Need help in Narrowing down the topic for my phd

I want to do PHD in telecommunications. But struggling to fix a topic. Now i am busy identifying problems in OFDM,CDMA,Reusable frequecies, coverage, capacity..................Plz help me out in narrowing down my topics.

Dear Nalina:

your request makes me VERY SAD! Studying for a PhD should mean that you are going to make a contribution to science or technology.

To do this, you should have not only the basic knowledge ( examinations are to prove it), but also that you have your own ideas!

Without ideas (and your request witnesses that) please do something else.

There is nothing worse than a Ph.D. who needs someone to get ideas!

I should know, I earned mine in 1975.

Good luck, Jiri