comm.OQPSKDemodulator

Demodulation using OQPSK method

Description

The comm.OQPSKDemodulator object applies pulse shape filtering to the input waveform and demodulates it using the offset quadrature phase shift keying (OQPSK) method. For more information, see Pulse Shaping Filter. The input is a baseband representation of the modulated signal.

For information about delays incurred by modulator-demodulator pair processing, see Modulation Delays.

To demodulate a signal that is OQPSK modulated:

1. Create the comm.OQPSKDemodulator 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

oqpskdemod = comm.OQPSKDemodulator

oqpskdemod = comm.OQPSKDemodulator(mod)

oqpskdemod = comm.OQPSKDemodulator(Name,Value)

oqpskdemod = comm.OQPSKDemodulator(phase,Name,Value)

Description

oqpskdemod = comm.OQPSKDemodulator creates a demodulator System object™. This object can jointly match-filter and decimate a waveform, and demodulate it using the offset quadrature phase shift keying (OQPSK) method.

oqpskdemod = comm.OQPSKDemodulator(mod) creates a demodulator System object with symmetric configuration to the OQPSK modulator object, mod. If the modulator object is an IIR filter (PulseShape='Custom' and FilterDenominator is not a scalar), then the demodulator is a Integrate and Dump (I&D) filter where PulseShape='Custom' and FilterNominator=1.

oqpskdemod = comm.OQPSKDemodulator(Name,Value) sets properties using one or more name-value pairs. Enclose each property name in single quotes.

Example: comm.OQPSKDemodulator('BitOutput',true)

oqpskdemod = comm.OQPSKDemodulator(phase,Name,Value) sets the PhaseOffset property of the created object to phase and sets any other specified Name, Value pairs.

Example: comm.OQPSKDemodulator(0.5*pi,'SamplesPerSymbol',2)

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 of zeroth point of signal constellation

0 (default) | scalar

Phase offset from π/4, specified as a scalar in radians. The phase offset is applied to the zeroth point of the signal constellation before delay of quadrature component. After the OQPSK imaginary-component delay the signal is normalized with unity power.

Example: 'PhaseOffset',pi/4 aligns the zeroth point of the QPSK signal constellation point on the axes, {(1,0), (0,j), (-1,0), (0,-j)}.

Data Types: double

BitOutput — Option to output data as bits

false (default) | true

Option to output data as bits, specified as false or true.

• When you set this property to false, the object outputs a column vector of integer values with a length equal to the number of demodulated symbols. The output values are integer representations of two bits and range from 0 to 3.

• When you set this property to true, the object outputs a binary column vector of bit values. The output vector length is twice as long as the number of input symbols.

Data Types: logical

SymbolMapping — Signal constellation bit mapping

'Gray' (default) | 'Binary' | custom 4-element numeric vector of integers with values from 0 to 3

Signal constellation bit mapping, specified as 'Gray', 'Binary', or a custom 4-element numeric vector of integers with values from 0 to 3.

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+π/4) + j*2*π*m/4).
Custom 4-element numeric vector of integers with values from 0 to 3			Elements [a b c d] must be composed of the set of values [0, 1, 2, 3] in any order.

Data Types: char | double

PulseShape — Filtering pulse shape

'Half sine' (default) | 'Normal raised cosine' | 'Root raised cosine' | 'Custom'

Filtering pulse shape, specified as 'Half sine', 'Normal raised cosine' | 'Root raised cosine', or 'Custom'.

Data Types: char

RolloffFactor — Raised cosine filter rolloff factor

0.2 (default) | scalar

Raised cosine filter rolloff factor, specified as a scalar from 0 to 1.

Dependencies

This property is enabled when PulseShape is 'Normal raised cosine' or 'Root raised cosine'.

Data Types: double

FilterSpanInSymbols — Filter length

10 (default) | scalar

Filter length in symbols, specified as a scalar. An ideal raised cosine filter has an infinite impulse response. However, to realize a practical implementation of this filter, the object truncates the impulse response to FilterSpanInSymbols symbols.

Dependencies

This property is enabled when PulseShape is 'Normal raised cosine' or 'Root raised cosine'.

Data Types: double

FilterNumerator — FIR filter numerator

[0.7071 0.7071] (default) | row vector

FIR filter numerator, specified as a row vector.

Dependencies

This property is enabled when PulseShape is 'Custom'.

Data Types: double
Complex Number Support: Yes

SamplesPerSymbol — Number of samples per symbol

4 (default) | positive even integer

Number of samples per symbol, specified as a positive even integer.

Data Types: double

OutputDataType — Data type assigned to output

'double' (default) | 'single' | 'uint8'

Data type assigned to output, specified as 'double', 'single', or 'uint8'.

Data Types: char

Usage

Syntax

outsignal = oqpskdemod(waveform)

Description

outsignal = oqpskdemod(waveform) returns the demodulated output signal. The object produces one output symbol for each input pulse.

Input Arguments

waveform — Received waveform

scalar | column vector

Received waveform, specified as a complex scalar or column vector.

This object accepts variable-size inputs. After the object is locked, you can change the size of each input channel, but you cannot change the number of channels. For more information, see Variable-Size Signal Support with System Objects.

Data Types: double
Complex Number Support: Yes

Output Arguments

outsignal — Demodulated signal

integer vector | bit vector

Demodulated signal, returned as an N_S-element integer vector or bit vector, where N_S is the number of samples.

The received waveform is pulse shaped according to the configuration properties PulseShape and SamplesPerSymbol. The setting of the BitOutput property determines the interpretation of the received waveform.

Data Types: double

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.OQPSKDemodulator

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

OQPSK Signal in AWGN

Create an OQPSK modulator and demodulator pair. Create an AWGN channel object having two bits per symbol.

oqpskmod = comm.OQPSKModulator('BitInput',true);
oqpskdemod = comm.OQPSKDemodulator('BitOutput',true);
channel = comm.AWGNChannel('EbNo',4,'BitsPerSymbol',2);

Create an error rate calculator. To account for the delay between the modulator and demodulator, set the ReceiveDelay property to 2.

errorRate = comm.ErrorRate('ReceiveDelay',2);

Process 300 frames of data looping through these steps.

• Generate vectors with 100 elements of random binary data.

• OQPSK-modulate the data. The data frames are processed as 50 sample frames of 2-bit binary data.

• Pass the modulated data through the AWGN channel.

• OQPSK-demodulate the data.

• Collect error statistics on the frames of data.

for counter = 1:300
    txData = randi([0 1],100,1);
    modSig = oqpskmod(txData);
    rxSig = channel(modSig);
    rxData = oqpskdemod(rxSig);
    errorStats = errorRate(txData,rxData);
end

Display the error statistics.

ber = errorStats(1)

ber = 
3.3336e-05

numErrors = errorStats(2)

numErrors = 
1

numBits = errorStats(3)

numBits = 
29998

OQPSK Signal with Root Raised Cosine Filtering

Perform OQPSK modulation and demodulation and apply root raised cosine filtering to a waveform.

System initialization

Define simulation parameters and create objects for OQPSK modulation and demodulation.

sps = 12; % samples per symbol
bits = randi([0, 1], 800, 1); % transmission data

modulator = comm.OQPSKModulator( ...
    'BitInput',true, ...
    'SamplesPerSymbol',sps, ...
    'PulseShape','Root raised cosine');
demodulator = comm.OQPSKDemodulator(modulator);

Waveform transmission and reception

Use the modulator object to apply OQPSK modulation and transmit filtering to the input data.

oqpskWaveform = modulator(bits);

Pass the waveform through a channel.

snr = 0;
rxWaveform = awgn(oqpskWaveform,snr);

Use the demodulator object to apply receive filtering and OQPSK demodulation to the waveform.

demodData = demodulator(rxWaveform);

Compute the bit error rate to confirm the quality of the data recovery.

delay = (1+modulator.BitInput)*modulator.FilterSpanInSymbols;
[~, ber] = biterr(bits(1:end-delay), demodData(delay+1:end))

ber = 
0

Soft-Decision OQPSK Modulation-Demodulation

Use the qamdemod function to simulate soft decision output for OQPSK-modulated signals.

Generate an OQPSK modulated signal.

sps = 4;
msg = randi([0 1],1000,1);
oqpskMod = comm.OQPSKModulator( ...
    SamplesPerSymbol=sps, ...
    BitInput=true);
oqpskSig = oqpskMod(msg);

Add noise to the generated signal.

impairedSig = awgn(oqpskSig,15);

Perform Soft-Decision Demodulation

Create QPSK equivalent signal to align in-phase and quadrature.

impairedQPSK = complex( ...
    real(impairedSig(1+sps/2:end-sps/2)), ...
    imag(impairedSig(sps+1:end)));

Apply matched filtering to the received OQPSK signal.

halfSinePulse = sin(0:pi/sps:(sps)*pi/sps);
matchedFilter = dsp.FIRDecimator(sps,halfSinePulse, ...
    DecimationOffset=sps/2);
filteredQPSK = matchedFilter(impairedQPSK);

To perform soft demodulation of the filtered OQPSK signal use the qamdemod function. Align symbol mapping of qamdemod with the symbol mapping used by the comm.OQPSKModulator, then demodulate the signal.

oqpskModSymbolMapping = [1 3 0 2];
demodulated = qamdemod(filteredQPSK,4,oqpskModSymbolMapping, ...
    OutputType='llr');

End-to-End Communication System Using Butterworth Pulses

Create a transmitter with the OQPSK Modulator.

bits = randi([0, 1], 800, 1);     % message signal
order = 6;       % CCSDS, TM
btProduct = 0.5; % CCSDS, TM
[num, denom] = butter(order, btProduct);
modulator = comm.OQPSKModulator('BitInput', true, PulseShape="Custom", ...
                         FilterNumerator=num, FilterDenominator=denom);
oqpskWaveform = modulator(bits);

Specify the channel properties.

snr = 10;
received = awgn(oqpskWaveform, snr);

Create a receiver with the OQPSK Demodulator.

demodulator = comm.OQPSKDemodulator(modulator); % FIR Integrate and Dump in Demod, for IIR pulses in Modulator
demodulated = demodulator(received);

Compute the BER.

delay = 1 + modulator.BitInput;
[~, ber] = biterr(bits(1:end-delay), demodulated(delay+1:end));
fprintf('Bit error rate: %f\n', ber);

Bit error rate: 0.051378

More About

Modulation Delays

Digital modulation and demodulation objects incur delays between their inputs and outputs that result in an offset in the arrival sample of the received data. When comparing transmitted data with received data, such as when plotting or computing error statistics, you must take system delays into account. As shown here, the OQPSK modulation-demodulation delay varies depending on the pulse shaping filter and the input/output settings of the object pairs.

Pulse Shape	Input/Output Data (*)	End-to-End Delay Incurred by ay an OQPSK Modulator-Demodulator Object Pair (in samples)
'Half sine' or 'Custom'	Integer	1
	Bit	2
'Normal raised cosine' or 'Root raised cosine'	Integer	FilterSpanInSymbols
	Bit	2*FilterSpanInSymbols
(*) Set the data type property (BitInput for modulation or BitOutput for demodulation) to false for integer data and true for bit data.

Pulse Shaping Filter

The OQPSK modulation scheme requires oversampling of two or greater in order to delay (or offset) the quadrature channel by 90 degrees. This oversampling is achieved through interpolation filtering implemented by pulse shaping.

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