802.11p Spectral Emission Mask Testing

This example shows how to perform spectrum emission mask tests for an IEEE® 802.11p™ transmitted waveform.

Introduction

IEEE 802.11p [ 2 ] is an approved amendment to the IEEE 802.11™ standard to enable support for wireless access in vehicular environments (WAVE). Using the half-clocked mode with a 10 MHz channel bandwidth, it operates at the 5.85-5.925 GHz bands for which additional spectral emission masks are defined [ Annex D of 1 ].

This example shows how spectral mask measurements can be performed on a transmitted waveform. The waveform is generated with WLAN Toolbox™ for simplicity, but a waveform captured with a spectrum analyzer could be used as well.

A waveform consisting of three 10 MHz IEEE 802.11p packets separated by a 32 microsecond gap is generated. Random data is used for each packet and 16QAM modulation is used. The baseband waveform is upsampled and filtered to reduce the out of band emissions thereby meeting the spectral mask requirements. A high power amplifier (HPA) model is used, which introduces inband distortion and spectral regrowth. The spectral emission mask measurement is performed on the upsampled waveform after the high power amplifier modeling. The test schematic is illustrated in the following diagram:

IEEE 802.11p non-HT Packet Configuration

In this example, an IEEE 802.11p waveform consisting of multiple non-HT format packets is generated. Format parameters of the non-HT waveform are described using a non-HT format configuration object. The object is created using the wlanNonHTConfig function. In this example, the object is configured for a 10 MHz bandwidth operation as used by IEEE 802.11p.

cfgNHT = wlanNonHTConfig;          % Create packet configuration
cfgNHT.ChannelBandwidth = 'CBW10'; % 10 MHz
cfgNHT.MCS = 4;                    % Modulation 16QAM, rate-1/2
cfgNHT.PSDULength = 1000;          % PSDU length in bytes

Baseband Waveform Generation

The waveform generator can be configured to generate one or more packets and add an idle time between each packet. In this example three packets with a 32 microsecond idle period will be created. Random bits for all packets data are created and passed as an argument to wlanWaveformGenerator along with the non-HT packet configuration object cfgNHT and additional waveform generation parameters. cfgNHT configures the waveform generator to create the IEEE 802.11p non-HT waveform.

% Set random stream for repeatability of results
s = rng(98765);

% Generate a multi-packet waveform
idleTime   = 32e-6;     % 32 microsecond idle time between packets
numPackets = 3;         % Generate 3 packets

% Create random data; PSDULength is in bytes
data = randi([0 1], cfgNHT.PSDULength*8*numPackets, 1);

genWaveform = wlanWaveformGenerator(data, cfgNHT, ...
                'NumPackets', numPackets,...
                'IdleTime', idleTime);

% Get the sampling rate of the waveform
fs = wlanSampleRate(cfgNHT);
disp(['Baseband sampling rate: ' num2str(fs/1e6) ' Msps']);

Baseband sampling rate: 10 Msps

Oversampling and Filtering

Spectral filtering is used to reduce the out of band spectral emissions due to the implicit rectangular pulse shaping in the OFDM modulation, and spectral regrowth caused by the high power amplifier in an RF chain. To model the effect of a high power amplifier on the waveform and view the out of band spectral emissions the waveform must be oversampled. In this example the waveform is oversampled with an interpolation filter which also acts as a spectral filter. This allows the waveform to meet spectral mask requirements. The waveform is oversampled and filtered using dsp.FIRInterpolator.

% Oversample the waveform
osf = 3;         % Oversampling factor
filterLen = 100; % Filter length
r = 50;          % Design parameter for Chebyshev window (attenuation, dB)

% Generate filter coefficients and interpolate
coeffs = osf.*firnyquist(filterLen, osf, chebwin(filterLen+1, r));
coeffs = coeffs(1:end-1);   % Remove trailing zero
interpolationFilter = dsp.FIRInterpolator(osf, 'Numerator', coeffs);
filtWaveform = interpolationFilter([genWaveform; zeros(filterLen/2,1)]);

% Plot the magnitude and phase response of the filter applied after
% oversampling
h = fvtool(interpolationFilter);
h.Analysis = 'freq';           % Plot magnitude and phase responses
h.FS = osf*fs;                 % Set sampling rate
h.NormalizedFrequency = 'off'; % Plot responses against frequency

High Power Amplifier Modeling

The high power amplifier introduces nonlinear behavior in the form of inband distortion and spectral regrowth. This example simulates the power amplifiers by using the Rapp model in 802.11ac [ 2 ], which introduces AM/AM distortion.

Model the amplifier by using comm.MemorylessNonlinearity object, and configure reduced distortion by specifying a back-off, hpaBackoff, such that the amplifier operates below its saturation point. You can increase the backoff to reduce EVM for higher MCS values.

pSaturation = 25; % Saturation power of a power amplifier in dBm
hpaBackoff = 16; % dB

% Create and configure a memoryless nonlinearity to model the amplifier
nonLinearity = comm.MemorylessNonlinearity;
nonLinearity.Method = 'Rapp model';
nonLinearity.Smoothness = 3; % p parameter
nonLinearity.LinearGain = -hpaBackoff; % dB
nonLinearity.OutputSaturationLevel = db2mag(pSaturation-30);

% Apply the model to the transmit waveform
txWaveform = nonLinearity(filtWaveform);

Transmit Spectrum Emission Mask Measurement

Stations are classified according to the allowed maximum transmit powers (in mW). For the four different classes of stations, four different spectral emission masks are defined [ Annex D of 1]. The spectral masks are defined relative to the peak power spectral density (PSD).

In this example the spectrum emission mask of the transmitted waveform after high power amplifier modeling is measured for a Class A station.

% IEEE Std 802.11-2012 Annex D.2.3, Table D-5: Class A STA
dBrLimits = [-40  -40 -28 -20  -10 0   0  -10 -20 -28 -40 -40];
fLimits   = [-Inf -15 -10 -5.5 -5 -4.5 4.5 5  5.5  10  15 Inf];

A time gated spectral measurement of the non-HT Data field is used for the transmitter spectrum emission mask test [ 3 ]. The non-HT Data field of each packet is extracted from the upsampled txWaveform using the start index of each packet. The extracted non-HT Data fields are concatenated in preparation for measurement.

% Indices for accessing each field within the time-domain packet
ind = wlanFieldIndices(cfgNHT);
startIdx = osf*(ind.NonHTData(1)-1)+1;   % Upsampled start of non-HT Data
endIdx = osf*ind.NonHTData(2);           % Upsampled end of non-HT Data
idleNSamps = osf*idleTime/(1/fs);        % Upsampled idle time samples
perPktLength = endIdx + idleNSamps;

idx = zeros(endIdx-startIdx+1, numPackets);
for i = 1:numPackets
    % Start of packet in txWaveform, accounting for the filter delay
    pktOffset = (i-1)*perPktLength+filterLen/2;
    % Indices of non-HT Data in txWaveform
    idx(:,i) = pktOffset+(startIdx:endIdx);
end
% Select the Data field for the individual packets
gatedNHTDataTx = txWaveform(idx(:),:);

The plot generated by the helper function helperSpectralMaskTest overlays the required spectral mask with the measured PSD. It checks the transmitted PSD levels to be within the specified mask levels and displays a pass/fail status after the test.

% Evaluate the PSD and check for compliance
helperSpectralMaskTest(gatedNHTDataTx, fs, osf, dBrLimits, fLimits);

% Restore default stream
rng(s);

   Spectrum mask passed

Conclusion and Further Exploration

The transmit spectral mask for Class A Stations at the 5.85-5.925 GHz bands for a 10 MHz channel spacing is shown in this example. It is also shown how the peak spectral density of the transmitted signal falls within the spectral mask to satisfy regulatory restrictions. A similar result can be generated for the 5 MHz channel spacing.

The high power amplifier model and the spectral filtering affect the out-of-band emissions in the spectral mask plot. For different station classes with higher relative dB values, try using different filters or filter lengths and/or increase the backoff for lower emissions.

For information on other transmitter measurements like modulation accuracy and spectral flatness, refer to the following example:

• 802.11ac Transmitter Measurements

Selected Bibliography

1. IEEE Std 802.11-2012: IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE, New York, NY, USA, 1999-2013.

2. IEEE Std 802.11p-2010: IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Amendment 6: Wireless Access in Vehicular Environments, IEEE, New York, NY, USA, 2010.

3. Archambault, Jerry, and Shravan Surineni. "IEEE 802.11 spectral measurements using vector signal analyzers." RF Design 27.6 (2004): 38-49.

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