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## Re: [Discuss-gnuradio] BER in bert- example

**From**: |
Daniel Bartel |

**Subject**: |
Re: [Discuss-gnuradio] BER in bert- example |

**Date**: |
Wed, 27 Apr 2011 08:49:54 +0200 |

>*>> But from my tests I see that*
>*>> 1 bit error in -> 7 bit errors out*
>*>> 2 consecutive bit errors in -> 2 errors in the output*
>*>> 3 consecutive bit errors in -> 7 errors in the output*
>*>> 4 consecutive bit errors in -> 4 errors in the output*
>*>> ...*
>*>> And so forth up to 7 (Length of the lfsr)*
>*>>*
>*>> The reason I ask is that if I want to change the scrambler and/or the*
>*>> modulation, I assume that this "magic number" will change as well.*
>*>*
>*> If you want a good BER measurement, I wouldn't use the method that you*
>*> describe here.*
>
>*You are correct. The scrambler introduces three output errors per*
>*input error for single channel errors that are farther apart than the*
>*length of the shift register. This is the case once the shift*
>*register has already achieved self-synchronization and at low channel*
>*error rates. For the purposes of the simple BERT example, this was*
>*sufficient. (The number 3 comes from the number of taps in the*
>*scrambler polynomial.)*
I know this thread is a bit old, but is it possible that there is an mistake in
the text?
I think it should be written, that 1 bit error in the scramber input introduces
4 errors in the output, because 1 error is passed directly through and 3
feedback errors are introduced by the taps in the scrambler polynomial of:
self._descrambler = gr.descrambler_bb(0x8A, 0x7F, 7) # CCSDS 7-bit descrambler
If I'm wrong, don't hesitate to correct me. Thanks.
Daniel

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*Daniel Bartel* **<=**