4.2.1 CRC attachment

25.2223GPPMultiplexing and channel coding (TDD)Release 17TS

Error detection is provided on transport blocks through a Cyclic Redundancy Check (CRC). The size of the CRC is 24, 16, 12, 8 or 0 bits and it is signalled from higher layers what CRC size that should be used for each transport channel.

4.2.1.1 CRC calculation

The entire transport block is used to calculate the CRC parity bits for each transport block. The parity bits are generated by one of the following cyclic generator polynomials:

g_CRC24(D) = D²⁴ + D²³ + D⁶ + D⁵ + D + 1

g_CRC16(D) = D¹⁶ + D¹² + D⁵ + 1

g_CRC12(D) = D¹² + D¹¹ + D³ + D² + D + 1

g_CRC8(D) = D⁸ + D⁷ + D⁴ + D³ + D + 1

Denote the bits in a transport block delivered to layer 1 by , and the parity bits by . A_i is the size of a transport block of TrCH i, m is the transport block number, and L_i is the number of parity bits. L_i can take the values 24, 16, 12, 8, or 0 depending on what is signalled from higher layers.

The encoding is performed in a systematic form, which means that in GF(2), the polynomial:

yields a remainder equal to 0 when divided by g_CRC24(D), polynomial:

yields a remainder equal to 0 when divided by g_CRC16(D), polynomial:

yields a remainder equal to 0 when divided by g_CRC12(D) and the polynomial:

yields a remainder equal to 0 when divided by g_CRC8(D).

If no transport blocks are input to the CRC calculation (M_i = 0), no CRC attachment shall be done. If transport blocks are input to the CRC calculation (M_i  0) and the size of a transport block is zero (A_i = 0), CRC shall be attached, i.e. all parity bits equal to zero.

4.2.1.2 Relation between input and output of the CRC attachment block

The bits after CRC attachment are denoted by , where B_i = A_i + L_i. The relation between a_imk and b_imk is:

k = 1, 2, 3, …, A_i

k = A_i + 1, A_i + 2, A_i + 3, …, A_i + L_i