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<div class="div">
<h1>Handover and Power Control Algorithms</h1>

<a name="mar241039223" shape="rect"></a>
<div class="topic">
<h2>Handover
strategy</h2>

<p>Handover and Power Control Algorithm is a feature designed
to make the mobility of the mobile stations possible (MS) during a call and
to maintain good speech/data quality. Furthermore, the battery lifetime of
an MS can be optimised. A versatile handover and power control algorithm enables
the efficient use of air interface resources.</p>


<p>The handover decisions made by the BSC are based on the
measurement results reported by the MS/BTS and various parameters set for
each cell. The standard handover algorithm is capable to start handover due
to following reasons:</p>


<ul>
<li><p>Decreased signal level of serving cell</p>
</li>
<li><p>Decreased signal quality of serving
cell</p>
</li>
<li><p>Umbrella criteria</p>
</li>
<li><p>Power budget criteria</p>
</li>
<li><p>O&amp;M reason</p>
</li>
<li><p>Distance reason (Timing Advance)</p>
</li>
<li><p>Traffic reason (see Traffic Reason
Handover).</p>
</li>
</ul>


<p>The crucial principle for the BSC selecting the target
cells for the handover caused by radio criteria (signal level / signal quality)
is that the adjacent cell must be better than the current serving cell in
order for the handover to be useful. If the handover is caused by other reasons
than radio criteria, it is not necessary for the target cell to be better
than the serving cell. It is enough that the target cell serves the call well
enough; for example, a handover from an umbrella cell to a microcell is performed
whenever the call can be maintained on the neighbouring microcell.</p>


<p>The BSC supports handovers, which occur between different
channels of TRX, between different TRXs of BTS, between different BTSs of
BSS, and between different BSSs of NSS(s). The handover can occur between
signalling channels (SDCCH to SDCCH) or between traffic channels (TCH to TCH).
In case of Directed Retry (133.0), the handover can occur from signalling
channel to traffic channel (SDCCH to TCH).</p>


<p>The handover can be synchronised or asynchronised, depending
upon whether the cells are synchronised or not.</p>
</div>


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<div class="topic">
<h2>RF
power control strategy</h2>

<p>The RF power control strategy employed by the BSC defines
the RF power command that is signalled to the MS and the RF power level that
is used by the BTS. The RF power control optimises the RF output power of
the MS and the BTS and simultaneously ensures that the signal level required
at the BTS/MS is sufficient to maintain an adequate speech/data quality.</p>


<p>The RF power level to be employed in each case is based
on the measurement results reported by the MS/BTS and on the various parameters
set for each cell. All parameters controlling the power control procedure
are administered on a cell-by-cell basis.</p>


<p>The power control, for both the BTS and the MS, runs independently
in parallel with the handover, but in such a way that the power control knows
the status of the handover and vice versa, that is, the BSC does not try to
adjust the MS/BTS power level in the current serving cell and execute a handover
to a neighbouring cell simultaneously.</p>


<p>The RF Power Control and Handover Algorithm controls the
intervals between handovers and handover attempts by means of timers:</p>


<ul>
<li><p>A minimum interval between handovers
related to the same connection</p>
</li>
<li><p>A minimum   interval   between an unsuccessful
handover attempt and the following handover attempt.</p>
</li>
</ul>


<p>The Bad Quality Experience Guard Time improvement concerns
the control of the intervals between handovers and handover attempts.</p>
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