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Re: [Discuss-gnuradio] raised cosine filter taps implementation

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] raised cosine filter taps implementation
Date: Mon, 14 Nov 2011 11:26:31 -0500

On Mon, Nov 14, 2011 at 10:40 AM, Nowlan, Sean <address@hidden> wrote:

Because I need a raised cosine filter. If I convolve root raised cosine filter coefficients with themselves (generated with gr_firdes), do I need to apply a scaling factor?

Ok, but my question was Why do you need a raised cosine filter? I'm just curious about what applications use this instead of RRC filters?




From: Tom Rondeau [mailto:address@hidden]
Sent: Monday, November 14, 2011 9:56 AM
To: Nowlan, Sean
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] raised cosine filter taps implementation


On Sun, Nov 13, 2011 at 6:56 PM, Nowlan, Sean <address@hidden> wrote:

I want to add a raised cosine filter to gr_firdes.* in gnuradio/gnuradio-core/src/lib/general/. I see there's already a root-raised cosine there. After looking through a few sources, I have the following time-domain response of an RC filter:

h(t) = [sin(pi * t / T_s) / (pi * t / T_s)] / [cos(pi * alpha * t / T_s) / (1 - (2 * alpha * t / T_s )^2 )]

As far as I can tell, I should be able to compute the taps using:

for n = -FLOOR(ntaps/2) to FLOOR(ntaps/2), do
    h(n) = [sin(pi * n) / (pi * n)] / [cos(pi * alpha * n) / (1 - (2 * alpha * n)^2 )]

The things I'll have to worry about:
1) if n is 0: h(n) = 1
2) if n is +/- 1/(2 * alpha): h(n) = (1 / (8 * alpha) ) * sin(pi / (2 * alpha) )    #I think I did this right...
3) rounding errors: not sure what to do here given we're operating with floats.

Any tips/suggestions?




We've gotten this question before, and I'm again curious why you want a raised cosine filter? If you really are using it for something, the easiest thing to do is just create a root raised cosine filter and convolve it with itself.




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