Selasa, 28 Juli 2009

Handover in 3G networks

Handover in 3G systems may be initiated by the network based on RF quality (signal level, connection quality, power level propagation delay) or traffic criteria (current traffic loading per cell, interference levels and maintenance requests). MS continuously gathers measurements from neighboring cells and reports such measurements to the serving cell on an event driven or regular basis. When a network determines a need for executing a handover the procedures given in 3GPP TS 08.08, 3GPP TS 25.303, 3GPP TS 25.331 are followed.


3G systems support both soft and hard handover. Soft handover is the preferred mode of operation since it exploits macrodiversity functionality essential for CDMA efficiency. In this mode of operation the mobile terminal is connected to a variety of nodes B and signals are combined at the RNC level.


The RNC responsible for handling the recombining function is called Serving RNC (SRNC) while the supporting controller is named Drift RNC (DRNC). The Iur interface between two adjacent RNCs is essential to support macrodiversity and therefore soft-handover.
The mobile terminal holds a list of nodes B it is connected with. This list, namely the active list, is continuously updated following the user’s movements. As soon as a node B signal decrease below a pre-defined threshold, this is removed from the active list. Conversely, as soon as the MT receives a new node B over a given threshold, this is added in the active list.

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