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Understanding LTE Transport Blocks and Data Transmission

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This article delves into the concept of Transport Blocks in LTE (Long-Term Evolution) systems, explaining how data from the upper layers is handled at the physical layer for transmission. We’ll also touch upon related LTE features and terminology.

What is a Transport Block?

In LTE, the data received by the physical layer from the MAC (Medium Access Control) layer is known as a Transport Block. For a single antenna transmission mode (SISO - Single-Input Single-Output), one transport block is created for each TTI (Transmission Time Interval).

The size of the transport block is determined by two key factors:

  • NPRB: The number of Physical Resource Blocks allocated.
  • MCS: The Modulation and Coding Scheme used.

Transport Block Size Calculation

Here’s how the Transport Block Size (TBS) is calculated:

Step 1: Determine the TBS index (ITBS) based on the MCS value. This is done using Table 7.1.7.1-1 specified in the 3GPP 36.213 document.

Step 2: Using the ITBS and the number of Physical Resource Blocks (NPRB), find the corresponding Transport Block Size from Table 7.1.7.2.1-1.

Example:

Let’s say we have an MCS of 14 and NPRB is 3. What’s the transport block size?

  • According to Table 7.1.7.1-1 (from 3GPP 36.213), an MCS of 14 gives an ITBS of 13.
  • Looking up ITBS = 13 and NPRB = 3 in Table 7.1.7.2.1-1, we find a TBS of 744 bits.

CRC Attachment: Ensuring Data Integrity

To ensure data integrity, a CRC (Cyclic Redundancy Check) is added to each transport block for error detection.

The process involves dividing the entire transport block by a Cyclic Generator Polynomial (as detailed in TS 36.212, section 5.1.1). This calculation generates 24 parity bits, which are then appended to the end of the transport block.

Segmentation and Concatenation

Before being processed by the channel coding and rate matching modules, the transport block is divided into smaller units called code blocks. After the rate matching module processes the code blocks, they are concatenated back together.