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Re: [Discuss-gnuradio] Cyclic Prefixer Rolloff Samples

From: Bastian Bloessl
Subject: Re: [Discuss-gnuradio] Cyclic Prefixer Rolloff Samples
Date: Fri, 23 Aug 2013 13:50:14 +0200
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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:


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!

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