Re: [Discuss-gnuradio] Questions

[Tom Rondeau]

On Sat, May 5, 2012 at 12:00 PM, Amr Youssef
<address@hidden> wrote:
> Hi , i have some questions :
>
> 1- In  the following  .cc file  : ./gr-digital/lib/digital_crc32.cc
>
> Regarding CRC-32  , why the generated polynomial  is of length 36 Bits
> instead of 32 Bits (as expected) !  " it is written as :
>   // Automatically generated CRC function
>  // polynomial: 0x104C11DB7
> the above two lines exist in the code  . "

Amr,
That code is old and I'm not sure where it's from, but I'll respond
like I know what I'm talking about, anyways (mostly from experience
with these kinds of registers).

That's not 36 bits, actually, it's 33 bits, and the 33rd bit is a 1.
It's typical to have always have a 1 in the last place for shift
registers like this. The algorithm itself is likely created using a
32-bit value.

>  2- In the following .py file  : ./gr-digital/python/ofdm_packet_utils.py
>
> How  "random_mask_tuple"  is generated through the 15-bit Linear Feedback
> Shift Register (LFSR) .
> Are they random ? if yes , why they are of length 8 bits (maximum number
>  255 ) instead of 15 bits ?!
>
> Thanks in-advance
> Amr,

I'd have to let the person who wrote that code answer.

Tom