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comm.PSKModulator System object

Package: comm

Modulate using M-ary PSK method

Description

The PSKModulator object modulates using the M-ary phase shift keying method. The output is a baseband representation of the modulated signal. The M-ary number parameter, M, is the number of points in the signal constellation.

To modulate a signal using phase shift keying:

  1. Define and set up your PSK modulator object. See Construction.

  2. Call step to modulate the signal according to the properties of comm.PSKModulator. 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.PSKModulator creates a modulator System object, H. This object modulates the input signal using the M-ary phase shift keying (M-PSK) method.

H = comm.PSKModulator(Name,Value) creates an M-PSK modulator object, H, with each specified property set to the specified value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

H = comm.PSKModulator(M,PHASE,Name,Value) creates an M-PSK modulator object, H. This object has the ModulationOrder property set to M, the PhaseOffset property set to PHASE, and the other specified properties set to the specified values.

Properties

ModulationOrder

Number of points in signal constellation

Specify the number of points in the signal constellation as a positive, integer scalar value. The default is 8.

PhaseOffset

Phase of zeroth point of constellation

Specify the phase offset of the zeroth point of the constellation, in radians, as a real scalar value. The default is pi/8.

BitInput

Assume bit inputs

Specify whether the input is bits or integers. When you set this property to true, the step method input must be a column vector of bit values. This vector must have a length that is an integer multiple of log2(ModulationOrder). This vector contains bit representations of integers between 0 and ModulationOrder-1. When you set the BitInput property to false, the step method input must be a column vector of numeric data type integer symbol values. These values must be between 0 and ModulationOrder-1. The default is false.

SymbolMapping

Constellation encoding

Specify how the object maps an integer or group of log2(ModulationOrder) input bits to the corresponding symbol as one of Binary | Gray | Custom. The default is Gray. When you set this property to Gray, the object uses a Gray-encoded signal constellation. When you set this property to Binary, the integer m, between 0 mModulationOrder1) maps to the complex value exp(j×PhaseOffset + j 2 π m/ModulationOrder). When you set this property to Custom, the object uses the signal constellation defined in the CustomSymbolMapping property.

CustomSymbolMapping

Custom constellation encoding

Specify a custom constellation symbol mapping vector. This property requires a row or column vector of size ModulationOrder and must have unique integer values in the range [0, ModulationOrder1]. The values must be of data type double. The first element of this vector corresponds to the constellation point at an angle of 0 + PhaseOffset, with subsequent elements running counterclockwise. The last element corresponds to the constellation point at an angle of – /ModulationOrder + PhaseOffset. This property applies when you set the SymbolMapping property to Custom. The default is 0:7.

OutputDataType

Data type of output

Specify the output data type as double | single | Custom. The default is double.

 Fixed-Point Properties

Methods

cloneCreate PSK modulator object with same property values
constellationCalculate or plot ideal signal constellation
getNumInputsNumber of expected inputs to step method
getNumOutputsNumber of outputs from step method
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
stepModulate using M-ary PSK method

Examples

expand all

Modulate an 8-PSK signal, add white Gaussian noise, and plot the signal to observe the effects of noise.

Create a PSK Modulator System object™. The default modulation order for the PSK modulator object is 8.

pskModulator = comm.PSKModulator;

Modulate the signal.

modData = pskModulator(randi([0 7],2000,1));

Add white Gaussian noise to the modulated signal by passing the signal through an AWGN channel.

channel = comm.AWGNChannel('EbNo',20,'BitsPerSymbol',3);

Transmit the signal through the AWGN channel.

channelOutput = channel(modData);

Plot the noiseless and noisy data using scatter plots to observe the effects of noise.

scatterplot(modData)
scatterplot(channelOutput)

Change the EbNo property to 10 dB to increase the noise.

channel.EbNo = 10;

Pass the modulated data through the AWGN channel.

channelOutput = channel(modData);

Plot the channel output. You can see the effects of increased noise.

scatterplot(channelOutput)

Algorithms

The block outputs a baseband signal by mapping input bits or integers to complex symbols according to the following:

sn(t)=exp(jπ(2n+1M));n{0,1,,M1}.

This applies when a natural binary ordering is used. Another common mapping is Gray coding, which has the advantage that only one bit changes between adjacent constellation points. This results in better bit error rate performance. For 8-PSK modulation with Gray coding, the mapping between the input and output symbols is shown.

InputOutput
0 0 (000)
1 1 (001)
2 3 (011)
3 2 (010)
4 6 (110)
5 7 (111)
6 5 (101)
7 4 (100)

The corresponding constellation diagram follows.

When the input signal is composed of bits, the block accepts binary-valued inputs that represent integers. The block collects binary-valued signals into groups of log2(M) bits.

Introduced in R2012a

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