# nrDLSCHDecoder

Apply DL-SCH decoder processing chain

Since R2019a

## Description

The nrDLSCHDecoder System object™ applies the downlink shared channel (DL-SCH) decoder processing chain to the soft bits corresponding to one or two DL-SCH-encoded transport blocks. The DL-SCH decoding process consists of rate recovery, low-density parity-check (LDPC) decoding, desegmentation, and cyclic redundancy check (CRC) decoding. The object implements the inverse operation of the DL-SCH encoding process specified in TS 38.212 Section 7.2 [1].

To apply the DL-SCH decoder processing chain:

1. Create the nrDLSCHDecoder object and set its properties.

2. Call the object with arguments, as if it were a function.

## Creation

### Description

decDL = nrDLSCHDecoder creates a DL-SCH decoder System object.

decDL = nrDLSCHDecoder(Name,Value) creates the object with properties set by using one or more name-value pairs. Enclose the property name inside quotes, followed by the specified value. Unspecified properties take default values.

Example: For example, nrDLSCHDecoder('MultipleHARQProcesses',true) creates the object and enables multiple hybrid automatic repeat-request (HARQ) processes.

## Properties

expand all

Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

Enable multiple HARQ processes, specified as false or true. When set to false, the object uses a single process. When set to true, the object uses multiple HARQ processes, at most 32. To enable soft combining of retransmissions before LDPC decoding, the object maintains a soft buffer for each HARQ process.

Data Types: logical

Target code rate, specified as a numeric scalar or a 1-by-2 numeric vector. The values must be in the interval (0, 1). The default value corresponds to 526/1024. If you specify TargetCodeRate as a scalar, the object applies scalar expansion when processing two transport blocks. To specify different target code rates for each transport block, specify TargetCodeRate as a vector.

Tunable: Yes

Data Types: double

Length of decoded transport block, or transport blocks, in bits, specified as a positive scalar integer or a 1-by-2 integer vector. If you specify TransportBlockLength as a scalar, the object applies scalar expansion when processing two transport blocks. To specify a different length for the decoded transport blocks, specify TransportBlockLength as a vector.

Tunable: Yes

Data Types: double

Limited buffer size used for rate recovery, specified as a positive integer. The default value corresponds to 384×66, which is the maximum coded length of a code block. The default value implies no limit on the buffer size.

Data Types: double

Maximum LDPC decoding iterations, specified as a positive integer. Since early termination is enabled, decoding stops once parity-checks are satisfied. In this case, fewer iterations take place than the maximum specified by this argument.

Data Types: double

LDPC decoding algorithm, specified as one of these values:

• 'Belief propagation' — Use this option to specify the belief-passing or message-passing algorithm.

• 'Layered belief propagation' — Use this option to specify the layered belief-passing algorithm, which is suitable for quasi-cyclic parity-check matrices (PCMs).

• 'Normalized min-sum' — Use this option to specify the layered belief propagation algorithm with normalized min-sum approximation.

• 'Offset min-sum' — Use this option to specify the layered belief propagation algorithm with offset min-sum approximation.

Data Types: char | string

Scaling factor for normalized min-sum decoding, specified as a real scalar in the range (0, 1].

#### Dependencies

To enable this property, set the LDPCDecodingAlgorithm property to 'Normalized min-sum'.

Data Types: double

Offset for offset min-sum decoding, specified as a nonnegative finite real scalar.

#### Dependencies

To enable this property, set the LDPCDecodingAlgorithm property to 'Offset min-sum'.

Data Types: double

## Usage

### Description

example

trblk = decDL(softbits,mod,nLayers,rv) applies the DL-SCH decoder processing chain to the input softbits and returns the decoded bits. mod specifies the modulation scheme. nLayers specifies the number of transmission layers. rv specifies the redundancy version of the transmission.

trblk = decDL(___,harqID) specifies the HARQ process number harqID used with the current transmission in addition to the input arguments in the previous syntax. To use this syntax, set the MultipleHARQProcesses property to true. When the property is set to false, the object uses HARQ process number 0.

When the object receives codewords with different redundancy version for an individual HARQ process, the object uses soft buffer state retention to enable soft combining of retransmissions. When you enable multiple HARQ processes, the object maintains independent buffers for each process.

[trblk,blkerr] = decDL(___) returns an error flag, using the input arguments in any of the previous syntaxes. A value of 1 in blkerr indicates an error during transport block decoding.

### Input Arguments

expand all

Approximate log-likelihood ratio (LLR) soft bits, corresponding to one or two DL-SCH-encoded transport blocks, specified as a real column vector or a cell array of two real column vectors.

Data Types: single | double

Modulation scheme, specified as 'QPSK', '16QAM', '64QAM', '256QAM', '1024QAM', a string array, or cell array of character vectors. This modulation scheme determines the modulation type and number of bits used per modulation symbol. For two transport blocks, the modulation scheme applies to both blocks. Alternatively, you can specify different modulation schemes for each transport block by using a string array or a cell array of character vectors.

Modulation SchemeNumber of Bits Per Symbol
'QPSK'2
'16QAM'4
'64QAM'6
'256QAM'8
'1024QAM'10

Data Types: char | string

Number of transmission layers, specified as an integer from 1 to 8. For nLayers > 4, the object expects two encoded transports blocks as input.

Data Types: double

Redundancy version, specified as one of these options.

• Integer from 0 to 3 — Use this option to specify the redundancy version for a single encoded transport block.

• 1-by-2 vector of integers from 0 to 3 — Use this option to specify the redundancy version for two encoded transport blocks.

Data Types: double

HARQ process number, specified as an integer from 0 to 31. To specify a value other than 0, set the MultipleHARQProcesses object property to true.

Data Types: double

### Output Arguments

expand all

Decoded DL-SCH transport blocks, returned as a binary column vector or cell array of two binary column vectors. The TransportBlockLength property specifies the length of the column vectors.

Result of DL-SCH transport block decoding for each transport block, returned as a logical scalar or logical vector of length 2. A value of 1 in blkerr indicates an error during transport block decoding.

Data Types: logical

## Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

expand all

 resetSoftBuffer Reset soft buffer for HARQ process in UL-SCH or DL-SCH decoder
 step Run System object algorithm clone Create duplicate System object isLocked Determine if System object is in use release Release resources and allow changes to System object property values and input characteristics reset Reset internal states of System object

## Examples

collapse all

Generate a random sequence of binary values corresponding to one transport block of length 5120.

trBlkLen = 5120;
trBlk = randi([0 1],trBlkLen,1,'int8');

Create and configure a DL-SCH encoder System object with the specified target code rate.

targetCodeRate = 567/1024;
encDL = nrDLSCH;
encDL.TargetCodeRate = targetCodeRate;

Load the transport block into the DL-SCH encoder.

setTransportBlock(encDL,trBlk);

Call the encoder with 64-QAM modulation scheme, 1 transmission layer, an output length of 10,240 bits, and redundancy version 0. The encoder applies the DL-SCH processing chain to the transport block loaded into the object.

mod = '64QAM';
nLayers = 1;
outlen = 10240;
rv = 0;
codedTrBlock = encDL(mod,nLayers,outlen,rv);

Create and configure a DL-SCH decoder System object.

decDL = nrDLSCHDecoder;
decDL.TargetCodeRate = targetCodeRate;
decDL.TransportBlockLength = trBlkLen;

Call the DL-SCH decoder on the soft bits representing the encoded transport block. Use the configuration parameters specified for the encoder. The error flag in the output indicates that the block decoding does not have errors.

rxSoftBits = 1.0 - 2.0*double(codedTrBlock);
[decbits,blkerr] = decDL(rxSoftBits,mod,nLayers,rv)
decbits = 5120x1 int8 column vector

1
1
0
1
1
0
0
1
1
1
⋮

blkerr = logical
0

Verify that the transmitted and received message bits are identical.

isequal(decbits,trBlk)
ans = logical
1

expand all

## References

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

[2] Gallager, Robert G. Low-Density Parity-Check Codes, Cambridge, MA, MIT Press, 1963.

[3] Hocevar, D.E. "A reduced complexity decoder architecture via layered decoding of LDPC codes." In IEEE Workshop on Signal Processing Systems, 2004. SIPS 2004. dois: 10.1109/SIPS.2004.1363033

[4] Chen, Jinghu, R.M. Tanner, C. Jones, and Yan Li. "Improved min-sum decoding algorithms for irregular LDPC codes." In Proceedings. International Symposium on Information Theory, 2005. ISIT 2005. doi: 10.1109/ISIT.2005.1523374

## Version History

Introduced in R2019a

expand all