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Convolutionally encode binary data and map using arbitrary QAM constellation


The GeneralQAMTCMModulator object implements trellis-coded modulation (TCM) by convolutionally encoding the binary input signal. The object then maps the result to an arbitrary signal constellation. The Signal constellation property lists the signal constellation points in set-partitioned order.

To modulate a signal using a trellis-coded, general quadrature amplitude modulator:

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

  2. Call step to modulate a signal according to the properties of comm.GeneralQAMTCMModulator. 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.GeneralQAMTCMModulator creates a trellis-coded, general quadrature amplitude (QAM TCM) modulator System object, H. This object convolutionally encodes a binary input signal and maps the result using QAM modulation with a signal constellation specified in the Constellation property.

H = comm.GeneralQAMTCMModulator(Name,Value) creates a general QAM TCM modulator System 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.GeneralQAMTCMModulator(TRELLIS,Name,Value) creates a general QAM TCM modulator System 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 if a structure is a valid trellis structure. The default is the result of poly2trellis([1 3], [1 0 0; 0 5 2]).


Termination method of encoded frame

Specify the termination method as one of 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 length y=N×(L+S)K, where S = constraintLength–1. In the case of multiple constraint lengths, S = sum(constraintLength(i)–1)). L represents 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 this additional reset input is a nonzero value, the internal states of the encoder reset to their initial conditions. This property applies when you set the TerminationMethod property to Continuous.


Signal constellation

Specify a double- or single-precision complex vector that lists the points in the signal constellation that were used to map the convolutionally encoded data. You must specify the constellation in set-partitioned order. See documentation for the General TCM Encoder block for more information on set-partitioned order. The length of the constellation vector must equal the number of possible input symbols to the convolutional decoder of the general QAM TCM demodulator object. This corresponds to 2N for a rate K/N convolutional code. The default corresponds to a set-partitioned order for the points of an 8-PSK signal constellation. This value is expressed exp(2×π×j×[04261537]8).


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 arbitrary QAM constellation
Common to All System Objects

Allow System object property value changes


Reset internal states of System object


collapse all

Modulate data using QAM TCM modulation with an arbitrary 4-point constellation. Display a scatter plot of the modulated data.

Create binary data.

data = randi([0 1],1000,1);

Use the trellis structure with generating polynomial [171 133] and 4-point arbitrary constellation { ejπ/4, ejπ/2, ej3π/4, ej3π/2 } to perform QAM TCM modulation.

t = poly2trellis(7,[171 133]);
hMod = comm.GeneralQAMTCMModulator(t,...
    'Constellation',exp(pi*1i*[1 2 3 6]/4));

Modulate and plot the data.

modData = step(hMod,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 General TCM Encoder block reference page. The object properties correspond to the block parameters.

Extended Capabilities

Version History

Introduced in R2012a