Dual-connectivity, Bearer split and other Release-12 small cell enhancements

3GPP TR 36.842 has some interesting details on the small cells enhancements for Release-12. There is too much details for me to go through at the moment but there is some discussions. One of them is on the Dual connectivity and another is Bearer split. The concept of Master eNB (MeNB) and Secondary eNB (SeNB) seems to have been introduced. Here are some interesting definitions:

Bearer Split: in dual connectivity, refers to the ability to split a bearer over multiple eNBs.

Dual Connectivity:Operation where a given UE consumes radio resources provided by at least two different network points (Master and Secondary eNBs) connected with non-ideal backhaul while in RRC_CONNECTED.

Master Cell Group: the group of the serving cells associated with the MeNB.

Master eNB: in dual connectivity, the eNB which terminates at least S1-MME and therefore act as mobility anchor towards the CN.

Secondary Cell Group: the group of the serving cells associated with the SeNB.

Secondary eNB: in dual connectivity, an eNB providing additional radio resources for the UE, which is not the Master eNB.

Xn: interface between MeNB and SeNB. Since the current E-UTRAN architecture was selected as baseline in this study, Xn in this TR means X2.

There is also the concept of RRC diversity which can help mobility. as per 36.842

RRC diversity is a potential solution for improving mobility robustness. With RRC diversity, the handover related RRC signalling could additionally be transmitted from or to a potential target cell as illustrated in Figure 7.1.3-1.  RLF could in this case be prevented as long as the UE is able to maintain a connection to at least one of the cells.  This will eventually lead to a more successful handover performance (i.e. avoiding UE RRC re-establishment procedure). The RRC diversity scheme could also be applied for handovers from the macro to pico cells, between macro or between pico cells.

Finally, the security aspects for this Bearer split and dual connectivity is also being discussed. See the latest Rel-12 security update here.