What is the Size of the Transport Block in LTE?
In LTE, the transport block size (TBS) is a key factor in determining the efficiency and performance of data transmission. Let me walk you through how the size of the transport block is defined and how it plays a crucial role in LTE network operations.
The transport block refers to the unit of data that is transmitted from the eNodeB (evolved Node B) to the user equipment (UE). This block contains the actual data that needs to be delivered to the device, and its size directly impacts the throughput and capacity of the LTE network. The TBS size varies depending on several factors, such as the modulation scheme, the coding rate, and the number of resource blocks assigned to the user.
The size of the transport block is determined by two primary factors:
- Number of Resource Blocks (RBs): In LTE, the available bandwidth is divided into smaller blocks called resource blocks. The more resource blocks allocated to the user, the larger the transport block can be. The size of the transport block increases as the number of allocated resource blocks increases.
- Modulation and Coding Scheme (MCS): The modulation scheme (QPSK, 16-QAM, 64-QAM, etc.) determines how many bits can be encoded in each symbol. A higher-order modulation scheme (such as 64-QAM) allows more bits to be transmitted in each symbol, thus increasing the transport block size. The coding rate, which is a measure of how much redundancy is added for error correction, also impacts the TBS.
The relationship between these factors is described in the Transport Block Size Table, which is part of the LTE standard. This table outlines the corresponding TBS values based on the number of resource blocks (RBs) and the MCS index. By referencing this table, the eNodeB can determine the appropriate transport block size to be used for each transmission based on the current network conditions.
Here’s how the TBS is typically calculated:
- Step 1: Determine the number of resource blocks (RBs) allocated to the user.
- Step 2: Identify the modulation and coding scheme (MCS) based on the radio link quality, which is typically represented by the Signal-to-Noise Ratio (SNR).
- Step 3: Use the TBS table to find the corresponding transport block size based on the RBs and MCS values.
The transport block size directly affects the throughput of the LTE connection. If the TBS is large, more data can be transmitted in a single transmission, leading to higher throughput. However, this also means that the system needs to handle larger amounts of data at once, and errors in transmission may have a more significant impact on the overall performance. Conversely, smaller transport blocks may lead to lower throughput but can be more robust in the presence of channel errors.
Let’s make this clearer with a simple example. If the system allocates 50 resource blocks to the user and uses a 64-QAM modulation scheme, the transport block size will be larger compared to a scenario where only 10 RBs are allocated and QPSK is used. The eNodeB uses the transport block size to organize and send the data, ensuring it matches the current network conditions and the user’s needs.
In previous articles, we’ve explored various LTE parameters that affect the overall data throughput and network efficiency. Understanding the transport block size is crucial for optimizing these factors, as it helps balance the system’s data rates and error handling capabilities.