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… 
so where I think I am now is the Inner Interleaver…
very many thank for listening
vincenzo