Re: RRC filter understanding

[Michael Carosino]

Yes there will be a difference, the RRC filter block is just a wrapper for a call to firdes to design an RRC with the specified sampling rate/symbol rate/excess bandwidth/ntap. The designed taps are then used in a fir filter block whose decimation or interpolation is determined by the field you provide. So If you put in the parameters you mentioned, you'd have designed a 4 Sample/symbol RRC and stuck it into a FIR filter doing 128x interpolation (128x zero insertion followed by filtering using your RRC) - this is probably not what you wanted. In most cases I'd agree you want the interp to match your SPS. I can imagine some usecases where you'd want to filter received data without downsampling it which this block allows you to do but I'm blanking on other uses for this functionality.

On Sat, May 29, 2021 at 9:33 AM Ivan Penchev Ivanov <ivan.pe.ivanov@gmail.com> wrote:

Having a background with Matlab, now I am working on QPSK transceiver in Gnuradio.

I have a question for understanding RRC filter parameters in Gnuradio.

Let's take an RRC interpolation filter in the transmitter. There are parameters for sample rate and for symbol rate. Suppose my sample rate is 32 kHz (typical default  value) and for the symbol rate I put 8 kHz. Isn't it obvious that interpolation is 4, so I have 4 samples per symbol. My question is why is there a dedicated parameter for interpolation? What function will it have? What would be the difference if instead of 4 in the field interpolation, I put 128?

Best regards.