# comm.PSKTCMDemodulator

Demodulate convolutionally encoded data mapped to M-ary PSK signal constellation

## Description

The `PSKTCMDemodulator` object uses the Viterbi algorithm to decode a trellis-coded modulation (TCM) signal that was previously modulated using a PSK signal constellation.

To demodulate a signal that was modulated using trellis-coded modulation:

1. Define and set up your PSK TCM demodulator object. See Construction.

2. Call `step` to demodulate the signal according to the properties of `comm.PSKTCMDemodulator`. 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.PSKTCMDemodulator` creates a trellis-coded, M-ary phase shift, keying (PSK TCM) demodulator System object, `H`. This object demodulates convolutionally encoded data that has been mapped to an M-PSK constellation.

`H = comm.PSKTCMDemodulator(Name,Value)` creates a PSK TCM 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.PSKTCMDemodulator(TRELLIS,Name,Value)` creates a PSK TCM demodulator System object, `H`. This object has the `TrellisStructure` property set to `TRELLIS` and the other specified properties set to the specified values.

## Properties

 `TrellisStructure` 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 the trellis structure is valid. The default is the result of `poly2trellis`(```[1 3], [1 0 0; 0 5 2]```). `TerminationMethod` 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 saves the internal state metric at the end of each frame. The next frame uses the same state metric. The object treats each traceback path independently. If the input signal contains only one symbol, use `Continuous` mode. When you set this property to `Truncated`, the object treats each input vector independently. The traceback path starts at the state with the best metric and always ends in the all-zeros state. When you set property to `Terminated`, the object treats each input vector independently, and the traceback path always starts and ends in the all-zeros state. `TracebackDepth` Traceback depth for Viterbi decoder Specify the scalar, integer number of trellis branches to construct each traceback path. The default is `21`. The traceback depth influences the decoding accuracy and delay. The decoding delay is the number of zero symbols that precede the first decoded symbol in the output. When you set the `TerminationMethod` property to `Continuous`, the decoding delay consists of TracebackDepth zero symbols or TracebackDepth$×$K zero bits for a rate K/N convolutional code. When you set the `TerminationMethod` property to `Truncated` or `Terminated`, no output delay occurs and the traceback depth must be less than or equal to the number of symbols in each input vector. `ResetInputPort` Enable demodulator 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 initial conditions. This property applies when you set the `TerminationMethod` property to `Continuous`. `ModulationOrder` 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. The number of points must be `4`, `8`, or `16`. The default is `8`. The `ModulationOrder` property value must equal the number of possible input symbols to the convolutional decoder of the PSK TCM demodulator object. The `ModulationOrder` property must equal 2N for a rate K/N convolutional code. `OutputDataType` Data type of output Specify output data type as `logical` | `double`. The default is `double`.

## Methods

 step Demodulate convolutionally encoded data mapped to M-ary PSK constellation
Common to All System Objects
`release`

Allow System object property value changes

`reset`

Reset internal states of System object

## Examples

collapse all

Modulate and demodulate data using 8-PSK TCM modulation in an AWGN channel. Estimate the resulting error rate.

Define a trellis structure with four input symbols and eight output symbols.

`t = poly2trellis([5 4],[23 35 0; 0 5 13]);`

Create modulator and demodulator System objects™ using trellis, `t`, having modulation order 8.

```hMod = comm.PSKTCMModulator(t,'ModulationOrder',8); hDemod = comm.PSKTCMDemodulator(t,'ModulationOrder',8, ... 'TracebackDepth',16);```

Create an AWGN channel object.

```hAWGN = comm.AWGNChannel('NoiseMethod','Signal to noise ratio (SNR)', ... 'SNR',7);```

Create an error rate calculator with delay in bits equal to `TracebackDepth` times the number of bits per symbol.

```hError = comm.ErrorRate('ReceiveDelay',... hDemod.TracebackDepth*log2(t.numInputSymbols));```

Generate random binary data and modulate with 8-PSK TCM. Pass the modulated signal through the AWGN channel and demodulate. Calculate the error statistics.

```for counter = 1:10 % Transmit frames of 250 2-bit symbols data = randi([0 1],500,1); % Modulate modSignal = step(hMod,data); % Pass through AWGN channel noisySignal = step(hAWGN,modSignal); % Demodulate receivedData = step(hDemod,noisySignal); % Calculate error statistics errorStats = step(hError,data,receivedData); end```

Display the BER and the number of bit errors.

```fprintf('Error rate = %5.2e\nNumber of errors = %d\n', ... errorStats(1),errorStats(2))```
```Error rate = 2.17e-02 Number of errors = 108 ```

## Algorithms

This object implements the algorithm, inputs, and outputs described on the M-PSK TCM Decoder block reference page. The object properties correspond to the block parameters.

## Extended Capabilities

Introduced in R2012a