I've thinking about this for some time now and I was wondering why do we need to increase smapling rate in the transmitter? I will explain a bit more.
From the point of view of a software-defined radio, you convert bits to symbols (let's suppose I'm doing QPSK) and after pulse shaping with 4 samples per symbol, (which I know it also increase sampling frequency by 4 since it's another interpolation after all) I've seen it's quite common to do some multi-stage interpolation (or a single-stage interpolation, it doesn't matter) to increase the sampling frequency even more. Why is this beneficial? I mean, I can only think of one reason and that'd be because you have more samples per symbol, but if after you're upconverting to passband and send it to the channel, what was that for?
Then, when you get the signal at the receiver, you decimate it to decrease sampling frequency. This one, I think I understand it better, after doing synchronization (for example in symbol sync you might need more than 1 sample per symbol), you want to decrease the sampling frequency so you don't have to deal with so many samples and is less computational costly.
So my question is, why doing interpolation in the transmitter?
Correct me if I said something wrong, please. Thank you.