Re: [Discuss-gnuradio] Cyclic Prefixer Rolloff Samples

[Bastian Bloessl]

On 08/23/2013 11:35 AM, Martin Braun (CEL) wrote:
> On Fri, Aug 23, 2013 at 10:10:13AM +0200, Bastian Bloessl wrote:
> > I wonder why the OFDM Cyclic Prefixer uses the samples at the
> > beginning of the symbol to calculate the roll off. I would have
> > expected that the samples from the end of the previous symbol are
> > used. So something like this:
> >
> > https://github.com/bastibl/gnuradio/commit/1a1f0f92cedac90ccadf5e42f4e09b3188b04352
> >
> > But since also the unit test expects the samples from the start of
> > the symbol I might get the concept totally wrong.

> Only we're not filtering--we're adding a tapering window in time domain.
> At the beginning of the symbol, we can simply multiply the samples with
> the up-ramp of our tapering window (we use a raised cosine flank).
>
> So what happens at the end? We could do the same: Multiply the last few
> samples with the down-ramp. But then we're simply throwing away energy.
> It would be much nicer if the down-ramp came *after* the symbol, during
> the up-ramp of the following symbol. This is achieved by adding samples
> from the beginning of the symbol (think of it as a "cyclic postfix") and
> multiply those with the down-ramp of the tapering window.

Ah OK, I see, taking the samples from the end is indeed stupid -.- 
Thanks for your explanation!