What is TB (transport block)

In the world of cellular communication networks, particularly LTE (Long-Term Evolution) and 5G, the Transport Block (TB) emerges as a fundamental unit of data transmission on the air interface. It acts as the data payload carried by radio waves between the base station and the user equipment (UE). Understanding TBs is essential for grasping how data flows efficiently within cellular networks.

The Bigger Picture: Lower Layers of Cellular Networks

• Cellular network communication relies on a layered architecture, with each layer handling specific functionalities.
• The lower layers, particularly the Medium Access Control (MAC) layer and the Physical (PHY) layer, are responsible for data transmission over the radio interface.

The Role of TBs:

• TBs reside at the interface between the MAC layer and the PHY layer.
• The MAC layer segments the higher-layer data received from applications (like web browsing or video streaming) into smaller, manageable units. These units are then packaged into TBs for efficient transmission over the radio channel.
• The PHY layer takes the TBs and applies channel coding and modulation techniques to convert them into radio signals suitable for transmission over the air.

Structure of a TB:

• A TB primarily consists of two parts:
  1. Payload: This is the actual user data that needs to be transmitted. The size of the payload can vary depending on factors like:
     • Modulation and Coding Scheme (MCS) used for transmission (higher MCS typically allows for larger payloads).
     • Resource allocation information (number of resource blocks assigned to the UE for transmission).
  2. Control Information: This includes additional bits that provide essential information for the receiving UE to decode the TB correctly. It might contain details like:
     • Redundancy bits for error correction.
     • Signaling information for channel decoding.

TB Size and Efficiency:

• The size of a TB is a critical factor for efficient data transmission.
• Larger TBs can potentially improve throughput by reducing the overhead associated with control information and signaling. However, larger TBs are also more susceptible to channel impairments like fading, which can lead to errors.
• The network dynamically chooses the optimal TB size based on factors like channel conditions, MCS, and desired throughput.

TB Scheduling and Multiplexing:

• The network can schedule and transmit multiple TBs from different UEs within the same time slot. This technique, known as Multiplexing, allows for efficient utilization of radio resources.
• Scheduling algorithms consider factors like UE channel quality, fairness, and Quality of Service (QoS) requirements to determine the order and number of TBs to be transmitted.

Evolution of TBs:

• The concept of TBs has evolved with advancements in cellular technologies.
• LTE introduced TBs with various sizes and coding schemes to cater to diverse data traffic types.
• 5G builds upon this foundation, offering larger TB sizes and more flexible scheduling mechanisms to support the ever-increasing demands for network capacity and efficiency.

Conclusion:

Transport Blocks (TBs) serve as the workhorses of data transmission in cellular networks. By efficiently packaging data and control information, TBs enable reliable and efficient communication between UEs and base stations. As network technologies continue to progress, TBs will likely remain a central element for ensuring seamless and high-performance data delivery in future cellular communication systems.