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

[Tom Rondeau]

On Mon, Nov 14, 2011 at 1:09 PM, Ben Reynwar <address@hidden> wrote:

On Mon, Nov 14, 2011 at 10:03 AM, Tom Rondeau <address@hidden> wrote:
> On Mon, Nov 14, 2011 at 12:01 PM, Ben Reynwar <address@hidden> wrote:
>>
>> PSK31 for ham radio uses a raised cosine filter rather than the RRC.
>
> Interesting. Thanks.
> Do you know why the do it? Do they just have the filter on one side?
> Tom
>

I don't think it's for any good reason.  The raised cosine filter is
on the transmit side and I guess you can put whatever you want on the
receive side.  I would have thought another raised cosine filter on
the receiver side was the best way to go to maximise the signal and
then deal with the ISI afterwards.  I haven't looked at other peoples
code to see how they're doing it.

I would have figured it was on the Tx side for bandwidth control, and you could do various things on the receiver side with that. It's of course why we normally use RRC filters though; restrict the transmitted signal bandwidth, then the receive RRC does both a matched filter and creates Nyquist pulses (ignoring multipath).

Tom

 

 

>>
>> On Mon, Nov 14, 2011 at 9:26 AM, Tom Rondeau <address@hidden>
>> wrote:
>> > 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?
>> > Tom
>> >
>> >>
>> >>
>> >>
>> >> 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 )]
>> >> end_for
>> >>
>> >> 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?
>> >>
>> >> Thanks,
>> >> Sean
>> >>
>> >>
>> >>
>> >> Sean,
>> >>
>> >> 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.
>> >>
>> >>
>> >>
>> >> Tom
>> >>
>> >>
>> >
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>> >
>> >
>
>