comm.PSKCoarseFrequencyEstimator

(To be removed) Estimate frequency offset for PSK signal

comm.PSKCoarseFrequencyEstimator will be removed in a future release. Use comm.CoarseFrequencyCompensator instead.

Description

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

To estimate frequency offset for a PSK signal:

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

Note

Starting in R2016b, instead of using the step method to perform the operation defined by the System object, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

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

step	(To be removed) Estimate frequency offset for PSK signal

Common to All System Objects
`release`	Allow System object property value changes
`reset`	Reset internal states of System object

When using reset, note comm.PSKCoarseFrequencyEstimator will be removed in a future release. Use comm.CoarseFrequencyCompensator instead.

Examples

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Compare Frequency Offset Estimation and Correction Methods for QPSK Signal

Estimate and correct for a frequency offset in a QPSK signal using the recommended comm.CoarseFrequencyCompensator System object. Compare frequency correction results to a workflow using the comm.PSKCoarseFrequencyEstimator System object.

Set example parameters.

nSym = 2048;     % Number of input symbols
M = 4;           % Modulation order
fs = 80000;      % Sampling frequency (Hz)
freqRez = 1;     % Frequency resolution (Hz)
freqOff = -2000; % Frequency offset

Create System objects for these operations:

% Square root raised cosine transmit filter
txfilter = comm.RaisedCosineTransmitFilter;
% Phase frequency offset - one to apply a frequency offset and a second
% that takes the frequency offset estimate as an input to correct the
% offset.
pfo = comm.PhaseFrequencyOffset('FrequencyOffset',freqOff, ...
    'SampleRate',fs);
pfoCorrect = comm.PhaseFrequencyOffset(...
    'FrequencyOffsetSource','Input port', ...
    'SampleRate',fs);
% PSK coarse frequency estimator
frequencyEst = comm.PSKCoarseFrequencyEstimator(...
    'SampleRate',fs,'FrequencyResolution',freqRez);

Warning: comm.PSKCoarseFrequencyEstimator will be removed in a future release.
Use comm.CoarseFrequencyCompensator instead.

% Coarse frequency compensatator
freqComp = comm.CoarseFrequencyCompensator('Modulation','QPSK', ...
    'SampleRate',fs,'FrequencyResolution',freqRez);

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

data = randi([0 M-1],nSym,1);
modData = pskmod(data,M,pi/4);  % Generate QPSK signal
txFiltData = txfilter(modData); % Apply Tx filter
offsetData = pfo(txFiltData);   % Apply frequency offset
rxData = awgn(offsetData,25);   % Pass through AWGN channel

This example does not apply receive filtering. In general, when the frequency offset is high, it is beneficial to apply coarse frequency compensation prior to receive filtering because filtering suppresses energy in the useful spectrum.

Compare the results for estimating and correcting the frequency offset by:

Using the frequencyEst object to estimate the frequency offset and pfoCorrect to compensate for the frequency offset.
Using the freqComp object estimate and apply compensation to the signal.

Observe the frequency offset estimate returned by both estimation methods.

estFreqOffset1
estFreqOffset2

estFreqOffset1 =
  -2.0000e+03
estFreqOffset2 =
  -2.0000e+03

Confirm the maximum resulting difference between the two compensation methods is negligible.

max(compensatedData1-compensatedData2)

ans =

  1.4710e-13 + 9.1149e-14i

Selected Bibliography

[1] Luise, M. 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.

Compatibility Considerations

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