comm.QPSKModulator

Modulate using QPSK method

Description

The comm.QPSKModulator object modulates using the quadrature phase shift keying method. The output is a baseband representation of the modulated signal.

To apply QPSK modulation:

1. Create the comm.QPSKModulator 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

qpskmod = comm.QPSKModulator

qpskmod = comm.QPSKModulator(Name,Value)

qpskmod = comm.QPSKModulator(phase,Name,Value)

Description

qpskmod = comm.QPSKModulator creates a modulator System object™. Use this object to modulate the input signal using the quadrature phase shift keying (QPSK) method.

qpskmod = comm.QPSKModulator(Name,Value) creates a QPSK modulator object, 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).

qpskmod = comm.QPSKModulator(phase,Name,Value) creates a QPSK modulator object, qpskmod. This object has the PhaseOffset property set to phase and the other specified properties set to the specified values. 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.

PhaseOffset — Phase offset of zeroth point in constellation

pi/4 (default) | scalar

Phase of zeroth point of the signal 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

BitInput — Option to provide input in bits

false (default) | true

Option to provide input in bits, specified as false or true.

• When this property is set to false, the input values must be integer representations of two-bit input segments and range from 0 to 3.

• When this property is set to true, the input must be a binary vector of even length. Element pairs are binary representations of integers.

Data Types: logical

SymbolMapping — Signal constellation bit mapping

'Gray' (default) | 'Binary'

Constellation encoding

Signal constellation bit mapping, specified as 'Gray' or 'Binary'.

Setting	Constellation Mapping for Integers	Constellation Mapping for Bits	Comment
Gray			The signal constellation mapping is Gray-encoded.
Binary			The signal constellation mapping for the input integer m (0 ≤ m ≤ 3) is the complex value e(j*(PhaseOffset) + j*2*π*m/4).

Data Types: char

OutputDataType — Data type assigned to output

'double' (default) | 'single' | 'Custom'

Data type assigned to output, specified as 'double', 'single', or 'Custom'.

Data Types: char

Fixed-Point Properties

CustomOutputDataType — Fixed-point data type of output

numerictype([],16) (default) | numerictype object

Fixed-point data type of output, specified as a numerictype (Fixed-Point Designer) object with a signedness of Auto. This property applies when you set the OutputDataType property to Custom.

Dependencies

This property applies when you set the OutputDataType property to 'Custom'.

Usage

Syntax

waveform = qpskmod(insignal)

Description

waveform = qpskmod(insignal) returns baseband-modulated output.

Input Arguments

insignal — Input signal

integer column vector | bit column vector

Input signal, specified as an N_S-element column vector of integers or bits, where N_S is the number of samples.

The setting of the BitInput property determines the interpretation of the input vector.

Data Types: double | int8 | logical | fi

Output Arguments

waveform — Output waveform

vector

Output waveform, returned as a complex-valued vector.

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.QPSKModulator

constellation

Calculate or plot ideal signal constellation

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

Plot QPSK Reference Constellation

Create a QPSK modulator.

mod = comm.QPSKModulator;

Determine the reference constellation points.

refC = constellation(mod)

refC = 4×1 complex

   0.7071 + 0.7071i
  -0.7071 + 0.7071i
  -0.7071 - 0.7071i
   0.7071 - 0.7071i

Plot the constellation.

constellation(mod)

Create a PSK demodulator having 0 phase offset.

demod = comm.QPSKDemodulator('PhaseOffset',0);

Plot its reference constellation. The constellation method works for both modulator and demodulator objects.

constellation(demod)

Phase Noise on QPSK Signal

Create a QPSK modulator object and a phase noise object.

qpskModulator = comm.QPSKModulator;
phNoise = comm.PhaseNoise('Level',-55,'FrequencyOffset',20,'SampleRate',1000);

Generate random QPSK data. Pass the signal through the phase noise object.

d = randi([0 3],1000,1);
x = qpskModulator(d);
y = phNoise(x);

Display the constellation diagram of the QPSK signal. The phase noise has introduced a rotational distortion on the constellation diagram.

constDiagram = comm.ConstellationDiagram;

constDiagram(y)

Create QPSK Modulator Object with Bit Input

Create QPSK modulator object setting the BitInput property to true. Display the properties.

qpskmod = comm.QPSKModulator('BitInput',true)

qpskmod = 
  comm.QPSKModulator with properties:

       PhaseOffset: 0.7854
          BitInput: true
     SymbolMapping: 'Gray'
    OutputDataType: 'double'

Determine the reference constellation points.

refC = constellation(qpskmod)

refC = 4×1 complex

   0.7071 + 0.7071i
  -0.7071 + 0.7071i
  -0.7071 - 0.7071i
   0.7071 - 0.7071i

Plot the constellation. Since BitInput is true, the constellation symbols are label with bit values.

constellation(qpskmod)

Create QPSK modulator object with default properties settings. Display the properties.

qpskmod2 = comm.QPSKModulator

qpskmod2 = 
  comm.QPSKModulator with properties:

       PhaseOffset: 0.7854
          BitInput: false
     SymbolMapping: 'Gray'
    OutputDataType: 'double'

Plot constellation with default settings. Since BitInput is false, the constellation symbols are label with integer values.

constellation(qpskmod2)

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).