How TBS Size Is Calculated in LTE?
Let me explain how the Transport Block Size (TBS) is calculated in LTE. TBS is a crucial concept in LTE that determines how much data can be transmitted in a given time period. The size of the TBS directly affects the throughput of the LTE network, as it dictates how much user data can be packed into a transport block for transmission between the eNodeB and the User Equipment (UE).
In LTE, the TBS is determined by factors like the modulation scheme, the number of Resource Blocks (RBs), and the coding rate. Let’s dive into how these factors come into play in the calculation of TBS.
Factors Affecting TBS Size
The size of the Transport Block depends on the following key parameters:
- Modulation Scheme: The modulation scheme used in the LTE transmission (QPSK, 16-QAM, 64-QAM) determines the number of bits that can be transmitted per symbol. Higher-order modulations like 64-QAM allow more bits to be transmitted in each symbol compared to QPSK.
- Number of Resource Blocks (RBs): The RBs are the smallest unit of resource allocation in LTE, and the more RBs allocated, the higher the potential TBS size. The total number of RBs allocated for a transmission can range from 1 to 110 (depending on the bandwidth).
- Coding Rate: The coding rate determines the level of error correction. A higher coding rate means more data is transmitted, but there is a greater risk of errors, so lower coding rates may be used for more reliable transmission, especially in poor signal conditions.
Steps to Calculate TBS Size
The calculation of the TBS involves a lookup procedure, where the parameters mentioned above are used to reference predefined tables. Here’s how the process works:
- Step 1: Determine the number of Resource Blocks (RBs) allocated for the transmission.
- Step 2: Choose the modulation scheme (QPSK, 16-QAM, 64-QAM) based on the channel conditions. This will dictate how many bits are transmitted per symbol.
- Step 3: Select the coding rate, which is the ratio of useful data to the total transmitted data. This is determined by the eNodeB based on the signal quality.
- Step 4: Use the above parameters to refer to a Transport Block Size table, which is defined in the LTE specifications (3GPP). The table provides the TBS size based on the number of RBs and the modulation/coding scheme used.
Transport Block Size Table Example
The LTE system uses predefined TBS tables that match the number of RBs with the modulation scheme and coding rate. These tables are typically stored in the eNodeB and UE. Here’s a simplified example:
| RBs | Modulation | Coding Rate | Transport Block Size (TBS) |
|---|---|---|---|
| 1 | QPSK | 1/3 | 150 bits |
| 2 | QPSK | 1/2 | 300 bits |
| 4 | 16-QAM | 3/4 | 1,000 bits |
| 6 | 64-QAM | 5/6 | 2,500 bits |
In this table, as the number of RBs increases, the TBS size also increases. Similarly, using a higher modulation scheme (like 16-QAM or 64-QAM) and a higher coding rate increases the TBS size, allowing more data to be transmitted within the same time frame.
In summary, the Transport Block Size in LTE is calculated by considering the number of allocated RBs, the modulation scheme, and the coding rate. These parameters are then referenced against predefined TBS tables to determine the exact size of the transport block for transmission. This ensures efficient use of available resources, improving network throughput and performance.