comm.GeneralQAMDemodulator

Demodulate using arbitrary QAM constellation

Description

The GeneralQAMDemodulator object demodulates a signal that was modulated using quadrature amplitude modulation. The input is a baseband representation of the modulated signal.

To demodulate a signal that was modulated using quadrature amplitude modulation:

1. Define and set up your QAM demodulator object. See Construction.

2. Call step to demodulate a signal according to the properties of comm.GeneralQAMModulator. The behavior of step 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(Name,Value) creates a general QAM demodulator object, H, 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).

H = comm.GeneralQAMDemodulator(CONST,Name,Value) creates a general QAM demodulator object, H. This object has the Constellation property set to CONST, and the other specified properties set to the specified values.

Properties

Constellation	Signal constellation Specify the constellation points as a real or complex, double-precision 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 BitOutput property to false, the step method outputs a vector with integer values. These integers are between 0 and M–1, where M is the length of this property vector. The length of the output vector equals the length of the input signal. When you set the BitOutput property to true, the output signal contains bits. For bit outputs, the size of the signal constellation requires an integer power of two and the output length is an integer multiple of the number of bits per symbol.
BitOutput	Output data as bits Specify whether the output consists of groups of bits or integer symbol values. The default is false. When you set this property to true the step method outputs a column vector of bit values with length equal to log2(M) times the number of demodulated symbols, where M is the length of the signal constellation specified in the Constellation property. The length M determines the modulation order. When you set this property to false, the step method outputs a column vector, of length equal to the input data vector. The vector contains integer symbol values between 0 and M–1.
DecisionMethod	Demodulation decision method Specify the decision method the object uses as one of Hard decision | Log-likelihood ratio | Approximate log-likelihood ratio. The default is Hard decision. When you set the BitOutput property to false the object always performs hard decision demodulation. This property applies when you set the BitOutput property to true.
VarianceSource	Source of noise variance Specify the source of the noise variance as one of Property | Input port. The default is Property. This property applies when you set the DecisionMethod property to Log-likelihood ratio or Approximate log-likelihood ratio.
Variance	Noise variance Specify the variance of the noise as a nonzero, real scalar value. The default is 1. The LLR algorithm involves computing exponentials of very large or very small numbers using finite precision arithmetic and would yield: Inf to -Inf if the variance is very high NaN if the variance and signal power are both very small In such cases, use approximate LLR because the algorithm does not involve computing exponentials. This property applies when you set the VarianceSource property to Property. This property is nontunable for fixed-point inputs. Tips The exact LLR algorithm computes exponentials using finite precision arithmetic. For computations involving very large positive or negative magnitudes, the exact LLR algorithm yields: Inf or -Inf if the noise variance is a very large value NaN if the noise variance and signal power are both very small values The approximate LLR algorithm does not compute exponentials. You can avoid Inf, -Inf, and NaN results by using the approximate LLR algorithm.
OutputDataType	Data type of output Specify the output data type as one of Full precision | Smallest unsigned integer | double | single | int8 | uint8 | int16 | uint16 | int32 | uint32. The default is Full precision . This property applies only when you set the BitOutput property to false or when you set the BitOutput property to true and the DecisionMethod property to Hard decision or Approximate log-likelihood ratio. In this case, when you set the OutputDataType property to Full precision, the output data type is the same as that of the input when the input data has a single or double-precision data type. When the input data is of a fixed-point type, the output data type works as if you had set the OutputDataType property to Smallest unsigned integer. When the input signal is an integer data type, you must have a Fixed-Point Designer™ user license to use this property in Smallest unsigned integer or Full precision mode. When you set the BitOutput property to true, and the DecisionMethod property to Hard Decision the data type logical becomes a valid option. When you set the BitOutput property to true and the DecisionMethod property to Approximate log-likelihood ratio you may only set this property to Full precision | Custom. If you set the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio, the output data has the same type as that of the input. In this case, that value can be only single or double precision.

 Fixed-Point Properties

FullPrecisionOverride	Full precision override for fixed-point arithmetic Specify whether to use full precision rules. If you set FullPrecisionOverride to true, which is the default, the object computes all internal arithmetic and output data types using full precision rules. These rules provide the most accurate fixed-point numerics. It also turns off the display of other fixed-point properties because they do not apply individually. These rules guarantee that no quantization occurs within the object. Bits are added, as needed, to ensure that no roundoff or overflow occurs. If you set FullPrecisionOverride to false, fixed-point data types are controlled through individual fixed-point property settings. For more information, see Full Precision for Fixed-Point System Objects.
RoundingMethod	Rounding of fixed-point numeric values Specify the rounding method as one of Ceiling | Convergent | Floor | Nearest | Round | Simplest | Zero. The default is Floor. This property applies when the object is not in a full precision configuration. This property does not apply when you set BitOutput to true and DecisionMethod to Log-likelihood ratio.
OverflowAction	Action when fixed-point numeric values overflow Specify the overflow action as one of Wrap | Saturate. The default is Wrap. This property applies when the object is not in a full precision configuration. This property does not apply when you set the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
ConstellationDataType	Data type of signal constellation Specify the constellation fixed-point data type as one of Same word length as input | Custom. The default is Same word length as input. This property does not apply when you set the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
CustomConstellationDataType	Fixed-point data type of signal constellation Specify the constellation fixed-point type as an unscaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],16). This property applies when you set the ConstellationDataType property to Custom.
Accumulator1DataType	Data type of accumulator 1 Specify the accumulator 1 fixed-point data type as one of Full precision | Custom. The default is Full precision. This property applies when you set the FullPrecisionOverride property to false. This property does not apply when you set the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
CustomAccumulator1DataType	Fixed-point data type of accumulator 1 Specify the accumulator 1 fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the Accumulator1DataType property to Custom.
ProductInputDataType	Data type of product Specify the product input fixed-point data type as one of Same as accumulator 1 | Custom. The default is Same as accumulator 1. This property applies when you set the FullPrecisionOverride property to false, the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
CustomProductInputDataType	Fixed-point data type of product Specify the product input fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the ProductInputDataType property to Custom.
ProductOutputDataType	Data type of product output Specify the product output fixed-point data type as one of Full precision | Custom. The default is Full precision . This property applies when you set the FullPrecisionOverride property to false, the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
CustomProductOutputDataType	Fixed-point data type of product output Specify the product output fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the ProductOutputDataType property to Custom.
Accumulator2DataType	Data type of accumulator 2 Specify the accumulator 2 fixed-point data type as one of Full precision | Custom. The default is Full precision . This property applies when you set the FullPrecisionOverride property to false, the BitOutput property to true and the DecisionMethod property to Log-likelihood ratio.
CustomAccumulator2DataType	Fixed-point data type accumulator 2 Specify the accumulator 2 fixed-point data type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the Accumulator2DataType property to Custom.
Accumulator3DataType	Data type of accumulator 3 Specify the accumulator 3 fixed-point data type as one of Full precision | Custom. The default is Full precision . This property applies when you set the FullPrecisionOverride property to false, the BitOutput property to true and the DecisionMethod property to Approximate log-likelihood ratio.
CustomAccumulator3DataType	Fixed-point data type of accumulator 3 Specify the accumulator 3 fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the Accumulator3DataType property to Custom.
NoiseScalingInputDataType	Data type of noise-scaling input Specify the noise-scaling input fixed-point data type as one of Same as accumulator 3 | Custom. The default is Same as accumulator 3. This property applies when you set the FullPrecisionOverride property to false, the BitOutput property to true and the DecisionMethod property to Approximate log-likelihood ratio.
CustomNoiseScalingInputDataType	Fixed-point data type of noise-scaling input Specify the noise-scaling input fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the NoiseScalingInputDataType property to Custom.
InverseVarianceDataType	Data type of inverse noise variance Specify the inverse noise variance fixed-point data type as one of Same word length as input | Custom. The default is Same word length as input. This property applies when you set the BitOutput property to true, the DecisionMethod property to Approximate log-likelihood ratio, and the VarianceSource property to Property.
CustomInverseVarianceDataType	Fixed-point data type of inverse noise variance Specify the inverse noise variance fixed-point type as a numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],16,8). This property applies when you set the InverseVarianceDataType property to Custom.
CustomOutputDataType	Data type of output Specify the output fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto. The default is numerictype([],32,30). This property applies when you set the FullPrecisionOverride property to false and the OutputDataType property to Custom.

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 three-point 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 50-symbol 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

Full Precision for Fixed-Point System Objects

FullPrecisionOverride is a convenience property that, when you set to true, automatically sets the appropriate properties for an object to use full-precision to process fixed-point input.

For System objects, full precision, fixed-point operation refers to growing just enough additional bits to compute the ideal full precision result. This operation has no minimum or maximum range overflow nor any precision loss due to rounding or underflow. It is also independent of any hardware-specific settings. The data types chosen are based only on known data type ranges and not on actual numeric values. Full precision for System objects does not optimize coefficient values. When you set the FullPrecisionOverride property to true, the other fixed-point properties it controls no longer apply and any of their non-default values are ignored. These properties are also hidden. To specify individual fixed-point properties, first set FullPrecisionOverride to false.

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

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The Variance property is nontunable when using code generation.

• See System Objects in MATLAB Code Generation (MATLAB Coder).

Version History

Introduced in R2012a