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

<p>The purpose of the Link Adaptation (LA) mechanism is to
provide the highest throughput and lowest delay available by adapting the
protection level of the transmitted information according to link quality.
LA uses various measurements of the past link to predict the upcoming channel
quality.</p>


<p>The link adaptation algorithm to work in cooperation with Incremental
Redundancy (IR). While IR improves throughput by adapting the total amount
of transmitted redundancy automatically to the radio channel conditions, LA
selects the amount of redundancy individually for each transmission. This
helps reduce the number of re-transmissions, and thus keeps the transfer delay
reasonably low.</p>


<p>Normally, LA adapts to path attenuation and slow fading
but not fast fading. This corresponds to the "ideal LA" curves in link level
simulations.</p>


<p>Link Adaptation can be enabled and disabled with the parameter <span class="parameter-sem">EGPRS link adaptation enabled (ELA)</span>. Other LA-related
parameters include:</p>


<ul>
<li><p><span class="parameter-sem">EGPRS Init MCS Ack Mode</span></p>
</li>
<li><p><span class="parameter-sem">EGPRS Init MCS Un Ack Mode</span></p>
</li>
<li><p><span class="parameter-sem">EGPRS Max Bler Ack Mode</span></p>
</li>
<li><p><span class="parameter-sem">EGPRS Max Bler Un Ack Mode </span></p>
</li>
<li><p><span class="parameter-sem">EGPRS Mean Bep Offset 8PSK</span></p>
</li>
<li><p><span class="parameter-sem">EGPRS Mean Bep Offset GMSK</span></p>
</li>
</ul>


<p>For more information, see BSS Radio Network Parameter Dictionary in
BSC/TCSM Product Documentation Set.</p>


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<h2>Link
quality measurements</h2>

<p>Enabling LA requires accurate link quality measurements
and a set of modulation and coding schemes (<a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id09745#id09745" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id09745#id09745');return false" shape="rect" target="_blank">MCS</a>s)
with different degrees of protection.</p>


<p>The efficient EGPRS measurements provide accurate predictions of the
upcoming link quality in several propagation channels that have various speeds
(for example, typical urban and rural areas and hilly terrain). The link adaptation
algorithm is based on Bit Error Probability (BEP) measurements performed at
the MS (downlink TBF) and the BTS (uplink TBF). In acknowledged mode, the
algorithm is designed to optimise channel throughput in different radio conditions.
In unacknowledged mode, the algorithm tries to keep below a specified Block
Error Rate (BLER) limit.</p>
</div>


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<h2>MCS selection</h2>

<p>LA selects the optimal MCS for each radio condition. The
LA procedure is based on static MCS selection tables in the <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id014238#id014238" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id014238#id014238');return false" shape="rect" target="_blank">PCU</a>.
The MCS selection can be divided in four classes:</p>


<ol class="type-odd">
<li type="1"><p>initial MCS to be used when entering
packet transfer mode</p>
</li>
<li type="1"><p>modulation selection</p>
</li>
<li type="1"><p>MCS selection for initial transmissions
of each RLC block in ACK mode</p>
</li>
<li type="1"><p>MCS to be used for re-transmissions.</p>
</li>
</ol>
</div>


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<h2>Related
topics</h2>

<p><a href="NED?action=retrieve&amp;identifier=dn02242662&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=nov22215133515#nov22215133515" onclick="sync(this);return true" shape="rect">Overview of Nokia EDGE</a></p>
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