In 5G New Radio (NR), the concept of Transport Block Size (TBS) is central to the efficient transmission of data across the network. It defines the amount of data that can be transmitted in one transmission time interval, which directly affects the speed, capacity, and reliability of the 5G network. Understanding TBS and how it is calculated is crucial for engineers, network planners, and anyone involved in the development of 5G networks.
What is Transport Block Size?
Transport Block Size (TBS) refers to the size of the data payload that is transmitted between the Medium Access Control (MAC) layer and the Physical (PHY) layer in 5G NR. It represents the data unit sent over the air interface in a 5G network, particularly over the Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH).
At the transmitter side, a transport block undergoes processing in the PHY layer, where it is mapped onto the PDSCH for transmission. This block is then segmented, and the Cyclic Redundancy Check (CRC) is added. The CRC ensures data integrity during transmission, and the segmentation helps optimize how large blocks of data are handled.
Importance of Transport Block Size in 5G NR
The Transport Block Size is a key factor that impacts several aspects of 5G NR performance, such as:
-
Data Transmission Efficiency: The larger the TBS, the more data can be transmitted within a given period, improving overall throughput.
-
Resource Allocation: The TBS calculation depends on the number of Resource Blocks (RBs), modulation scheme, and coding rate. This affects how efficiently the available resources are utilized.
-
Error Control and Correction: The TBS also determines the number of code blocks and the need for error correction, which impacts the reliability of data transmission.
-
Latency and Scheduling: A dynamic TBS allows for better handling of variable network conditions, reducing latency and improving scheduling flexibility.
Read Also: Unveiling the Future: Top Trends in Science and Technology
Factors Affecting Transport Block Size
Several factors influence the size of the transport block. These include:
-
Number of Layers (v): This refers to the number of Multiple Input Multiple Output (MIMO) layers. More layers can improve data throughput, but require more resources.
-
Modulation Order (Qm): The modulation scheme used, such as QPSK, 16-QAM, or 64-QAM, directly affects the TBS. Higher modulation orders allow more bits to be transmitted per symbol, resulting in larger transport blocks.
-
Coding Rate (R): The coding rate defines how efficiently the data is encoded. A higher coding rate results in a larger TBS, as more bits are used for data transmission instead of error correction.
-
Number of Physical Resource Blocks (PRBs): PRBs are units of bandwidth allocated to the transport block. More PRBs mean more resources for transmitting data, which increases the TBS.
-
Transmission Duration: The time duration allocated for transmission can also influence the TBS. Longer transmission windows allow for larger transport blocks.
How is Transport Block Size Calculated?
Calculating the Transport Block Size (TBS) involves multiple steps, and understanding these steps is essential for optimizing 5G NR performance.
Step 1: Determine the Number of Resource Elements (REs)
The first step in calculating TBS is to determine the number of Resource Elements (REs) available for data transmission. REs are the basic units of data in the frequency-time grid of a physical resource block.
The formula for determining the number of REs within a single Resource Block is:
REs per RB=12×14=168 REs (with normal cyclic prefix)\text{REs per RB} = 12 \times 14 = 168 \text{ REs (with normal cyclic prefix)}
However, in cases where the number of REs exceeds 156, the value is rounded down to 156 to avoid inefficient resource allocation.
Step 2: Account for MIMO and Modulation Scheme
Once the number of available REs is determined, the next step involves considering the modulation scheme and coding rate. The modulation scheme (e.g., QPSK, 16-QAM) affects how much data can be transmitted in each RE, while the coding rate determines how much data can be used for transmission versus error correction.
The formula for converting REs into information bits takes these parameters into account:
Information Bits=Number of REs×Modulation Order (Qm)×Number of Layers (v)×Coding Rate (R)\text{Information Bits} = \text{Number of REs} \times \text{Modulation Order (Qm)} \times \text{Number of Layers (v)} \times \text{Coding Rate (R)}
Step 3: Apply Segmentation (If Necessary)
When the transport block exceeds 3824 bits, segmentation is applied. This means the transport block is split into smaller segments, each requiring additional overhead for CRC, thus increasing the total size. The segmentation process is important for managing very large transport blocks that cannot be processed in a single step due to limitations in the encoding scheme.
Read Also: Robots Dot to Dot Nattapong: Proven Fun Learning
Formula-Based Approach for TBS Calculation in 5G NR
Unlike in 4G LTE, where a table-based approach was used for TBS calculation, 5G NR uses a more flexible formula-based approach. This allows for a wider range of transport block sizes, especially given the larger bandwidth and dynamic transmission conditions in 5G networks.
For small transport blocks (less than 3824 bits), the TBS is determined using a fixed table based on the Modulation and Coding Scheme (MCS) index and the Resource Block Allocation. For larger blocks, a combination of formulas and tables is used to calculate the TBS.
Step 4: Determining the CRC Size
The size of the Cyclic Redundancy Check (CRC) added to the transport block is another factor in calculating the TBS. If the transport block size is less than or equal to 3824 bits, a 16-bit CRC is added. For blocks larger than 3824 bits, a 24-bit CRC is applied, and segmentation may occur.
Example Calculation
Let’s consider an example where a transport block needs to be transmitted with the following parameters:
-
Number of Resource Blocks (RBs): 20
-
Modulation Scheme: 16-QAM (Qm = 4)
-
Coding Rate (R): 0.75
-
Number of Layers (v): 2
Using the formulas, we first calculate the number of REs available:
REs per RB=168 REs\text{REs per RB} = 168 \text{ REs}
For 20 RBs, the total number of REs is:
Total REs=168×20=3360 REs\text{Total REs} = 168 \times 20 = 3360 \text{ REs}
Now, using the modulation scheme and coding rate:
Information Bits=3360×4×2×0.75=20160 bits\text{Information Bits} = 3360 \times 4 \times 2 \times 0.75 = 20160 \text{ bits}
This results in a Transport Block Size (TBS) of 20160 bits.
FAQs about Transport Block Size
1. What is the maximum Transport Block Size in 5G NR?
The maximum transport block size in 5G NR is dependent on the resource block allocation, modulation scheme, and coding rate. It can exceed 8000 bits in some cases.
2. How does the number of MIMO layers affect the TBS?
Increasing the number of MIMO layers (v) can increase the TBS, as more layers allow more data to be transmitted simultaneously, thus improving throughput.
3. Why is segmentation necessary for large transport blocks?
Segmentation is required for transport blocks that exceed 3824 bits because larger blocks cannot be processed as a single entity due to the limits of the encoding scheme and channel coding methods.
4. How does the modulation order impact the TBS?
Higher modulation schemes, such as 64-QAM, allow more bits per symbol, increasing the TBS. Conversely, lower modulation schemes, like QPSK, reduce the TBS as fewer bits are transmitted per symbol.
5. What role does the Coding Rate play in calculating TBS?
The coding rate determines how many bits are used for error correction versus actual data transmission. A higher coding rate means more bits can be used for actual data, resulting in a larger TBS.
6. What is a resource block in 5G NR?
A resource block in 5G NR is the smallest unit of radio resources, consisting of 12 subcarriers and spanning one slot in the time domain.
7. What is the block size of 5G LDPC?
The block size of 5G LDPC (Low-Density Parity Check) can range up to 8448 bits, depending on the transport block size.
8. How long is a 5G NR subframe?
A 5G NR subframe is 1 millisecond long, consisting of 14 symbols in time.
9. What is the size of the LTE code block?
In LTE, the maximum size of a code block is 3840 bits, before segmentation is applied.
Conclusion
The Transport Block Size (TBS) in 5G NR is a critical factor for efficient data transmission across the network. By understanding the various parameters that influence TBS, such as the number of layers, modulation order, coding rate, and the resource block allocation, network engineers can optimize the performance of 5G systems. As 5G continues to evolve, understanding and calculating TBS will remain crucial for maximizing data throughput and network efficiency.
For broader information, visit Daily Trend Times
I’m Salman Khayam, founder of Daily Trend Times. I write well-researched, trustworthy content on business, tech, lifestyle, entertainment, travel, and more. My goal is to provide practical insights and tips to keep you informed, inspired, and empowered every day.