Capture Wideband Spectrum by Combining Data from Multiple Antennas

This example shows how to configure a software-defined radio (SDR) as a baseband receiver to capture a wideband spectrum by combining received data from multiple antennas using a multiband combiner. The example also plots the combined wideband spectrogram of the captured data.

Introduction

In this example, you use the basebandReceiver object to capture a wide frequency band by combining data from two radio antennas with different center frequencies. Because each antenna captures data at a different center frequency, you can capture a wider frequency band than using a single antenna. To view the whole frequency band, you then use the comm.MultibandCombiner System object™.

Set Up Radio

Call the radioConfigurations function. The function returns all available radio setup configurations that you saved using the Radio Setup wizard. For more information, see Connect and Set Up NI USRP Radios.

savedRadioConfigurations = radioConfigurations;

To update the dropdown menu with your saved radio setup configuration names, click Update. Then select the radio to use with this example.

savedRadioConfigurationNames = [string({savedRadioConfigurations.Name})];
radio = savedRadioConfigurationNames(1) ;

Configure Baseband Receiver

Create a baseband receiver object with the specified radio. Because the object requires exclusive access to radio hardware resources, before running this example for the first time, clear any other object associated with the specified radio. In subsequent runs, to speed up the execution time of the example, reuse your new workspace object.

if ~exist("bbrx","var")
    bbrx = basebandReceiver(radio);
end

To capture the largest combined bandwidth, set the SampleRate property to the maximum supported value on the SDR. Choose a single antenna bandwidth less than or equal to the maximum instantaneous bandwidth of your radio and less than the chosen sample rate to reduce aliasing effects.

To obtain the maximum sample rate and instantaneous bandwidth available for your radio, call the hMaxSampleRate and hMaxBandwidth helper functions. Alternatively, you can set custom values.

maxSampleRate = hMaxSampleRate(radio);
bbrx.SampleRate = maxSampleRate;
maxBandwidth = hMaxBandwidth(radio);
singleAntennaBandwidth = maxBandwidth;

Set the Antennas object property to a value that corresponds to two antennas that support different center frequencies on the SDR.

To update the dropdown menu with antennas that support different center frequencies, call the hIndependentFrequencyCaptureAntennas helper function. Then select the antennas to use with this example.

[firstAntennaSelection, secondAntennaSelection] = hIndependentFrequencyCaptureAntennas(radio);
bbrx.Antennas = [firstAntennaSelection(1),secondAntennaSelection(1)];

Because the multiband combining operation (after capture) requires double-precision or single-precision input data, configure the baseband receiver to return the captured data in single precision.

bbrx.CaptureDataType = "single";

Set the RadioGain object property according to the local signal strength.

bbrx.RadioGain = 30;

Set the center frequency for each antenna by shifting the center frequency of the combined band by half of the single antenna bandwidth.

centerFrequency = 2.55e9;
bbrx.CenterFrequency = [centerFrequency - singleAntennaBandwidth/2, ...
    centerFrequency + singleAntennaBandwidth/2];

Capture IQ Data

To capture IQ data from the specified antennas, call the capture function on the baseband receiver object. Specify the length of the capture.

captureLength = milliseconds(100);
data = capture(bbrx,captureLength);

Filter and Combine Captured Data

To reduce aliasing effects, filter the captured data to the selected bandwidth. Create a comm.MultibandCombiner System object and combine the captured data from each antenna.

filteredData = lowpass(data,singleAntennaBandwidth/2,bbrx.SampleRate);
mbc = comm.MultibandCombiner( ...
    InputSampleRate=bbrx.SampleRate, ...
    FrequencyOffsets=(bbrx.CenterFrequency-centerFrequency), ...
    OutputSampleRateSource="Auto");
combinedData = mbc(filteredData);
mbcInfo = info(mbc);
resampledData = resample(combinedData,2*singleAntennaBandwidth,mbcInfo.OutputSampleRate);

Plot Spectrogram

Plot the spectrogram of the resampled data. The example uses 4096 FFT points and a 50% overlapping Hann window of length equal to the length of the resampled data divided by 4096. Alternatively, you can experiment with custom values.

window = hann(floor(length(resampledData)/4096));
nOverlap = floor(length(window)/2);
nFFT = 4096;
spectrogram(resampledData,window,nOverlap,nFFT,2*singleAntennaBandwidth,"centered");

To call the capture function again and to update the spectrum analyzer by rerunning the current section, click Capture and plot frequency spectrum.