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<div class="div">
<h1>Link Adaptation (LA) for GPRS</h1>

<p>From BSS11.5 onwards, there are two GPRS Link Adaptation algorithms,
the use of which depends on the PCU type (PCU1 or PCU2).</p>


<a name="dec174135412" shape="rect"></a>
<dl class="note-notification">
<dt class="note-caption">Note</dt>
<dd>Although the CS-3 and CS-4 coding scheme functionality is licence-based,
the new LA algorithm is nevertheless provided with PCU2.</dd>
</dl>


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<div class="topic">
<h2>Link Adaptation algorithm for PCU1</h2>

<p>The GPRS Link Adaptation (LA) algorithm selects the optimum channel
coding scheme (CS-1 or CS-2) for a particular RLC connection and it is based
on detecting the occurred RLC block errors and calculating the block error
rate (BLER).</p>


<p>The BSC level parameters <span class="parameter-sem">coding scheme no hop</span>
(<span class="parameter-sem">COD</span>) and <span class="parameter-sem">coding scheme
hop</span> (<span class="parameter-sem">CODH</span>) define whether a fixed CS value
(CS-1 or CS-2) is used or if the coding scheme changes dynamically according
to the LA algorithm. When the LA algorithm is deployed, the initial CS value
at the beginning of a TBF is CS-2.</p>


<a name="dec1741228291" shape="rect"></a>
<dl class="note-notification">
<dt class="note-caption">Note</dt>
<dd>Regardless of the parameter values, CS-1 is always used in unacknowledged
RLC mode.</dd>
</dl>
</div>


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<div class="topic">
<h2>Link Adaptation algorithm for PCU2</h2>

<p>A new Link Adaptation algorithm is introduced in BSS11.5, which replaces
the current GPRS LA algorithm and covers the following coding schemes:</p>


<ul>
<li><p><em>CS-1 and CS-2</em> if CS-3 and CS-4 support is disabled
in the territory in question</p>
</li>
<li><p><em>CS-1, CS-2, CS-3, and CS-4</em> if CS-3 and CS-4 support
is enabled in the territory in question</p>
</li>
</ul>


<p>The following BTS-level parameters define, whether the fixed CS value
(CS-1 - CS4) is used or if the changes dynamically according to the LA algorithm.
They can also be used to define the initial CS value at the beginning of a
TBF:</p>


<ul>
<li><p><span class="parameter-sem">DL coding scheme in acknowledged mode</span>
(<span class="parameter-sem">DCSA</span>)</p>
</li>
<li><p><span class="parameter-sem">UL coding scheme in acknowledged mode</span>
(<span class="parameter-sem">UCSA</span>)</p>
</li>
<li><p><span class="parameter-sem">DL coding scheme in unacknowledged mode</span>
(<span class="parameter-sem">DCSU</span>)</p>
</li>
<li><p><span class="parameter-sem">UL coding scheme in unacknowledged mode</span>
(<span class="parameter-sem">UCSU</span>)</p>
</li>
</ul>


<p>The LA algorithm measures the signal quality for each TBF in terms of
the received signal quality (RXQUAL). RXQUAL is measured for each received
RLC block, which makes it a more accurate estimate than BLER.</p>


<p>The PCU determines the average BLER value separately for each BTS by
continuously collecting statistics from all the connections in the territory
in question. Based on the estimates, the LA algorithm determines which coding
scheme will give the best performance.</p>


<p>The new LA algorithm can be used in both RLC acknowledged and unacknowledged
modes in both uplink and downlink direction. </p>
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