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Rectangular QAM TCM Decoder - Decode trellis-coded modulation data, modulated using QAM method

Library

Trellis-Coded Modulation

Description

The Rectangular QAM TCM Decoder block uses the Viterbi algorithm to decode a trellis-coded modulation (TCM) signal that was previously modulated using a QAM signal constellation.

The M-ary number parameter is the number of points in the signal constellation, which also equals the number of possible output symbols from the convolutional encoder. (That is, log₂(M-ary number) is the number of output bit streams from the convolutional encoder.)

The Trellis structure and M-ary number parameters in this block should match those in theRectangular QAM TCM Encoder block, to ensure proper decoding.

Input and Output Signals

The input signal must be a frame-based column vector containing complex numbers.

If the convolutional encoder described by the trellis structure represents a rate k/n code, then the Rectangular QAM TCM Decoder block's output is a frame-based binary column vector whose length is k times the vector length of the input signal.

Operation Modes

The block has three possible methods for transitioning between successive frames. The Operation mode parameter controls which method the block uses. This parameter also affects the range of possible values for the Traceback depth parameter, D.

In Continuous mode, the block initializes all state metrics to zero at the beginning of the simulation, waits until it accumulates D symbols, and then uses a sequence of D symbols to compute each of the traceback paths. D can be any positive integer. At the end of each frame, the block saves its internal state metric for use with the next frame.
If you select the Enable the reset input check box, the block displays another input port, labeled Rst. This port receives an integer scalar signal. Whenever the value at the Rst port is nonzero, the block resets all state metrics to zero and sets the traceback memory to zero.
In Truncated mode, the block treats each frame independently. The traceback path starts at the state with the lowest metric. D must be less than or equal to the vector length of the input.
In Terminated mode, the block treats each frame independently. The traceback path always starts at the all-zeros state. D must be less than or equal to the vector length of the input. If you know that each frame of data typically ends at the all-zeros state, then this mode is an appropriate choice.

Decoding Delay

If you set Operation mode to Continuous, then this block introduces a decoding delay equal to Traceback depth*k bits, for a rate k/n convolutional code. The decoding delay is the number of zeros that precede the first decoded bit in the output.

The block incurs no delay for other values of Operation mode.

Dialog Box

Trellis structure: MATLAB structure that contains the trellis description of the convolutional encoder.
M-ary number: The number of points in the signal constellation.
Traceback depth: The number of trellis branches (equivalently, the number of symbols) the block uses in the Viterbi algorithm to construct each traceback path.
Operation mode: The operation mode of the Viterbi decoder. Choices are Continuous, Truncated, and Terminated.
Enable the reset input port: When you check this box, the block has a second input port labeled Rst. Providing a nonzero input value to this port causes the block to set its internal memory to the initial state before processing the input data. This option appears only if you set Operation mode to Continuous.