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<h2>WCDMA–GSM cell re-selection and inter-system handover</h2>

<p>The handover control of the RAN supports inter-system handovers,
both from WCDMA to GSM and from GSM to WCDMA. Inter-system handover is required
so that the coverage areas of GSM and WCDMA can complement each other. When
the coverage areas of WCDMA and GSM are overlapping each other, an inter-system
handover can be used to control the load and/or services between the systems.
The inter-system handover is a hard handover, which means that an inter-system
handover causes a short disconnection of a real-time radio access bearer.</p>


<p>The handover control within the Nokia RAN supports also
cell re-selection. Both circuit-switched (CS) and packet-switched (PS) services
can also be maintained between GSM/GPRS and WCDMA networks.</p>


<p>The inter-system handover of packet-switched services between
WCDMA and <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08914#id08914" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08914#id08914');return false" shape="rect" target="_blank">GPRS</a> is based on the cell re-selection
procedure. The Handover Control of the RAN supports the network initiated
cell re-selection from WCDMA to GPRS in CELL_DCH state of connected mode.
In the CELL_PCH and URA_PCH states of connected mode, the cell re-selection
from WCDMA to GPRS is initiated by the mobile station.</p>


<p>The network initiated cell re-selection (for packet data)
from WCDMA to GPRS and inter-system handover (for circuit-switched data) from
WCDMA to GSM are network-evaluated handovers (NEHO). The decision algorithm
of the inter-system handover and network initiated cell re-selection is located
in the Radio Network Controller (<a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id010096#id010096" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id010096#id010096');return false" shape="rect" target="_blank">RNC</a>). The RNC
makes the decision on the basis of periodical inter-system measurement reports
received from the mobile station and relevant control parameters. The RNC
orders the mobile station to start the periodical reporting of inter-system
measurement results only when an inter-system handover or cell re-selection
is needed. The measurement object information (cells and frequencies) for
the inter-system measurement is determined by the RNC.</p>


<p>Unless the mobile station is equipped with dual receivers
it can only be tuned to one frequency at a time. Therefore, compressed mode
must be used at the physical layer of the radio interface to allow the MS
to make the required inter-system (GSM) measurements while maintaining its
existing connection.</p>
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