comm.QPSKModulator

Modulate using QPSK method

Description

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

To modulate using the QPSK method:

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 System object™ to modulate input signals using the QPSK method.

qpskmod = comm.QPSKModulator(Name,Value) sets properties using one or more name-value arguments. For example, 'OutputDataType'='single' specifies output of the modulated signal values in single precision data type.

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

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

0 or false (default) | 1 or true

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

• When this property is set to false, the input values must be integer representations of two-bit input segments in the range [0, 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 — Symbol encoding

'Gray' (default) | 'Binary'

Symbol encoding mapping of constellation bits, specified as 'Gray' or 'Binary'.

Setting	Constellation Mapping for Integers	Constellation Mapping for Bits	Comment
Gray			Map symbols using Gray-coded ordering.
Binary			Map symbols using natural binary-coded ordering. The signal constellation maps to the complex value ej(PhaseOffset + (2πm/4)), where m is an integer in the range [0, 3].

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.

Dependencies

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

Usage

Syntax

y = qpskmod(x)

Description

y = qpskmod(x) returns baseband-modulated output.

Input Arguments

x — 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

y — Output QPSK-modulated signal

vector

Output QPSK-modulated signal, 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)

Reconfigure the object for bit input and plot the constellation to show the binary values of the Gray-encoded mapping.

release(mod)
mod.BitInput = true; 
constellation(mod)

Create a QPSK demodulator having phase offset set to 0.

demod = comm.QPSKDemodulator(0);

Plot the 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.

qpskmod = 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 = qpskmod(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).

HDL Code Generation

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

double and single data types are supported 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