ETSI TS 126 093 V16.1.0 (2021-04)

Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; 5G; Mandatory speech codec speech processing functions Adaptive Multi-Rate (AMR) speech codec; Source controlled rate operation (3GPP TS 26.093 version 16.1.0 Release 16)

RTS/TSGS-0426093vg10

General Information

• Status: Not Published
• Technical Committee: 3GPP SA 4 - Codec
• Current Stage: 12 - Completion
• Completion Date: 15-Apr-2021

Standards Content (Sample)

TECHNICAL SPECIFICATION
Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS);
LTE;
5G;
Mandatory speech codec speech processing functions
Adaptive Multi-Rate (AMR) speech codec;
Source controlled rate operation
(3GPP TS 26.093 version 16.1.0 Release 16)

3GPP TS 26.093 version 16.1.0 Release 16 1 ETSI TS 126 093 V16.1.0 (2021-04)

Reference
RTS/TSGS-0426093vg10
Keywords
5G,GSM,LTE,UMTS
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ETSI
3GPP TS 26.093 version 16.1.0 Release 16 2 ETSI TS 126 093 V16.1.0 (2021-04)
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ETSI
3GPP TS 26.093 version 16.1.0 Release 16 3 ETSI TS 126 093 V16.1.0 (2021-04)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions, symbols and abbreviations . 6
3.1 Definitions . 6
3.2 Symbols . 7
3.3 Abbreviations . 7
4 General . 7
4.1 General organisation. 7
5 AMR SCR operation . 8
5.1 Transmit (TX) side . 8
5.1.1 General operation . 8
5.1.2 Functions of the TX SCR handler . 9
5.1.2.1 AMR SCR Timing procedures . 9
5.1.3 The TX part of the AN . 10
5.2 Receive (RX) side . 11
5.2.1 General operation . 11
5.2.3 Demands on the RX SCR handler . 11
5.3 AMR SID Information format . 12
Annex A (normative): AMR DTX handler for the GSM system (corresponding to GSM
06.93) . 13
A.1 Scope . 13
A.2 References . 13
A.3 Definitions, symbols and abbreviations . 14
A.3.1 Definitions . 14
A.3.2 Symbols . 14
A.3.3 Abbreviations . 15
A.4 General . 15
A.4.1 General organisation. 15
A.5 Transmit (TX) side . 16
A.5.1 General operation . 16
A.5.1.1 Functions of the TX DTX handler . 17
A.5.1.2 Functions of the TX Radio Subsystem . 18
A.5.1.2.1 Functions of the TX Radio Subsystem for TCH/AFS . 18
A.5.1.2.2 Functions of the TX Radio Subsystem for TCH/AHS . 18
A.5.1.2.3 Functions of the Downlink TX Radio Subsystem for TFO . 19
A.5.1.2.4 Functions of the TX Radio Subsystem for RATSCCH . 20
A.6 Receive (RX) side . 20
A.6.1 General operation . 20
A.6.1.1 Functions of the RX radio subsystem . 21
A.6.1.2 Functions of the RX DTX handler . 21
Annex B (normative): ETSI GSM-EFR SCR handler . 22
B.1 Transmit (TX) side . 22
B.1.1 General operation . 22
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3GPP TS 26.093 version 16.1.0 Release 16 4 ETSI TS 126 093 V16.1.0 (2021-04)
B.1.2 Functions of the TX SCR handler . 22
B.1.2.1 GSM-EFR SCR Timing procedures . 22
B.1.3 The TX part of the AN . 22
B.1.3.1 Demands on the TX part of the Access Network . 22
B.2 Receive (RX) side . 23
B.3 SID Information format . 23
Annex C (normative): TIA IS-641 SCR Handler . 24
C.1 TX-side . 24
C.2 RX-side. 24
C.3 SID Information format . 24
Annex D (normative): TIA TDMA-US1 SCR Handler . 25
D.1 TX-side . 25
D.2 RX-side. 25
D.3 SID Information format . 25
Annex E (normative): ARIB PDC-EFR SCR Handler . 26
E.1 Transmit (TX) side . 26
E.1.1 General operation . 26
E.1.2 Functions of the TX SCR handler . 26
E.1.2.1 PDC_EFR SCR Timing procedures . 26
E.1.3 The TX part of the AN . 27
E.1.3.1 Demands on the TX part of the Access Network . 27
E.2 Receive (RX) side . 27
E.3 SID information format . 27
Annex F (informative): Change history . 28
History . 29

ETSI
3GPP TS 26.093 version 16.1.0 Release 16 5 ETSI TS 126 093 V16.1.0 (2021-04)
Foreword
This Technical Specification has been produced by the 3GPP.
The present document describes the operation of the Adaptive Multi Rate speech codec during Source Controlled Rate
(SCR) operation within the 3GPP system.
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of this TS, it will be re-released by the TSG with an identifying
change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the specification;
ETSI
3GPP TS 26.093 version 16.1.0 Release 16 6 ETSI TS 126 093 V16.1.0 (2021-04)
1 Scope
This document describes the Source Controlled Rate (SCR)operation of the Adaptive Multi-Rate speech Codec in
Codec Types UMTS_AMR and UMTS_AMR2 for the UMTS system. The implementation of this SCR operation is
mandatory in all UMTS equipment.
The description is structured according to the block diagram in figure 1. This structure of distributing the various
functions between system entities is not mandatory for implementation, as long as the operation on the speech decoder
output remains the same.
Annex A describes the Discontinuous Transmission (DTX) operation of the Adaptive Multi-Rate speech Codec in
Codec Types FR_AMR, HR_AMR and OHR_AMR for GERAN. This annex is the former GSM 06.93 (Release 98).
Annexes B to E describe the SCR operation of the Adaptive Multi-Rate speech Codec in Codec Types GSM_EFR,
TDMA_EFR, TDMA_US1 and PDC_EFR for the UMTS system.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TS 26.071 : "AMR Speech Codec; General description".
[2] 3GPP TS 26.073 : "AMR Speech Codec; ANSI-C code".
[3] 3GPP TS 26.074 : "AMR Speech Codec; Test sequences".
[4] 3GPP TS 26.090 : "AMR Speech Codec; Transcoding functions".
[5] 3GPP TS 26.091 : "AMR Speech Codec; Error concealment of lost frames".
[6] 3GPP TS 26.092 : "AMR Speech Codec; Comfort noise aspects".
[7] 3GPP TS 26.094 : "AMR Speech Codec; Voice Activity Detector (VAD)".
[8] 3GPP TS 26.101 : "AMR Speech Codec; Frame structure".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purpose of this document, the following definitions apply.
frame: Time interval of 20 ms, corresponding to the time segmentation of the Adaptive Multi Rate speech Codec, also
used as a short term for a traffic frame.
traffic frame: Block of 95.244 information bits transmitted on the speech traffic channels.
SID frame: Frame that conveys information about the acoustic background noise.
speech frame: Traffic frame that has been classified as SPEECH_GOOD or SPEECH_BAD frame.
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VAD flag: Boolean flag, generated by the VAD algorithm indicating the presence ("1") or the absence ("0") of a speech
frame.
RX_TYPE: classifies the received frame.
TX_TYPE: classifies the frame to be transmitted.
hangover period: A period of frames added at the end of a speech burst in which VAD flag ="0" and TX_TYPE is
="SPEECH_GOOD", this period provides the encoder with an extra window to derive the Comfort Noise parameters .
3.2 Symbols
For the purpose of this document, the following symbols apply.
N Number of elapsed frames since the last updated SID frame.
elapsed
3.3 Abbreviations
For the purpose of this document , the following abbreviations apply.
AN Access Network
SCR Source Controlled Rate operation
TS Telecommunication Standard, Technical Specification
GSM Global System for Mobile Telecommunication
GSM-EFR GSM Enhanced Full Rate speech Codec
UE User Equipment
PDC-EFR ARIB PDC-EFR 6.7 kBit/s speech Codec
RAN Radio Access Network
RX Receive
SID Silence Descriptor
TDMA-EFR TIA IS-641 Enhanced speech Codec
TDMA-US1 TIA TDMA-US1 (12.2 kBit/s Codec, similar to GSM-EFR)
TX Transmit
VAD Voice Activity Detector
4 General
Source Controlled Rate operation (SCR) is a mechanism for the AMR Speech Codec, which allows to encode the input
signal at a lower average rate by taking speech inactivity into account. The SCR scheme may be used for the following
purposes:
- to save power in the User Equipment;
- to reduce the overall interference and load in the networks.
SCR in the transmitting path (uplink) shall be in operation in UEs, if commanded so by the network. Note that for
UMTS AMR and UMTS AMR2 codec types, the uplink SCR operation shall always be activated. The UE shall handle
SCR in the receiving path (downlink) at any time, regardless, whether SCR in the transmitting path is enabled or not.
4.1 General organisation
The default SCR mechanism described in this document requires the following functions:
- a Voice Activity Detector (VAD) on the transmit (TX) side;
- evaluation of the background acoustic noise on the transmit (TX) side, in order to transmit characteristic
parameters to the receive (RX) side;
- generation on the receive (RX) side of a similar noise, called comfort noise, during periods where the
transmission is switched off.
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3GPP TS 26.093 version 16.1.0 Release 16 8 ETSI TS 126 093 V16.1.0 (2021-04)
The Voice Activity Detector (VAD) is defined in [7] and the AMR-mode comfort noise functions in [6]. Both are based
partly on the speech Codec and its internal variables, defined in [4].
In addition to these functions, if the parameters arriving at the RX side are detected to be seriously corrupted by errors,
the speech or comfort noise must be generated from substituted data in order to avoid seriously annoying effects for the
listener. These functions are defined in [5].
An overall description of the speech processing parts can be found in [1]. An overview of one link SCR operation is
shown in Figure 1.
TX SCR handler “Network” RX SCR handler
Speech
Speech
Information bits
Encoder Information bits
Decoder
Information
packeting,
Voice Mode Indication
Mode Indication
Error
transport and
Activity
Concealment
classification
Detector
RX_TYPE
TX_TYPE
Comfort Noise
Comfort
Parameter
Noise
Computation
Generation
Figure 1: Block diagram of one link SCR operation
5 AMR SCR operation
5.1 Transmit (TX) side
A block diagram of the transmit side SCR functions is shown in Figure 2.
TX SCR handler TX Access Network
Framing
T
Speech
S
Encoder Information bits
Information bits
Voice
Mode Indication
Frame Type TX of
.
Activity
information
Detector
TX_TYPE 0
FQI
Comfort Noise
Parameter
Computation
Figure 2: Block diagram of SCR functions at the TX side
5.1.1 General operation
The TX SCR handler passes traffic frames, individually marked by TX_TYPE, to the Framing unit. Each frame consists
of bit fields containing the information bits, the codec mode indication, and the TX_TYPE. TX_TYPE shall be used to
specify the contents of the frame. The table below provides an overview of the different TX_TYPEs used and explains
the required contents in the information bit and the mode indication bit fields.
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3GPP TS 26.093 version 16.1.0 Release 16 9 ETSI TS 126 093 V16.1.0 (2021-04)
Table 1: SCR TX_TYPE identifiers for UMTS_AMR and UMTS_AMR2
TX_TYPE Information Bits Mode Indication
SPEECH_GOOD Speech frame, size 95.244 bits, Current codec mode
depending on codec mode
SPEECH_BAD Corrupt speech frame (bad CRC), Current codec mode
size 95.244 bits, depending on
codec mode
SID_FIRST Marker for the end of talkspurt, no The codec mode that would have been used if
further information, all 35 comfort
TX_TYPE had been "SPEECH_GOOD"

noise bits set to "0"
SID_UPDATE 35 comfort noise bits The codec mode that would have been used if
TX_TYPE had been "SPEECH_GOOD"

SID_BAD Corrupt SID update frame (bad The codec mode that would have been used if
CRC) TX_TYPE had been "SPEECH_GOOD"
NO_DATA No useful information, nothing to No useful information
be transmitted
TX_TYPE = "NO_DATA" indicates that the Information Bit and Codec Mode fields do not contain any useful data
(and should not be transmitted over AN). The purpose of this TX_TYPE is to provide the option to save network
transmission between the transcoder and AN. Note, the TX_TYPEs "SPEECH_BAD" and "SID_BAD" may occur in
TFO and TrFO situations.
The scheduling of the frames for transmission on the Access Network is controlled by the TX SCR handler by the use
of the TX_TYPE field.
5.1.2 Functions of the TX SCR handler
If TX SCR operation is disabled, the TX SCR handler continuously generates speech frames, i.e. frames marked with
TX_TYPE="SPEECH_GOOD".
If the TX SCR operation is enabled, the VAD flag controls the TX SCR handler operation as described in the following
paragraphs.
Note that the TX SCR operation is always enabled on the UE side for UMTS AMR and UMTS AMR2 codec types.
5.1.2.1 AMR SCR Timing procedures
To allow an exact verification of the TX SCR handler functions, all frames before the reset of the system are treated as
if there were speech frames of an infinitely long time. Therefore, and in order to ensure the correct estimation of
comfort noise parameters at RX SCR side, the first 7 frames after the reset or after enabling the SCR operation shall
always be marked with TX_TYPE= "SPEECH_GOOD", even if VAD flag ="0" (hangover period, see figure 3).
The Voice Activity Detector (VAD) shall operate all the time in order to assess whether the input signal contains speech
or not. The output is a binary flag (VAD flag ="1" or VAD flag ="0", respectively) on a frame by frame basis (see [7]).
The VAD flag controls indirectly, via the TX SCR handler operations described below, the overall SCR operation on
the transmit side.
Whenever VAD flag ="1", the speech encoder output frame along with mode information shall be passed directly to the
AN, marked with TX_TYPE =" SPEECH_GOOD "
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3GPP TS 26.093 version 16.1.0 Release 16 10 ETSI TS 126 093 V16.1.0 (2021-04)
At the end of a speech burst (transition VAD flag ="1" to VAD flag ="0"), it takes eight consecutive frames to make a
new updated SID analysis available (see [6]). Normally, the first seven speech encoder output frames after the end of
the speech burst shall therefore be passed directly to the AN, marked with TX_TYPE =" SPEECH_GOOD "
("hangover period").
The end of the speech is then indicated by passing frame eight after the end of the speech burst to the AN, marked with
TX_TYPE = "SID_FIRST" (see figure 3). SID_FIRST frames do not contain data.
last speech frame
end of speech burst
first pause frame Frame
(20 ms)
Hangover
VAD flag
N e.g.     35   36   37   38   39   40   41    42    43   44   45    0    1
elapsed
S S S S S S S S S S F N N U N
Frames
to AN
TX Type
TX Types: “S” = SPEECH; “F” = SID_FIRST; “U” = “SID_UPDATE; “N” = NO DATA
N : No. of elapsed frames since last SID_UPDATE
elapsed
Figure 3: Normal hangover procedure for AMR (N > 23)
elapsed
If, however, at the end of the speech burst, less than 24 frames have elapsed since the last SID_UPDATE frame was
computed, then this last analysed SID_UPDATE frame should be passed to the AN whenever a SID_UPDATE frame
is to be produced, until a new updated SID analysis is available (8 consecutive frames marked with VAD flag
="0").This reduces the load on the network in cases where short background noise spikes are taken for speech, by
avoiding the "hangover" waiting for the SID frame computation.
Once the SID_FIRST frame has been passed to the AN, the TX SCR handler shall at regular intervals compute and pass
updated SID_UPDATE (Comfort Noise) frames to the AN as long as VAD flag = "0". SID_UPDATE frames shall be
th
generated every 8 frame. The first SID_UPDATE shall be sent as the third frame after the SID_FIRST frame.
The speech encoder is operated in full speech modality if TX_TYPE = " SPEECH_GOOD " and otherwise in a
simplified mode, because not all encoder functions are required for the evaluation of comfort noise parameters and
because comfort noise parameters are only to be generated at certain times.
5.1.3 The TX part of the AN
The TX part of the AN has the following overall functionality. The transmission is cut after the transmission of a
SID_FIRST frame when the speaker stops talking. During speech pauses the transmission is resumed at regular
intervals for transmission of one SID_UPDATE frame, in order to update the generated comfort noise on the RX side.
The operation of
the TX part of the AN is controlled by the TX SCR handler via the TX_TYPE.
All frames, marked with SPEECH_GOOD, SID_FIRST or SID_UPDATE shall be transmitted by the TX part of the
AN.
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5.2 Receive (RX) side
A block diagram of the receive side SCR functions is shown in Figure 4 below.
RX SCR handler De-framing RX Access Network
T
Speech
S
AMR Frame
Decoder Information bits
Information bits
Type
Detection
Mode Indication
Error
. Frame Type
Concealment
Error
RX_TYPE
FQI
Correction &
Comfort 1
Detection
Noise
Generation
Figure 4: Block diagram of the receive side SCR functions
5.2.1 General operation
The AN passes all the received traffic frames to the RX SCR handler, classified with RX_TYPE, as described in Table
2 (see TS 26.102). The RX SCR handles the frame accordingly.
Table 2: RX_TYPE identifiers for UMTS_AMR and UMTS_AMR2
RX_TYPE Information Bits
Speech frame without detected errors.
SPEECH_GOOD
SPEECH_BAD (likely) speech frame with bad CRC (or estimated to be very bad by
the RX part of the AN )
This SID-frame marks the beginning of a comfort noise period.
SID_FIRST
SID_UPDATE Correct SID update frame
SID_BAD Corrupt SID update frame (bad CRC; applicable only for
SID_UPDATE frames)
NO_DATA Nothing useable was received. The synthesis mode of the previous
frame type is used.
5.2.3 Demands on the RX SCR handler
The RX SCR handler is responsible for the overall SCR operation on the RX side. It consists of two main modes:
SPEECH and COMFORT_NOISE. The initial mode shall be SPEECH.
The SCR operation on the RX side shall be as follows:
- The RX SCR handler shall enter mode SPEECH, when a frame classified as SPEECH_GOOD is received.
- whenever a frame classified as SPEECH_GOOD is received the RX SCR handler shall pass it directly on to the
speech decoder;
- if the RX SCR handler is in mode SPEECH, then frames classified as SPEECH_BAD or NO_DATA shall be
substituted and muted as defined in [5]. Frames classified as NO_DATA shall be handled like SPEECH_BAD
frames without valid speech information;
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- frames classified as SID_FIRST, SID_UPDATE or SID_BAD shall bring the RX SCR handler into mode
COMFORT_NOISE and shall result in comfort noise generation, as defined in [6]. SID_BAD frames shall be
substituted and muted as defined in [5];;
- in mode COMFORT_NOISE the RX SCR handler shall ignore all unusable frames (NO_DATA,
SPEECH_BAD); comfort noise generation shall continue, until timeout may apply ([5]).
5.3 AMR SID Information format
The SID_UPDATE frame format is according to [5]. This is the default and only mandatory operating mode of the SCR
handler.
ETSI
3GPP TS 26.093 version 16.1.0 Release 16 13 ETSI TS 126 093 V16.1.0 (2021-04)
Annex A (normative):
AMR DTX handler for the GSM system (corresponding to
GSM 06.93)
A.1 Scope
The present document gives a description of the general baseband operation of Adaptive Multi-Rate speech traffic
channels in the transmitter and in the receiver of GSM Mobile Stations (MS)s and Base Station Systems (BSS)s during
Discontinuous Transmission (DTX).
For clarity, the description is structured according to the block diagrams in figures 1 and 3. Except in the case described
next, this structure of distributing the various functions between system entities is not mandatory for implementation, as
long as the operation on the air interface and on the speech decoder output remains the same.
In the case of BSSs where the speech transcoder is located remote from the Base Transceiver Station (BTS), the
implementation of the interfaces between the DTX handlers and the Radio Sub System (RSS) as described in the
present document together with all their flags is mandatory, being part of the A-bis interface as described in GSM 08.60
and GSM 08.61.
The DTX functions described in this technical specification are mandatory for implementation in the GSM MSs. The
receiver requirements are mandatory for implementation in all GSM BSSs, the transmitter requirements only for those
where downlink DTX or Tandem Free Operation will be used.
A.2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905:"Vocabulary for 3GPP Specifications".
[2] 3GPP TS 24.008: "Digital cellular telecommunication system (Phase 2+); Mobile radio interface
layer 3 specification".
[3] 3GPP TS 45.003: "Digital cellular telecommunication system (Phase 2+); Channel coding".
[4] 3GPP TS 45.005: "Digital cellular telecommunication system (Phase 2+); Radio transmission and
reception".
[5] 3GPP TS 45.008: "Digital cellular telecommunication system (Phase 2+); Radio subsystem link
control".
[6] 3GPP TS 45.009: "Digital cellular telecommunication system (Phase 2+); Link adaptation".
[7] 3GPP TS 26.071: "Digital cellular telecommunications system (Phase 2+); Adaptive Multi-Rate
(AMR) speech processing functions; General description".
[8] 3GPP TS 26.073: "Digital cellular telecommunications system (Phase 2+); ANSI-C code for the
GSM Adaptive Multi-Rate speech codec".
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[9] 3GPP TS 26.074: "Digital cellular telecommunications system (Phase 2); Test vectors for the
GSM Adaptive Multi-Rate speech codec".
[10] 3GPP TS 26.090: "Digital cellular telecommunications system (Phase 2+); Adaptive Multi-Rate
speech transcoding".
[11] 3GPP TS 26.091: "Digital cellular telecommunications system (Phase 2+); Substitution and
muting of lost frame for Adaptive Multi-Rate speech traffic channels".
[12] 3GPP TS 26.092: "Digital cellular telecommunications system (Phase 2+); Comfort noise aspects
for Adaptive Multi-Rate speech traffic channels".
[13] 3GPP TS 26.094: "Digital cellular telecommunications system (Phase 2+); Voice Activity
Detector (VAD) for Adaptive Multi-Rate speech traffic channels".
[14] 3GPP TS 28.060: "Digital cellular telecommunication system (Phase 2+); Inband control of
remote transcoders and rate adaptors for Full Rate traffic channels".
[15] 3GPP TS 28.061: "Digital cellular telecommunication system (Phase 2+); Inband Control of
Remote Transcoders and Rate Adaptors for Half Rate traffic channels".
[16] 3GPP TS 28.062: " Digital cellular telecommunications system; Inband Tandem Free Operation
(TFO) of Speech Codecs".
A.3 Definitions, symbols and abbreviations
A.3.1 Definitions
For the purpose of the present document, the following definitions apply.
frame: Time interval of 20 ms, corresponding to the time segmentation of the Adaptive Multi Rate speech transcoder
(3GPP TS 26.090 [9]), also used as a short term for a traffic frame.
traffic frame: Block of 95.244 information bits transmitted on the TCH/AFS or TCH/AHS speech traffic channels.
SID frame: Frame characterised by the SID (Silence Descriptor) gross bit patterns. It may convey information on the
acoustic background noise.
speech frame: Traffic frame that has been classified as a SPEECH frame.
VAD flag: Boolean flag, generated by the VAD algorithm defined in 3GPP TS 26.094 indicating the presence ("1") or
the absence ("0") of a speech frame.
RX_TYPE: flag with eight values, generated by the RX radio subsystem, indicating to the RX DTX handler the type of
data in the current frame. Refer to Table 2.
TX_TYPE: flag with eight values, generated by the TX DTX handler, indicating to the TX radio subsystem the type of
data in the current frame. Refer to Table 1.
hangover period: A period of 7 frames added at the end of a speech burst in which VAD flag ="0" and TX_TYPE is
"SPEECH".
A.3.2 Symbols
For the purpose of the present document, the following symbols apply.
N Number of elapsed frames since the last updated SID frame.
elapsed
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A.3.3 Abbreviations
For the purpose of the present document, the following abbreviations apply.
BSC Base Station Controller
BSS Base Station System
BTS Base Transceiver Station
CHD Channel Decoder
CHE Channel Encoder
DTX Discontinuous Transmission
ETS European Telecommunication Standard
FACCH Fast Associated Control CHannel
GSM Global System for Mobile Telecommunications
MS Mobile Station
RATSCCH Robust Amr Traffic Synchronised Control CHannel
RSS Radio Sub System
RX Receive
SACCH Slow Associated Control CHannel
SID SIlence Descriptor
TX Transmit
VAD Voice Activity Detector
For abbreviations not given in this subclause, see 3GPP TS 21.004.
A.4 General
Discontinuous Transmission (DTX) is a mechanism, which allows the radio transmitter to be switched off most of the
time during speech pauses for the following two purposes:
to save power in the Mobile Station (MS);
to reduce the overall interference level over the air interface.
DTX in uplink shall be in operation within the GSM MS, if commanded so by the network, see 3GPP TS 204.08. The
MS shall handle DTX in downlink at any time, regardless, whether DTX in uplink is commanded or not.
A.4.1 General organisation
The overall DTX mechanism described in the present document requires the following functions:
a Voice Activity Detector (VAD) on the transmit (TX) side;
evaluation of the background acoustic noise on the transmit (TX) side, in order to transmit characteristic parameters
to the receive (RX) side;
generation on the receive (RX) side of a similar noise, called comfort noise, during periods where the radio
transmission is switched off.
The Voice Activity Detector (VAD) is defined in 3GPP TS 26.094 and the comfort noise functions in 3GPP TS 26.092.
Both are based partly on the speech transcoder and its internal variables, defined in 3GPP TS 26.090.
In addition to these functions, if the parameters arriving at the RX side are detected to be seriously corrupted by errors,
the speech or comfort noise must be generated from substituted data in order to avoid seriously annoying effects for the
listener. This function is defined in 3GPP TS 26.091.
An overall description of the speech processing parts can be found in 3GPP TS 26.071.
The description for Tandem Free Operation is given in 3GPP TS 28.062.
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A.5 Transmit (TX) side
A block diagram of the transmit side DTX functions is shown in figure 5.
TX DTX handler TX Radio subsystem
Speech
encoder
Information bits
Channel
Encoder
Voice
Mode Indication
Activity
Detector
TX_TYPE
TX_TYPE
Monitoring
Comfort
Noise
NSYNC
Computation
Figure 5: Block diagram of the transmit side DTX functions
A.5.1 General operation
The TX DTX handler passes traffic frames, individually marked by TX_TYPE, to the Radio Subsystem (RSS). Each
frame passed to the RSS consists of bit fields containing the information bits, the codec mode indication, and the
TX_TYPE. TX_TYPE is used to specify the contents of the frame. The table below provides an overview of the
different TX_TYPEs used and explains the required contents in the information bit and the mode indication bit fields. In
case of ongoing Tandem Free Operation (see 3GPP TS 28.062) frames with errors may arrive in downlink in the BTS.
Table 3: TX TYPE identifiers
TX_TYPE Legend Information Bits Mode Indication
SPEECH_GOOD speech frame, size 95.244 bits depending current code mode

on codec mode;
no errors known.
SPEECH_DEGRADED Speech frame, size 95.244 bits, depending current codec mode
(only in downlink in on codec mode;
TFO) there might be errors in class 2 bits.
SPEECH_BAD Speech frame, size 95.244 bits, depending current codec mode
(only in downlink in on codec mode;
TFO) there are errors in class 1 bits.
SID_FIRST
marks the end of a talkspurt, respectively the codec mode that would have
the beginning of a speech pause; does not been used if TX_TYPE had been
contain information bits. SPEECH
SID_UPDATE comfort noise, 35 bits; the codec mode that would have
no errors known been used if TX_TYPE had been
SPEECH
SID_BAD comfort noise, 35 bits; the codec mode that would have
(only in downlink in errors detected, parameters unusable been used if TX_TYPE had been
TFO) SPEECH
ONSET announces the beginning of a speech
...