5G NR Downlink ACLR Measurement

This example shows how to measure the adjacent channel leakage power ratio (ACLR) for 5G NR test models (NR-TMs) in frequency range 1 (FR1) and FR2 using 5G Toolbox™.

Introduction

The ACLR is the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency. This example performs ACLR measurements for a NR downlink waveform, as defined in TS 38.104 Section 6.6.3.

Generate NR-TM Waveform

Use the MATLAB class hNRReferenceWaveformGenerator to generate 5G NR-TMs for FR1 and FR2. You can generate the NR-TM waveforms by specifying these parameters:

  • NR-TM name

  • Channel bandwidth

  • Subcarrier spacing

  • Duplexing mode

For more information, see the 5G NR-TM and FRC Waveform Generation example.

% Select the NR-TM waveform parameters
nrtm = "NR-FR1-TM1.1";  % NR-TM name and properties
bw   = "20MHz";  % Channel bandwidth
scs  = "15kHz";  % Subcarrier spacing
dm   = "FDD";  % Duplexing mode

% Create generator object for the above NR-TM
tmwavegen = hNRReferenceWaveformGenerator(nrtm,bw,scs,dm);

% Generate waveform
[tmwaveform,tmwaveinfo] = generateWaveform(tmwavegen);
samplingrate = tmwaveinfo.Info.SamplingRate;   % Waveform sampling rate (Hz)

% Visualize the associated PRB and subcarrier resource grids
displayResourceGrid(tmwavegen);

Calculate ACLR Parameters

The helper function hACLRParametersNR.m calculates the parameters required for ACLR measurement.

The function determines the required oversampling. If the input waveform sampling rate (samplingrate) is not sufficient to span the entire bandwidth (aclr.BandwidthACLR) of the adjacent channels (allowing for a maximum of 85% bandwidth occupancy), an upsampled version of the waveform is used for ACLR calculations. The upsampling factor aclr.OSR.

aclr = hACLRParametersNR(tmwavegen.Config);

Filter Waveform to Improve ACLR

The generated waveform has no filtering, so there are significant out of band spectral emissions owing to the implicit rectangular pulse shaping in the OFDM modulation (each OFDM subcarrier has a sinc shape in the frequency domain). In order to achieve a good ACLR performance, filtering must be applied to the waveform. A filter is designed with a transition band that starts at the edge of the occupied transmission bandwidth (aclr.BandwidthConfig) and stops at the edge of the overall channel bandwidth (aclr.Bandwidth). This filter involves no rate change, it just shapes the spectrum within the original bandwidth of the waveform. The filter is first designed, then applied to the waveform.

% Design filter
lpFilt = designfilt('lowpassfir',...
    'PassbandFrequency', aclr.BandwidthConfig/2,...
    'StopbandFrequency', aclr.Bandwidth/2,...
    'PassbandRipple', 0.1,...
    'StopbandAttenuation', 80,...
    'SampleRate', samplingrate);

% Apply filter
filtWaveform = filter(lpFilt,tmwaveform);

Calculate NR ACLR

The hACLRMeasurementNR.m helper function measures the NR ACLR using a square window on adjacent channels. A DFT of the measurement signal is taken and the energy of the appropriate bins used to calculate the adjacent channel powers.

% Apply required oversampling
resampled = resample(filtWaveform,aclr.OSR,1);

% Calculate NR ACLR
aclr = hACLRMeasurementNR(aclr,resampled);

Display Results

The ACLR measurement results are returned in a structure with these fields:

  • Bandwidth: The channel bandwidth associated with tmwaveform, in Hertz. This is the overall bandwidth of the assigned channel.

  • BandwidthConfig: The transmission bandwidth configuration associated with tmwaveform, in Hertz. This is the bandwidth within the channel bandwidth that contains active subcarriers.

  • BandwidthACLR: The bandwidth required to represent 1st and 2nd adjacent carriers; the sampling rate used internally for ACLR measurements will support this bandwidth with at most 85% bandwidth occupancy.

  • OSR: The integer oversampling ratio of the input waveform required to create a signal capable of representing 1st and 2nd adjacent carriers i.e. to represent aclr.BandwidthACLR with at most 85% bandwidth occupancy.

  • SamplingRate: The sampling rate of the internal measurement signal from which the ACLR is calculated. If OSR=1, this signal is the input waveform; if OSR>1, this signal is the input waveform upsampled by OSR. Therefore: aclr.SamplingRate = OSR*samplingrate.

  • SignalPowerdBm: The power, in decibels relative to 1mW in 1ohm, of the input within the NR channel of interest i.e. in a square filter of bandwidth aclr.BandwidthConfig centered at 0Hz.

  • ACLRdB: A vector of NR ACLRs, in decibels relative to aclr.SignalPowerdBm, measured for adjacent channels [-2, -1, 1, 2].

  • CarrierFrequency: A vector of NR center frequencies, in Hertz, for adjacent channels [-2, -1, 1, 2].

The helper function hACLRResultsNR.m displays the NR ACLR and plots the adjacent channel powers.

hACLRResultsNR(aclr);
            Bandwidth: 20000000
    SubcarrierSpacing: 15000
      BandwidthConfig: 19080000
        BandwidthACLR: 100000000
                  OSR: 4
         SamplingRate: 122880000
     CarrierFrequency: [-40000000 -20000000 20000000 40000000]
       SignalPowerdBm: -5.1358
               ACLRdB: [78.5721 72.1461 72.0420 78.5704]

Appendix

This example uses the following helper functions and classes:

References

[1] 3GPP TS 38.104. “NR; Base Station (BS) radio transmission and reception.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

Related Topics