7.1 Broadcast channel

38.2123GPPMultiplexing and channel codingNRRelease 17TS

Data arrives to the coding unit in the form of a maximum of one transport block every 80ms. The following coding steps can be identified:

– Payload generation

– Scrambling

– Transport block CRC attachment

– Channel coding

– Rate matching

7.1.1 PBCH payload generation

Denote the bits in a transport block delivered to layer 1 by , where is the payload size generated by higher layers. The lowest order information bit is mapped to the most significant bit of the transport block as defined in Clause 6.1.1 of [8, TS 38.321].

Generate the following additional timing related PBCH payload bits , where:

– are the 4^th, 3^rd, 2^nd, and 1^st LSB of SFN, respectively;

– is the half frame bit ;

– if as defined in Clause 4.1 of [5, TS38.213],

is the MSB of as defined in Clause 7.4.3.1 of [4, TS 38.211].

is reserved.

is the MSB of candidate SS/PBCH block index.

– else if as defined in Clause 4.1 of [5, TS38.213],

is the MSB of as defined in Clause 7.4.3.1 of [4, TS 38.211].

, are the 5^th and 4^th bits of the candidate SS/PBCH block index, respectively.

– else if as defined in Clause 4.1 of [5, TS38.213],

, , are the 6^th, 5^th, and 4^th bits of the candidate SS/PBCH block index, respectively.

– else

is the MSB of as defined in Clause 7.4.3.1 of [4, TS 38.211].

, are reserved.

– end if

Let ; ; ; ; ;

for to

if is an SFN bit

;

;

elseif is the half radio frame bit

elseif

;

;

else

;

;

end if

end for

where is the number of candidate SS/PBCH blocks in a half frame according to Clause 4.1 of [5, TS38.213], and the value of is given by Table 7.1.1-1.

Table 7.1.1-1: Value of PBCH payload interleaver pattern

0	16	4	8	8	24	12	3	16	9	20	14	24	21	28	27
1	23	5	30	9	7	13	2	17	11	21	15	25	22	29	28
2	18	6	10	10	0	14	1	18	12	22	19	26	25	30	29
3	17	7	6	11	5	15	4	19	13	23	20	27	26	31	31

7.1.2 Scrambling

For PBCH transmission in a frame, the bit sequence is scrambled into a bit sequence , where for and is generated according to the following:

;

;

while

if corresponds to any one of the bits belonging to the candidate SS/PBCH block index, the half frame index, and 2^nd and 3^rd least significant bits of the system frame number

;

else

;

;

end if

;

end while

The scrambling sequence is given by Clause 5.2.1of [4, TS38.211] and initialized with at the start of each SFN satisfying ; for or , for , for , and for , where is the number of candidate SS/PBCH blocks in a half frame according to Clause 4.1 of [5, TS38.213]; and is determined according to Table 7.1.2-1 using the 3^rd and 2^nd LSB of the SFN in which the PBCH is transmitted.

Table 7.1.2-1: Value of for PBCH scrambling

(3rd LSB of SFN, 2nd LSB of SFN)	Value of
(0, 0)	0
(0, 1)	1
(1, 0)	2
(1, 1)	3

7.1.3 Transport block CRC attachment

Error detection is provided on BCH transport blocks through a Cyclic Redundancy Check (CRC).

The entire transport block is used to calculate the CRC parity bits. The input bit sequence is denoted by , and the parity bits by, where is the payload size and is the number of parity bits.

The parity bits are computed and attached to the BCH transport block according to Clause 5.1 by setting to 24 bits and using the generator polynomial , resulting in the sequence, where .

The bit sequence is the input bit sequence to the channel encoder, where for and .

7.1.4 Channel coding

Information bits are delivered to the channel coding block. They are denoted by , where is the number of bits, and they are encoded via Polar coding according to Clause 5.3.1, by setting , , , and .

After encoding the bits are denoted by , where is the number of coded bits.

7.1.5 Rate matching

The input bit sequence to rate matching is .

The rate matching output sequence length .

Rate matching is performed according to Clause 5.4.1 by setting .

The output bit sequence after rate matching is denoted as .