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<h1>Common BCCH and handover</h1>

<p>In a Common <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=id04445#id04445" 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=id04445#id04445');return false" shape="rect" target="_blank">BCCH</a> segment
environment, a new type of handover is introduced: inter-BTS intra-segment
handover, that is, a handover between different <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=id03816#id03816" 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=id03816#id03816');return false" shape="rect" target="_blank">BTSs</a>
inside a segment. The inter-BTS handover can occur within the same frequency
band or between different frequency bands</p>


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<h2>Handover
causes</h2>

<ul>
<li><p>Power Budget Handover</p>


<p>The standard <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=id016179#id016179" 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=id016179#id016179');return false" shape="rect" target="_blank">PBGT</a> (power
budget) calculation is applied for an <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=id03944#id03944" 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=id03944#id03944');return false" shape="rect" target="_blank">MS</a> on the PGSM
900, EGSM 900, or GSM 800 layer. If an MS is on the <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=id012006#id012006" 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=id012006#id012006');return false" shape="rect" target="_blank">GSM 1800</a>/<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=id012007#id012007" 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=id012007#id012007');return false" shape="rect" target="_blank">1900</a> layer of the multi band Common BCCH segment,
the decision on the PBGT handover is based on the measurement of the segment's
own BCCH frequency that the MS measures when on the GSM 1800 band. The BCCH
measurements are compared with each other to decide the superiority between
the serving and an adjacent segment.</p>
</li>
<li><p>Load based <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=id04070#id04070" 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=id04070#id04070');return false" shape="rect" target="_blank">TCH</a>
handover</p>


<p>In addition to the standard TCH – TCH handovers, when the
Common BCCH is active, the BSC can command an additional handover to balance
the load between different frequency bands.</p>


<p>During the call setup procedure the load cannot necessarily
be kept under the <span class="parameter-sem">BTS Load In SEG</span> limit for each
BTS, because not all terminals support all the resource types. Furthermore,
due to their propagation properties, the GSM 1800/1900 resources may not be
available in all TCH allocation cases.</p>


<p>When deciding on initiating a handover to balance the load
between the BTSs of a segment, the triggering load limit L is defined with
the formula:</p>


<p>L = (<span class="parameter-sem">BTS Load In SEG</span> + 100)
/ 2</p>


<p>If the <span class="parameter-sem">BTS Load In SEG</span> parameter
is set to value 1%..100%, the load limit is calculated according to the formula
above. The value 0 of the parameter <span class="parameter-sem">BTS Load In SEG</span>
is a special case. When the operator has set the value to be 0, the BSC tries
to hand all calls fullfilling signal strength criteria over from the BCCH
layer BTS to non BCCH layer. This results in the resources on the BCCH layer
to be kept free for as long as possible.</p>


<p>In most cases the main purpose of the handover is to move
TCH load from the BCCH band to the other band in order to free BCCH band traffic
channels for single-band mobile stations.</p>


<p> The BSC checks the load of BTSs in a multi band segment
every time it receives a TCH request for the segment in question. When the
BSC selects target BTSs for a load based intra cell handover, it only accepts
BTSs whose load is below the respective <span class="parameter-sem">BTS Load In
SEG</span> value.</p>
</li>
</ul>
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<h2>SDCCH
handover</h2>

<p>In addition to the standard <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=id011274#id011274" 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=id011274#id011274');return false" shape="rect" target="_blank">SDCCH</a>–SDCCH
handovers, a new inter band (intra-segment) SDCCH handover has been implemented
to avoid long SDCCH reservations, thus reducing the SDCCH pressure on the
BCCH resource layer.</p>


<p>This handover is triggered when the length of an SDCCH
reservation on the BCCH layer equals the value of the <span class="parameter-sem">Intra
Seg SDCCH Ho Guard</span> parameter.</p>


<p>The handover is performed if there are available SDCCH
resources outside the BCCH band and the MS has the required capability.</p>
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<p><a href="NED?action=retrieve&amp;identifier=dn02222031&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=ned_top#ned_top" onclick="sync(this);return true" shape="rect">Overview of Common BCCH</a></p>
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