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PDCCH Conformance Test

Example showing how to generate the PDCCH/PCFICH RMCs defined in TS 36.101 and use in conformance tests.

Introduction

This example shows the demodulation performance of PDCCH/PCFICH and determination of the probability of miss-detection of the downlink scheduling grant (Pm-dsg) test as defined in TS 36.101/TS 36.521-1 Section 8.4. The PDCCH and PCFICH are tested jointly and a miss-detection of PCFICH implies a miss-detection of PDCCH. The 1x2 transmit/receive antenna port performance test in TS 36.101 Section 8.4.1.1 Table 8.4.1.1-1[ 1 ] for R.15 PDCCH/PCFICH reference channel (FDD duplex mode) defines a required Pm-dsg of 1% at an SNR of -1.7 dB.

The LTE System Toolbox™ allows configuration of the downlink waveform generator (lteRMCDLTool) for PDCCH conformance tests. Working on a frame by frame basis when simulating at a particular SNR, a populated resource grid is generated and OFDM modulated to create a transmit waveform. The generated waveform is passed through a noisy Extended Typical Urban (ETU) channel. Channel estimation, equalization, demodulation and decoding are performed at receiver. The average miss-detection probability of the PDCCH/PCFICH is determined using the combination of the received CFI and received DCI message.

Simulation Setup

The example is executed for a simulation length of 1 frame to keep the simulation time low. SNR values include -1.7 dB specified for the R.15 RMC single port test. A large number of frames, NFrames, should be used to produce meaningful results. SNRIn can be an array of values or a scalar.

nFrames = 1;                      % Number of frames to simulate
snrIn = [-3.1 -2.4 -1.7 -1 -0.3]; % SNR points to simulate

PDCCH/PCFICH Configuration

The downlink waveform generator lteRMCDLTool generates the PDSCH, PDCCH, PCFICH and other physical channels and signals. Here we perform the conformance testing for the R.15 FDD RMC by configuring the PCFICH and PDCCH channels and generating the waveform. The parameters for the generator are specified via a structure rmc. The lteRMCDL function generates PDSCH RMC configurations including associated PDCCH/PCFICH and we can update the PDCCH/PCFICH parameters to generate the PDCCH/PCFICH RMCs. Here we start by using a PDSCH RMC that closely matches the PDSCH setup mentioned in TS 36.521-1 Section 8.4.1.1.4 [ 2 ] for the R.15 FDD RMC and then setting the DCI and PDCCH parameters. No data and control is transmitted on subframe 5. Therefore it is not decoded and does not count towards the missed detection calculation.

% Setup the PDSCH according to TS 36.521-1 Table A.3.5.1-2 for single port
% which corresponds to R.2 FDD PDSCH RMC
rmc = lteRMCDL('R.2','FDD');

% Setup DCI and PDCCH according to TS 36.101 Section 8.4.1.1 for R.15 FDD
% RMC which requires the CFI, DCI format, HICH group multiplier,
% aggregation level and power to be set as defined for the test
rmc.CFI = 2;                     % OFDM symbols for PDCCH
rmc.Ng = 'One';                  % HICH group multiplier
rmc.PDSCH.DCIFormat = 'Format1'; % Set the DCI Format
rmc.PDSCH.PDCCHFormat = 3;       % Set the aggregation level to be 8
rmc.PDSCH.PDCCHPower = 0;        % Relative power is 0dB for single port

% Setup the OCNG to fill all unused data and control region REs
rmc.OCNGPDSCHEnable = 'On';
rmc.OCNGPDSCHPower = 0;
rmc.OCNGPDCCHEnable = 'On';
rmc.OCNGPDCCHPower = 0;

Propagation Channel Model Configuration

cfg = struct;                    % Initialize channel config structure
cfg.Seed = 6;                    % Channel seed chosen at random
cfg.NRxAnts = 2;                 % 2 receive antennas
cfg.DelayProfile ='ETU';         % Delay profile
cfg.DopplerFreq = 70;            % Doppler frequency in Hz
cfg.MIMOCorrelation = 'Low';     % Multi-antenna correlation
cfg.NTerms = 16;                 % Oscillators used in fading model
cfg.ModelType = 'GMEDS';         % Rayleigh fading model type
cfg.InitPhase = 'Random';        % Random initial phases
cfg.NormalizePathGains = 'On';   % Normalize delay profile power
cfg.NormalizeTxAnts = 'On';      % Normalize for transmit antennas

Channel Estimator Configuration

The variable perfectChannelEstimator controls channel estimator behavior. Valid values are true or false. When set to true a perfect channel estimate is used otherwise an imperfect estimate is used, based on the values of received pilot signals.

% Channel estimator behavior
perfectChannelEstimator = false;

The imperfect channel estimator is configured with a structure cec. An ETU delay profile causes the channel to change quickly over time. Therefore only time averaging is performed over pilot estimates by setting the frequency window to 1 Resource Element (RE). A Doppler frequency of 70 Hz causes the channel fading to be slow enough to allow a large time averaging window. Pilots in the whole frame are included in the average by setting the time window to 31 REs.

% Configure channel estimator
cec.PilotAverage = 'UserDefined';   % Type of pilot symbol averaging
cec.FreqWindow = 1;                 % Frequency window size in REs
cec.TimeWindow = 31;                % Time window size in REs
cec.InterpType = 'Cubic';           % 2D interpolation type
cec.InterpWindow = 'Centered';      % Interpolation window type
cec.InterpWinSize = 3;              % Interpolation window size

Processing

For each SNR point the following operations are performed for each frame:

  • RMC Generation and OFDM Modulation: Generate the OFDM modulated waveform as defined for the PDCCH/PCFICH RMC.

  • Propagation Channel Model: The OFDM modulated waveform is transmitted through the propagation channel. The output of the channel rxWaveform has two columns, one per receive antenna.

  • Add Channel Noise: The channel noise is modeled by AWGN.

  • Receiver Synchronization and OFDM Demodulation: Determine the delay suffered during propagation. This is calculated by calling lteDLFrameOffset, which uses the primary and secondary synchronization signals. OFDM demodulation is performed after synchronization.

  • Channel Estimation: Provides an estimate of the channel response at each element of the resource grid for a transmit/receive antenna pair. This estimate is then used to remove the effect of the channel on the transmitted signal. The channel estimation also aims to reduce the channel noise experienced during transmission by averaging the reference signals (pilot symbols).

  • Miss-Detection Measurement: PCFICH and PDCCH data is analyzed on a subframe by subframe basis. A specific frame worth of data for all receive antennas is extracted from the array of received grids, the same defined frame worth of information is extracted from the estimate of channel response for all transmit and receive antenna pairs. These, along with the noise estimate, are input into the function ltePCFICHDecode, which decodes the CFI data. If the CFI is correctly detected, the PCFICH reception is successful and the next stage is to receive the DCI. Using the channel estimate and noise estimate, the PDCCH message is recovered using the ltePDCCHDecode function; blind search is then performed on this decoded bits to recover the DCI. If the DCI message is also correctly received, the transmission is successful and used to determine the detection of the system.

% dims is a three element vector [K L P]: where K = no of subcarriers,
% L = no of OFDM symbols and P = no of transmit antennas
dims = lteDLResourceGridSize(rmc);  % Determine resource grid dimensions
L = dims(2);                        % Number of OFDM symbols per subframe
P = dims(3);                        % Number of transmit antennas

% Initialize the variables used in the simulation and analysis
resultIndex = 1;                    % Initialize SNR index
initOffsets = 0;                    % Initialize frame offset

% Set the random number generator to default value
rng('default');

% Number of subframes carrying transport blocks with data and control
% within a frame. For RMC R.15 this should be 9, as out of the 10 subframes
% in a frame subframe 5 does not carry any data or control
nDataTBSPerFrame = sum(rmc.PDSCH.TrBlkSizes(:) ~= 0);

% Total Pmdsg vector
totalPmdsg = zeros(numel(snrIn),nDataTBSPerFrame*nFrames);

for snrdb = snrIn
    fprintf('\nSimulating at %gdB SNR for a total %d Frame(s)\n',...
    snrdb,nFrames);

    % Initialize result vector for each SNR point
    totalPmdsgSNR = zeros(nFrames,nDataTBSPerFrame);% Total block CRC

    % Initialize offset vector for each frame
    offsets = initOffsets;

    for FrameNo = 1:nFrames

        % Set the subframe number
        rmc.NSubframe = 0;

        % Generate the waveform
        [txWaveform,txGrid,info] = lteRMCDLTool(rmc,[1 0 0 1]);

        % Initialize result store for frame
        pmdsg = zeros(nDataTBSPerFrame,1);
        dataSubframeIndex = 1;

        % Set sampling rate of channel to that of OFDM modulation
        cfg.SamplingRate = info.SamplingRate;

        % Set channel offset to current frame (1 frame = 10ms)
        cfg.InitTime = (FrameNo-1)*(rmc.TotSubframes)/1000;

        % Pass data through the fading channel model.
        % An additional 25 samples are added to the end of the waveform.
        % These are to cover the range of delays expected from the channel
        % modeling (a combination of implementation delay and channel
        % delay spread).
        rxWaveform = lteFadingChannel(cfg,[txWaveform ; zeros(25,P)]);

        % Noise setup
        SNR = 10^((snrdb-rmc.PDSCH.PDCCHPower)/20);    % Linear SNR

        % Normalize noise power to take account of sampling rate, which is
        % a function of the IFFT size used in OFDM modulation, and the
        % number of antennas
        N0 = 1/(sqrt(2.0*rmc.CellRefP*double(info.Nfft))*SNR);

        % Create additive white Gaussian noise
        noise = N0*complex(randn(size(rxWaveform)), ...
                            randn(size(rxWaveform)));

        % Add AWGN to the received time domain waveform
        rxWaveform = rxWaveform + noise;

        % Perform receiver synchronization
        offset = lteDLFrameOffset(rmc,rxWaveform);

        % Determine if frame synchronization was successful
        if (offset > 25)
            offset = offsets(end);
        else
            offsets = [offsets offset]; %#ok
        end
        if (offset>0)
            rxWaveform = rxWaveform(1+offset:end,:);
        end

        % Perform OFDM demodulation on the received data to recreate the
        % resource grid
        rxGrid = lteOFDMDemodulate(rmc,rxWaveform);

        % Channel estimation
        if(perfectChannelEstimator)
            estChannelGrid = lteDLPerfectChannelEstimate(rmc, cfg, offset);  %#ok<UNRCH>
            n = lteOFDMDemodulate(rmc, noise(1+offset:end ,:));
            noiseest = var(n(:));
        else
            [estChannelGrid, noiseest] = lteDLChannelEstimate( ...
                rmc, cec, rxGrid);
        end

        % Process subframes 0 to 9 within received frame. Test for PCFICH
        % and if successful, test for PDCCH. If both PCFICH and PDCCH are
        % successfully received, the transmission is successful
        for sf = 0:rmc.TotSubframes-1

            % Increment subframe number for correct decoding of data
            rmc.NSubframe = mod(sf,10);

            % No data is transmitted in subframe 5, so skip this
            if(rmc.NSubframe ~= 5)

                % Extract one subframe for analyzing at a time from the
                % received grid
                rxSubframe = rxGrid(:,L*sf+1:L*(sf+1),:);

                % Extract the estimated channel response for the subframe
                % being analyzed
                chSubframe = estChannelGrid(:,L*sf+1:L*(sf+1),:,:);

                % Perform PCFICH decoding of received data using the estimate
                % of the channel
                pcfichIndices = ltePCFICHIndices(rmc);
                [rxPcfichSym,pcfichHestSym] = lteExtractResources(pcfichIndices,rxSubframe,chSubframe);
                rxPcfich = ltePCFICHDecode(rmc,rxPcfichSym,pcfichHestSym,noiseest);
                rxCFI = lteCFIDecode(rxPcfich);

                if rxCFI ~= rmc.CFI
                    rxFail = 1;
                else
                   % CFI decoded fine, now check if DCI can be decoded

                   % Extract and decode PDCCH bits
                   pdcchIndices = ltePDCCHIndices(rmc);
                   [rxPdcchSym,pdcchHestSym] = lteExtractResources(pdcchIndices,rxSubframe,chSubframe);
                   rxPdcchBits = ltePDCCHDecode(rmc,rxPdcchSym,pdcchHestSym,noiseest);

                   % PDCCH blind search, demask PDCCH candidate using RNTI
                   ueConfig.RNTI = rmc.PDSCH.RNTI;
                   ueConfig.ControlChannelType = 'PDCCH';
                   ueConfig.EnableCarrierIndication = 'Off';
                   ueConfig.SearchSpace = 'UESpecific';
                   ueConfig.EnableMultipleCSIRequest = 'Off';
                   ueConfig.EnableSRSRequest = 'Off';
                   ueConfig.NTxAnts = 1;
                   [rxDCI,rxDCIBits] = ltePDCCHSearch(rmc,ueConfig,rxPdcchBits);

                   if ~isempty(rxDCI) && isfield(rxDCI{1},'DCIFormat') && isequal(rxDCI{1}.DCIFormat,rmc.PDSCH.DCIFormat)
                       rxFail = 0;
                   else
                       rxFail = 1;
                   end

                end
                % Store the values and increment the subframe index
                pmdsg(dataSubframeIndex, :) = rxFail;
                dataSubframeIndex = dataSubframeIndex + 1;
            end
        end
        % Store resulting pmdsg results
        totalPmdsgSNR(FrameNo,:) = pmdsg(:);

    end
    % Record the missed detection for the total number of frames simulated
    % at a particular SNR
    totalPmdsg(resultIndex,:) = totalPmdsgSNR(:); % CRC
    resultIndex = resultIndex + 1;

end
Simulating at -3.1dB SNR for a total 1 Frame(s)

Simulating at -2.4dB SNR for a total 1 Frame(s)

Simulating at -1.7dB SNR for a total 1 Frame(s)

Simulating at -1dB SNR for a total 1 Frame(s)

Simulating at -0.3dB SNR for a total 1 Frame(s)

Plot Results

% Pm-dsg should be less than 1% at -1.7 dB SNR for single port
pmdsgVal = 100*mean(totalPmdsg,2);
figure
plot(snrIn,pmdsgVal,'-*')
title(['Pm-dsg for ', num2str(nFrames) ' frame(s)'] )
xlabel('SNR(dB)'); ylabel('Pm-dsg (%)');
grid on;
hold on;
plot(snrIn, ones(1,numel(snrIn)),'--rs');
legend('Simulation Result','Target 1% Pm-dsg','Location',...
       'NorthEast');
ylim([fix(min(pmdsgVal)-2) fix(max(pmdsgVal)+2)]);

Further Exploration

You can modify parts of this example to generate other PDCCH/PCFICH RMCs (as per TS 36.101 Section 8.4 [ 1 ] and 36.521-1 Section 8.4 [ 2 ]). For example, to generate the R.16 FDD RMC from TS 36.101 Section 8.4.2.2.1 [ 1 ] which is defined for transmit diversity performance with 2x2 transmit/receive antenna configuration, update the rmc structure as below:

rmc.CellRefP = 2;                   % Number of transmit antenna ports
rmc.CFI = 2;                        % OFDM symbols for PDCCH
rmc.Ng = 'One';                     % HICH group multiplier
rmc.PDSCH.DCIFormat = 'Format2';    % Set the DCI Format
rmc.PDSCH.PDCCHFormat = 2;          % Set the aggregation level to be 4
rmc.PDSCH.PDCCHPower = -3;          % Relative power is -3dB
rmc.PDSCH.TxScheme = 'TxDiversity'; % PDSCH transmission scheme
rmc.PDSCH.NLayers = 2;              % Number of transmission layers
rmc.OCNGPDSCH.TxScheme = 'TxDiversity'; % PDSCH OCNG transmission scheme
rmc.OCNGPDCCHPower = -3;            % Relative power is -3dB for PDCCH OCNG

Selected Bibliography

  1. 3GPP TS 36.101 "User Equipment (UE) radio transmission and reception"

  2. 3GPP TS 36.521-1 "User Equipment (UE) conformance specification; Radio transmission and reception; Part 1: Conformance testing"

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