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Handover from 3G to 4G while UE in connected state?

+2 votes
3,558 views

I know its possible to handover to 3G from 4G in connected state.

But do networks implemented the backwards, I mean 3G to 4G while UE in connected state?? Reselection happens for me but not Handover ?? When UE goes to Cell_PCH or IDLE state it moves back to 4G, not when I am streaming or downloading.

posted Jul 21, 2013 by Nora Jones

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1 Answer

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support for 3G to LTE handover (i.e. direct transition from 3G RRC connected state to LTE RRC connected state) is optional in both the network and the UE (modem/chipset) side. Handover is not the only mechanism to move the UE from one RAT (in this case 3G) to another (in this case, LTE). In many discussions, the term handover is used "loosely" as a general expression for moving from one cell or system to another, whereas in the 3GPP specifications lingo the terms are more explicit.

Coming back to the point even without support for 3G->LTE handover, there are mechanisms for the UE to move (on its own or commanded by the network) to move from 3G to LTE even when the UE has a RRC connection in 3G. For example:

  • 3G to LTE redirection: the 3G network can send a RRC connection release message to the UE , redirecting the UE to LTE. This is mandatory functionality for the UE, so the network can always use this if it chooses to do so.

  • 3G to LTE cell reselection: in 3G CELL_PCH and URA_PCH states, the UE can support autonomous cell reselection to LTE. In addition, the UE can also support cell reselection from 3G CELL_FACH state, if the network supports this feature.

In 3G idle mode, of course the UE shall support cell reselection to LTE as per the inter-RAT cell reselection algorithms (see 3GPP TS 25.304).

answer Jul 21, 2013 by Kevin Peterson
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