comm.GeneralQAMDemodulator
Demodulate using arbitrary QAM constellation
Description
The comm.GeneralQAMDemodulator
object demodulates a signal that was modulated using
quadrature amplitude modulation. The input is a baseband representation of the modulated
signal.
This object accepts variablesize 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 VariableSize Signal Support with System Objects.
To demodulate a signal that was modulated using quadrature amplitude modulation:
Define and set up your QAM demodulator object. See Construction.
Call
step
to demodulate a signal according to the properties ofcomm.GeneralQAMModulator
. The behavior ofstep
is specific to each object in the toolbox.
Note
Starting in R2016b, instead of using the step
method
to perform the operation defined by the System object™, you can
call the object with arguments, as if it were a function. For example, y
= step(obj,x)
and y = obj(x)
perform
equivalent operations.
Construction
H = comm.GeneralQAMDemodulator
creates
a demodulator System object, H
. This object
demodulates the input signal using a general quadrature amplitude
modulation (QAM) method.
H = comm.GeneralQAMDemodulator(
creates
a general QAM demodulator object, Name
,Value
)H
, with each
specified property set to the specified value. You can specify additional
namevalue pair arguments in any order as (Name1
,Value1
,...,NameN
,ValueN
).
H = comm.GeneralQAMDemodulator(CONST,
creates
a general QAM demodulator object, Name
,Value
)H
. This object
has the Constellation
property set to CONST
,
and the other specified properties set to the specified values.
Properties

Signal constellation Specify the constellation points as a real or complex, doubleprecision data type vector. The default is exp($$2\times \pi \times 1i\times \raisebox{1ex}{$(0:7)$}\!\left/ \!\raisebox{1ex}{$8$}\right.$$). The length of the vector determines the modulation order. When you set the When you set the 

Output data as bits Specify whether the output consists of groups of bits or integer
symbol values. The default is When you set this property to When you set this property to 

Demodulation decision method Specify the decision method the object uses as one of 

Source of noise variance Specify the source of the noise variance as one of 

Noise variance Specify the variance of the noise as a nonzero, real scalar value. The default is
The LLR algorithm involves computing exponentials of very large or very small numbers using finite precision arithmetic and would yield:
In such cases, use approximate LLR because the algorithm does not involve computing exponentials. This property applies when you set the TipsThe exact LLR algorithm computes exponentials using finite precision arithmetic. For computations involving very large positive or negative magnitudes, the exact LLR algorithm yields:
The approximate LLR algorithm does not compute exponentials. You can avoid


Data type of output Specify the output data type as one of This property applies only when you set the When the input data is of a fixedpoint type, the output data
type works as if you had set the When the input signal is an integer data type, you must have
a FixedPoint Designer™ user license to use this property in When you set the When you set the If you set the 
Methods
step  Demodulate using arbitrary QAM constellation 
Common to All System Objects  

release  Allow System object property value changes 
Examples
Modulate and demodulate data using an arbitrary threepoint constellation.
% Setup a three point constellation c = [1 1i 1]; hQAMMod = comm.GeneralQAMModulator(c); hAWGN = comm.AWGNChannel('NoiseMethod', ... 'Signal to noise ratio (SNR)','SNR',15,'SignalPower',0.89); hQAMDemod = comm.GeneralQAMDemodulator(c); %Create an error rate calculator hError = comm.ErrorRate; for counter = 1:100 % Transmit a 50symbol frame data = randi([0 2],50,1); modSignal = step(hQAMMod, data); noisySignal = step(hAWGN, modSignal); receivedData = step(hQAMDemod, noisySignal); errorStats = step(hError, data, receivedData); end fprintf('Error rate = %f\nNumber of errors = %d\n', ... errorStats(1), errorStats(2))
More About
Algorithms
This object implements the algorithm, inputs, and outputs described on the General QAM Demodulator Baseband block reference page. The object properties correspond to the block parameters.
Extended Capabilities
Version History
Introduced in R2012a