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<div class="div">
<h1>Overview of Adaptive Multi Rate Codec (AMR)</h1>

<p>The Adaptive Multi Rate (AMR) codec, standardised for GSM
during 1998-1999, optimises speech quality in various radio channel conditions
by adapting its bit rate allocation between speech and channel coding. This
provides the next step in the improvement of speech quality in GSM after the
introduction of 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=id09091#id09091" 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=id09091#id09091');return false" shape="rect" target="_blank">Enhanced Full Rate (EFR) codec</a> in 1996,
the first codec to provide wireline speech quality. The AMR codec brings further
quality enhancements, especially in terms of high error robustness in the
full rate channel. It also provides the first codec with quality comparable
to wireline for the half rate channel in good channel conditions. All previous
GSM codecs operate with fixed partitioning between speech and channel coding
(error protection) bit rates. These bit rates have been chosen as compromises
between performance in error-free and high-error channels. The AMR codec operates
in either the GSM full or half rate channel and selects the optimum bit rate
trade-off between speech and channel coding, according to the channel quality,
to deliver the best possible overall speech quality. To achieve overall good
speech quality, the quality degradation caused by speech coding and the errors
engendered by the transmission channel have to be carefully balanced.</p>


<p>AMR is a technology that enables operators smoothly and
cost-efficiently to add voice capacity in their networks. In the Nokia AMR
solution, this requires only a software upgrade. AMR is one of the voice capacity
enhancement technologies (the others are: half rate, frequency hopping, intelligent
frequency hopping, further development of handover algorithm).</p>


<p>AMR consists of eight different speech codec modes with
a total of 14 channel codec modes (see Table <em>Channel and
speech codec modes available for AMR</em>). All the speech codecs are defined
for the full rate channel, while the six lowest ones are defined for the half
rate channel. The net bit rate is 0.10 kbit/s (in-band channel). The channel
coding bit rate (in-band) is 0.30 kbit/s.</p>


<a name="nov1421222532" shape="rect"></a>
<div>
<p>Table: Channel and speech codec modes available for AMR</p>


<table border="1">
<thead>
<tr align="left" char="" charoff="50" valign="bottom">
<th colspan="1" rowspan="1" valign="top"><p>Channel mode</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Channel codec </p>


<p>mode</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Source coding bit rate, speech</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Channel coding bit rate, speech</p>
</th>
</tr>
</thead>
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td align="center" colspan="1" rowspan="1"><p>TCH/FR</p>
</td>
<td colspan="1" rowspan="1"><p>CH0-FS</p>


<p>CH1-FS</p>


<p>CH2-FS</p>


<p>CH3-FS</p>


<p>CH4-FS</p>


<p>CH5-FS</p>


<p>CH6-FS</p>


<p>CH7-FS</p>
</td>
<td colspan="1" rowspan="1"><p>12.20kbit/s (GSMEFR)</p>


<p>10.20 kbit/s</p>


<p>7.95 kbit/s</p>


<p>7.40 kbit/s (IS-641)</p>


<p>6.70 kbit/s</p>


<p>5.90 kbit/s</p>


<p>5.15 kbit/s</p>


<p>4.75 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p>10.20 kbit/s</p>


<p>12.20 kbit/s</p>


<p>14.45 kbit/s</p>


<p>15.00 kbit/s</p>


<p>15.70 kbit/s</p>


<p>16.50 kbit/s</p>


<p>17.25 kbit/s</p>


<p>17.65 kbit/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>TCH/HR</p>
</td>
<td colspan="1" rowspan="1"><p>CH8-HS</p>


<p>CH9-HS</p>


<p>CH10-HS</p>


<p>CH11-HS</p>


<p>CH12-HS</p>


<p>CH13-HS</p>
</td>
<td colspan="1" rowspan="1"><p>7.95 kbit/s(*)</p>


<p>7.40 kbit/s (IS-641)</p>


<p>6.70 kbit/s</p>


<p>5.90 kbit/s</p>


<p>5.15 kbit/s</p>


<p>4.75 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p>3.25 kbit/s</p>


<p>3.80 kbit/s</p>


<p>4.50 kbit/s</p>


<p>5.30 kbit/s</p>


<p>6.05 kbit/s</p>


<p>6.45 kbit/s</p>
</td>
</tr>
</tbody>
</table>


<div>
<p>(*) Requires 16 kbit/s TRAU. Therefore it is not seen as
a feasible codec mode and is not supported by Nokia BSS.</p>
</div>
</div>


<p>A mobile station must implement all the codec modes. However,
the network can support any combination of them.</p>


<p>Each codec mode provides a different level of error protection
through a different distribution between speech and channel coding.</p>


<p>The link adaptation process measures the channel quality.
Depending on the quality and possible network constraints (for example network
load), mode adaptation selects the optimal speech and channel codecs. The
mobile station (MS) and the base transceiver station (BTS) both perform channel
quality estimation for their own receive paths. Based on the channel quality
measurements, the MS sends a Codec Mode Request (Mode requested to be used
in the downlink) to the BTS. This signalling is sent in-band, along with the
speech data. The in-band signalling has been designed to allow fast adaptation
to rapid channel variations.</p>


<p>Codec mode adaptation for AMR is based on received channel
quality estimation in both MS and BTS, followed by a decision on the most
appropriate speech and channel codec mode to apply at a given time. In high-error
conditions more bits are used for error correction to obtain error robust
coding, while in good transmission conditions a lower amount of bits are needed
for sufficient error protection and more bits can therefore be allocated for
the source coding.</p>


<p>An in-band signalling channel is defined for AMR that enables
the MS and the BTS to exchange messages on applied or requested speech and
channel codec modes. The selected speech codec mode mentioned above is then
sent to the transmitting side by using the in-band signalling channel, where
it is applied for the other link. The BTS commands the MS to apply a particular
speech codec mode in the uplink by Codec Mode Command. The MS sends a Codec
Mode Request (Mode requested to be used in the downlink) to the BTS. The BTS
has an option to override the MS' request. The codec mode in the uplink may
be different from the one used in the downlink, but the channel mode (full
rate or half rate) must be the same.</p>


<p>Mobile stations must support all speech codec modes, although
only a set of up to 4 speech codec modes is used during a call. <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=id04439#id04439" 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=id04439#id04439');return false" shape="rect" target="_blank">BSC</a>
supports all of speech codec modes, except 7.95 kbit/s on HR channel, and
it has one default set for each channel mode. The default codec sets also
include a default set of decision thresholds and hysteresis. The initial codec
mode and codec set with thresholds and hysteresis are transferred between
network elements and MS by using the existing layer 3 signalling. Only a few
add-ons are needed.</p>


<p>EFR, AMR full rate (FR), and AMR half rate (HR) are application
software in the BSC. You can use either AMR FR, AMR HR, or both.</p>


<p>AMR HR requires a valid licence in the BSC. For more information, see <a href="NED?action=retrieve&amp;identifier=dn04149427&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=oct1841615431#oct1841615431" onclick="sync(this);return true" shape="rect">BSS Licensing</a>.</p>


<p></p>


<a name="nov1421256224" shape="rect"></a>
<div class="topic">
<h2>Benefits
of Adaptive Multi Rate Codec (AMR)</h2>

<p>GSM speech codecs (full rate - FR, half rate - HR and enhanced
full rate - EFR) operate at a fixed coding rate. The channel protection (against
errors) is also added at a fixed rate. The coding rates are chosen as a compromise
between the best clear channel performance and robustness to channel errors.</p>


<p>The AMR system exploits the implied performance compromises
by adapting the speech and channel coding rates according to the quality of
the radio channel. This gives better and clearer channel quality and better
robustness to errors. These benefits are realised regardless of whether operating
in full rate or half rate channels.</p>


<a name="dec1221745441" shape="rect"></a>
<div>
<h6>Example</h6>


Consider the situation where the mobile is in a zone of
the cell border where you have a bad C/I (for example 7dB). With EFR you have
a degradation of the quality of the speech due to interference. With AMR,
however, similar quality can be achieved with a reduced number of speech coded
bits which allows more bits to be used for error protection and correction
(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=nov1921524421#nov1921524421" onclick="sync(this);return true" shape="rect">ETSI Mean Opinion Score test results for current EFR/FR
and AMR FR</a>).</div>


<p>Together with quality improvements, the need to enhance
capacity by allocating half rate channels to some or all mobiles in the network
is also recognised. The radio resource algorithm, enhanced to support AMR
operation, allocates a half rate or full rate channel according to channel
quality and the traffic load on the cell to obtain the best balance between
quality and capacity.</p>


<a name="dec1221746502" shape="rect"></a>
<div>
<h6>Example</h6>


Increase in capacity: in normal C/I condition two voice
channels can use a single timeslot in the case of Half rate coding (HR) with
little or no compromise in voice quality compared to EFR (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=nov1921524421#nov1921524421" onclick="sync(this);return true" shape="rect">ETSI
Mean Opinion Score test results for current EFR/FR and AMR FR</a>).</div>


<p>Optimal interworking with power control and handover algorithms
together with enhanced quality measurements (FER Measurement feature) provides
full benefits and interworking with prior Nokia capacity features including <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=id09018#id09018" 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=id09018#id09018');return false" shape="rect" target="_blank">Intelligent Frequency Hopping (IFH)</a>.</p>
</div>


<a name="nov1921524421" shape="rect"></a>
<div>
<img alt="urn:mars:dn02236314:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02236314&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: ETSI Mean Opinion Score test results for current EFR/FR and AMR FR</p>
</div>


<a name="dec122182513" shape="rect"></a>
<div class="topic">
<h2>Related
topics</h2>

<ul>
<li><p><a href="NED?action=retrieve&amp;identifier=dn03297833&amp;edition=4&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=feb20312473511#feb20312473511" onclick="sync(this);return true" shape="rect">System impact of Adaptive
Multi Rate Codec (AMR)</a></p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=dn02244115&amp;edition=3&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=nov2521414571#nov2521414571" onclick="sync(this);return true" shape="rect">Overview of implementing AMR </a></p>
</li>
</ul>
</div>


<p><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=ned_2#ned_2" onclick="sync(this);return true" shape="rect">Link adaptation</a></p>


<a name="dec221411522" shape="rect"></a>
<div class="div">
<h2>Channel allocation</h2>

<p>HR and EFR principles are applied with one exception: an
AMR call may be started in full rate channel in a new cell. It depends on
the parameter <span class="parameter-sem">Initial AMR Channel Rate</span> which has
a default value <span class="parameter-sem">Any Rate</span>. This value
means that the chosen channel rate is defined by taking into account the currently
used information (Channel Type IE, resource situation on radio interface,
circuit pool, current channel rate, HO parameters). The other option is <span class="parameter-sem">AMR FR</span> which means that full rate channel is allocated
despite the values of the currently used information. If AMR FR codec is not
present in the Channel Type element or it cannot be allocated (for example
AMR FR set is disabled in the target cell), allocation continues with the
currently used information. The parameter is valid in call setup (except in <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=id011221#id011221" 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=id011221#id011221');return false" shape="rect" target="_blank">FACCH</a> call setup), internal inter cell handover and
external handover. The reason for this parameter is that quality may not be
sufficient for HR AMR call setup (radio measurement is done on SDCCH).</p>


<p>With the use of the TRP parameter, you can direct AMR calls
primarily to non-BCCH TRX and non-AMR calls primarily to BCCH TRX.</p>
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