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List:       gnuradio-discuss
Subject:    Re: [Discuss-gnuradio] fir filter parameters
From:       Tom Rondeau <tom () trondeau ! com>
Date:       2012-06-29 14:36:46
Message-ID: CA+SzT6i-pJQCbUauiOKTWUcCARCu4i+umTafvRBWFyJT=WYdtQ () mail ! gmail ! com
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On Thu, Jun 28, 2012 at 10:53 PM, signalswdm <signalswdm@yeah.net> wrote:
> Dear everyone:
>    In ofdm_receiver.py there is chan_coeffs =
> gr.fihan_coeffs(1.0,1.0,tb+bw,tb,gr.firdes.WIN_HAMMING), which is to
> determine the parameters of fir low-pass filters. My question is about the
> third parameter, which denotes the center of transition band, and it is
> computed as tb= bw*0.08. So can you explain to me why we should set the
> center of transition band tb+bw and how tb is computed? Besides in
> benchmark_rx case, midpoint of trans. band is directly set 1 and width of
> trans. band 0.5. So in general how do we set the bandwith and midpoint of
> band?
>
> Best regards

Any frequency shaping on (standard) OFDM signals is bad. We probably
just did this to make sure that the filter bandwidth was plenty over
the bandwidth of the signal, but there's no real science behind why we
did it that way.

Tom


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