Estimate frequency offset for PSK signal
The PSKCoarseFrequencyEstimator System object™ estimates frequency offset for a PSK signal.
To estimate frequency offset for a PSK signal:
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).
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.
Estimation algorithm to object uses
Specify the estimation algorithm as one of FFT-based or Correlation-based. The default is FFT-based.
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.
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.
Sample rate (Hz)
Specify the sample rate in samples per second as a positive, real scalar of data type double. The default is 1.
|clone||Create PSKCoarseFrequencyEstimator object with same property values|
|isLocked||Locked status for input attributes and nontunable properties|
|release||Allow property value and input characteristics changes|
|reset||Reset states of the PSKCoarseFrequencyEstimator object|
|step||Estimate frequency offset for PSK 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);
 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.