Encoding/Decoding CCSDS (255,223) messages with reedsolomon #76
Comments
|
Thank you for your extensive detailing of this issue! Could you also please provide the python code you wrote to run these tests? One possibility is that the encoding is mangling the input message somehow? Also are you using the low level functions? Because otherwise if you use the high level RSCodec object then it automatically pads input messages so this may be the culprit. There can also be a difference in the implementation which is not obvious and may change results a lot. |
|
With some code i will be able to reproduce the issue and test faster. |
|
Roger. Just a second. |
|
Here you go, thanks for waiting! (For some I had to rename it to |
|
If you are interested this is the code I wrote to generate the data using #include "fec.h"
#include <memory.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define MSG_SIZE 255
#define MAX_ERRORS 16
#define MSG_CODE MAX_ERRORS * 2
#define MSG_PAYLOAD MSG_SIZE - MSG_CODE
int main() {
srand(time(NULL)); // use current time as seed for random generator
unsigned char message[MSG_PAYLOAD] = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B,
0x0C, 0x0D, 0x0E, 0x0F, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20, 0x21, 0x22, 0x23,
0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E, 0x2F,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A, 0x3B,
0x3C, 0x3D, 0x3E, 0x3F, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
0x48, 0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50, 0x51, 0x52, 0x53,
0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5A, 0x5B, 0x5C, 0x5D, 0x5E, 0x5F,
0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6A, 0x6B,
0x6C, 0x6D, 0x6E, 0x6F, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77,
0x78, 0x79, 0x7A, 0x7B, 0x7C, 0x7D, 0x7E, 0x7F, 0x80, 0x81, 0x82, 0x83,
0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8A, 0x8B, 0x8C, 0x8D, 0x8E, 0x8F,
0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9A, 0x9B,
0x9C, 0x9D, 0x9E, 0x9F, 0xA0, 0xA1, 0xA2, 0xA3, 0xA4, 0xA5, 0xA6, 0xA7,
0xA8, 0xA9, 0xAA, 0xAB, 0xAC, 0xAD, 0xAE, 0xAF, 0xB0, 0xB1, 0xB2, 0xB3,
0xB4, 0xB5, 0xB6, 0xB7, 0xB8, 0xB9, 0xBA, 0xBB, 0xBC, 0xBD, 0xBE, 0xBF,
0xC0, 0xC1, 0xC2, 0xC3, 0xC4, 0xC5, 0xC6, 0xC7, 0xC8, 0xC9, 0xCA, 0xCB,
0xCC, 0xCD, 0xCE, 0xCF, 0xD0, 0xD1, 0xD2, 0xD3, 0xD4, 0xD5, 0xD6, 0xD7,
0xD8, 0xD9, 0xDA, 0xDB, 0xDC, 0xDD, 0xDE};
/*
for (size_t i = 0; i < MSG_PAYLOAD; ++i) {
message[i] = rand() % 256;
}
*/
const int fcr = 128 - MAX_ERRORS;
struct rs *rs = (struct rs *)init_rs_char(8, 0x187, fcr, 11, MSG_CODE, 0);
printf("CCSDS(255,223) internals:\n");
print_details_ccsds_239(rs);
unsigned char parity[MSG_CODE];
encode_rs_ccsds(message, parity, 0);
printf("Message:");
for (size_t i = 0; i < MSG_PAYLOAD; i++) {
if ((i % 16) == 0)
printf("\n\t");
printf("0x%02x,", message[i]);
};
printf("\n\n");
printf("Parity");
for (size_t i = 0; i < MSG_CODE; i++) {
if ((i % 16) == 0)
printf("\n\t");
printf("0x%02x,", parity[i]);
}
printf("\n");
return 0;
}; |
|
I read a bit more around (especially the wiki books entry about ReedSolomon) and tested out a bit more. import galois
rs_field = galois.GF(2**8, irreducible_poly='x^8 + x^7 + x^2 + x + 1', primitive_element=_GEN_ALPHA)
reed_solomon = galois.ReedSolomon(_CHUNK_SIZE, _CHUNK_PAYLOAD, c=_FCR, field=rs_field)
galois_out = reed_solomon.encode(msg_data)So I went back to the definition book to read more, given I had a bit more knowledge. This is the part that threw me off:
The fact that I made (log, exp, field_char) = crs.init_tables(0x187)
gen = bytearray([0x01,0x5b,0x7f,0x56,0x10,0x1e,0x0d,0xeb,0x61,0xa5,0x08,0x2a,0x36,0x56,0xab,0x20,0x71,0x20,0xab,0x56,0x36,0x2a,0x08,0xa5,0x61,0xeb,0x0d,0x1e,0x10,0x56,0x7f,0x5b,0x01])
conv_message = crs.rs_encode_msg(conv_data,32,fcr=112,gen=gen)This allows me to move forward, but if I can use this without using the "low-level" functionalities it would be great! Let me know if I can give you more data to work on. |
|
Ouch. def rs_calc_syndromes(msg, nsym, fcr=0, generator=2):
return [0] + [crs.gf_poly_eval(msg, crs.gf_pow(generator, (i+fcr)*11)) for i in range(nsym)]This makes the function return all zeroes in case of a correct message. |
|
Oh, wait a second, I did it. I should have used |
|
Dear @Jazzinghen , thank you so much for investigating this issue and providing so much details, this certainly helps me a LOT! :D I think I can implement a fix and will do so shortly, but bear in mind this is a workaround for a specific implementation that doesn't follow the original model. Indeed, there is no reason there is such an exponent here, it's not a parameter described anywhere (I mean it is in the book you cited - great finding! But it's not a common parameter and it's redundant with the j exponent when generating - it seems to me it's kinda like a salt or a shift to make their implementation unique). |
|
No problem, I am sorry I flooded you with messages, I was very excited to have understood what was wrong. It surely is a very specific detail here (albeit for a "famous" implementation), so I am not sure if it calls for an API change or just a mention in the readme :D |
I have been trying to use the library to encode/decode messages using the CCSDS standard for the last couple days and I cannot seem to be able to get results in line with a reference implementation. I am already handling conventional to double-base conversion, but it seems impossible to set the proper parameters for the encoder.
The parameters for the CCSDS encoder are the following:
8255320x187(x^8 + x^7 + x^2 + x + 1)11211I tried using the library with this configuration but it doesn't seem to generate the correct output:
Which has the following members:
I have been also trying to debug the issue by checking all the steps, but due to my lack of knowledge on Reed-Solomon and the differences in implementation between the two libraries it is almost impossible for me.
I also tried to use
detect_reedsolomon_parametersfrompyFileFixty, and it cannot find the proper parameters:As an example, this is the output of an application using
libfecto compute the CCSDS parity:This is the output of reedsolo:
Did I miss something? It is entirely possible given that I am not an expert in this field.
The text was updated successfully, but these errors were encountered: