General TCM Decoder
Decode trellis-coded modulation data, mapped using arbitrary constellation
Library
TCM, in Digital Baseband sublibrary of Modulation
Description
The General TCM Decoder block uses the Viterbi algorithm to decode a trellis-coded modulation (TCM) signal that was previously modulated using an arbitrary signal constellation.
The Trellis structure and Signal constellation parameters in this block should match those in the General TCM Encoder block, to ensure proper decoding. In particular, the Signal constellation parameter must be in set-partitioned order.
Input and Output Signals
This block accepts a column vector input signal containing complex numbers. The
input signal must be double or single. The
reset port signal must be double or Boolean.
For information about the data types each block port supports, see Supported Data Types.
If the convolutional encoder described by the trellis structure represents a rate k/n code, then the General TCM Decoder block's output is a 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
Continuousmode, 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 Enable the reset input port, the block displays another input port, labeled
Rst. This port receives an integer scalar signal. Whenever the value at theRstport is nonzero, the block resets all state metrics to zero and sets the traceback memory to zero.In
Truncatedmode, 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
Terminatedmode, 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.
Parameters
- Trellis structure
MATLAB structure that contains the trellis description of the convolutional encoder.
- Signal constellation
A complex vector that lists the points in the signal constellation in set-partitioned order.
- 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. The choices are
Continuous,Truncated, andTerminated.- Enable the reset input port
When you select this check box, the block has a second input port labeled
Rst. Providing a nonzero value to this port causes the block to set its internal memory to the initial state before processing the input data. This field appears only if you set Operation mode toContinuous.- Output data type
Select the data type for the block output signal as
booleanorsingle. By default, the block sets this todouble.
Supported Data Types
| Port | Supported Data Types |
|---|---|
Input |
|
Reset |
|
Output |
|
Pair Block
References
[1] Biglieri, E., D. Divsalar, P. J. McLane, and M. K. Simon, Introduction to Trellis-Coded Modulation with Applications, New York, Macmillan, 1991.
[2] Proakis, John G., Digital Communications, Fourth edition, New York, McGraw-Hill, 2001.