Suggest me a suitable topic for PhD in RF IC design

depend on your interest, any field can be a doctor thesis of the rf ic field

is it better to build the whole receiver / transceiver system ? or just focus on LNA or power amplifier?

as per my opinion full tx-rx will be better option

First of all, do you want a PhD just because you want the title or because of interest in field? What I mean is that people who are thinking about a PhD should have some idea of topics to research. I find that there are too many people today looking for a PhD position just because they want status. Anyway I'm not judging you or anyone else, just think through it first.

So in RFIC there are many topics. First you should try to think if there is some specific area you have more interest in. For example, are you more into system design architectures or do you want to deal with circuit and IC implementation issues? Or maybe you have some specific interest in filters or mixers.

I think the best way to go is to first have some working experience in the area of interest and then go for a PhD.