comm.DQPSKDemodulator

Demodulate using DQPSK method

Description

The comm.DQPSKDemodulator System object™ demodulates a signal that was modulated using the differential quadrature phase shift keying method. The input is a baseband representation of the modulated signal.

To demodulate a signal that was modulated using differential quadrature phase shift keying:

1. Create the comm.DQPSKDemodulator 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

dqpskdemod = comm.DQPSKDemodulator

dqpskdemod = comm.DQPSKDemodulator(Name=Value)

dqpskdemod = comm.DQPSKDemodulator(phase,Name=Value)

Description

dqpskdemod = comm.DQPSKDemodulator creates a demodulator System object, dqpskdemod. This object demodulates the input differential quadrature phase shift keying (DQPSK) modulated data.

dqpskdemod = comm.DQPSKDemodulator(Name=Value) sets properties using one or more name-value arguments.

dqpskdemod = comm.DQPSKDemodulator(phase,Name=Value) creates a DQPSK demodulator with the PhaseRotation property set to phase and the other specified properties set to the specified values.

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.

PhaseRotation — Additional phase shift

pi/4 (default) | real scalar

Additional phase difference between previous and current modulated symbols in radians, specified as a real scalar. This value corresponds to the phase difference between previous and current modulated symbols when the input is zero.

BitOutput — Output data as bits

false or 0 (default) | true or 1

Option to return output in bits, specified as a numeric or logical 0 (false) or 1 (true).

• When you set this property to true, the object method outputs a column vector of bit values that is equal in length to twice the number of demodulated symbols.

• When you set this property to false, the object method outputs a column vector that is equal in length to the input data vector and contains integer symbol values between 0 and 3.

Data Types: logical

SymbolMapping — Constellation encoding

"Gray" (default) | "Binary"

Constellation encoding, specified as "Gray" or "Binary". This property controls how the object maps an integer or group of two input bits to the corresponding modulated symbol. When you set this property to "Gray", the object uses a Gray-encoded signal constellation. When you set this property to "Binary", the input integer m, between $$0\le m\le 3$$, shifts the output phase. This shift is (PhaseRotation + $$2\times \pi \times \raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$4$}\right.$$) radians from the previous output phase. The output symbol is exp(j$$\times$$PhaseRotation + j$$\times$$$$2\times \pi \times \raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$4$}\right.$$)$$\times$$(previously modulated symbol).

Data Types: char | string

OutputDataType — Data type of output

"Full precision" (default) | "Smallest unsigned integer" | "double" | "single" | "int8" | "uint8" | "int16" | "uint16" | "int32" | "uint32"

Data type of output, specified as Full precision", "Smallest unsigned integer", "double", "single", "int8", "uint8", "int16", "uint16", "int32", or "uint32". When you set this property to "Full precision", the output has the same data type as the input. In this case, the input data type is single- or double-precision value. When you set the BitOutput property to true, logical data type becomes a valid option.

Data Types: char | string

Usage

Syntax

y = mskdemod(x)

Description

y = mskdemod(x) applies DQPSK demodulation to the input signal and returns the demodulated signal.

Input Arguments

x — DQPSK-modulated signal

scalar | column vector

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

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. The output data type is determined by 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)

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 and Demodulate using DQPSK Method

Create a DQPSK modulator and demodulator pair.

dqpskmod = comm.DQPSKModulator(BitInput=true);
dqpskdemod = comm.DQPSKDemodulator(BitOutput=true);

Create an error rate calculator. Set the ComputationDelay property to 1 to account for the one bit transient caused by the differential modulation.

errorRate = comm.ErrorRate(ComputationDelay=1);

Run the processing loop. It consists of these main steps:

1. Generate 50 2-bit frames.

2. DQPSK-modulate.

3. Pass the signal through an AWGN channel with a signal-to-noise ratio of 9.

4. DQPSK-demodulate.

5. Collect error statistics.

for counter = 1:100
    txData = randi([0 1],100,1);
    modSig = dqpskmod(txData);
    rxSig = awgn(modSig,9);
    rxData = dqpskdemod(rxSig);
    errorStats = errorRate(txData,rxData);
end

Display the error statistics.

ber = errorStats(1)

ber = 
0.0183

numErrors = errorStats(2)

numErrors = 
183

numBits = errorStats(3)

numBits = 
9999

Algorithms

Outputs and Constellation Types

When you set the BitOutput property to false, the object maps a phase difference of θ + πm/2 to m, where θ represents the PhaseRotation property and m is 0, 1, 2, or 3.

When you set the BitOutput property to true, the output contains pairs of binary values. The reference page for the comm.DQPSKModulator System object shows which phase differences map to each binary pair when the SymbolMapping property is either "Binary" or "Gray".

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