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PDSCH Error Vector Magnitude (EVM) Measurement

This example measures the EVM within a downlink Reference Measurement Channel (RMC) signal, according to the EVM measurement requirements specified in TS 36.104, Annex E [ 1 ].

Introduction

This example creates an RMC signal and applies some noise to the transmission to model transmitter EVM. Frequency offset and IQ offset are also applied. The impaired signal is then processed according to the EVM measurement requirements specified in TS36.104, Annex E [ 1 ]. This example measures peak and RMS EVM averaged across the 2 frames of the input signal.

The average EVM is measured at two locations in time (low and high), where the low and high locations correspond to the alignment of the FFT window within the start and end of the cyclic prefix. LTE System Toolbox™ requires the low and high locations to be specified as a fraction of the cyclic prefix length.

Note that for multi-antenna RMCs, the EVM measurement assumes each receive signal antenna is directly connected to each transmit signal antenna, as shown in TS36.141 Annex I.1.1 [ 2 ]. In accordance with the EVM measurement requirements defined in TS 36.104 Annex E [ 1 ], the PDSCH decoding uses only Zero Forcing equalization. For an illustration of PDSCH reception that includes complete MIMO decoding, see the Cell Search, MIB and SIB1 Recovery example.

Transmitter

Setup the transmitter as per TS36.101 RMC [ 3 ].

% eNodeB Configuration
rng('default');         % Set the default random number generator
rmc = lteRMCDL('R.5');  % Configure RMC
rmc.PDSCH.RVSeq = 0;    % Redundancy version indicator
rmc.TotSubframes = 20;  % Total number of subframes to generate

% Create eNodeB transmission with random PDSCH data
txWaveform = lteRMCDLTool(rmc,randi([0 1],rmc.PDSCH.TrBlkSizes(1),1));

Impairment Modeling

Model the transmitter EVM and add frequency and IQ offsets.

% Model EVM with additive noise
ofdmInfo   = lteOFDMInfo(rmc);
txEVMpc    = 1.2;          % Transmit EVM in per cent
evmModel   = txEVMpc/(100*sqrt(double(ofdmInfo.Nfft)))* ...
    complex(randn(size(txWaveform)),randn(size(txWaveform)))/sqrt(2);
rxWaveform = txWaveform+evmModel;

% Add frequency offset impairment to received waveform
foffset    = 33.0;         % Frequency offset in Hertz
t = (0:length(rxWaveform)-1).'/ofdmInfo.SamplingRate;
rxWaveform = rxWaveform.*repmat(exp(1i*2*pi*foffset*t),1,rmc.CellRefP);

% Add IQ offset
iqoffset   = complex(0.01,-0.005);
rxWaveform = rxWaveform+iqoffset;

Receiver

The receiver synchronizes with the received signal and computes and displays the measured EVM.

% Apply frequency estimation and correction for the purposes of performing
% timing synchronization
foffset_est = lteFrequencyOffset(rmc,rxWaveform);
rxWaveformFreqCorrected = lteFrequencyCorrect(rmc,rxWaveform,foffset_est);

% Synchronize to received waveform
offset = lteDLFrameOffset(rmc,rxWaveformFreqCorrected,'TestEVM');
rxWaveform = rxWaveform(1+offset:end,:);

% Use 'TestEVM' pilot averaging
cec.PilotAverage = 'TestEVM';

Perform Measurements

The PDSCH EVM is calculated by calling hPDSCHEVM.

The average EVM for a downlink RMC is displayed. First the results for low and high edge EVM are calculated for each subframe within a frame and their averages are displayed at the command window. The max of these averages is the EVM per frame. The final EVM for the downlink RMC is the average of the EVM across all frames. A number of plots are also produced:

  • EVM versus OFDM symbol

  • EVM versus subcarrier

  • EVM versus resource block

  • EVM versus OFDM symbol and subcarrier (i.e. the EVM resource grid)

Note that the EVM measurement displayed at the command window is only calculated across allocated PDSCH resource blocks, in accordance with the LTE standard. The EVM plots are shown across all resource blocks (allocated or unallocated), allowing the quality of the signal to be measured more generally. In unallocated resource blocks, the EVM is calculated assuming that the received resource elements have an expected value of zero.

The EVM of each E-UTRA carrier for QPSK, 16QAM and 64QAM modulation schemes on PDSCH should be better than the required EVM of 17.5%, 12.5% and 8% respectively as per TS36.104 Table 6.5.2-1 [ 1 ].

% Compute and display EVM measurements
[evmmeas, plots] = hPDSCHEVM(rmc,cec,rxWaveform);
Low edge EVM, subframe 0: 1.289%
High edge EVM, subframe 0: 1.290%
Low edge EVM, subframe 1: 1.397%
High edge EVM, subframe 1: 1.392%
Low edge EVM, subframe 2: 1.330%
High edge EVM, subframe 2: 1.324%
Low edge EVM, subframe 3: 1.233%
High edge EVM, subframe 3: 1.237%
Low edge EVM, subframe 4: 1.234%
High edge EVM, subframe 4: 1.228%
Low edge EVM, subframe 6: 1.292%
High edge EVM, subframe 6: 1.290%
Low edge EVM, subframe 7: 1.348%
High edge EVM, subframe 7: 1.342%
Low edge EVM, subframe 8: 1.338%
High edge EVM, subframe 8: 1.335%
Low edge EVM, subframe 9: 1.332%
High edge EVM, subframe 9: 1.320%
Averaged low edge EVM, frame 0: 1.312%
Averaged high edge EVM, frame 0: 1.308%
Averaged EVM frame 0: 1.312%
Low edge EVM, subframe 0: 1.239%
High edge EVM, subframe 0: 1.241%
Low edge EVM, subframe 1: 1.229%
High edge EVM, subframe 1: 1.228%
Low edge EVM, subframe 2: 1.218%
High edge EVM, subframe 2: 1.220%
Low edge EVM, subframe 3: 1.216%
High edge EVM, subframe 3: 1.220%
Low edge EVM, subframe 4: 1.240%
High edge EVM, subframe 4: 1.240%
Low edge EVM, subframe 6: 1.218%
High edge EVM, subframe 6: 1.207%
Low edge EVM, subframe 7: 1.244%
High edge EVM, subframe 7: 1.243%
Low edge EVM, subframe 8: 1.252%
High edge EVM, subframe 8: 1.247%
Low edge EVM, subframe 9: 1.253%
High edge EVM, subframe 9: 1.250%
Averaged low edge EVM, frame 1: 1.234%
Averaged high edge EVM, frame 1: 1.233%
Averaged EVM frame 1: 1.234%
Averaged overall EVM: 1.274%

Appendix

This example uses the following helper functions:

Selected Bibliography

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

  2. 3GPP TS 36.141 "Base Station (BS) conformance testing"

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

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