Re: How to add fec in GMSK

Hello Quenten:

"Hamming Code" is a type of an forward-error correction code (FEC). See: https://en.wikipedia.org/wiki/Hamming_code . It is not related to low-pass filtering (you really should not have that there).  I mentioned Hamming Code because it is usually the first code that you learn in the school.

For an example, Hamming(7,4) takes a 4-bit data, and generates a 7-bit code. That means, if you use Hamming(7,4), the payload size of the "Oacket Encoder" (which you really shoudn't be using, but I guess you already know that) must be an integer multiple of 7 (bits). If you don't do that, you will need to implement additional handling processes to fix that.

In your flow graph, you are using Convolutional Code (which is not a linear code), instead of Hamming Code (which makes sense, as Hamming Code usually makes little sense on wireless communication system). Still, you have to take similar issues into your consideration. I don't remember the exact details (I didn't use Convolutional code much), but you may have to add paddings and tails on your packets and fix the paylod size to a certain number to get things working (go through WiFi OFDM PLCP and WiFi convolutional code tutorials, if you are sticking with Convolutional code).

Also, I do not see the "Packet Decoder" in your flowgraph.

WX GUI is no longer supported, and some error messages that you posted are about WX GUI, so consider using QT GUI instead.

Finally, please note that it will still miss some packets (mostly during the beginning of the transmission).

(I added discuss-gnuardio@gnu.org back to the recipant list.)

Regards,

Kyeong Su Shin

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보낸 사람: Quenten . <quentenswitten@gmail.com>
보낸 날짜: 2020년 4월 29일 수요일 오후 4:57
받는 사람: Kyeong Su Shin <ksshin@postech.ac.kr>
제목: Re: How to add fec in GMSK

 

hello, Kyeong Su Shin,

I added hamming with a low pass filter(only way I know how to add Hamming in flowgraph) but when I run this is get the following results from GNU:

bits per symbol = 1
Gaussian filter bt = 0.35
bits per symbol = 1
M&M clock recovery omega = 4.000000
M&M clock recovery gain mu = 0.175000
M&M clock recovery mu = 0.500000
M&M clock recovery omega rel. limit = 0.005000
frequency error = 0.000000
Warning: the blks2.packet_encoder block is deprecated.         (I know why these are here)  
Warning: the blks2.packet_decoder block is deprecated.        (I know why these are here)
gr::pagesize: no info; setting pagesize = 4096
C:\Program Files\GNURadio-3.7\gr-python27\lib\site-packages\numpy-1.12.0-py2.7-win-amd64.egg\numpy\core\_methods.py:116: RuntimeWarning: overflow encountered in multiply
  x = um.multiply(x, x, out=x)
C:\Program Files\GNURadio-3.7\lib\site-packages\gnuradio\wxgui\common.py:272: RuntimeWarning: invalid value encountered in add
  return mean - factor*ampl, mean + factor*ampl
C:\Program Files\GNURadio-3.7\lib\site-packages\gnuradio\wxgui\common.py:272: RuntimeWarning: invalid value encountered in subtract
  return mean - factor*ampl, mean + factor*ampl

For the result ofthe graphs see file attached.

I added the Low Pass in the original Grc before the Gmsk Demod, as can be seen below:

Best regards,

Q

Op ma 27 apr. 2020 om 17:50 schreef Kyeong Su Shin <ksshin@postech.ac.kr>:

Hello Quenten:

Just to elaborate a bit further:  If you did something that is suggested in the example files, and failed to get your flowgraph working, it is probably because your data is no longer frame-synchronized after the mod / demod blocks (yes, that happens, both in simulations and in real-world environments), and error correction codes usually require them to be frame-synchronized.  So, what you will have to look into is frame sync or packet design.

Also, you will have to design your frames with your FEC algorithm in your mind. Simple linear-algebraic algorithms, like Hamming code, will require you to set your frame length to a multiple of some integer. Convolutional code additionally requires you to pad some zeros to your frames, but I forgot the details about it. 

Regards,

Kyeong Su Shin

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보낸 사람: Kyeong Su Shin <ksshin@postech.ac.kr>
보낸 날짜: 2020년 4월 28일 화요일 오전 12:23
받는 사람: Quenten . <quentenswitten@gmail.com>; discuss-gnuradio@gnu.org <discuss-gnuradio@gnu.org>
제목: Re: How to add fec in GMSK

 

Hello Quenten:

You may want to look at some example codes at https://github.com/gnuradio/gnuradio/tree/maint-3.7/gr-fec/examples  (from maint-3.7 branch since you are apparently using GR3.7).  

Please note that most error correction codes require frame synchronization (at least on real-world situations).

Regards,

Kyeong Su Shin

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보낸 사람: Quenten . <quentenswitten@gmail.com> 대신 Discuss-gnuradio <discuss-gnuradio-bounces+ksshin=postech.ac.kr@gnu.org>
보낸 날짜: 2020년 4월 27일 월요일 오후 10:31
받는 사람: discuss-gnuradio@gnu.org <discuss-gnuradio@gnu.org>
제목: How to add fec in GMSK

 

Hello all,

I have previous posted a question concerning frame drops in GNU radio when using GMSK. (https://lists.gnu.org/archive/html/discuss-gnuradio/2020-04/msg00148.html)

On this mail someone answered that I should use OFMD (what I normally should consider because this is better), but my prof told me to focus on GMSK and also told me to include FEC. I know with one I should use, it is either the FEC en-/decoder or the Extended FEC en-/decode, because I shall stream data cte. 

So my question is how do you include this because I have tried but didn't succeed.

In the file attached you will find the grc with the FEC en-/decoder. Help would be appreciated.

Best regards,

Q