Reporting of Channel Quality Indicator (CQI) Conformance Test

Open Script

This example demonstrates how to measure the Channel Quality Indicator (CQI) reporting performance using the LTE Toolbox™ under conformance test conditions as defined in TS36.101 Section 9.3.2.1.1.

Introduction

This example highlights the use of the lteCQISelect function which provides estimation of the CQI. The performance of the CQI estimation is also tested. This example provides a testbench which shows that the LTE Toolbox can satisfy the CQI reporting performance test defined in TS36.101 Section 9.3.2.1.1 [ 1 ]. The performance requirements of the test are as follows:

a CQI index not in the set {median CQI - 1, median CQI + 1} shall be reported at least 20% of the time;

the ratio of the throughput obtained when transmitting the transport format indicated by each reported wideband CQI index and that obtained when transmitting a fixed transport format configured according to the wideband CQI median shall be >= 1.05;

when transmitting the transport format indicated by each reported wideband CQI index, the average BLER for the indicated transport formats shall be greater than or equal to 0.02.

This example tests that these requirements are met.

Simulation Configuration

The example is executed for a simulation length of 10 frames at an SNR of 6.0dB. A large number of NFrames should be used to produce meaningful results.

NFrames = 10;
SNRdB = 6.0;

eNodeB Configuration

eNodeB settings are specified in a structure enb. This includes a substructure PDSCH to configure the PDSCH according to the conformance test requirements: HARQ is disabled by setting the RV sequence to zero and the value of CSIMode is configured according to TS36.101 Table 9.3.2.1.1-1 [ 1 ].

enb = struct('RC','R.3');         % Set up parameters of RMC R.3
enb = lteRMCDL(enb);
enb.CFI = 3;                      % Reconfigure Control Format Indicator
enb.OCNGPDSCHEnable = 'On';       % Enable OCNG for unallocated PDSCH REs
enb.TotSubframes = 1;             % Reconfigure for a single subframe
enb.PDSCH.RVSeq = 0;              % Disable HARQ
enb.PDSCH.CSIMode = 'PUCCH 1-0';  % Configure the CSI reporting mode
enb.PDSCH.CSI = 'On';             % CSI scaling of soft bits

Propagation Channel Model Configuration

The structure, channel, contains the channel model configuration parameters.

channel.Seed = 10;                  % Channel seed
channel.NRxAnts = 2;                % 2 receive antennas
channel.DelayProfile = 'EPA';       % Delay profile
channel.DopplerFreq = 5.0;          % Doppler frequency
channel.MIMOCorrelation = 'High';   % Multi-antenna correlation
channel.ModelType = 'GMEDS';        % Rayleigh fading model type
channel.NormalizeTxAnts = 'On';     % Normalize for transmit antennas
channel.NormalizePathGains = 'On';  % Normalize delay profile power
channel.InitPhase = 'Random';       % Random initial phases
channel.NTerms = 16;                % Oscillators used in fading model

 % Set channel model sampling rate
ofdmInfo = lteOFDMInfo(enb);
channel.SamplingRate = ofdmInfo.SamplingRate;

Channel Estimator Configuration

The channel estimator is configured with a structure cec. The variable perfectChanEstimator controls channel estimator behavior. Valid values are true or false. When set to true a perfect channel estimator is used otherwise an imperfect estimate of the channel is used, based on the values of received pilot signals. In this example, we enable the perfect channel estimator.

% Configure channel estimator
cec.PilotAverage = 'UserDefined';   % Type of pilot symbol averaging
cec.FreqWindow = 9;                 % Frequency window size in REs
cec.TimeWindow = 9;                 % Time window size in REs
cec.InterpType = 'Cubic';           % 2D interpolation type
cec.InterpWindow = 'Centered';      % Interpolation window type
cec.InterpWinSize = 1;              % Interpolation window size

% Channel estimator behavior
perfectChanEstimator = true;

Set CQI Delay

Set the CQI delay in subframes. This is the delay in a CQI being passed from UE to eNodeB as defined in TS36.101 Table 9.3.2.1.1-1 [ 1 ]. Note that the feedback of the CQI is assumed to be perfect, with the values being fed back in a buffer rather than being fed back in an uplink transmission.

cqiDelay = 8; % subframes

System Processing

The main processing is split into two phases, configured via the cqiConfig loop variable. These phases implement the two measurements required in the performance test defined in TS36.101 Section 9.3.2.1.1 [ 1 ]:

UE reported CQI. The first phase (cqiConfig=1) performs PDSCH transmission and reception where the Modulation and Coding Scheme (MCS) is selected on the basis of the UE reported CQI, with the reported CQI being updated every 2 subframes and fed back with a delay of 8 subframes. The final throughput, BLER and median CQI are recorded, and the BLER (measuredBLER) and deviation from the median CQI (measuredAlpha, in percent) are checked against the specified performance requirements.

Median CQI. In the second phase (cqiConfig=2), PDSCH transmission and reception are performed using the median CQI (medianCQI) determined in the first phase. The final throughput is recorded and the throughput ratio (measuredGamma) between using the UE reported CQI phase and the median CQI phase is reported and checked against the specified performance requirement.

The processing is performed on a subframe by subframe basis using the following steps:

Select CQI. For UE reported CQI, the current CQI is read from the oldest value in the CQI buffer cqiBuffer; for median CQI, the CQI is always set to medianCQI (this is achieved by filling the CQI buffer with the median CQI value and the buffer will not be updated).

Select MCS according to CQI. The Modulation and Coding Scheme (MCS) index corresponding to the CQI is selected by means of a lookup table defined by TS36.101 Table A.4-1 CSI RMC RC.1 FDD (MCS.1).

Determine Transport Block Size and modulation order. The MCS index is passed to the lteMCS function which calculates the corresponding Transport Block Size (TBS) index and modulation order; the lteTBS function is then used to calculate the TBS from the TBS index and the number of resource blocks allocated to the PDSCH.

Transmit and receive waveform. Transport block data is generated and passed to lteRMCDLTool to create a transmitted downlink waveform. This waveform is then passed through a fading channel and AWGN noise is added. The received signal is synchronized and OFDM demodulated and channel estimation is performed.

Measure PDSCH throughput. The PDSCH and DL-SCH are decoded and the CRC pass/fail is recorded to determine the data throughput.

Update CQI. If a CQI update is scheduled in this subframe, use the channel estimate to update the CQI with the lteCQISelect function. The updated CQI value is recorded in the CQI buffer. If a CQI update is not scheduled in this subframe, the previous CQI value is reused.

% Initialize variables used for results recording
CQIReport = [];     % reported CQI values
SINRReport = [];    % corresponding SINR values
xaxis = [];         % corresponding subframe numbers

% For each CQI configuration (UE reported and median):
for cqiConfig = 1:2

    if (cqiConfig==1)
        cqiConfigStr = 'UE reported';
    else
        cqiConfigStr = 'median';
    end
    fprintf('\nSimulating with %s CQI at %gdB SNR for %d Frame(s)\n', ...
        cqiConfigStr,SNRdB,NFrames);

    % Initialize CQI values: for UE reported, set to all ones; for median,
    % set to the median of the CQI values for the UE reported run
    if (cqiConfig==1)
        cqiBuffer = ones(1,cqiDelay);
    else
        cqiBuffer = ones(1,cqiDelay)*medianCQI;
    end

    % Initialize variables
    rng('default');     % Default random number generator seed
    totalCRC = [];      % CRC values, used for throughput calculation
    totalTBS = [];      % TBS values, used for throughput calculation
    offsets = 0;        % Initialize frame offset value

    % For each subframe:
    for subframeNo = 0:(NFrames*10-1)

        % Update subframe number
        enb.NSubframe = mod(subframeNo,10);

        % Select CQI, reading the oldest value from the CQI buffer
        cqiPtr = mod(subframeNo,cqiDelay);
        CQI = cqiBuffer(cqiPtr+1);

        % Select MCS according to CQI using TS36.101 Table A.4-1 CSI RMC
        % RC.1 FDD (MCS.1), which defines the relationship between CQI
        % indices and MCS indices
        IMCSTable = [-1 0 0 2 4 6 8 11 13 16 18 21 23 25 27 27];
        IMCS = IMCSTable(CQI+1);

        % Determine TBS and modulation order
        [ITBS,modulation] = lteMCS(IMCS);
        enb.PDSCH.Modulation = {modulation};
        if (mod(enb.NSubframe,5)==0)
            TBS = 0;
        else
            TBS = double(lteTBS(size(enb.PDSCH.PRBSet,1),ITBS));
        end
        enb.PDSCH.TrBlkSizes(enb.NSubframe+1) = TBS;

        % Determine if a CQI update is required in this subframe, according
        % to reporting periodicity N_pd = 2ms and configuration index
        % cqi-pmi-ConfigurationIndex = 1 from TS36.101 Table 9.3.2.1.1-1
        cqiPeriod = 2; % periodicity N_pd
        cqiOffset = 1; % offset deriving from cqi-pmi-ConfigurationIndex
        cqiUpdate = (mod(subframeNo,cqiPeriod)==cqiOffset);

        % Establish if this subframe actually needs executed for PDSCH
        % reception, CQI estimation or initial timing offset estimation:
        if((TBS~=0 && subframeNo>=(cqiDelay+cqiOffset)) || ...
                (cqiConfig==1 && cqiUpdate) || subframeNo==0)

            % Generate random bits for the subframe
            data = randi([0 1],TBS,1);

            % Create OFDM resource grid containing RMC transmission and
            % perform OFDM modulation.
            txWaveform = lteRMCDLTool(enb,data);

            % The initialization time for channel modeling is set each
            % subframe to simulate a continuously varying channel
            channel.InitTime = subframeNo/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(channel, ...
                            [txWaveform ; zeros(25,size(txWaveform,2))]);

            % Calculate noise gain including compensation for downlink
            % power allocation
            SNR = 10^((SNRdB-enb.PDSCH.Rho)/20);

            % 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*enb.CellRefP*double(ofdmInfo.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 synchronization
            % An offset within the range of delays expected from the
            % channel modeling (a combination of implementation delay and
            % channel delay spread) indicates success
            if (mod(subframeNo,10)==0)
                offset = lteDLFrameOffset(enb,rxWaveform);
                if (offset > 25)
                    offset = offsets(end);
                end
                offsets = [offsets offset]; %#ok<AGROW>
            end
            rxWaveform = rxWaveform(1+offset:end,:);

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

            % Channel estimation
            if (perfectChanEstimator)
                chEstGrid = ...
                   lteDLPerfectChannelEstimate(enb,channel,offset);
                n = lteOFDMDemodulate(enb,noise(1+offset:end,:));
                noiseEst = var(reshape(n,numel(n),1));
            else
                [chEstGrid,noiseEst] = ...
                    lteDLChannelEstimate(enb,enb.PDSCH, ...
                        cec,rxGrid); %#ok<UNRCH>
            end

            % If this subframe requires PDSCH reception:
            if(TBS~=0 && subframeNo>=(cqiDelay+cqiOffset))

                % Decode the PDSCH
                ind = ltePDSCHIndices(enb,enb.PDSCH,enb.PDSCH.PRBSet);
                pdschRx = lteExtractResources(ind,rxGrid) * ...
                        (10^(-enb.PDSCH.Rho/20));
                pdschChEst = lteExtractResources(ind,chEstGrid);
                [rxBits,rxSymbols] = ltePDSCHDecode(enb,enb.PDSCH, ...
                                        pdschRx,pdschChEst,noiseEst);

                % Decode the DL-SCH
                [decbits,crc] = lteDLSCHDecode(enb,enb.PDSCH,TBS,rxBits);

                % Record the CRC and TBS values for final throughput
                % calculation
                totalCRC = [totalCRC crc]; %#ok<AGROW>
                totalTBS = [totalTBS TBS]; %#ok<AGROW>

            end

            % Update CQI:
            if (cqiConfig==1 && cqiUpdate)

                % Perform CQI selection
                [thisCQI,thisSINR] = ...
                    lteCQISelect(enb,enb.PDSCH,chEstGrid,noiseEst);

                % Feed the CQI value back to UE (in a buffer)
                cqiBuffer(cqiPtr+1) = thisCQI;

                % Record values for plotting
                CQIReport = [CQIReport thisCQI];    %#ok<AGROW>
                SINRReport = [SINRReport thisSINR]; %#ok<AGROW>
                xaxis = [xaxis subframeNo];         %#ok<AGROW>

            end

        end

        % For subframes where CQI was not updated, re-use the previous
        % value in the buffer
        if (cqiConfig==1 && ~cqiUpdate)
            cqiBuffer(cqiPtr+1) = cqiBuffer(mod(cqiPtr-1,cqiDelay)+1);
        end

    end

    % Display results for the current CQI configuration
    fprintf('\nResults with %s CQI:\n',cqiConfigStr);
    tputTotal = sum(totalTBS);
    if (cqiConfig==1)

        % Compute and display throughput
        tputUEReported = sum(totalTBS.*(1-totalCRC));
        fprintf('Throughput: %d bits (%0.2f%%)\n', ...
            tputUEReported,tputUEReported/tputTotal*100);

        % Compute and display BLER
        measuredBLER = mean(totalCRC);
        fprintf('BLER: %0.3f (requirement is >= 0.02)\n',measuredBLER);

        % Compute and display median CQI
        medianCQI = ceil(median(CQIReport));
        fprintf('Median CQI: %d\n',medianCQI');

        % Compute and display proportion of CQI values
        % outside +/- 1 of the median
        measuredAlpha = (sum(CQIReport<(medianCQI-1)) + ...
             sum(CQIReport>(medianCQI+1)))/length(CQIReport)*100;
        fprintf(['Percentage of CQI indices outside +/- 1 of median:' ...
            ' %0.2f%% (requirement is >= 20%%)\n'],measuredAlpha);

    else

        % Compute and display throughput
        tputMedian = sum(totalTBS.*(1-totalCRC));
        fprintf('Throughput: %d bits (%0.2f%%)\n', ...
            tputMedian,tputMedian/tputTotal*100);

        % Compute and display throughput ratio
        measuredGamma = tputUEReported/tputMedian;
        fprintf(['Throughput ratio (gamma): %0.3f' ...
            ' (requirement is >= 1.05)'],measuredGamma);

    end

end

Simulating with UE reported CQI at 6dB SNR for 10 Frame(s)

Results with UE reported CQI:
Throughput: 980384 bits (77.98%)
BLER: 0.219 (requirement is >= 0.02)
Median CQI: 10
Percentage of CQI indices outside +/- 1 of median: 30.00% (requirement is >= 20%)

Simulating with median CQI at 6dB SNR for 10 Frame(s)

Results with median CQI:
Throughput: 722304 bits (60.27%)
Throughput ratio (gamma): 1.357 (requirement is >= 1.05)

Plot Results

A figure with two subplots is produced. The first subplot shows the estimated SINR for each subframe; the second subplot shows the reported CQI for each subframe. This illustrates how the SINR and corresponding reported CQI vary over time due to the fading channel.

figure;
subplot(2,1,1);
plot(xaxis,SINRReport,'rx-');
xlabel('Subframe number');
ylabel('Estimated SINR (dB)');
title('Estimated SINR versus subframe number');
hold on;
subplot(2,1,2);
plot(xaxis,CQIReport,'bo-');
xlabel('Subframe number');
ylabel('Selected CQI Index');
title('Selected CQI Index versus subframe number');

Selected Bibliography

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