Fir filter parameters

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

On Thu, Jun 28, 2012 at 10:53 PM, signalswdm [email protected]
wrote:

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