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Convolutionally encode binary data and map using rectangular QAM signal constellation


The RectangularQAMTCMModulator object implements trellis-coded modulation (TCM) by convolutionally encoding the binary input signal and mapping the result to a rectangular QAM signal constellation.

To convolutionally encode binary data and map the result using a rectangular QAM constellation:

  1. Define and set up your rectangular QAM TCM modulator object. See Construction.

  2. Call step to modulate the signal according to the properties of comm.RectangularQAMTCMModulator. The behavior of step is specific to each object in the toolbox.


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.


H = comm.RectangularQAMTCMModulator creates a trellis-coded, rectangular, quadrature amplitude (QAM TCM) System object, H. This object convolutionally encodes a binary input signal and maps the result to a rectangular QAM constellation.

H = comm.RectangularQAMTCMModulator(Name,Value) creates a rectangular QAM TCM modulator 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.RectangularQAMTCMModulator(TRELLIS,Name,Value) creates a rectangular QAM TCM modulator object, H. This object has the TrellisStructure property set to TRELLIS and the other specified properties set to the specified values.



Trellis structure of convolutional code

Specify trellis as a MATLAB structure that contains the trellis description of the convolutional code. Use the istrellis function to check whether a structure is a valid trellis. The default is the result of poly2trellis([3 1 1], [ 5 2 0 0; 0 0 1 0; 0 0 0 1]).


Termination method of encoded frame

Specify the termination method as Continuous | Truncated | Terminated. The default is Continuous.

When you set this property to Continuous, the object retains the encoder states at the end of each input vector for use with the next input vector.

When you set this property to Truncated, the object treats each input vector independently. The encoder is reset to the all-zeros state at the start of each input vector.

When you set this property to Terminated, the object treats each input vector independently. For each input vector, the object uses extra bits to set the encoder to the all-zeros state at the end of the vector. For a rate K/N code, the step method outputs the vector with a length given by y=N×(L+S)K, where S = constraintLength–1 (or, in the case of multiple constraint lengths, S = sum(constraintLength(i)–1)). L is the length of the input to the step method.


Enable modulator reset input

Set this property to true to enable an additional input to the step method. The default is false. When you set the reset input to the step method to a nonzero value, the object resets the encoder to the all-zeros state. This property applies when you set the TerminationMethod property to Continuous.


Number of points in signal constellation

Specify the number of points in the signal constellation used to map the convolutionally encoded data as a positive integer scalar value equal to 4, 8, 16, 32, or 64. The default is 16. The value of the ModulationOrder property must equal the number of possible output symbols from the convolutional encoder of the QAM TCM modulator. Thus, the value for the ModulationOrder property must equal 2N for a rate K/N convolutional code.


Data type of output

Specify the output data type as one of double | single. The default is double.


stepConvolutionally encode binary data and map using rectangular QAM constellation
Common to All System Objects

Allow System object property value changes


Reset internal states of System object


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Modulate data using rectangular 16-QAM TCM modulation and display the scatter plot.

Generate random binary data. The length of the data vector must be an integer multiple of the number of input streams into the encoder, log2(8) = 3.

data = randi([0 1],3000,1);

Create a modulator System object™ and use its step function to modulate the data.

hMod = comm.RectangularQAMTCMModulator;
modData = step(hMod,data);

Plot the modulated data.


Figure Scatter Plot contains an axes object. The axes object with title Scatter plot, xlabel In-Phase, ylabel Quadrature contains a line object which displays its values using only markers. This object represents Channel 1.


This object implements the algorithm, inputs, and outputs described on the Rectangular QAM TCM Encoder block reference page. The object properties correspond to the block parameters.

Extended Capabilities

Version History

Introduced in R2012a