Forum: GNU Radio Root rasied cosine parameters issue

8d8a4dc47ea64675b37d810212889b66?d=identicon&s=25 Irfan Ullah (Guest)
on 2014-05-15 13:57
(Received via mailing list)
Hi all,
    I have problem with pulse shaping RRC filter parameters when I set
the
parameters as

interpolation=4    Gain=nfilts    sample rate=1   symbol rate=1
Alpha=0.5
Num Taps=11*sps*nfilts

(nfilts = 32,  sps=4)

but my output constellation have much ISI please any one tell me the
reason. check the parameters uf there is any problem then inform me.
 my flowgraph is attached with this mail
C539637020fd56193dd6daec746c4a84?d=identicon&s=25 Tom Rondeau (Guest)
on 2014-05-15 14:27
(Received via mailing list)
On Thu, May 15, 2014 at 7:55 AM, Irfan Ullah <irfanlumsian@gmail.com>
wrote:

> reason. check the parameters uf there is any problem then inform me.
>  my flowgraph is attached with this mail
>

Irfan,

You seem to have confused the RRC taps in a normal FIR filter and the
polyphase filterbank resampler implementation that we've used in our PSK
mod/demod blocks. When just using the RRC filter as a filter, like the
Root
Raised Cosine Block in your example, you want to define it like you
normally would:

interpolation: 4
gain: 4 (to account for the loss when interpolating)
sample rate: samples_per_symbol
symbol rate: 1
alpha: (whatever between 0 and 1; generally <= 0.5)
num taps: ~45

Your use of nfilts in your email suggests you've pulled those ideas out
of
using the PFB resampler. That resampler block takes in a large prototype
filter, which we define based on the number of filters in the PFB
filterbank (32 is default and usually enough). Using this filterbank
allows
us to set an arbitrary real number of samples per symbol. Using just the
interpolating FIR filter means we can only use integer values for this.

Also, remember that using an RRC filter /will/ result in ISI. You have
to
use a secondary RRC filter to remove the ISI. In another posting of
yours,
I also saw that you used both an RRC decimating filter and the PFB clock
sync, which implements this for us. You don't need both. Read the clock
sync documentation closely:

http://gnuradio.org/doc/doxygen/classgr_1_1digital...

Tom
This topic is locked and can not be replied to.