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comm.MSKModulator

Modulate using MSK method

Description

The comm.MSKModulator System object™ modulates using the minimum shift keying method. The output is a baseband representation of the modulated signal. For more information, see Algorithms.

To modulate a signal using minimum shift keying:

  1. Create the comm.MSKModulator object and set its properties.

  2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Creation

Description

example

mskmodulator = comm.MSKModulator creates a modulator System object that modulates the input signal using the minimum shift keying (MSK) modulation method.

example

mskmodulator = comm.MSKModulator(Name=Value) sets Properties using one or more name-value arguments. For example, mskmodulator = comm.MSKModulator(InitialPhaseOffset=pi/2) specifies an initial phase of pi/2 radians for the modulated waveform.

Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Option to provide input in bits, specified as a numeric or logical 0 (false) or 1 (true).

  • When you set this property to false, the input to the System object call requires a double-precision or signed integer data type column vector with values of -1 or 1.

  • When you set this property to true, the input to the System object call requires a double-precision or logical data type column vector of 0s and 1s.

Data Types: logical

Initial phase offset of the modulated waveform in radians, specified as a numeric scalar.

Number of samples per output symbol, specified as a positive integer. The number of samples per symbol represents the upsampling factor from input samples to output samples.

Data Types: double

Output data type, specified as either "double" or "single".

Usage

Description

y = mskmodulator(x) applies MSK modulation to the input data and returns the modulated MSK baseband signal.

Input Arguments

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Input data, specified as an integer or column vector of integers or bits.

The setting of the BitInput property determines the interpretation of the input data. If the property is set to false, the input must take values of 1 or -1.

Data Types: double | logical

Output Arguments

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MSK-modulated baseband signal, returned as a column vector.

The length of the vector is equal to the number of input samples times the SamplesPerSymbol property. For more information about the output data type, see the OutputDataType property.

Data Types: single | double
Complex Number Support: Yes

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

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stepRun System object algorithm
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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Create an MSK modulator, an AWGN channel, and an MSK demodulator. Use a phase offset of π/4.

mskmodulator = comm.MSKModulator( ...
    BitInput=true, ...
    InitialPhaseOffset=pi/4);
awgn = comm.AWGNChannel( ...
    NoiseMethod='Signal to noise ratio (SNR)', ...
    SNR=0);
mskdemodulator = comm.MSKDemodulator( ...
    BitOutput=true, ...
    InitialPhaseOffset=pi/4);

Create an error rate calculator. Account for the delay caused by the Viterbi algorithm.

    ber = comm.ErrorRate(ReceiveDelay=mskdemodulator.TracebackDepth);

Transmit 100 3-bit words.

    for counter = 1:100
      data = randi([0 1],300,1);
      modSignal = mskmodulator(data);
      noisySignal = awgn(modSignal);
      receivedData = mskdemodulator(noisySignal);
      errorStats = ber(data, receivedData);
    end
    fprintf('Error rate = %f\nNumber of errors = %d\n', ...
      errorStats(1), errorStats(2))
Error rate = 0.000000
Number of errors = 0

Map binary sequences of zeros and ones to the output of an MSK modulator. This mapping also applies for GMSK modulation.

Create an MSK modulator that accepts binary inputs and has a samples per symbol value of 1.

mskmodulator = comm.MSKModulator(BitInput=true, SamplesPerSymbol=1);

Create an input sequence of all zeros. Modulate the sequence.

x = zeros(5,1);
y = mskmodulator(x)
y = 5×1 complex

   1.0000 + 0.0000i
   0.0000 - 1.0000i
  -1.0000 - 0.0000i
  -0.0000 + 1.0000i
   1.0000 + 0.0000i

Determine the phase angle for each point. Use the unwrap function to show the trend.

theta = unwrap(angle(y))
theta = 5×1

         0
   -1.5708
   -3.1416
   -4.7124
   -6.2832

A sequence of zeros causes the phase to shift by -π/2 between samples.

Reset the modulator. Modulate an input sequence of all ones.

reset(mskmodulator)
x = ones(5,1);
y = mskmodulator(x)
y = 5×1 complex

   1.0000 + 0.0000i
   0.0000 + 1.0000i
  -1.0000 + 0.0000i
  -0.0000 - 1.0000i
   1.0000 + 0.0000i

Determine the phase angle for each point. Use the unwrap function to show the trend.

theta = unwrap(angle(y))
theta = 5×1

         0
    1.5708
    3.1416
    4.7124
    6.2832

A sequence of ones causes the phase to shift by +π/2 between samples.

Compare Gaussian minimum shift keying (GMSK) and minimum shift keying (MSK) modulation schemes by plotting the eye diagram for GMSK with different pulse lengths and for MSK.

Set the samples per symbol variable.

sps = 8;

Generate random binary data.

data = randi([0 1],1000,1);

Create GMSK and MSK modulators that accept binary inputs. Set the PulseLength property of the GMSK modulator to 1.

gmskMod = comm.GMSKModulator('BitInput',true,'PulseLength',1, ...
    'SamplesPerSymbol',sps);
mskMod = comm.MSKModulator('BitInput',true,'SamplesPerSymbol',sps);

Modulate the data using the GMSK and MSK modulators.

modSigGMSK = gmskMod(data);
modSigMSK = mskMod(data);

Pass the modulated signals through an AWGN channel having an SNR of 30 dB.

rxSigGMSK = awgn(modSigGMSK,30);
rxSigMSK = awgn(modSigMSK,30);

Use the eyediagram function to plot the eye diagrams of the noisy signals. With the GMSK pulse length set to 1, the eye diagrams are nearly identical.

eyediagram(rxSigGMSK,sps,1,sps/2)

{"String":"Figure Eye Diagram contains 2 axes objects. Axes object 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes object 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.","Tex":["Eye Diagram for In-Phase Signal","Eye Diagram for Quadrature Signal"],"LaTex":[]}

eyediagram(rxSigMSK,sps,1,sps/2)

{"String":"Figure Eye Diagram contains 2 axes objects. Axes object 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes object 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.","Tex":["Eye Diagram for In-Phase Signal","Eye Diagram for Quadrature Signal"],"LaTex":[]}

Set the PulseLength property for the GMSK modulator object to 3. Because the property is nontunable, the object must be released first.

release(gmskMod)
gmskMod.PulseLength = 3;

Generate a modulated signal using the updated GMSK modulator object and pass it through the AWGN channel.

modSigGMSK = gmskMod(data);
rxSigGMSK = awgn(modSigGMSK,30);

With continuous phase modulation (CPM) waveforms, such as GSMK, the waveform depends on values of the previous symbols as well as the present symbol. Plot the eye diagram of the GMSK signal to see that the increased pulse length results in an increase in the number of paths in the eye diagram.

eyediagram(rxSigGMSK,sps,1,sps/2)

{"String":"Figure Eye Diagram contains 2 axes objects. Axes object 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes object 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.","Tex":["Eye Diagram for In-Phase Signal","Eye Diagram for Quadrature Signal"],"LaTex":[]}

Experiment by changing the PulseLength parameter of the GMSK modulator object to other values. If you set the property to an even number, you should set gmskMod.InitialPhaseOffset to pi/4 and update the offset argument of the eyediagram function from sps/2 to 0 for a better view of the modulated signal. In order to more clearly view the Gaussian pulse shape, you must use scopes that display the phase of the signal, as described in the View CPM Phase Tree Using Simulink example.

Algorithms

Differentially encoded minimum shift keying modulation uses pulse shaping to smooth the phase transitions of the modulated signal. The function q(t) is the phase response obtained from the frequency pulse, g(t), through this relation:q(t)=tg(t)dt.

The specified frequency pulse shape corresponds to this rectangular pulse shape expression for g(t).

Pulse ShapeExpression
Rectangularg(t)={12LT,0tLT0otherwise

  • L is the main lobe pulse duration in symbol intervals.

  • The duration of the pulse, LT, is the pulse length in symbol intervals.

Extended Capabilities

Version History

Introduced in R2012a