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<h2>Background Interference Matrix (BIM) update process</h2>

<p>Background Interference Matrix (BIM) is used for identifying all potentially
interfering DFCA neighbour cells. The BSC generates two BIM tables for each
DFCA cell: the incoming interference BIM table lists the potentially interfering
surrounding cells and their interference levels, while the outgoing interference
BIM table lists the potentially interfered surrounding cells and the interference
caused to them by a new connection. The BIM tables are automatically created
and updated by the BSC.</p>


<p>The BIM update process collects statistical C/I data from all the cells
that are reported by the mobiles in each DFCA BTS. The BSC collects these
C/I statistics for the DFCA algorithm from the active TCH connections of a
DFCA BTS as the signal levels are reported by the mobiles. Statistical data
is collected from all connections in the BTS and not only from those in DFCA
TRXs.</p>


<p>For each neighbour cell that has a sufficient amount of samples (at
least 1000) collected at the end of the BIM update period, the BSC determines
a general cell-level view about the interference from the neighbour cell in
question. The result is a C/I level that the mobile stations of the DFCA cell
experience in general towards the neighbour cell. It is the C/I that is exceeded
in a certain percentage of the samples for this neighbour in a DFCA cell.
This percentage can be adjusted by the operator.</p>


<p>In order for a neighbour cell to be added to the incoming interference
BIM table, its C/I value has to be lower than a threshold defined by the operator.
In this way, any unncessary processing load for the BSC is avoided.</p>


<p>If the C/I from the latest BIM update period and the old value saved
in the incoming interference BIM table differ from each other, the information
of the two values is combined by a statistical filtering process, and the
updated C/I related to the neighbour cell in question is saved in the incoming
interference BIM table of the interfered cell. The BSC also updates the related
BIM entry in the outgoing interference BIM table of the interfering cell,
if this is controlled by the same BSC.</p>


<p>In a constantly changing radio network environment, it can happen that
a cell that was previously found to be a relevant interference source may
cease to be significant enough to be considered in DFCA C/I estimations. In
such cases, the BSC removes the related BIM entry from the incoming interference
BIM table of the interfered cell. The BSC also removes the related BIM entry
from the outgoing interference BIM table of the interfering cell, if this
is controlled by the same BSC.</p>


<p>To collect reliable C/I statistics and create complete BIM tables where
all the relevant potentially interfering cells are listed, the mobiles should
be able to measure the BCCH frequencies of all surrounding cells. This minimises
the risk of some potential interfering cells being excluded from the decisions
made by the DFCA channel allocation algorithm. The operator should create
a BCCH frequency list that includes all the BCCH frequencies used in the area
and attach this list as the BCCH frequency list to be used for the active
state mobile stations in the DFCA cells.</p>
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