LDPC Processing for DL-SCH and UL-SCH

This example highlights the low-density parity-check (LDPC) coding chain for the 5G NR downlink and uplink shared transport channels (DL-SCH and UL-SCH).

Shared Channel Parameters

The example uses the DL-SCH to describe the processing, which also applies to the UL-SCH.

Select parameters for a transport block transmitted on the downlink shared (DL-SCH) channel.

rng(210);              % Set RNG state for repeatability

A = 10000;             % Transport block length, positive integer
rate = 449/1024;       % Target code rate, 0<R<1
rv = 0;                % Redundancy version, 0-3
modulation = 'QPSK';   % Modulation scheme, QPSK, 16QAM, 64QAM, 256QAM
nlayers = 1;           % Number of layers, 1-4 for a transport block

Based on the selected transport block length and target coding rate, DL-SCH coding parameters are determined using the nrDLSCHInfo function.

% DL-SCH coding parameters
cbsInfo = nrDLSCHInfo(A,rate);
disp('DL-SCH coding parameters')
disp(cbsInfo)

DL-SCH coding parameters
    CRC: '24A'
      L: 24
    BGN: 1
      C: 2
    Lcb: 24
      F: 244
     Zc: 240
      K: 5280
      N: 15840

DL-SCH supports multi-codeword transmission (i.e. two transport blocks) while UL-SCH supports only a single codeword. UL-SCH also supports pi/2-BPSK modulation in addition to those listed above for DL-SCH.

Transport Block Processing Using LDPC Coding

Data delivered from the MAC layer to the physical layer is termed as a transport block. For the downlink shared channel (DL-SCH), a transport block goes through the processing stages of:

• CRC attachment,

• Code block segmentation and code block CRC attachment,

• Channel coding using LDPC,

• Rate matching and code block concatenation

before being passed on to the physical downlink shared channel (PDSCH) for scrambling, modulation, layer mapping and resource/antenna mapping. Each of these stages is performed by a function as shown next.

% Random transport block data generation
in = randi([0 1],A,1,'int8');

% Transport block CRC attachment
tbIn = nrCRCEncode(in,cbsInfo.CRC);

% Code block segmentation and CRC attachment
cbsIn = nrCodeBlockSegmentLDPC(tbIn,cbsInfo.BGN);

% LDPC encoding
enc = nrLDPCEncode(cbsIn,cbsInfo.BGN);

% Rate matching and code block concatenation
outlen = ceil(A/rate);
chIn = nrRateMatchLDPC(enc,outlen,rv,modulation,nlayers);

The output number of bits from the rate matching and code block concatenation process must match the bit capacity of the PDSCH, based on the available resources. In this example, as the PDSCH is not modeled, this is set to achieve the target code rate based on the transport block size previously selected.

Similar processing applies for the UL-SCH, where the physical uplink shared channel (PUSCH) is the recipient of the UL-SCH codeword. The following schematics depict the processing for the two channels.

Refer to nrDLSCH and nrULSCH System objects that encapsulate the processing per transport block, with additional support for retransmissions.

Channel

A simple bipolar channel with no noise is used for this example. With the full PDSCH or PUSCH processing, one can consider fading channels, AWGN and other RF impairments as well.

chOut = double(1-2*(chIn));

Receive Processing Using LDPC Decoding

The receive end processing for the DL-SCH channel comprises of the corresponding dual operations to the transmit end that include

• Rate recovery

• LDPC decoding

• Code block desegmentation and CRC decoding

• Transport block CRC decoding

Each of these stages is performed by a function as shown next.

% Rate recovery
raterec = nrRateRecoverLDPC(chOut,A,rate,rv,modulation,nlayers);

% LDPC decoding
decBits = nrLDPCDecode(raterec,cbsInfo.BGN,25);

% Code block desegmentation and CRC decoding
[blk,blkErr] = nrCodeBlockDesegmentLDPC(decBits,cbsInfo.BGN,A+cbsInfo.L);

disp(['CRC error per code-block: [' num2str(blkErr) ']'])

% Transport block CRC decoding
[out,tbErr] = nrCRCDecode(blk,cbsInfo.CRC);

disp(['Transport block CRC error: ' num2str(tbErr)])
disp(['Recovered transport block with no error: ' num2str(isequal(out,in))])

CRC error per code-block: [0  0]
Transport block CRC error: 0
Recovered transport block with no error: 1

As the displays indicate, there are no CRC errors at both the code-block and transport block levels. This leads to the transport block being recovered and decoded with no errors, as expected, for a noiseless channel.

Refer to nrDLSCHDecoder and nrULSCHDecoder System objects that encapsulate the receive processing per codeword, with additional soft-combining of retransmissions for improved performance.