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<h2>DFCA channel allocation algorithm</h2>

<p>The DFCA channel allocation algorithm is used to obtain the C/I estimations
and perform channel allocation decisions based on those estimations.</p>


<p>The C/I estimation relies on the fact that the interference relations
in a DFCA network are stable and predictable due to BSS synchronisation and
the controlled usage of cyclic frequency hopping. The C/I estimation is done
by combining information from several sources such as the mobile station measurement
reports, background interference matrix (BIM), real time DFCA channel usage
information, and near real time information on UL/DL TX power reductions.</p>


<p>Several C/I estimations are produced for each radio channel. The incoming
C/I describes the interference coming from existing connections that would
affect the new connection for which the channel assignment is being done.
The new connection can also potentially cause interference to existing connections
using the same or adjacent frequencies. This is examined by determining the
outgoing C/I for every potentially affected existing connection. Furthermore,
both downlink and uplink C/I are estimated separately for both outgoing and
incoming interference.</p>


<p>The flowchart of the channel selection process is shown in the figure
below.</p>


<a name="jan1251246383" shape="rect"></a>
<div>
<img alt="urn:mars:dn056167:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn056167&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: DFCA channel selection process</p>
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<ol class="type-odd">
<li type="1"><p><em>C/N Check for cell access</em></p>


<p>The C/N check with the connection type specific C/N soft blocking limit
parameters allows a flexible signal level based cell access control by taking
into account the different properties of different connection types. C/N check
also ensures that the use of half rate channels is not allowed in a low field
strength situation.</p>


<p>The BSC checks the radio link signal level situation at the beginning
of the DFCA channel allocation procedure. The reported UL and DL signal levels
are compared to the UL noise level and DL noise level parameters as defined
by the operator.</p>
</li>
<li type="1"><p><em>Calculate BIM scaling factor</em></p>


<p>Since the C/I values in the BIM tables are based on long-term statistics
of all connections in the cell, BIM scaling must be used for correcting the
statistical C/I values so that they better correspond to the user's actual
situation. BIM scaling factor is the average difference between the measured
and statistical C/I values and it is applied to all C/I values taken from
the BIM table for C/I estimations for each particular connection.</p>
</li>
<li type="1"><p><em>Pick TSL, MA and MAIO</em></p>


<p>In the actual C/I estimation process, the DFCA algorithm determines
the interference that the new user would have with each available time slot,
DFCA MA list, and MAIO combination. The DFCA algorithm uses the round robin
method to circulate time slots, MA lists, MAIOs so that the estimations are
always started from the MA, MAIO and time slot combination following the one
which was allocated the previous time.</p>
</li>
<li type="1"><p><em>Incoming DL &amp; UL C/I estimation and transmit power
settings</em></p>


<p>In the incoming DL C/I estimation, the algorithm identifies the interfering
co- and adjacent channel connections in the neighbour cells and estimates
the level of interference coming from each potentially interfering neighbour
cell.</p>


<p>The most significant incoming DL interference source is identified,
using the C/I values either in the latest MS measurement report or in the
incoming interfererence BIM table (scaled by the BIM scaling factor). If the
incoming DL C/I for this interference source is above the target C/I set for
the connection type of the new connection, the initial DL transmit power is
reduced so that the final C/I corresponds to the target C/I. The difference
between the C/I target and the final C/I caused by the dominant interference
source is recorded.</p>


<p>The incoming UL C/I estimation is based purely on the statistical C/I
values available in the outgoing interference BIM tables, since only DL measurements
of the neighbour cell signal levels are available. The estimate can be improved
by accounting the relative location of the new connection in the cell that
is reflected in the incoming interference BIM scaling factor.</p>


<p>Although the BIM tables are based on the DL measurements, a reasonable
estimation of the UL interference situation can still be derived, based on
the fact that the propagation path of the incoming UL interference is the
same as the propagation path of the outgoing DL interference.</p>


<p>As in incoming DL C/I estimation, if the incoming UL C/I for the most
significant interference source is above the target C/I set for the connection
type of the new connection, the initial UL transmit power level for the new
connection is reduced so that the final C/I corresponds to the target C/I.
The difference between the C/I target and the final C/I caused by the dominant
interference source is recorded.</p>
</li>
<li type="1"><p><em>Outgoing DL &amp; UL C/I estimation</em></p>


<p>In the outgoing DL C/I estimation, the algorithm goes through the cells
listed in the outgoing interference BIM table and checks the DFCA RR table
for ongoing co- and adjacent channel connections in these cells.</p>


<p>The algorithm identifies the maximum caused outgoing DL interference
by calculating the C/I difference for each interfered connection, using either
the measured or statistical C/I values, scaled by the BIM scaling factor and
adjusted by the actual DL transmit power reduction determined in incoming
DL estimation. The difference between the C/I target and the C/I of the maximum
caused outgoing DL interference is recorded.</p>


<p>In the outgoing UL C/I estimation, the algorithm goes through the cells
listed in the incoming interference BIM table, since the interference propagation
path is the same for both outgoing UL and incoming DL interference. The maximum
caused outgoing UL interference is identified by calculating the C/I difference
for each interfered connection, using the statistical C/I values, scaled by
the BIM scaling factor and adjusted by the actual UL transmit power reduction
determined in incoming UL estimation. The difference between the C/I target
and the C/I of the maximum caused outgoing UL interference is recorded.</p>


<a name="jan1251250364" shape="rect"></a>
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<img alt="urn:mars:dn056194:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn056194&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: DFCA C/I estimations</p>
</div>
</li>
<li type="1"><p><em>Determine C/I difference for a TSL, MA and MAIO combination</em></p>


<p>After calculating the incoming and outgoing DL and UL C/I estimations
for a particular TSL, MA and MAIO combination, the algorithm determines the
most restrictive (lowest) minimum C/I difference and records it.</p>
</li>
<li type="1"><p><em>Choose the most suitable channel</em></p>


<p>After going through the TSL, MA and MAIO combinations, the algorithm
selects the most suitable channel on the basis of the recorded C/I differences.
The operator can choose between two DFCA channel allocation methods for selecting
a traffic channel from the set of available candidate channels.</p>


<p>In the primary method, the target is to have a channel with the highest
minimum C/I difference. To find the channel with the highest C/I, the minimum
C/I difference has to be determined for all the available channels. This makes
the method computationally intensive.</p>


<p>However, the operator can also use a secondary method to search for
a suitable channel in order to reduce the BSC processing load. In this method,
the search is stopped as soon as a channel on the target C/I level is found.
If such a channel cannot be found, the secondary target is to have a channel
with C/I as close above the target level as possible. Channels below the target
C/I level are the last choice.</p>


<p>In both methods, the channel search is stopped as soon as an interference
free channel is found.</p>
</li>
<li type="1"><p><em>Soft blocking C/I</em></p>


<p>In addition to the C/I target parameter, each connection type (full
rate (FR), half rate (HR), AMR FR, AMR HR, 14.4 data) has a soft blocking
C/I threshold parameter. In the C/I estimation phase, if any of the four C/I
estimates calculated for a radio channel candidate goes below the soft blocking
limit of the interfered connection, then the radio channel candidate is regarded
as soft-blocked and unsuitable for allocation.</p>


<p>If there are no acceptable candidate channels, the request is directed
towards the regular resources of the cell. If there are no free TSLs on any
regular TRX of the cell, the channel allocation request is rejected in the
examined cell (DFCA soft blocking situation). Directed retry can still be
used to move the call setup request to another cell.</p>
</li>
<li type="1"><p><em>Determine TSC</em></p>


<p>After a suitable channel has been found, the BSC determines the most
suitable training sequence code by examining all the interfering connections.
The BSC searches for the training sequence code (TSC) that has been used by
an interfering connection with the highest C/I. This means that for the selected
MA, MAIO, and time slot combination, the minimum C/I differences are checked
TSC by TSC and the one with the highest C/I difference is selected.</p>


<p>The method of the training sequence code selection aims at avoiding
the worstcase situation where a significant interference source uses the same
training sequence code as the new connection. A conflict with a significant
interferer would cause the receiver to obtain an incorrect channel estimate
and, therefore, lead to a link level performance degradation and a poor AMR
link adaptation performance.</p>
</li>
<li type="1"><p><em>Allocate channel</em></p>


<p>The selected channel together with the respective MA, MAIO, time slot
and transmit power information is sent to the BTS and MS.</p>
</li>
</ol>
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