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<div class="div">
<h1>Half Rate</h1>

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<h2>General
description</h2>

<p>Half Rate is a feature designed to maximise the spectrum
efficiency by doubling the amount of radio resources as compared the use of
the Full Rate traffic resources only.</p>


<p>Each radio time slot of the BTS TRX can be configured to
be a Full Rate, Half Rate, or Dual Rate TCH resource. In the last case, the
BSC will be able to allocate an idle radio time slot either for Half Rate
or Full Rate coding dynamically on a call basis.</p>


<p>Full Rate speech and data are coded and transferred by
using 16 kbit/s channels in the BSS. With the half rate coding, an 8 kbit/s
transmission can be used on the Abis interface. The A interface will support
different types of transcoders capable of Full Rate coding or Half Rate coding
or both. The utilisation of a Half Rate (HR) and a Full Rate (FR) coding and
corresponding transmission bit rates in the Nokia system is introduced in
the following picture:</p>


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<img alt="urn:mars:dn0468963:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0468963&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Utilisation of a Half Rate (HR) and a Full Rate (FR) coding and corresponding
transmission bit rates</p>
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<p>It will be possible to introduce the Half Rate coding to
the existing Full Rate GSM 900/1800 network gradually. The BSS is able to
co-operate with both old Full Rate mobiles and with new mobiles, which support
both Full and Half Rate.</p>
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<h2>Traffic
channel allocation</h2>

<p>In cases when the MSC does not determine the channel type
uniquely, the BSC always first decides on the channel rate of the TCH to be
allocated. Though the A interface circuit allocated by the MSC must enable
the BSC to allocate a radio channel of requested type, the actual A interface
circuit pool configuration determines how the BSC can select the channel rate.
In those cases when the A interface circuit, which the MSC has allocated,
belongs to a pool which does not support the type of TCH the BSC wishes to
allocate, the BSC can request the MSC to switch the circuit to the appropriate
pool.</p>


<p>Radio Resource Management of the BSC is able to optimise
the allocation of the half rate traffic channel resources of the cell in such
a way that they are primarily allocated from such radio timeslots in which
a half rate call is already maintained.</p>


<p>There are two main factors which are determining the type
of TCH or - more strictly speaking - the speech (codec) version in the traffic
channel allocation done by the BSC: the traffic load of the cell and, in handover
case, also the speech version which the call is using in the source cell of
the handover.</p>
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<h2>Traffic
channel allocation based on the traffic load of BTS</h2>

<p>The type of traffic channel - Half Rate or Full Rate -
to be allocated can be determined according to the actual traffic load of
a cell. Full Rate TCHs are allocated from the BTS until the number of free
full rate resources are reduced below a particular lower limit, the Half Rate
resources shall then be allocated. When the number of the free Full Rate resources
of the cell increases above a particular upper limit, Full Rate TCHs shall
be allocated again.</p>


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<img alt="urn:mars:dn0468975:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0468975&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Traffic channel allocation</p>
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