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<h1>Intelligent Underlay Overlay (IUO)</h1>

<p>Intelligent underlay overlay is a feature designed to allow
the operator to reuse frequencies more intensively and hence achieve a higher
radio network capacity.</p>


<p>In order to avoid interference due to the increased level
of frequency reuse, the BSC estimates the degree of interference on different
frequencies and directs the mobile stations to those frequencies that are
‘clean’ enough to sustain a good radio connection quality.</p>


<p>The interference estimation made by the BSC is based on
the downlink measurement results reported by the MS via the BTS and on various
adjustable parameters.</p>


<p>In order to achieve a higher radio network capacity by
means of the intelligent underlay-overlay feature, the operating spectrum
of the network is divided into regular frequencies and super-reuse frequencies.
A continuous coverage area is provided by the overlay network, which utilises
regular frequencies.</p>


<p>Frequency planning of the overlay network is based on conventional
planning criteria using safe handover margins and requiring low co- and adjacent
channel interference probabilities. Regular frequencies are intended to serve
mobile stations mainly at cell boundary areas and other locations where the
C/I ratio is the worst. The overlay network also provides interference-free
service in the overlapping cell areas required for handover control and neighbouring
cell measurements by mobile stations. The underlay network is formed from
the super-reuse frequencies, which are reused very intensively to provide
the extended capacity. The super-reuse frequencies are intended to serve mobile
stations, which are close to the BTS, inside buildings, and other locations
where the radio conditions are less vulnerable to interference.</p>


<p>The BSC controls the traffic division on regular and super-reuse
frequencies by means of the radio resource allocation at the call set-up phase
and later on during the call by means of handover procedures.</p>


<p>The direct access to super reuse TRXs that will give extra
capacity is determined by signal level measurements during call set-up and
in handovers. The access is only accepted if the signal level is above a given
threshold (Rx level). The direct access to super reuse TRX will also help
in situations where the MS is near to the BTS. In this case, the unnecessary
TCH reservation from the regular TRX will be avoided.</p>


<p>The BSC continuously monitors the downlink C/I ratio on
each super-reuse frequency of the cell for every ongoing call. The call is
always handed over from a regular frequency to a super-reuse frequency when
the C/I ratio on the super-reuse frequency is good enough. If the C/I ratio
on a super-reuse frequency becomes worse, the call is handed over from the
super-reuse frequency back to a regular frequency. Based on the profile of
interference each mobile is exposed to, the BSC determines the most appropriate
frequency to be assigned for the call connection. A new type of handover is
introduced between frequency groups as a direct handover from a super-reuse
frequency group to another super-reuse frequency group.</p>


<p>The BSC uses different handover decision algorithms for
handovers caused by traffic control between regular and super-reuse frequencies,
handovers caused by conventional radio criteria such as power budget, and
handovers caused by other reasons than radio criteria such as directed retry.</p>


<p>The signal strength may vary and drop rapidly in cases
where the MS moves fast through the coverage area of one cell. This drop can
happen also in street corner cases. When the IUO is used and the call in handled
by the super reuse TRX, in these rapid field drop cases both bad C/I and power
budget thresholds can trigger at the same time. This can happen especially
when the call is in the edge of the dominance area of the super reuse TRX.
In this case, it is preferred to do a handover to an adjacent cell due to
power budget instead of handing the call over to regular TRX and to perform
the power budget handover to an adjacent cell after the IUO handover from
the super reuse TRX to a regular TRX.</p>


<p>The Handover &amp; Power Control algorithm is modified
for the simultaneous use of Intelligent Underlay Overlay and Dual Band. The
behaviour of the dual band mobiles in IUO network is indicated with one parameter.
This parameter indicates whether dual band mobiles are allowed to use super
reuse TCHs in a dual band cell. This parameter is a BSC specific parameter.</p>


<p>The intelligent overlay-underlay feature does not have
any special requirements for the RF power control strategy. The service area
of super-reuse frequencies is controlled by the C/I calculation method together
with quality report analysis, not by limiting the maximum allowed MS/BTS TX
power levels. The algorithms allow mobile stations to use high power levels
in difficult conditions, such as inside buildings.</p>


<p>Frequency hopping can be applied in an Intelligent Underlay
Overlay cell. It can be defined on the IUO layer basis in order to further
increase the interference performance. By combining the benefits of Intelligent
Underlay Overlay and frequency hopping, it is possible to have a powerful
radio spectrum efficiency feature available.</p>


<p>There are following improvements:</p>


<ul>
<li><p>Inter super-reused layer handover</p>
</li>
<li><p>Additional reference cells</p>
</li>
<li><p>Direct access to super reuse TRX in
IUO</p>
</li>
<li><p>Handover cause prioritisation in IUO</p>
</li>
<li><p>Dual Band MS Access to IUO-layer.</p>
</li>
</ul>
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