nrBCHDecode

Broadcast channel (BCH) decoding

Syntax

scrblk = nrBCHDecode(softbits,L)

[scrblk,errFlag] = nrBCHDecode(softbits,L)

[scrblk,errFlag,trblk,lsbofsfn,hrf,msbidxoffset] = nrBCHDecode(softbits,L,lssb,ncellid)

Description

scrblk = nrBCHDecode(softbits,L) decodes the log-likelihood ratios (LLRs) softbits in accordance with TS 38.212, Section 7.1 [1]. The function returns the decoded scrambled BCH transport block scrblk. The input argument L is the list length used for polar decoding.

[scrblk,errFlag] = nrBCHDecode(softbits,L) also returns an error flag, errFlag, to indicate whether scrblk contains an error after decoding.

[scrblk,errFlag,trblk,lsbofsfn,hrf,msbidxoffset] = nrBCHDecode(softbits,L,lssb,ncellid) also returns the decoded and unscrambled BCH transport block trblk. The additional input arguments are the number of candidate synchronization signal / physical broadcast channel (SS/PBCH) blocks, lssb, and the physical layer cell identity number, ncellid. The function also returns these information elements:

• lsbofsfn, the four least significant bits (LSBs) of the system frame number

• hrf, the half frame bit

• msbidxoffset, the most significant bits (MSBs) of the index offset

Examples

Decode Scrambled BCH Transport Block

Generate a random sequence of binary values corresponding to a BCH transport block of 24 bits.

trblk = randi([0 1],24,1,'int8');  

Specify the physical layer cell identity number as 321, the system frame number as 10, and the second half frame.

nid = 321;  
sfn = 10;   
hrf = 1;     

Specify the number of candidate SS/PBCH blocks as 8. When you specify the number of candidate SS/PBCH blocks as 4 or 8, you can specify the subcarrier offset kssb as an input argument to the BCH encoder.

lssb = 8;                     
kssb = 18;               

Encode the BCH transport block using the specified arguments.

bch = nrBCH(trblk,sfn,hrf,lssb,kssb,nid);

Decode the encoded transport block and recover information by using a polar decoding list length of 8 bits.

listLen = 8;
[~,errFlag,rxtrblk,rxSFN4lsb,rxHRF,rxKssb] = nrBCHDecode( ...
   double(1-2*bch),listLen,lssb,nid);

Verify that the decoding has no errors.

errFlag

errFlag = uint32

0

isequal(trblk,rxtrblk)

ans = logical
   1

isequal(bit2int(rxSFN4lsb,4),mod(sfn,16))

ans = logical
   1

[isequal(hrf,rxHRF) isequal(int2bit(floor(kssb/16),1,false),rxKssb)]

ans = 1x2 logical array

   1   1

Input Arguments

softbits — Approximate log-likelihood ratio (LLR) soft bits

864-by-1 real-valued column vector

Approximate log-likelihood ratio (LLR) soft bits, specified as an 864-by-1 real-valued column vector.

Data Types: single | double

L — Polar decoding list length

power of 2

Polar decoding list length, specified as a power of 2.

Data Types: double

lssb — Number of candidate SS/PBCH blocks in a half frame

4 | 8 | 64

Number of candidate SS/PBCH blocks in a half frame, specified as 4, 8, or 64.

Data Types: double

ncellid — Physical layer cell identity number

integer

Physical layer cell identity number, specified as an integer from 0 to 1007.

Data Types: double

Output Arguments

scrblk — Decoded scrambled BCH transport block

32-by-1 binary column vector

Decoded scrambled BCH transport block, returned as a 32-by-1 binary column vector.

Data Types: int8

errFlag — Error flag

0 | 1

Error flag to indicate whether scrblk contains an error, returned as 0 or 1. If errFlag is 1, then an error has occurred.

Data Types: uint32

trblk — Decoded and unscrambled BCH transport block

24-by-1 binary column vector

Decoded and unscrambled BCH transport block, returned as a 24-by-1 binary column vector. The output trblk is the BCCH-BCH-Message, as defined in TS 38.331 Section 6.2.1 [2]. The BCCH-BCH-Message contains the master information block (MIB), as defined in TS 38.331 Section 6.2.2.

Data Types: logical

lsbofsfn — LSBs of the system frame number

4-by-1 column vector

The four LSBs of the system frame number, returned as a 4-by-1 column vector.

Data Types: logical

hrf — Half frame bit in SS/PBCH block transmissions

0 | 1

Half frame bit in SS/PBCH block transmissions, returned as 0 indicating the first half of a frame or 1 indicating the second half of a frame. For more information, see TS 38.214 Section 4.1 [3].

Data Types: logical

msbidxoffset — MSBs of index offset

scalar | 3-by-1 column vector

MSBs of index offset, returned as a scalar or 3-by-1 column vector.

• If lssb is 4 or 8, msbidxoffset is the decoded MSB of the subcarrier index, returned as a scalar.

• If lssb is 64, the entries of msbidxoffset are the three decoded MSBs of the SSB index, returned as a 3-by-1 column vector.

Data Types: logical

References

[1] 3GPP TS 38.212. “NR; Multiplexing and channel coding.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[2] 3GPP TS 38.331. “NR; Radio Resource Control (RRC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[3] 3GPP TS 38.214. “NR; Physical layer procedures for data.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

The input argument L must be less than or equal to 64.

Version History

Introduced in R2018b

R2023b: C/C++ code generation updates

For C/C++ code generation, the limitation that the input argument L must be a compile-time constant has been removed, however, L must be less than or equal to 64.

R2020a: Polar decoding metric update

In releases R2019b and before, polar decoding uses the exact form of the expression $$\log (1+e^x)$$ for internal metric evaluation. Starting in release R2020a, because the exact form leads to numerical instability for high SNR ranges, polar decoding approximates $$\log (1+e^x)$$ as 0 for $$x<0$$ and as $$x$$ for $$x\ge 0$$. This approximation affects the results of the nrBCHDecode function, resulting in a marginal degradation of the BLER performance in a link-level simulation.