According to the requirement for LTE-Advanced to be backwards compatible to legacy LTE, when using the standard legacy carrier (radio frame structure), both the PDCCH and E-PDCCH shall coexist. This is due to the fact that the carrier need to support all the Rel. 8, Rel. 10 and Rel. 11 UEs. Because of the Rel. 8 UE, that does not recognize E-PDCCH, in order to serve them, the PDCCH need to be there. UE searches for the configured PDCCHs, i.e. if configured, it may use only PDCCH, or PDCCH (e.g. for SI reading, i.e. common search space) together with E-PDCCH (for dedicated resource scheduling – i.e. dedicated search space). For Rel. 11 the E-PDCCH is configured to be used for dedicated search space only (i.e. there’s no E-PDCCH including common search space – types of messages).
Taking a step further into the future, there are concepts for Rel. 12 and beyond to define a new carrier type (i.e. Lean Carrier), that would not include legacy Control Region with PDCCH, but only E-PDCCH with both common and dedicated search spaces. This concept is however under discussion.