LTE Advanced: 3GPP Solution for IMT-Advanced

14.9 LTE Interworking

The introduction of LTE on top of HSPA network requires interworking features in both radio networks. The interworking includes idle mode, voice handovers and data handovers. In the first phase of LTE deployment LTE capable UE makes reselection from HSPA to LTE. The reselection is allowed only in idle and in PCH states but not in FACH states. Therefore, it is important to set the HSPA parameters so that UE can get quickly to PCH state in order to enable smooth access to LTE network. In the later phase also redirection and packet handover will be supported which allows moving UE from HSPA to LTE also from Cell_DCH state. If UE runs out of LTE network, the LTE network can move it to HSPA network by redirection or handover. The shortest connection break is obtained with packet handover. The first phase interworking is illustrated in Figure 14.14.

Figure 14.14 Early phase interworking between LTE and HSPA.

The early phase voice service for LTE smartphones uses CS fallback handovers where UE is moved from LTE to 3G during the call setup phase. The network switching can utilize redirection or handover. When the voice call is over, UE returns back to LTE by reselection or by redirection.