hii...

yes i thnk you shud focus on communication system because its an easy chapter and quite short... it may not take a vry long tym...

the best way to prepare it is from NCERT...because it is sufficient to do dese short chapters from it...

and the questions are also not very tough...just based on simple facts lyk they wiil ask the frequency of ground wave propagation... so i thnk you cud easily get marks so dont leave this chapter....