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<h2>Link adaptation</h2>

<p>Link adaptation is the capacity of AMR feature to vary
the codec used according to the link conditions. Both networks, for uplink,
and MS, for downlink, measure the radio conditions in each link and decide
which codec to apply to each way.</p>


<p>Two different types of link adaptation algorithms are defined: <em>codec mode adaptation</em> and <em>channel mode
adaptation</em>. The channel mode adaptation algorithm decides whether speech
can be handled by a full rate channel or by a half rate channel according
to the link conditions, whereas for the channel selected, the codec mode adaptation
algorithm decides which codec is the one that provides the best speech quality
for the current radio conditions. Because each codec has a different channel
protection and speech encoding performance, the idea of the codec mode adaptation
is to select the codec that provides the best speech quality for the radio
conditions that the receivers are submitted to.</p>


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<h3>Codec
mode adaptation</h3>

<p>There are two link adaptation (LA) modes; the standardised
fast LA and the Nokia-proprietary slow LA. Fast LA BTS allows in-band codec
mode changes on every other TCH frame, but in Nokia proprietary slow LA BTS
allows in-band codec mode changes only on <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=id011252#id011252" 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=id011252#id011252');return false" shape="rect" target="_blank">SACCH</a>
frame interval.</p>


<p>The choice of the LA mode is done on BSC-basis with the
parameter <span class="parameter-sem">Slow Amr La Enabled</span>: if it is set to <span class="parameter-sem">N</span> (default), fast LA is used; if it is
set to <span class="parameter-sem">Y</span>, Nokia slow LA is used.
With slow LA, BTS allows in-band codec mode changes only on the SACCH frame
interval of 480 ms and this option gives better flexibility with HO and PC
algorithms. During both LA modes, the BTS indicates the first and the last
used codec during the last measurement interval and the average quality. The
BTS commands the MS to apply a particular speech codec mode in the uplink
connection, but the MS can only request the BTS to apply a particular speech
codec mode downlink, because the BTS has an option to override the MS's request
(see Figure <a href="NED?action=retrieve&amp;identifier=dn02222016&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=dec221458481#dec221458481" onclick="sync(this);return true" shape="rect">AMR Link Adaptation</a>).</p>


<p>The codec mode bit rate, that is, the bit rate partitioning
between the speech and channel coding for a given channel mode, may be varied
rapidly (see Figure <a href="NED?action=retrieve&amp;identifier=dn02222016&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=dec22151572#dec22151572" onclick="sync(this);return true" shape="rect">Example of AMR Codec mode link adaptation</a>).
The codec mode can be switched one up or one down at the time so that it is
not possible to switch from the mode 12.2 kbit/s to 4.75 kbit/s when for example
the modes 5.9 kbit/s and 7.4 kbit/s are included to the mode set. Also, it
should be noted that codec changes do not take place immediately after the
Codec Mode Command/Request is sent: there is a delay until a frame is received
with the new codec.</p>


<p>Codec mode adaptation operates independently on the uplink
and downlink. It is transparent to the channel allocation and operates without
of it. Control depends mainly on measurements of the quality of the respective
links.</p>
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<h3>Channel
mode adaptation</h3>

<p>The channel mode (FR or HR) is switched to achieve the
optimum balance between speech quality and capacity enhancements. The uplink
and downlink use the same channel mode. The channel mode is selected by the
network based on measurements of the quality of the uplink and downlink.</p>


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<img alt="urn:mars:dn02250199:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02250199&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: AMR link adaptation</p>
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<img alt="urn:mars:dn02250206:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02250206&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Example of AMR Codec mode link adaptation</p>
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