Modeling Downlink Control Information
This example describes the downlink control information (DCI) processing for the 5G New Radio communications system. Starting from a random DCI message, it models the message encoding followed by the physical downlink control channel (PDCCH) processing on the transmit end. Corresponding receiver components recover the transmitted control information elements.
Set parameters for a UE-specific search space.
rng(211); % Set RNG state for repeatability nID = 23; % pdcch-DMRS-ScramblingID rnti = 100; % C-RNTI for PDCCH in a UE-specific search space K = 64; % Number of DCI message bits E = 288; % Number of bits for PDCCH resources
The DCI message bits based on a downlink format are encoded using the nrDCIEncode function, which includes the stages of CRC attachment, polar encoding and rate matching.
dciBits = randi([0 1],K,1,'int8'); dciCW = nrDCIEncode(dciBits,rnti,E);
PDCCH Symbol Generation
The encoded DCI bits (a codeword) are mapped onto the physical downlink control channel (PDCCH) using the nrPDCCH function which generates the scrambled, QPSK-modulated symbols. The scrambling accounts for the user-specific parameters.
sym = nrPDCCH(dciCW,nID,rnti);
For NR, the PDCCH symbols are then mapped to the resource elements of an OFDM grid which also has PDSCH, PBCH and other reference signal elements. These are followed by OFDM modulation and transmission over a channel. For simplicity, we directly pass the PDCCH symbols over an AWGN channel next.
The PDCCH symbols are transmitted over an AWGN channel with a specified SNR, accounting for the coding rate and QPSK modulation.
EbNo = 3; % in dB bps = 2; % bits per symbol, 2 for QPSK EsNo = EbNo + 10*log10(bps); snrdB = EsNo + 10*log10(K/E); rxSym = awgn(sym,snrdB,'measured');
The received symbols are demodulated with known user-specific parameters and channel noise variance using the nrPDCCHDecode function. The soft output is the log-likelihood ratio for each bit in the codeword.
noiseVar = 10.^(-snrdB/10); % assumes unit signal power rxCW = nrPDCCHDecode(rxSym,nID,rnti,noiseVar);
An instance of the received PDCCH codeword is then decoded by the nrDCIDecode function. This includes the stages of rate recovery, polar decoding and CRC decoding to recover the transmitted information bits.
listLen = 8; % polar decoding list length [decDCIBits,mask] = nrDCIDecode(rxCW,K,listLen); isequal(mask,rnti) isequal(decDCIBits,dciBits)
ans = logical 1 ans = logical 1
The output mask matches the C-RNTI used for the UE, thereby confirming the message recipient. For the chosen system parameters, the decoded information matches the transmitted information bits.