# Using HDL Optimized RS Encoder/Decoder Library Blocks

This example shows how to implement encoder and decoder for the IEEE® 802.16 standard [ 1 ] using the HDL Optimized Reed-Solomon (RS) Encoder and Decoder library blocks.

## Contents

## Introduction

The RS code is a nonbinary block code. A RS code that maps information symbols into a codeword of symbol length is denoted as RS(, ) code. The symbols for the code are integers between and , which represent elements of the finite field GF(). The IEEE 802.16 Broadband Wireless Access standard [ 1 ] employs a shortened version of the RS(255,239) code generated on GF(256), i.e., , , and . RS encoder introduces parity symbols, which are used by the RS decoder to detect and correct symbol errors. The code can correct up to symbol errors in each codeword.

This model shows how to use HDL Optimized RS Encoder and Decoder library blocks for simulation and HDL Code generation. It implements the encoding and error correction for the IEEE 802.16 standard. For details about HDL support for HDL Optimized RS Encoder and Decoder blocks, refer to encoder reference page or decoder reference page. To learn more about the algorithm used in the blocks refer to [ 2 ].

To open this example model, run the following commands:

```
modelname = 'commrshdl';
open_system(modelname);
```

## Source

The **Source** subsystem generates the information symbols for the RS Encoder. To open the **Source** subsystem, run the following commands:

```
systemname = [modelname '/Source'];
open_system(systemname);
```

One of the messages (information symbols) employed by the IEEE 802.16 standard contains the following 36 bytes (Randomized data specified on page 827 of [ 1 ]).

message = [D4 BA A1 12 F2 74 96 30 27 D4 88 9C 96 E3 A9 52 B3 15 AB FD 92 53 07 32 C0 62 48 F0 19 22 E0 91 62 1A C1 00].

The **Source** repeatedly transmits the message followed by a guard interval. The model has parameters **messagelength**, for the number of symbols in the message to encode; and **period**, which includes the **messagelength** and the length of the guard interval. The guard interval between messages accommodates the latency of the encoder adding parity check symbols to the message, and the decoder performing a Chien search. In the initFcn callback of the model, the **messagelength** is set to 36 and **period** is set to 236 (which suggest that the guard interval has a length of 200 symbols).

Note that the values of **messagelength** and **period** can be varied as desired.

The top-level **RS Subsystem** contains the HDL Optimized RS Encoder and Decoder blocks. To open the **RS subsystem**, run the following commands:

```
systemname = [modelname '/RS Subsystem'];
open_system(systemname);
```

The values of and are set in the InitFcn callback of the model and are used to configure the HDL Optimized RS Encoder and Decoder blocks. The values of and cannot be changed in this model.

The RS encoder infers a shortened code if the message length is less than symbols. In this case, it will pad the input message with zeros, encodes the padded message, and appends 16 parity check symbols. The block then removes the added zeros symbols, creating a symbol output.

The field generator polynomial employed by IEEE 802.16 standard is . Accordingly, for both RS encoder and decoder, the **Source of primitive polynomial** is set as **Property**, the **Primitive polynomial** is set as [1 0 0 0 1 1 1 0 1], the **Source of B which is the starting power for roots of the primitive polynomial** is set as **Property**, and the **B value** is set as **0**. The code generator polynomial used by IEEE 802.16 standard is , where .

Restrictions on and the codeword length are detailed here. The **ErrorGen** subsystem adds noise to the RS encoded message. To open the **ErrorGen** subsystem, run the following commands:

```
systemname = [modelname '/RS Subsystem/ErrorGen'];
open_system(systemname);
```

The **ErrorGen** subsystem implements the logic to add noise to the codewords at locations specified in the **Noise Locations** constant. The location can be changed as desired. In this example, the noise will be added to the 5th, 23rd, 34th, and 12th codewords, corresponding to the symbols F2, 07, 1A, and 9C. The MATLAB® function block outputs logical true only at these four time instances for each packet, and activates a bitwise XOR operation between the original symbols and the noise.

## Output Signals

Run the model using the following command:

sim(modelname);

## Viewing the Signals

The Logic Analyzer can be used to view multiple signals in one window and viewing signals this way makes it easier to observe transitions. Signals in this model at various stages, namely, before encoding, after encoding, after adding noise, and after decoding are streamed. The blue icon in the model indicates streamed signals. Launch the Logic Analyzer from the model's toolbar.

## Analysis of Results

In the Logic Analyzer output the inputdata signal represents the input of the RS encoder block and this is the 36 byte message given in the IEEE 802.16 specification. The encoded data shows the output of the RS encoder block. Note that the IEEE 802.16 specification performs puncturing of the parity bytes and retains only the first four bytes of the 16 bytes. In this demo all 16 bytes of parity are used and the first four bytes of parity are 49, 31, 40, and BF, matching the IEEE 802.16 specification.

The errdata signal represents the encoded data with noise added in the specified noise locations. These noise locations are marked with 1s in the inserterr signal.

The decoded and corrected message out of the RS decoder block is shown by the outputdata signal. Note that the RS decoder block introduces about 3 period lengths of latency. Observe outputdata to see that the errors induced by noise are corrected.

## Generate HDL Code and Test Bench

To check and generate HDL code for this example, you must have an HDL Coder™ license.

Get a unique temporary directory name for the generated files,

workingdir = tempname;

To check whether there are any issues with the model for HDL code generation, you can run the following command:

checkhdl('commrshdl/RS Subsystem','TargetDirectory',workingdir);

Enter the following command to generate HDL code:

makehdl('commrshdl/RS Subsystem','TargetDirectory',workingdir);

Enter the following command to generate the test bench:

makehdltb('commrshdl/RS Subsystem','TargetDirectory',workingdir);

## ModelSim Output

The following figure shows the ModelSim HDL simulator after running the generated .do file scripts for the test bench. Compare the ModelSim result with the Simulink result as plotted before.

## Selected References

1. IEEE 802.16: IEEE Standard for Air Interface for Broadband Wireless Access Systems(Revision of IEEE Std 802.16-2009). IEEE-SA. 8 June 2012. http://standards.ieee.org/getieee802/download/802.16-2012.pdf

2. George C. Clark Jr, J. Bibb Cain, Error-Correction Coding for Digital Communications, New York: Springer, 1981.