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<h3>Two-layer dual band network</h3>

<p>This first strategy is the simplest one to use. It works
when the network uses only 2 layers: a GSM 900/800 macro layer and a GSM 1800/1900
macro layer.</p>


<p>The GSM 1800/1900 layer is defined as a <em>LOWER</em>
layer for the GSM 900/800 layer and the handovers used are:</p>


<ul>
<li><p>Umbrella handover to go from 900/800
to 1800/1900, with a typical threshold of  -85 dBm.</p>
</li>
<li><p>Radio reason (Level or Quality) handover
to go from 1800/1900 to 900/800.</p>
</li>
<li><p>Power budget handover within the same
layer, with a margin of 6 dB.</p>
</li>
</ul>


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<p>In this strategy, we assume that there are only two layers
in the network, which means that micro cells are not defined as another lower
layer. This case will be studied later.</p>


<p>Dual band mobiles are directed to the 1800/1900 layer during
the call by using an umbrella handover and are kept in the suitable band with
a power budget handover. When the quality or the level becomes poor, the mobile
returns to the 900/800 layer (or stays in the 1800/1900 layer). When the 1800/1900
and 900/800 cells are co-located, only one 1800/1900 neighbour - the co-located
cell - can be defined from each 900/800 cell (in the case of a coverage limited
macro cell). This is justified by the low capacity needed in the beginning
and the level is always better in that cell than in the other ones.</p>
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