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LTE Transport Block Concept

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The data from the upper layer (or MAC) given to the physical layer in LTE system is basically referred as transport block.

For single antenna transmission mode viz. SISO , there will be 1 transport block which will be generated for each TTI(Transmission Time Interval). The transport block size is decided by the number of Physical Resource Blocks (NPRB) and the MCS( Modulation and Coding Scheme).

Transport Block Size calculation

Step-1: Based on MCS, ITBS or TBS index is determined based on table no. 7.1.7.1-1. Step-2: Now based on ITBS and NPRB (No.of Physical resource block) transport size is derived from the table no. 7.1.7.2.1-1.

Example: Given that MCS is 14 and NPRB is 3, determine transport block size(TBS).
•   MCS of 14 gives ITBS equal to 13 from table 7.1.7.1-1 as mentioned in 3GPP 36.213 document.
•   Now ITBS of 13 and NPRB of 3 gives TBS of 744 bits from 7.1.7.2.1-1.

CRC Attachment

CRC ( cyclic redundancy check) is appended to each transport block which provides error detection. Entire transport block is divided by Cyclic Generator Polynomial as specified in TS 36.212 (section-5.1.1). This generates 24 parity bits. These 24 bits of parity is appended at the end of transport block.

Transport block is segmented into code blocks before being passed to the channel coding and rate matching modules. After the code blocks are outputted by rate matching module they are concatenated.

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