Re: [Discuss-gnuradio] Root rasied cosine parameters issue

[Tom Rondeau]

On Thu, May 15, 2014 at 7:55 AM, Irfan Ullah <address@hidden> wrote:

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

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_1_1pfb__clock__sync__ccf.html

Tom