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:

Define and set up your QAM demodulator object. See Construction.
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( $2 \times π \times 1 i \times \frac{(0 : 7)}{8}$ ). 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`. If this value is very small (i.e., SNR is very high), log-likelihood ratio (LLR) computations may yield Inf or -Inf. This result occurs because the LLR algorithm would compute the exponential of very large or very small numbers using finite-precision arithmetic. In such cases, using approximate LLR is recommended because its algorithm does not compute exponentials. This property applies when you set the `VarianceSource` property to `Property`. This property is tunable.
`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

expand all

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.

Documentation

comm.GeneralQAMDemodulator

Description

Construction

Properties

Methods

Examples

More About

Full Precision for Fixed-Point System Objects

Algorithms

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

See Also

Functions

Objects

Communications Toolbox Documentation

Support

Documentation

comm.GeneralQAMDemodulator

Description

Construction

Properties

Methods

Examples

More About

Full Precision for Fixed-Point System Objects

Algorithms

Extended Capabilities

C/C++ Code Generation Generate C and C++ code using MATLAB® Coder™.

See Also

Functions

Objects

Communications Toolbox Documentation

Support

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.