From a recent NTT Docomo presentation (embedded below). Whereas right now 3GPP has only been working on FDD or TDD scenarios, this proposal is a combination of FDD as P-Cell and TDD as S-Cell. Inter-Technology carrier aggregation is another possible option. Anyway, the complete presentation is below.
Updated on 29/10/2013
3GPP has already started working on this work item. See RP-131399 for details.