Contents

comm.PSKCoarseFrequencyEstimator System object

Package: comm

Estimate frequency offset for PSK signal

Description

The PSKCoarseFrequencyEstimator System object™ estimates frequency offset for a PSK signal.

To estimate frequency offset for a PSK signal:

  1. Define and set up your PSK coarse frequency estimator object. See Construction.

  2. Call step to estimate frequency offset for a PSK signal according to the properties of comm.PSKCoarseFrequencyEstimator. The behavior of step is specific to each object in the toolbox.

Construction

H = comm.PSKCoarseFrequencyEstimator creates a PSK coarse frequency offset estimator object, H. This object uses an open-loop technique to estimate the carrier frequency offset in a received PSK signal.

H = comm.PSKCoarseFrequencyEstimator(Name,Value) creates a PSK coarse frequency offset estimator object, H, with the specified property Name set to the specified Value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

ModulationOrder

Modulation order the object uses

Specify the modulation order of the PSK signal as a positive, real scalar of data type double. This value must be a positive power of 2. The default is 4.

Algorithm

Estimation algorithm to object uses

Specify the estimation algorithm as one of FFT-based or Correlation-based. The default is FFT-based.

FrequencyResolution

Desired frequency resolution (Hz)

Specify the desired frequency resolution for offset frequency estimation as a positive, real scalar of data type double. This property establishes the FFT length used to perform spectral analysis, and must be less than or equal to half the SampleRate property. This property applies only if the Algorithm property is FFT-based. The default is 0.001.

MaximumOffset

Maximum measurable frequency offset (Hz)

Specify the maximum measurable frequency offset as a positive, real scalar of data type double. The default is 0.05.

The value of this property must be less than SampleRate/ ModulationOrder. It is recommended that MaximumOffset be less than or equal to SampleRate/(4*ModulationOrder). This property is active only if the Algorithm property is Correlation-based.

SampleRate

Sample rate (Hz)

Specify the sample rate in samples per second as a positive, real scalar of data type double. The default is 1.

Methods

cloneCreate PSKCoarseFrequencyEstimator object with same property values
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
resetReset states of the PSKCoarseFrequencyEstimator object
stepEstimate frequency offset for PSK signal

Examples

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Correct For a Frequency Offset in a QPSK Signal

Estimate and correct for a -250 Hz frequency offset in a QPSK signal using the PSK Coarse Frequency Estimator System object™.

Create a QPSK modulator System object.

hMod = comm.QPSKModulator;

Create a square root raised cosine transmit filter System object.

hTxFilter = comm.RaisedCosineTransmitFilter;

Create a phase frequency offset object, where the FrequencyOffset property is set to -250 Hz and SampleRate is set to 4000 Hz using name-value pairs.

hPFOError = comm.PhaseFrequencyOffset(...
    'FrequencyOffset',-250, ...
    'SampleRate',4000);

Create an AWGN channel object with a 25 dB signal-to-noise ratio.

hAWGN = comm.AWGNChannel(...
    'NoiseMethod','Signal to noise ratio (SNR)', ...
    'SNR',25);

Create a PSK coarse frequency estimator System object with a sample rate of 4 kHz and a frequency resolution of 1 Hz.

hFreqEst = comm.PSKCoarseFrequencyEstimator(...
    'SampleRate',4000, ...
    'FrequencyResolution',1);

Create a second phase frequency offset object to correct the offset. Set the FrequencyOffsetSource property to Input port so that the frequency correction estimate is an input argument to the step function.

hPFOCorrect = comm.PhaseFrequencyOffset(...
    'FrequencyOffsetSource','Input port', ...
    'SampleRate',4000);

Create a spectrum analzyer object to view the frequency response of the signals.

hSA = dsp.SpectrumAnalyzer('SampleRate',4000);

Generate a QPSK signal, filter the signal, apply the frequency offset, and pass the signal through the AWGN channel.

modData = step(hMod,randi([0 3],4096,1));     % Generate QPSK signal
txFiltData = step(hTxFilter,modData);         % Apply Tx filter
offsetData = step(hPFOError,txFiltData);      % Apply frequency offset
noisyData = step(hAWGN,offsetData);           % Pass through AWGN channel

Plot the frequency response of the noisy, frequency-offset signal using the spectrum analyzer. The signal is shifted 250 Hz to the left.

hSA.Title = 'Received Signal';
step(hSA,noisyData);

Estimate the frequency offset using the step function associated with hFreqEst. Observe that the estimate is close to the -250 Hz target.

estFreqOffset = step(hFreqEst,noisyData)
estFreqOffset =

 -249.7559

Correct for the frequency offset using hPFOCorrect and the inverse of the estimated frequency offset.

compensatedData = step(hPFOCorrect,noisyData,-estFreqOffset);

Plot the frequency response of the compensated signal using the spectrum analyzer. The signal is now properly centered.

hSA.Title = 'Frequency-Compensated Signal';
step(hSA,compensatedData);

Selected Bibliography

[1] Marco, L. and R. Regiannini. "Carrier recovery in all-digital modems for burst-mode transmissions", IEEE® Transactions on Communications, Vol. 43, No. 2, 3, 4, Feb/Mar/April, 1995, pp. 1169–1178.

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