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Re: [Discuss-gnuradio] really need suggestions for operating on big bloc

From: Achilleas Anastasopoulos
Subject: Re: [Discuss-gnuradio] really need suggestions for operating on big blocks of data
Date: Wed, 30 May 2007 09:48:34 -0400
User-agent: Mozilla Thunderbird 1.0.2 (Windows/20050317)


regarding your first question I am not sure what you mean by "wise".
In terms of speed? in terms of ease of coding?
you can take a look at the code implementing the block; is very simple.
The basic idea is to set the output_multiple to the size of the ineterleaver, so whenever work() is called it asks for a number of outputs being a multiple of the ineterleaver size. This means that
the entire interleaver block is available at the input and you just
permute it...

Regarding your second question the answer is: barely OK; I guess you might experience some aliasing at the outer carriers.


Vincenzo Pellegrini wrote:
Yes Achilleas, the explanation is very clear and useful.
I'll probably go this way.

just two questions:
Yesterday night, I think Ive succeded in passing vector input to my
block, and collecting vector output from it.
so now I do think that I could also perform permutation this way (i.e.
operating ona a big single item rather than on many bytes flowing in and
out). this eases a bit the process, in consideration of the fact that my
interleaver is made up of two parts, one being called the "bit
interleaver" and another called "sybmbol interleaver", both involving
also some demultiplexing of the serial input stream.

so my first question is: is permuting vector coefficients a wise way to perform interleaving? /
does the block you were telling me about internally implement anything
like this?

the second is: can I expect a 8 MHz wide (2048 carriers) OFDM signal to
travel safely over the USB2.0 bus (I recall its bandwidth is exactly 8
complex Msps @16 bits) and be interpolated the right way by the USRP?

very many thanks

On Tue, 2007-05-29 at 17:47 -0400, Achilleas Anastasopoulos wrote:


what you want is already implemented in gnuradio.
Take a look at the following classes/blocks in gr-trellis:

Brief summary:
1) generate a text file (see below) describing your permutation (or have your favorite program generate it automatically)
2) Instantiate an "interleaver" object with this file
3) instantiate two "permutation" blocks (one for
the interleaver and one for the deinterleaver) in your code.

inter = trellis.permutation(interleaver.K(),interleaver.INTER(),1,1)
deinter = trellis.permutation(interleaver.K(),interleaver.DEINTER(),1,1)

the meaning of the last two parameters can be seen from looking at
the above files/examples, but in your case (where you interleave bytes)
should be just 1 and 1.

the text file has the following format (in your case):
6048   (the size of the permutation)
3 1 0 5 .... (N numbers in {0,1,...,6047} describing what goes where)

eg, if I want to permute 5 items at a time you would generate the following file:
4 3 2 1 0
(this will just reverse the order of incoming symbols, ie
the first becomes fifth, the second fourth, etc).

I hope this is clear. Let me know if you have any problems with this,

Vincenzo Pellegrini wrote:

Thanks Achilleas Thanks Eric,

Yes Achilleas, what you have described is right exactly what I need.
the constant permutation is indeed a vector hard coded in my block.

my problem is that I could neither set the step of the cycle

for (int i = 0; i < noutput_items; i+= STEP){

as far as 1512, nor as far as 6048.. (well'if I do it runs but the
output has no meaning)

another thing I've tried is to demultiplex my input stream into many
output streams in order to do interleaving among them, but I don't think
it's a good idea.. and actually works only as far as 1512.

Yesterday I realized how to do that by passing just big vectors to my
block but I'm still failing to get it to work on vectors longer than
1512 bytes.. :)

what I'm working upon is a dvbt transmitter, as described in http://www.com-tech.it/downloads/en_300744.pdf

I'm (slowly, because I also have to keep up with my academic duties)
climbing up the functional block chain towards the aerial.. :D

so where I think I am now is the Inner Interleaver..

very many thank for listening


On Tue, 2007-05-29 at 10:48 -0400, Achilleas Anastasopoulos wrote:


let me make sure I understand what you want to implement:

You want a block that processes N=6048 BYTES at a time.
The input is x1,x2,x3....,xN
and the output is another N=6048 bytes which are a
permutation of the input block, ie,
such that
Next time another block of N BYTES is input and permuted in the same way, etc...
The "permutation" function is prespecified and constant
throughout the operation.

Is this correct?

If not, could you please specify exactly the functionality
you want to implement.


Achilleas Anastasopoulos                        
Associate Professor
EECS Department               Voice : (734)615-4024
UNIVERSITY OF MICHIGAN        Fax   : (734)763-8041
Ann Arbor, MI 48109-2122      E-mail: address@hidden
URL: http://www-personal.engin.umich.edu/~anastas/      

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