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info

Characteristic information about coarse frequency compensator

Description

example

infostruct = info(coarseFreqComp) returns characteristic information for the specified coarse frequency compensator.

Examples

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Compensate for a 4 kHz frequency offset imposed on a noisy QPSK signal.

Set up the example parameters.

nSym = 2048;       % Number of input symbols
sps = 4;           % Samples per symbol
nSamp = nSym*sps;  % Number of samples
fs = 80000;        % Sampling frequency (Hz)

Create a square root raised cosine transmit filter.

txfilter = comm.RaisedCosineTransmitFilter( ...
    RolloffFactor=0.2, ...
    FilterSpanInSymbols=8, ...
    OutputSamplesPerSymbol=sps);

Create a phase frequency offset object to introduce the 4 kHz frequency offset.

freqOffset = comm.PhaseFrequencyOffset( ...
    FrequencyOffset=-4000, ...
    SampleRate=fs);

Create a coarse frequency compensator object to compensate for the offset.

freqComp = comm.CoarseFrequencyCompensator( ...
    Modulation="qpsk", ...
    SampleRate=fs, ...
    FrequencyResolution=1);

Generate QPSK symbols, filter the modulated data, pass the signal through an AWGN channel, and apply the frequency offset.

data = randi([0 3],nSym,1);
modData = pskmod(data,4,pi/4);
txSig = txfilter(modData);
rxSig = awgn(txSig,20,"measured");
offsetData = freqOffset(rxSig);

Compensate for the frequency offset using the coarse frequency compensator. When the frequency offset is high, applying coarse frequency compensation prior to receive filtering is benefitial because filtering suppresses energy in the useful spectrum.

[compensatedData,estFreqOffset] = freqComp(offsetData);

Display the estimate of the frequency offset.

estFreqOffset
estFreqOffset = -4.0001e+03

Return information about the coarse frequency compensator System object. To obtain the FFT length, you must call coarse frequency compensator System object prior to calling the info object function.

freqCompInfo = info(freqComp)
freqCompInfo = struct with fields:
    FFTLength: 131072
    Algorithm: 'FFT-based'

Create a spectrum analyzer object and plot the offset and compensated spectra. Verify that the compensated signal has a center frequency at 0 Hz and that the offset signal has a center frequency at -4 kHz.

sa = spectrumAnalyzer( ...
    SampleRate=fs, ...
    ShowLegend=true, ...
    ChannelNames=["Offset Signal","Compensated Signal"]);
sa([offsetData compensatedData])
release(sa)

Input Arguments

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Coarse frequency compensator specified as a comm.CoarseFrequencyCompensator System object.

Output Arguments

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Characteristic information about coarse frequency compensator, returned as a structure containing these fields.

Number of fast Fourier transform (FFT) samples, returned as a scalar. Appears only when Algorithm is FFT-based.

Dependency

To enable this field, set the Algorithm property of the coarseFreqComp input to 'FFT-based'.

Algorithm used to estimate frequency offset, returned as 'FFT-based' or 'Correlation-based'. This value matches the Algorithm property of the coarseFreqComp input.

Number of samples used to estimate the autocorrelation, returned as a positive integer.

Dependency

To enable this field, set the Algorithm property of the coarseFreqComp input to 'Correlation-based'.

Data Types: struct

Version History

Introduced in R2015b