MATLAB Examples

PUSCH HARQ-ACK Detection Conformance Test

This example shows how to measure the false detection probability and missed detection probability of Hybrid Automatic Repeat Request ACK (HARQ-ACK) multiplexed on the Physical Uplink Shared Channel (PUSCH) using the LTE System Toolbox™ under conformance test conditions as defined in TS36.104, Section 8.2.4.1 [ 1 ].

Contents

Introduction

Uplink Control Information (UCI) can be carried on the Physical Uplink Shared Channel (PUSCH) and may contain Hybrid Automatic Repeat Request ACK (HARQ-ACK) information. This example uses the LTE System Toolbox to perform the "HARQ-ACK Multiplexed on PUSCH" conformance test specified in TS36.104, Section 8.2.4.1 [ 1 ].

Two performance requirements are defined for HARQ-ACK multiplexed on the PUSCH:

  • ACK false detection probability is the probability that ACK is detected when data is only sent on symbols where HARQ-ACK information can be allocated.
  • The ACK missed detection probability is the conditional probability of not detecting an ACK when it was sent on PUSCH resources.

The conditions specified for this test are two receive antennas, normal cyclic prefix, ETU70 channel, using Fixed Reference Channel (FRC) A4-3. The target defined in TS36.104, Section 8.2.4.1 [ 1 ] for 1.4 MHz bandwidth (6RB) is an false detection probability and missed detection probability of 1% at 13.8 dB.

The test is run on a subframe by subframe basis at each SNR test point. For each subframe an uplink waveform is generated with and without ACK information, passed through a fading channel and the HARQ-ACK decoded. False detections and missed detections are recorded and the probability of error displayed for the range of SNRs tested.

Simulation Configuration

The example is executed for a simulation length of 1 frame at a number of SNRs including the required 13.8 dB at which the test requirements for false and missed detection rates (1% in each case) are to be achieved as per TS36.104, Table 8.2.4.1-1 [ 1 ]. A large number of numSubframes should be used to produce meaningful throughput results. SNRdB can be an array of values or a scalar.

numSubframes = 10;  % Number of frames to simulate at each SNR
SNRdB = [4.8 7.8 10.8 13.8 16.8]; % SNR points to simulate

UE Configuration

To configure the transmitter, a few desired parameter fields are set in the structure frc, which is the passed to lteRMCUL which will set all the other required parameter fields. At this point, the structure frc represents the complete configuration of the required transmission.

frc.TotSubframes = 1;   % Total number of subframes to generate
frc.NCellID = 10;       % Cell identity
frc.RC = 'A4-3';        % FRC number

% Populate FRC configuration structure with default values for A4-3
frc = lteRMCUL(frc);

Propagation Channel Model Configuration

The propagation channel is configured via the structure chcfg, with settings per the test requirements. Note that the sampling rate of the propagation is determined from the sampling rate of the transmitted waveform, which can be established using lteSCFDMAInfo.

chcfg.NRxAnts = 2;               % Number of receive antennas
chcfg.DelayProfile = 'ETU';      % Delay profile
chcfg.DopplerFreq = 70;          % Doppler frequency
chcfg.MIMOCorrelation = 'Low';   % MIMO correlation
chcfg.Seed = 91;                 % Channel seed
chcfg.NTerms = 16;               % Oscillators used in fading model
chcfg.ModelType = 'GMEDS';       % Rayleigh fading model type
chcfg.InitPhase = 'Random';      % Random initial phases
chcfg.NormalizePathGains = 'On'; % Normalize delay profile power
chcfg.NormalizeTxAnts = 'On';    % Normalize for transmit antennas

% Set channel model sampling rate
info = lteSCFDMAInfo(frc);
chcfg.SamplingRate = info.SamplingRate;

Channel Estimator Configuration

The channel estimator is configured using the structure cec. An ETU delay profile causes the channel to change quickly over time. Therefore a small pilot averaging frequency window of 9 Resource Elements (REs) is used. The Demodulation Reference Signal (DRS) is contained in only one symbol per slot, therefore a time averaging window of 1 RE is used. This will not include any pilots from an adjacent slot when averaging.

cec.PilotAverage = 'UserDefined'; % Type of pilot averaging
cec.FreqWindow = 9;               % Frequency averaging windows in REs
cec.TimeWindow = 1;               % Time averaging windows in REs
cec.InterpType = 'cubic';         % Interpolation type
cec.Reference = 'Antennas';       % Reference for channel estimation

Loop for SNR Values

A loop is used to run the simulation for a set of SNR points, given by the vector SNRdB.

The noise added before SC-FDMA demodulation will be amplified by the IFFT. The amplification is the square root of the size of the IFFT ($N_{\mathrm{FFT}}$), thus to ensure the power of the noise added is normalized after demodulation, to achieve the desired SNR the desired noise power is divided by $N_{\mathrm{FFT}}$. In addition, as real and imaginary parts of the noise are created separately before being combined into complex Additive White Gaussian Noise (AWGN), the noise amplitude must be scaled by $1/\sqrt{2}$ so the generated noise power is 1.

At each SNR test point the probability of false and missed detection is calculated. These probabilities are calculated on subframe by subframe basis using the following steps:

  • Create Transmit Waveform: The Uplink Reference Measurement Channel (RMC) Tool lteRMCULTool is used to generate an uplink waveform containing random transport data and, every odd subframe an HARQ-ACK bit.
  • Noisy Channel Modeling: The waveform is passed through a fading channel and AWGN added.
  • Perform Synchronization and SC-FDMA Demodulation: The received symbols are synchronized to account for a combination of implementation delay and channel delay spread. The symbols are then SC-FDMA demodulated.
  • Perform Channel and Noise Power Spectral Density Estimation and Equalization: The channel and noise power spectral density are estimated and the received PUSCH symbols equalized.
  • Decode the PUSCH: The UpLink Shared Channel (UL-SCH) coding is determined and used to decode the PUSCH to recover the interleaved UL-SCH.
  • Recover UCI ACK Bit: Deinterleave the PUSCH and decode the HARQ-ACK information to recover the UCI HARQ-ACK bit and detect HARQ-ACK false or missed detection cased on whether an HARQ-ACK bit was encoded in the transmitted subframe or not.
% Initialize variables used in the simulation and analysis
pFalse = zeros(size(SNRdB));  % Probability of false detection at each SNR
pMissed = zeros(size(SNRdB)); % Probability of missed detection at each SNR

for nSNR = 1:length(SNRdB)

    % Initialize the random number generator stream
    rng('default');

    % Extract SNR to test
    SNR = 10^(SNRdB(nSNR)/20);

    % Scale noise to ensure the desired SNR after SC-FDMA demodulation
    N = 1/(SNR*sqrt(double(info.Nfft)))/sqrt(2.0);

    offsetUsed = 0;
    falseCount = 0;
    falseTests = 0;
    missCount = 0;
    missTests = 0;

    for subframeNo = 0:(numSubframes-1)

        % Updating subframe number
        frc.NSubframe = mod(subframeNo, 10);

        % Transmit ACK on every odd subframe
        if (mod(subframeNo, 2)==0)
            ACK = [];
            falseTests = falseTests + 1;
        else
            ACK = 1;
            missTests = missTests + 1;
        end

        % Create random data to transmit
        trblklen = frc.PUSCH.TrBlkSizes(frc.NSubframe+1);
        trblk = randi([0 1], trblklen, 1);

        % Transmit a waveform with an additional 25 samples to cover the
        % range of delays expected from the channel modeling (a
        % combination of implementation delay and channel delay spread)
        txWaveform = [lteRMCULTool(frc, trblk, [], [], ACK); zeros(25, 1)];

        % Pass waveform through fading channel model
        chcfg.InitTime = subframeNo/1000;
        rxWaveform = lteFadingChannel(chcfg, txWaveform);

        % Add noise
        noise = N*complex(randn(size(rxWaveform)), ...
            randn(size(rxWaveform)));
        rxWaveform = rxWaveform + noise;

        % Synchronization
        offset = lteULFrameOffset(frc, frc.PUSCH,rxWaveform);
        if (offset < 25)
            offsetUsed = offset;
        end

        % SC-FDMA demodulation
        rxSubframe = lteSCFDMADemodulate(frc, ...
            rxWaveform(1+offsetUsed:end, :));

        % Channel Estimation
        [estChannelGrid, noiseEst] = lteULChannelEstimate( ...
            frc, frc.PUSCH, cec, rxSubframe);

        % PUSCH indices for given subframe
        puschIndices = ltePUSCHIndices(frc,frc.PUSCH);

        % Minimum Mean Squared Error (MMSE) equalization
        rxSymbols = lteEqualizeMMSE(rxSubframe(puschIndices), ...
                        estChannelGrid(puschIndices), noiseEst);

        % Obtain UL-SCH coding information for current transport block and
        % ACK, and concatenate this information with the PUSCH / UL-SCH
        % configuration
        frc.PUSCH = lteULSCHInfo(frc, frc.PUSCH, trblklen, ...
            0, 0, 1, 'chsconcat');

        % Perform deprecoding, demodulation and descrambling on the
        % received data
        rxEncodedBits = ltePUSCHDecode(frc, frc.PUSCH, rxSymbols);

        % UL-SCH channel deinterleaving
        [deinterleavedData, ccqi, cri, cack] = ...
            lteULSCHDeinterleave(frc, frc.PUSCH, rxEncodedBits);

        % HARQ-ACK decoding
        rxACK = lteACKDecode(frc.PUSCH, cack);

        % Detect false or missed HARQ-ACK
        if (isempty(ACK) && ~isempty(rxACK))
            falseCount = falseCount + 1;
        end
        if (~isempty(ACK) && ~isequal(ACK,rxACK))
            missCount = missCount + 1;
        end

    end

    % Calculate false or missed HARQ-ACK probability
    pFalse(nSNR) = falseCount/falseTests;
    pMissed(nSNR) = missCount/missTests;

end

Analysis

Plot the simulated results against the target performance as stipulated in the standard using hHARQACKResults.m.

hHARQACKResults(SNRdB, pFalse, pMissed);

Appendix

This example uses the helper functions:

Selected Bibliography

  1. 3GPP TS 36.104 "Base Station (BS) radio transmission and reception"