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

Syntax

mskmodulator = comm.MSKModulator

mskmodulator = comm.MSKModulator(Name=Value)

Description

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

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

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.

BitInput — Option to provide input as bits

false or 0 (default) | true or 1

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

• When you set this property to false, input X must be a column vector with values of -1 or 1.

• When you set this property to true, input X must be a column vector of 0s and 1s.

Data Types: logical

InitialPhaseOffset — Initial phase offset

0 (default) | numeric scalar

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

SamplesPerSymbol — Number of samples per output symbol

8 (default) | positive integer

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

OutputDataType — Data type of output

'double' (default) | 'single'

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

Usage

Syntax

Y = mskmodulator(X)

Description

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

Input Arguments

X — Input data

integer | column vector

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

• When you set BitInput to false, the input data must be double-precision or signed integer.

• When you set BitInput to true, the input data must be double-precision or logical.

This object accepts variable-size inputs. After the object is locked, you can change the size of each input channel, but you cannot change the number of channels. For more information, see Variable-Size Signal Support with System Objects.

Data Types: double | logical | int8 | int16 | int32

Output Arguments

Y — MSK-modulated baseband signal

column vector

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: double | single

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)

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

Modulate MSK Signals with Bit Inputs and Phase Offset

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

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 Data to MSK Signal

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 $$-\pi /2$$ 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 $$+\pi /2$$ between samples.

Compare GMSK and MSK Modulation

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)

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

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)

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:$$q(t)={\displaystyle {\int }_{-\infty }^{t}g(t)dt}$$.

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

Pulse Shape	Expression
Rectangular	$$g(t)=\{\begin{array}{cc}\frac{1}{2LT},& 0\le t\le LT\\ 0& \text{otherwise}\end{array}$$

• 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

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

See System Objects in MATLAB Code Generation (MATLAB Coder).

Version History

Introduced in R2012a

R2024a: Variable-size support

This support enables you to vary the length of input signal each time you call the object.