AMR - WB
AMR - WB
(Adaptive Multi Rate – Wide Band)
Skill Center Team
September 2012
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AMR - WB
1.
Introduction...................................................................................................................................... 3
2.
AMR Overview ............................................................................................................................... 3
3.
AMR Narrow Band.......................................................................................................................... 6
4.
AMR WB technical description....................................................................................................... 8
5.
AMR WB active set & codec rate adjustment ................................................................................. 9
6.
AMR WB parameter settings for activation .................................................................................. 10
7.
TFO (Tandem Free Operation) ...................................................................................................... 12
8.
TrFO (Transcoder Free Operation)................................................................................................ 17
9.
Conclusion ..................................................................................................................................... 19
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AMR - WB
1. Introduction
This document gives a brief description on the AMR WB feature. It describes AMR, AMR NB, WB AMR
speech coding/decoding, rate adjustment threshold adaptation, Tandem Free Operation (TFO) & TrFO
(Transcoder Free Operation).
2. AMR Overview
Adaptive Multi Rate (AMR) is a speech and channel codec for both half rate and full rate GERAN
channels and for UTRAN. By adapting the codec rate to the radio conditions the speech quality is
enhanced. AMR requires support in all network nodes, that is MSC, BSC, BTS and MS and AMR is only
supported in cells where all radio units are AMR capable.
The feature AMR makes it possible to offer an enhanced speech quality for AMR mobiles in a
network. The enhanced speech quality also provides better coverage at the edges of the cell, thus
making it possible to increase the coverage area. AMR also tolerates more interference than the old
speech codec.
The speech quality enhancement is divided in two parts and can therefore be of differe nt interest to
the operator.
1. The robust FR channel that provides high speech quality at low C/I and this makes it possible
to tighten the cell planning in a network with AMR mobiles.
2. Offers better HR codec than HR speech version one and this will increase the capacity in the
networks and reduce transmission costs.
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 At low C/I large amount of channel coding is applied and less speech coding.
 At high C/I the speech coding is increased and the channel coding is decreased.
C/I
CHANNEL
CODING
SPEECH
CODING
GOOD RADIO CONDITIONS
BAD RADIO CONDITIONS
C/I
SPEECH
CODING
CHANNEL
CODING
The below chart shows how the AMR UL codec mode is adjusted by the BTS and the AMR DL codec
mode is adjusted by the MS
The C/I is detected from
the Measurement results
sent by the MS on the UL.
The Codec Mode
adaptation is done and
the Codec Mode to be
used on the UL is selected
and sent for the MS to be
the rate in the UL
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BTS
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UL
DL
MS
The C/I is detected from
the Measurement
results sent by the BTS
on the DL The Codec
Mode adaptation is
done and the Codec
Mode to be used on the
DL is selected and sent
for the BTS to be the
rate in the DL
AMR - WB
The below figure shows the model inside the BTS and the MS which is responsible for the codec
mode control entity which selects the codec mode .Also the channel and speech encoder and
decoder are shown below .
The transcoder is shown in the figure below which is responsible for the speech transcoding over the
A interface.
Transcoder: It is a device that converts the encoding of information from one particular scheme to
a different one
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AMR - WB
3. AMR Narrow Band
What are the AMR Codec Rates and ACS?
AMR is classified into AMR FR and AMR HR. AMR makes it possible to change codec rate during a
call. There are 8 different codec rates available in the AMR transcoder:








12.2 kbps (Only used in FR channels)
10.2 kbps (Only used in FR channels)
7.95 kbps (Both FR and HR channels)
7.40 kbps (Both FR and HR channels)
6.70 kbps (Both FR and HR channels)
5.90 kbps (Both FR and HR channels)
5.15 kbps (Both FR and HR channels)
4.75 kbps (Both FR and HR channels)
ACS is the Active Set defined on the system when activating the AMR feature; it contains a maximum
of four codec modes (One codec mode corresponds to one speech coding rate)

For an AMR FR call, the ACS contains a maximum of four speech coding rates of the eight
rates listed above

For an AMR HR call, the ACS contains a maximum of four speech coding rates of the six
rates listed above
In Huawei network we have two ACS, one for the FR and one for the HR. The AMR FR and AMR HR
are termed full-rate speech version 3 and half-rate speech version 3.
 AMR FR ACS contains the following rates: 4.75KBIT/S & 5.90KBIT/S & 7.40KBIT/S &
12.2KBIT/S
 AMR HR ACS contains the following rates: 4.75KBIT/S & 5.90KBIT/S & 7.40KBIT/S
When AMR is enabled on the network, the BSC selects an ACS and then a codec mode from the ACS
during the call establishment or the handover procedure. Meanwhile, the MS and the BTS
continuously measure the receive level, receive quality, and C/I ratio.
According to the measurement results, the MS and the BTS continuously evaluate the interference
level in the radio environment. The BTS then adjusts the speech coding rates (within the same ACS)
of the MS and the BTS according to the evaluated interference level through the in band signaling.
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AMR - WB
The below flow chart shows the procedure of the codec modes change
If the currently used CODEC_MODE_3, when the C/I ratio is greater than THR_3 + HYST_3 then the
codec mode is changed to CODEC_MODE_4. When the C/I ratio is smaller than THR_2, then the
codec mode is changed to CODEC_MODE_2.
.
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AMR - WB
4. AMR WB technical description
Why AMR WB?
With the development of communication technologies, users have higher requirements for speech
quality. The narrowband speech signals, however, cannot always meet users' requirements for
speech quality.
Wide Band Adaptive Multi Rate Codec (WB AMR) is introduced to provide high-grade speech quality.
What is the difference between the AMR WB and AMR Narrow Band?
The sampling frequency of the traditional narrowband speech signals is 8 kHz, and the speech
frequency ranges from 200 Hz to 3400 Hz
The sampling frequency of WB AMR is 16 kHz, and the speech frequency ranges from 50 Hz to
7000Hz. Compared with AMR, WB AMR has wide high frequency extension and low frequency
extension. Thus, WB AMR can provide better speech quality.
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5. AMR WB active set & codec rate adjustment
Currently, Huawei WB AMR supports only one Active Codec Set (ACS).
The ACS contains three speech coding rates: 6.60 Kbit/s, 8.85 Kbit/s, and 12.65 Kbit/s.
The BSC automatically selects an appropriate speech coding rate from the ACS.
WB AMR speech coding rates
Coding Rate
WB AMR
6.60 Kbit/s
√
8.85 Kbit/s
√
12.65 Kbit/s
√
The call will adjust the speech rate on the channel frequently from the AMR WB codec modes to
adapt to the variations of the interference level in the radio environment.
Each WB AMR codec mode has an adjustment threshold, which is used to select the codec mode
that best suits the C/I ratio of the interference level in the radio environment. To avoid the constant
changes of the codec mode, the hysteresis is introduced.
The value of the threshold ranges from 0 to 63. Value 1 indicates 0.5 dB, and value 2 indicates 1 dB.
The rest may be deduced by analogy. The value of the hysteresis ranges from 0 to 15. The hysteresis
value complements the threshold.
The Codec rates must be set from the core side, same as the codec rate settings done when
activating the narrow band feature.
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AMR - WB
6. AMR WB parameter settings for activation
AMR WB Parameters and Values:
Below are the parameters to be set when activating this feature and their recommended values by
Huawei (the default values)

VOICEVER:

INITCDMDWB: 2

ULTHWB1: 21

ULHYSTWB1: 2

ULTHWB2: 25

ULHYSTWB2: 2

DLTHWB1: 12

DLHYSTWB1: 2

DLTHWB2: 18

DLHYSTWB2: 2
FULL_RATE_V ER5 (Full-rate VER 5)
AMR WB Parameter description:

VOICEVER: (MML Command: SET GCELLCCACCESS)
This parameter represents the speech version supported by the cell
Note: The AMR WB occupies only full-rate channels.
The WB AMR corresponds to full-rate speech version 5
 INITCDMDWB : (MML Command: SET GCELLCCAMR)
This parameter represents the Initial coding mode used for broadband AMR calls. The three values 0,
1 and 2 of this parameter respectively represent the lowest, low and highest coding rates in the ACS
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AMR - WB
AMR WB Thresholds and Hysteresis:
Based on the RQI in the call measurement report, the BTS and MS automatically adjust the current
speech coding rate according to the related algorithm. The coding rate adjustment threshold is the
threshold of RQI. The RQI indicates the carrier-to-interference ratio (CIR) of the call.
If RQI equals 1, the CIR is 0.5 dB; if RQI equals 2, the CIR is 1 dB; and so forth. Since there are
multiple coding rates in the ACS, there is an adjustment threshold and an adjustment hysteresis
between the neighboring coding rates.

ULTHWB1 :
(MML Command: SET GCELLCCAMR)
Unit: dB
Actual Value Range: 0~63
Recommended Value: 21
 ULHYSTWB1:.
(MML Command: SET GCELLCCAMR)
Actual Value Range: 0~15
Recommended Value: 2
 ULTHWB2 : (MML Command: SET GCELLCCAMR)
Unit: dB
Actual Value Range: 0~63
Recommended Value: 25
 ULHYSTWB2: (MML Command: SET
GCELLCCAMR)
Actual Value Range: 0~15
Recommended Value: 2
 DLTHWB1:
(MML Command: SET GCELLCCAMR)
Unit: dB
Actual Value Range: 0~63
Recommended Value: 12
 DLHYSTWB1: (MML Command: SET GCELLCCAMR)
Actual Value Range: 0~15
Recommended Value: 2
 DLTHWB2: (MML Command: SET GCELLCCAMR)
Unit: dB
Actual Value Range: 0~63
Recommended Value: 18
 DLHYSTWB2: (MML Command: SET GCELLCCAMR)
Actual Value Range: 0~15
Recommended Value: 2
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AMR - WB
7. TFO (Tandem Free Operation)
What is Tandem Operation?
Tandem Operation is a process in which the speech signals of an MS-to-MS call is transcoded twice.
In a classic MS to MS call configuration the speech signal is first encoded in the originating MS, sent
over the air interface, coded into G.711 PCM format (64Kbit/sec), carried over the fixed network,
transcoded again in the distant transcoder, sent over the distant air interface and finally decoded in
the terminating MS.
The figure below shows the Tandem Operation where the speech signal passes through two
transcoder, each transcoder at the BSC of each end user. The Tandem Operation degrades the
speech quality.
In the tandem operation both radio channels are totally independent from each other. This means
that codec mode adaptation is done separately in respective radio channel
The below figure shows the scenario that takes place when activating the AMR NB or AMR WB
feature in our network without enabling the TFO (this is the real case in our network nowadays).
Each link can have a different Codec mode (Codec rate) than the other three links. Every link adapts
its rate to the rate that suites its radio conditions
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What is Tandem Free Operation?
To improve the quality of speech signals, Tandem Free Operation (TFO) is introduced to pass by the
TC decoder when the originating MS and the terminating MS use the same speech version. Tandem
Free Operation is activated and controlled by the Transcoder Units after the completion of the call
set-up phase at both ends of the call configuration.
The TFO protocol is fully handled and terminated in the Transcoder Units. For this reason, the
Transcoder Units cannot be bypassed in Tandem Free Operation. This is the key difference with the
feature called Transcoder Free Operation (TrFO).
The speech signal is coded at the originating MS and decoded at the terminating MS, are
transparently transmitted between the TCs at the two ends. Thus, the process of encoding and
decoding by the TC is eliminated to improve the quality of the speech signal.
TFO is used in a configuration with two transcoders that are connected by a traditional 64 kbps
traffic channel with G.711 PCM coded speech. Both transcoding functions can be bypassed under
the following conditions:
 The two transcoders must support the TFO protocol
 Their coding schemes must be compatible
 The path between these transcoders must be TFO transparent
Under these circumstances TFO enables compressed speech parameters to pass between the
transcoders, embedded into the least significant bits (LSBs) of the 64 kbps PCM signal
Tandem Free Operation is activated and controlled by the Transcoder Units after the completion of
the call set-up phase at both ends of a call configuration. The TFO protocol is fully handled and
terminated in the Transcoder Units. For this reason, the Transcoder Units cannot be bypassed in
Tandem Free Operation. This is the key difference with the feature called Transcoder Free Operation
(TrFO)
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AMR - WB
The below figure shows the codec mode adaptation in case of activating the TFO.
The Codec Mode is the same on the UL of MS1 and the DL of MS2, while the Codec mode is the
same between the UL of MS2 and the DL of MS1.
When does a TFO operation start?
The TFO operation starts if compatible Speech Codec Types and Configurations are used at both
ends of the call configuration, the Transcoders automatically activate TFO.
If incompatible Speech Codec Types and/or Configurations are used at both ends, then a codec
mismatch situation exists. TFO cannot be activated until the codec mismatch is resolved.
A common codec type should be found so as to solve the codec mismatch and can start the TFO
operation.

When TFO is activated between two end connections using the GSM_HR speech codec, the
TFO Frames are carried over 8 Kbit/s channels mapped onto the least significant bit (LSB) of
the 64 Kbit/s PCM speech samples.
When TFO is activated between two end connections using the GSM_FR or GSM_EFR speech
codec’s, the TFO Frames are carried over 16 Kbit/s channels mapped onto the two least
significant bits of the 64 Kbit/s PCM speech samples.
When TFO is activated between two end connections using the AMR speech codec, the TFO
Frames are carried over 8 or 16 Kbit/s (dependant on the chosen codec rate) channels
mapped onto the least or two least significant bits of the 64 Kbit/s PCM speech samples. The
format depends on the codec configuration (Optimized Active Codec Set)


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TFO with AMR Feature enabled
The feature TFO offers an enhanced speech quality due to less transcoding within the end-to-end
speech path. The speech quality gain will however be most noticeable for the lower codec rates of
the AMR codec types
For AMR, TFO interworking between two AMR transcoders will of course be established if the two
transcoders use the same codec set. But TFO interworking is also possible if different codec sets are
used in the transcoders. In that case the common codec modes from the different codec sets must
be contiguous. Moreover the common codec modes must contain the lowest mode of the different
codec sets.
Why activating the TFO when enabling the AMR WB?
For WB_AMR, in A over TDM mode, the sampling frequency of the PCM code stream (64 Kbit/sec)
must be decreased to 8 kHz before being sent over the A interface. The spectrum information on
speech signals thus suffers a loss. In this case, the speech quality of a WB AMR call is worse than the
speech quality of an AMR FR call. After TFO is established, the PCM code stream decrease can be
passed by. In this way, the spectrum information on speech signals retains and the speech quality is
improved.
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AMR - WB
TFO on Two TDM- Based A interfaces:
The below figure shows the TFO connectivity between two 2G terminals where the call is over two
TDM -based A interfaces.
The TFO connectivity is established between the two BSC’s when the codec used between BSC 1 and
MS 1 is the same as or compatible with the codec used between BSC 2 and MS 2,then TFO is started
between BSC 1 and BSC 2 when TRAUs agree on the codec that will be used.
TFO between one TDM-based A interface and one A over IP:
The below figure shows the TFO connectivity between two 2G terminals where the call is over one
TDM -based A interfaces and one A over IP.
If the codec used between the MGW and BSC 2 is the same as or compatible with the codec used
between BSC 1 and MS 1, TFO is started when the TRAU on BSC 1 and the TC on the MGW agree on
the codec that will be used
When the TFO is activated in the below case (The TFO will be activated between BSC1 and the MGW
only where the transcoder exists) no transcoding will take place on BSC1 which contains the
transcoder. The speech rate will be sent embedded in the 64kbit/sec (PCM mode) reaching the
MGW and then sent on the IP interface with the same speech rate. The speech rate will be sent to
the end user on BSC2 and no transcoding will take place on BSC2 (In case of A over IP no hardware
transcoders exists so no transcoding takes place).
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AMR - WB
8. TrFO (Transcoder Free Operation)
The Transcoder Free Operation (TrFO) feature enables setup of speech calls without using
Transcoders (TCs) for speech transmission. It helps to achieve end-to-end high fidelity and low delay.
After successful negotiation, a network can set up a call between two mobile subscribers without
the assistance of TCs. This improves voice quality.
One of the TrFO advantages is that there is no need for expensive TC resources and associated
power consumption is not required.
Note: When activating the TrFO the speech codec will be send with the same rate to the other end.
The speech can be transferred over the core network at the rate of compressed codec’s, for
example, AMR 12.2 kbit/s.
The TrFO will not be activated over the TDM as the TDM use the G.711 signaling that must send the
Speech codec on the 64 Kbit/sec. The TrFO is activated over the IP interface where the speech codec
can be send as it is.
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TrFO between IP-based A interface:
If the A interface is IP-based and the BSC and the MGW use the same codec or compatible codec’s,
TrFO is started upon successful codec negotiation as shown below
The TrFO will be activated all the way between the 2 BSCs
TrFO between TDM -based A interface:
If the A interface is time division multiplexing (TDM)-based, TrFO is started between MGWs upon
successful codec negotiation, as shown below
The TrFO will be activated all the way between the 2 MGWs and the TFO if activated will send the
compressed speech codec over the TDM to the MGW then the speech codec will be transferred
using the Trfo feature between the 2 MGWs.
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9. Conclusion
 The AMR narrow band is activated on all Huawei network
 The AMR WB in 2G cannot be implemented nowadays on Huawei network as the core does
not support the AMR WB in 2G and needs to be upgraded (Nowadays version is R8C03)
 The AMR WB in 3G is supported by Huawei network nowadays.
 A trial was made to test the AMR WB in 3G at the beginning of year 2012.
 The AMR WB feature 2G will be tested with the values mentioned above once the Core
network is upgraded to version CS09
 When activating the AMR WB in Huawei network the TFO will not be activated as the A
interface is A over IP
 When activating the AMR WB we will activate the TrFO feature
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