CRC.

The transport block is closely tied to HARQ (Hybrid Automatic Repeat Request) operations. Each transport block is associated with a HARQ process, and if decoding fails, a retransmission is requested. The redundancy version (RV) and HARQ process ID are signalled to the receiver to enable soft combining. In NR, the transport block can be transmitted with different redundancy versions to improve reliability. The transport block also plays a key role in link adaptation: the base station adjusts the MCS and TBS based on channel quality feedback (CQI) to maximize throughput while maintaining a target block error rate (BLER).

In summary, the transport block is the fundamental unit of data exchange between MAC and PHY, encapsulating user data and control information. Its size, format, and transmission parameters are dynamically controlled to adapt to varying radio conditions, making it central to the efficient operation of 3GPP radio access networks.

Purpose & Motivation

The Transport Block concept exists to provide a well-defined interface between the MAC and physical layers, enabling efficient and flexible data transmission over the radio interface. Before its formalization, early radio systems lacked a standardized unit for data exchange, leading to interoperability issues and inefficient use of radio resources. The transport block solves this by defining a clear payload unit that can be dynamically sized and adapted to channel conditions, which is essential for link adaptation and HARQ.

Historically, the transport block was introduced in 3GPP Release 4 as part of the UMTS specifications, but its roots trace back to earlier GSM and GPRS systems. The need for a flexible data unit became more pronounced with the introduction of HSDPA (Release 5) and HSUPA (Release 6), where fast scheduling and link adaptation required a variable-size transport block. In LTE (Release 8), the transport block became even more central, with a well-defined TBS calculation and HARQ operation. In 5G NR (Release 15), the transport block concept was extended to support new numerologies, flexible TTI durations, and massive MIMO.

The transport block addresses the limitations of fixed-size data units by allowing the scheduler to adapt the payload size to instantaneous channel conditions, thereby maximizing spectral efficiency. It also enables error protection through CRC and HARQ, ensuring reliable delivery. Without the transport block abstraction, the MAC and PHY layers would be tightly coupled, hindering modularity and evolution. Thus, the transport block is a cornerstone of modern cellular systems.