# comm.PSKDemodulator

(To be removed) Demodulate using M-ary PSK method

**comm.PSKDemodulator will be removed in a future release. Use pskdemod instead. For information on updating your code, see Version History.**

## Description

The `comm.PSKDemodulator`

object demodulates a signal that was modulated using
the M-ary phase shift keying (M-PSK) method. The input is a baseband representation of
the modulated signal.

To demodulate a signal that was modulated using the M-PSK method:

Create the

`comm.PSKDemodulator`

object and set its properties.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

### Description

`mpskdemod = comm.PSKDemodulator`

creates a System object™ to demodulate input M-PSK signals.

`mpskdemod = comm.PSKDemodulator(`

sets properties using one or more name-value arguments. For example,
`Name`

=`Value`

)`DecisionMethod="Hard decision"`

specifies demodulation
using the hard-decision method.

`mpskdemod = comm.PSKDemodulator(M,`

sets the `Name`

=`Value`

)`ModulationOrder`

property to `M`

and optional name-value arguments.

`mpskdemod = comm.PSKDemodulator(M,phase,`

sets the `Name`

=`Value`

)`ModulationOrder`

property to `M`

, the `PhaseOffset`

property
to `phase`

, and optional name-value arguments. Specify
`phase`

in radians.

## 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.

`ModulationOrder`

— Number of points in signal constellation

`8`

(default) | positive integer

Number of points in signal constellation, specified as a positive integer.

**Data Types: **`double`

`PhaseOffset`

— Phase of zeroth point in constellation

`pi/8`

(default) | scalar

Phase of the zeroth point in the constellation in radians, specified as a scalar.

**Example: **`PhaseOffset=0`

aligns the QPSK signal
constellation points on the axes {(1,0), (0,j), (-1,0),
(0,-j)}.

**Data Types: **`double`

`BitOutput`

— Option to output data as bits

`0`

or `false`

(default) | `1`

or `true`

Option to output data as bits, specified as a logical `0`

(`false`

) or `1`

(`true`

).

Set this property to

`false`

to output symbols as integer values in the range [`0`

, (*M*– 1)] with length equal to the input data vector.*M*represents the value of the`ModulationOrder`

.Set this property to

`true`

to output a column vector of bit values with length equal to log2(*M*) times the number of demodulated symbols. Groups of log_{2}(`M`

) bits are mapped onto a symbol, with the first bit representing the MSB and the last bit representing the LSB.

**Data Types: **`logical`

`SymbolMapping`

— Symbol encoding mapping

`'Gray'`

(default) | `'Binary'`

| `'Custom'`

Symbol encoding mapping of constellation bits, specified as
`'Gray'`

, `'Binary'`

, or
`'Custom'`

. Each integer or group of log2(`ModulationOrder`

) bits corresponds to one symbol.

When you set this property to

`'Gray'`

, the object map symbols to a Gray-encoded signal constellation.When you set this property to

`'Binary'`

, the object map symbols to a natural binary-encoded signal constellation. Specifically, the complex value e^{j(PhaseOffset + (2πm/ModulationOrder))}, where*m*is an integer in the range [`0`

, (`ModulationOrder`

–`1`

)].When you set this property to

`'Custom'`

, the object map symbols to the signal constellation defined in the`CustomSymbolMapping`

property.

`CustomSymbolMapping`

— Custom symbol encoding

`0:7`

(default) | integer vector

Custom symbol encoding, specified as an integer vector with length equal
to the value of `ModulationOrder`

and unique values in the range
[`0`

, (`ModulationOrder`

– `1`

)]. 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 –2π/`ModulationOrder`

+ `PhaseOffset`

.

#### Dependencies

To enable this property, set the `SymbolMapping`

property to `'Custom'`

.

**Data Types: **`double`

`DecisionMethod`

— Demodulation decision method

`'Hard decision'`

(default) | `'Log-likelihood ratio'`

| `'Approximate log-likelihood ratio'`

Demodulation decision method, specified as ```
'Hard
decision'
```

, `'Log-likelihood ratio'`

, or
`'Approximate log-likelihood ratio'`

. When you set the
`BitOutput`

property
to `false`

, the object always performs hard-decision
demodulation.

#### Dependencies

To enable this property, set the `BitOutput`

property to `true`

.

`VarianceSource`

— Source of noise variance

`'Property'`

(default) | `'Input port'`

Source of noise variance, specified as `'Property'`

or
`'Input port'`

.

#### Dependencies

To enable this property, set the `BitOutput`

property to `true`

and the `DecisionMethod`

property to ```
'Log-likelihood
ratio'
```

or ```
'Approximate log-likelihood
ratio'
```

.

`Variance`

— Noise variance

`1`

(default) | positive scalar

Noise variance, specified as a positive scalar.

**Tunable: **Yes

#### Tips

The exact LLR algorithm computes exponentials using finite precision arithmetic. For computations involving very large positive or negative magnitudes, the exact LLR algorithm yields:

`Inf`

or`-Inf`

if the noise variance is a very large value`NaN`

if the noise variance and signal power are both very small values

The approximate LLR algorithm does not compute exponentials. You can avoid
`Inf`

, `-Inf`

, and `NaN`

results by using
the approximate LLR algorithm.

#### Dependencies

To enable this property, set the `BitOutput`

property to `true`

, the `DecisionMethod`

property to ```
'Log-likelihood
ratio'
```

or ```
'Approximate log-likelihood
ratio'
```

, and the `VarianceSource`

property to
`'Property'`

.

**Data Types: **`double`

`OutputDataType`

— Data type of output

`'Full precision'`

(default) | `'Smallest unsigned integer'`

| `'double'`

| ...

Data type of the output, specified as `'Full precision'`

,
`'Smallest unsigned integer'`

,
`'double'`

, `'single'`

,
`'int8'`

, `'uint8'`

,
`'int16'`

, `'uint16'`

,
`'int32'`

, or
`'uint32'`

,`'logical'`

.

When the input data type is single or double precision and you set the

`BitOutput`

property to`true`

, the`DecisionMethod`

property to`'Hard decision'`

, and the`OutputDataType`

property to`'Full precision'`

, the output has the same data type as that of the input.When the input data is of a fixed-point type, the output data type behaves as if you had set the

`OutputDataType`

property to`'Smallest unsigned integer'`

.When you set

`BitOutput`

to`true`

and the`DecisionMethod`

property to`'Hard Decision'`

, then`'logical'`

data type is a valid option.When you set the

`BitOutput`

property to`true`

and the`DecisionMethod`

property to`'Log-likelihood ratio'`

or`'Approximate log-likelihood ratio'`

, the output data type is the same as that of the input and the input data type must be single or double precision.

#### Dependencies

To enable this property, set the `BitOutput`

property to `false`

or set the
`BitOutput`

property to `true`

and the `DecisionMethod`

property to ```
'Hard
decision'
```

.

**Fixed-Point Properties**

`DerotateFactorDataType`

— Data type of derotate factor

`'Same word length as input'`

(default) | `'Custom'`

Data type of the derotate factor, specified as ```
'Same word length
as input'
```

or `'Custom'`

. The object uses the
derotate factor in the computations only when the `ModulationOrder`

property is `2`

,
`4`

, or `8`

, the input signal is a
fixed-point type, and the `PhaseOffset`

property has a nontrivial value.

For

`ModulationOrder`

=`2`

, the phase offset is trivial if it is a multiple of π/2.For

`ModulationOrder`

=`4`

, the phase offset is trivial if it is an even multiple of π/4.For

`ModulationOrder`

=`8`

, there are no trivial phase offsets.

#### Dependencies

To enable this property, set the `BitOutput`

property to `false`

or set the `BitOutput`

property to `true`

and the
`DecisionMethod`

property to ```
'Hard
decision'
```

.

`CustomDerotateFactorDataType`

— Fixed-point data type of derotate factor

`numerictype([],16)`

(default) | unscaled `numerictype`

object

Fixed-point data type of the derotate factor, specified as an unscaled
`numerictype`

(Fixed-Point Designer) object with a
`Signedness`

of `Auto`

. The word
length must be a value in the range [2, 128].

#### Dependencies

To enable this property, set the `DerotateFactorDataType`

property to
`'Custom'`

.

**Data Types: **`numerictype object`

## Usage

### Description

uses soft decision demodulation and noise variance `y`

= mpskdemod(`x`

,`var`

)`var`

. This
syntax applies when you set the `BitOutput`

property to
`true`

, the `DecisionMethod`

property to `'Approximate log-likelihood ratio'`

or
`'Log-likelihood ratio'`

, and the `VarianceSource`

property to `'Input port'`

.

### Input Arguments

`x`

— M-PSK-modulated signal

scalar | column vector

M-PSK-modulated signal, specified as a scalar or column vector.

#### Dependencies

The object accepts inputs with a signed integer data type or
signed fixed point (`sfi`

(Fixed-Point Designer)) objects when
you set the `ModulationOrder`

property to a value less than or
equal to `8`

and you set the `BitOutput`

property to `false`

or you set the `DecisionMethod`

property to ```
'Hard
decision'
```

and the `BitOutput`

property to
`true`

.

**Data Types: **`double`

| `single`

| `int`

| `fi`

`var`

— Noise variance

scalar

Noise variance, specified as a scalar.

#### Dependencies

To enable this argument, set the `VarianceSource`

property to ```
'Input
port'
```

, the `BitOutput`

property to `true`

, and the `DecisionMethod`

property to ```
'Approximate
log-likelihood ratio'
```

or ```
'Log-likelihood
ratio'
```

.

**Data Types: **`single`

| `double`

### Output Arguments

`y`

— Output signal

scalar | column vector

Output signal, returned as a scalar or column vector. To specify
whether the object outputs values as integers or bits, use the `BitOutput`

property. To specify the output data type, use the `OutputDataType`

property.

## 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)

### Specific to `comm.PSKDemodulator`

`constellation` | Calculate or plot ideal signal constellation |

## Examples

### 16-PSK with Custom Symbol Mapping

Create 16-PSK modulator and demodulator System objects that use custom symbol mapping. Estimate the BER in an AWGN channel and compare the performance to a theoretical Gray-coded PSK system.

Create a custom symbol mapping for the 16-PSK modulation scheme. The 16 integer symbols must have values in the range [0, 15].

custMap = [0 2 4 6 8 10 12 14 15 13 11 9 7 5 3 1];

Create a 16-PSK modulator and demodulator pair having custom symbol
mapping defined by the array `custMap`

.

pskModulator = comm.PSKModulator(16,'BitInput',true, ... 'SymbolMapping','Custom','CustomSymbolMapping',custMap); pskDemodulator = comm.PSKDemodulator(16,'BitOutput',true, ... 'SymbolMapping','Custom','CustomSymbolMapping',custMap);

Create an AWGN channel System object for use with 16-ary data.

`awgnChannel = comm.AWGNChannel('BitsPerSymbol',log2(16));`

Create an error rate object to track the BER statistics.

errorRate = comm.ErrorRate;

Initialize the simulation vectors. Vary ${\mathit{E}}_{\mathrm{b}}/{\mathit{N}}_{0}$ from 6 to 18 dB in 1 dB steps.

ebnoVec = 6:18; ber = zeros(size(ebnoVec));

Estimate the BER by modulating binary data, passing it through an AWGN channel, demodulating the received signal, and collecting the error statistics.

for n = 1:length(ebnoVec) % Reset the error counter for each Eb/No value reset(errorRate) % Reset the array used to collect the error statistics errVec = [0 0 0]; % Set the channel Eb/No awgnChannel.EbNo = ebnoVec(n); while errVec(2) < 200 && errVec(3) < 1e7 % Generate a 1000-symbol frame data = randi([0 1],4000,1); % Modulate the binary data modData = pskModulator(data); % Pass the modulated data through the AWGN channel rxSig = awgnChannel(modData); % Demodulate the received signal rxData = pskDemodulator(rxSig); % Collect the error statistics errVec = errorRate(data,rxData); end % Save the BER data ber(n) = errVec(1); end

Generate theoretical BER data for an AWGN channel using the `berawgn`

function.

berTheory = berawgn(ebnoVec,'psk',16,'nondiff');

Use this code to plot the simulated and theoretical results. The 16-PSK modulation BER performance of the simulated custom symbol mapping is not as good as the theoretical prediction curve for Gray codes.

figure semilogy(ebnoVec,[ber; berTheory]) xlabel('Eb/No (dB)') ylabel('BER') grid legend('Simulation','Theory','location','ne')

## More About

### Hard-Decision BPSK Demodulation

The signal preprocessing required for BPSK demodulation depends on the configuration.

This figure shows the hard-decision BPSK demodulation signal diagram for the trivial phase offset (multiple of π/2) configuration.

This figure shows the hard-decision BPSK demodulation floating-point signal diagram for the nontrivial phase offset configuration.

This figure shows the hard-decision BPSK demodulation fixed-point signal diagram for the nontrivial phase offset configuration.

### Hard-Decision QPSK Demodulation

The signal preprocessing required for QPSK demodulation depends on the configuration.

This figure shows the hard-decision QPSK demodulation signal diagram for the trivial phase offset (odd multiple of π/4) configuration.

This figure shows the hard-decision QPSK demodulation floating-point signal diagram for the nontrivial phase offset configuration.

This figure shows the hard-decision QPSK demodulation fixed-point signal diagram for the nontrivial phase offset configuration.

### Hard-Decision Higher-Order PSK

The signal preprocessing required for higher order PSK demodulation depends on the configuration.

This figure shows the hard-decision 8-PSK demodulation signal diagram for the trivial phase offset (odd multiple of π/8) configuration.

This figure shows the hard-decision 8-PSK demodulation fixed-point signal diagram for trivial phase offset (odd multiple of π/8) configuration.

This figure shows the hard-decision M-PSK demodulation floating-point signal diagram for the nontrivial phase offset configuration.

For *M* > 8, to improve speed and implementation costs, no derotation
arithmetic is performed for trivial case (specifically, when phase offset is 0, π/2, π, or
3π/2).

Also, for *M* > 8, only `double`

and
`single`

input types are supported.

### Soft-Decision M-PSK Demodulation

For soft demodulation, two soft-decision log-likelihood ratio (LLR) algorithms are available: exact LLR and approximate LLR. The exact LLR algorithm is more accurate but has slower execution speed than the approximate LLR algorithm. For further description of these algorithms, see the Hard- vs. Soft-Decision Demodulation topic.

**Note**

The exact LLR algorithm computes exponentials using finite precision arithmetic. For computations involving very large positive or negative magnitudes, the exact LLR algorithm yields:

`Inf`

or`-Inf`

if the noise variance is a very large value`NaN`

if the noise variance and signal power are both very small values

The approximate LLR algorithm does not compute exponentials. You can avoid
`Inf`

, `-Inf`

, and `NaN`

results by using
the approximate LLR algorithm.

## References

[1] Proakis, John G. *Digital Communications*.
4th ed. New York: McGraw Hill, 2001.

## 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).

### HDL Code Generation

Generate Verilog and VHDL code for FPGA and ASIC designs using HDL Coder™.

The software supports `double`

and
`single`

data types for simulation, but not for HDL code generation.

To generate HDL code from predefined System objects, see HDL Code Generation from Viterbi Decoder System Object (HDL Coder).

## Version History

**Introduced in R2012a**

### R2023a: To be removed

comm.PSKDemodulator will be removed in a future release. Use the `pskdemod`

function to M-ary PSK demodulate the input signal.

## See Also

### Functions

### Objects

### Blocks

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