# Documentation

### This is machine translation

Translated by
Mouseover text to see original. Click the button below to return to the English verison of the page.

To view all translated materals including this page, select Japan from the country navigator on the bottom of this page.

# comm.HDLRSDecoder System object

Package: comm

Decode data using a Reed-Solomon decoder

## Description

The HDL-optimized `HDLRSDecoder` System object™ recovers a message vector from a Reed-Solomon codeword vector. For proper decoding, the property values for this object should match those in the corresponding `HDLRSEncoder` System object.

To recover a message vector from a Reed-Solomon codeword vector optimized for HDL code generation:

1. Define and set up your HDL RS decoder object. See Construction.

2. Call `step` to recover a message vector from a Reed-Solomon codeword vector according to the properties of `comm.HDLRSDecoder`. 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.

### Troubleshooting

• Each input frame must contain more than `(N-K)*2` symbols, and fewer than or equal to `N` symbols. A shortened code is inferred when the number of valid data samples between `startIn` and `endIn` is less than `N`. A shortened code still requires `N` cycles to perform the Chien search. If the input is less than `N` symbols, leave a guard interval of at least `N-size` inactive cycles before starting the next frame.

• The decoder can operate on up to 4 messages at a time. If the object receives the start of a fifth message before completely decoding the first message, the object drops data samples from the first message. To avoid this issue, increase the number of inactive cycles between input messages.

• The generator polynomial is not specified explicitly. However, it is defined by the code word length, the message length, and the B value for the starting exponent of the roots. To get the value of B from a generator polynomial, use the `genpoly2b` function.

## Construction

`H = comm.HDLRSDecoder` creates an HDL-optimized RS decoder System object, `H`, that performs Reed-Solomon (RS) decoding.

`H = comm.HDLRSDecoder(Name,Value)` creates an HDL-optimized RS decoder System object, `H`, with additional options specified by one or more `Name,Value` pair arguments, where Name is a property name and Value is the corresponding value. Name must appear inside single quotes (`''`). You can specify several name-value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

`H = comm.HDLRSDecoder(N,K,Name,Value)` creates an HDL-optimized RS decoder System object, `H`, with the `CodewordLength` property set to `N`, the `MessageLength` property set to `K`, and other specified property names set to the specified values.

## Properties

 `B` B value for polynomial generation `BSource` Source of B, the starting power for roots of the primitive polynomial Specify the source of the B value as one of these values: Auto: B=0Property Default: Auto `CodewordLength` Codeword length Specify the codeword length of the RS code as a double-precision, positive, integer scalar value. The default is `7`. If you set the `PrimitivePolynomialSource` property to `Auto`, `CodewordLength` must be in the range 3 < `CodewordLength` $\le$ 216–1. When you set the `PrimitivePolynomialSource` property to `Property`, `CodewordLength` must be in the range 3 $\le$ `CodewordLength` $\le$ 2M–1. M is the degree of the primitive polynomial that you specify with the `PrimitivePolynomialSource` and `PrimitivePolynomial` properties. M must be in the range 3$\le$ M $\le$ 16. The difference (`CodewordLength` –`MessageLength`) must be an even integer. The value of this property is rounded up to 2M–1. If the value of this property is less than 2M–1, the object assumes a shortened RS code. `MessageLength` Message length Specify the message length as a double-precision, positive integer scalar value. The default is `3`. The difference (`CodewordLength` – `MessageLength`) must be an even integer. `NumErrorsOutputPort` Enable number of errors output When you set this property to true, the step method outputs number of corrected errors. The number of corrected errors is not valid when errOut is asserted, since there were more errors than could be corrected. The default is false. `PrimitivePolynomialSource` Source of primitive polynomial Specify the source of the primitive polynomial as `Auto` | `Property`. The default is `Auto`. When you set this property to `Auto`, the object uses a primitive polynomial of degree M = `ceil`(log2(`CodewordLength`+1)), which is the result of `fliplr`(`de2bi`(`primpoly`(M))). When you set this property to `Property`, you can specify a polynomial using the `PrimitivePolynomial` property. `PrimitivePolynomial` Primitive polynomial Specify the primitive polynomial that defines the finite field `GF`(2M) corresponding to the integers that form messages and codewords. You must set this property to a double-precision, binary row vector that represents a primitive polynomial over `GF`(2) of degree M in descending order of powers. This property applies when you set the `PrimitivePolynomialSource` property to `Property`.

## Methods

 clone Create HDLRSDecoder System object with same property values isLocked Locked status for input attributes and nontunable properties release Allow property value and input characteristics change step Perform Reed-Solomon decoding

## Examples

expand all

Create an HDLRSEncoder object with RS(255,239) code. This is the code used in the IEEE802.16 Broadband Wireless Access standard.

B is the starting power of the roots of the primitive polynomial.

```rsEnc = comm.HDLRSEncoder(255,239,'BSource','Property','B',0) ```
```rsEnc = comm.HDLRSEncoder with properties: CodewordLength: 255 MessageLength: 239 PrimitivePolynomialSource: 'Auto' PuncturePatternSource: 'None' BSource: 'Property' B: 0 ```

Create a random message to encode. This message is smaller than the codeword length to demonstrate the shortened-code capability of the objects. Pad the message with zeros to accomodate the Chien search in the decoder and the decoder latency.

```messageLength = 188; dataIn = [randi([0,255],1,messageLength,'uint8') zeros(1,1024-messageLength)]; for ii = 1:1024 messageStart = (ii==1); messageEnd = (ii==messageLength); validIn = (ii<=messageLength); [encOut(ii), startOut(ii), endOut(ii), validOut(ii)] = step(rsEnc, dataIn(ii), messageStart, messageEnd, validIn); end ```

Inject errors at random locations in the encoded message. Reed-Solomon can correct up to (N-K)/2 errors in each N symbols. So, in this example the error correction capability is (255-239)/2=8 symbols.

```numErrors = 8; loc = randperm(messageLength, numErrors); % encOut is qualified by validOut, use an offset for injecting errors vi = find(validOut==true,1); for i = 1:numErrors idx = loc(i)+vi; symbol = encOut(idx); encOut(idx) = randi([0 255],'uint8'); fprintf('Symbol(%d), was 0x%x now 0x%x.\n', loc(i), symbol, encOut(idx)) end ```
```Symbol(147), was 0x1f now 0x82. Symbol(16), was 0x6b now 0x82. Symbol(173), was 0x3 now 0xd1. Symbol(144), was 0x66 now 0xcb. Symbol(90), was 0x13 now 0xa4. Symbol(80), was 0x5a now 0x60. Symbol(82), was 0x95 now 0xcf. Symbol(56), was 0xf5 now 0x88. ```

Create an RS Decoder to detect and correct errors in the message. It must have the same code and polynomial as the encoder.

```rsDec = comm.HDLRSDecoder(255,239,'BSource','Property','B',0) for ii = 1:1024 [decOut(ii), decStartOut(ii), decEndOut(ii), decValidOut(ii), decErrOut(ii)] = step(rsDec, encOut(ii), startOut(ii), endOut(ii), validOut(ii)); end ```
```rsDec = comm.HDLRSDecoder with properties: CodewordLength: 255 MessageLength: 239 PrimitivePolynomialSource: 'Auto' BSource: 'Property' B: 0 NumErrorsOutputPort: false ```

Select the valid decoder output and compare decoded symbols to the original message.

```decOut = decOut(decValidOut==1); originalMessage = dataIn(1:messageLength); if all(originalMessage==decOut) fprintf('All %d message symbols were correctly decoded.\n', messageLength) else for jj = 1:messageLength if dataIn(jj)~=decOut(jj) fprintf('Error in decoded symbol(%d). Original 0x%x Decoded 0x%x.\n',jj,dataIn(jj),decOut(jj)) end end end ```
```All 188 message symbols were correctly decoded. ```