SOURCEFORGE » RALF » 2015 Erasure codes for Cubesat missions

h1. III - Advanced Project.

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h2. 1. Adapting project to elementary task.

See [[Wiki#"3. Creation of a simple MPLAB X IDE project"|Tutorial page]] for project bases.

In this part, you will have to modify particular files of the previous folder Test/.

h3. 1.1 Integration of C Source files into task(s).

* *task_name.c* stating _void task_name (void) { ... }_.

Here you can either call the C main function in the task_name function or adapting your main file into one or several tasks (for simultaneity during process).

* *task_name.h* stating _extern void task_name (void);_ and other intern functions.

h3. 1.2 Definition of your task(s) to be called.

* *tasks.h* defining OSTCBP of all predifined tasks i.e. the way of specifying a pointer to a particular Salvo _Task Control Block_. More information is available on Salvo User Manual.

* *task_cmd.c* which is a tool provided by Test/ that allows to resume/stop your task(s).

!task_cmd.png!

* *main.c* i.e. the basic implementation to call your task(s). All statements are essential for Salvo and CSK to work.

!main.png!

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h2. 2. Integration of a Reed-Solomon encoder/decoder.

A highly convenient implementation of a Reed-Solomon code is available on "RS Code Website":http://rscode.sourceforge.net/ and will be integrated in the following.

h3. 2.1 Presentation of Reed-Solomon.

This is an implementation of a Reed-Solomon code with 8 bit bytes, and a configurable number of parity bytes. The maximum sequence length (codeword) that can be generated is 255 bytes, including parity bytes. It provides the following C/H - files :

(1) *rs.c*, (2) *berlekamp.c*, (3) *crcgen.c*, (4) *ecc.h* and (5) *example.c*

The *rs.c* file contains an algorithm to generate the encoder polynomial for any number of bytes of parity, configurable as the _NPAR_ constant in the file *ecc.h*. For this RS code, G(x) = (x-a^1)(x-a^2)(x-a^3)(x-a^4)...(x-a^NPAR) where 'a' is a primitive element of the Galois Field GF(256).

*rs.c* contains also a decoder that generates four syndrome bytes, which will be all zero if the message has no errors. 

The more general error-location algorithm is the Berlekamp-Massey algorithm (implemented in *berlekamp.c*), which will locate up to four errors, by iteratively solving for the error-locator polynomial.

More information is available on attachment:rs.doc and attachment:config.doc.

h3. 2.2 Task Example.

The example of Reed-Solomon code *example.c* can be used to implement our encoder/decoder in a simple task. See [[III - Advanced Project#"1.1 Integration of C Source files into task(s)."|1.1]] and [[III - Advanced Project#"1.2 Definition of your task(s) to be called."|1.2]] for the definition and integration of *task_example.c*

* The following message (or information word) is considered at the encoder :

!CodingWin3.png!

*Message of 87 characters*

* Then, errors due to a fictional channel are added to the codeword :

!CodingWin2.png!

*Code of adding errors*

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h2. 3. Displaying results.

h3. 3.1 In simulation.

First of all, we want to simulate the behavior of the dsPIC33 and get an output message window in order to watch our results :

1. Right-click on your project > Properties > Chose Simulator as Hardware Tool > Apply

2. Right-click on your project > Properties > Simulator > Chose Uart IO Options > Check Enable Uart1 IO > Apply

!UART1.png!

*Checking Output UART Window*

After building and debugging the program (through Debug Main Project), results are printed in the UART 1 Output window :

!CodingWin1_red.png!

*Displayed result*

It is now possible to see the codeword, the errors and the correction.

h3. 3.2 On board.

In this part, we are focusing the last byte of the message.

|_.-                    |_.Char       |_.Code ASCII  |_.Hexadecimal |_. Binary       |

|_.Codeword             |=. 2         |=. 50         |=. 0x32       |   0b00110010   |

|_.Erroneous Codeword   |=. 3         |=. 51         |=. 0x33       |   0b00110011   |

A first function _void display_6bits(unsigned char* string, int index)_ has been written in order to light up the six LEDs according to the most right six bits of the binary representation of the character indicated by the _index_ in _string_. A second function _void reset_leds(void)_ switchs off the LEDs.

!LEDs_resized.png! !LEDs_error_resized.png! !LEDs_resized.png!

__________________ *a. Codeword : 0bXX110010* ________________________ *b. Erroneous codeword : 0bXX110011* ____________________ *c. Corrected codeword : 0bXX110010* ______________

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h2. 4. Measurement of encoding/decoding time.

POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.

!Clock1.png!

*1000 ticks per milliseconds*

!Clock2.png!

*Time measurement of data encoding*

!Clock3.png!

*Printing clock data*

p=. !Time_RS_resized.PNG!

*Chart of Reed Solomon time measurement*