Content for TS 46.041 Word version: 18.0.0

3.1 Definitions 3.2 Symbols 3.3 Abbreviations 4.1 General Organisation
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1 Scope p. 5

The present document gives a description of the general baseband operation of half 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 4. Except in the case described below, 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 remotely in the Base Station Controller (BSC), 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 a part of the A-bis interface as described in GSM 08.61 [10].

The DTX functions described in the present document are mandatory for implementation in all GSM MSs. The receiver requirements are mandatory for implementation in all GSM BSSs, the transmitter requirements only for those where downlink DTX will be used.

2 References p. 5

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]

GSM 01.04: "Digital cellular telecommunication system (Phase 2+); Abbreviations and acronyms".

[2]

GSM 04.08: "Digital cellular telecommunication system (Phase 2+); Mobile radio interface layer 3 specification".

[3]

GSM 05.05: "Digital cellular telecommunications system (Phase 2+); Radio transmission and reception".

[4]

GSM 05.08: "Digital cellular telecommunication system (Phase 2+); Radio subsystem link control".

[5]

GSM 06.02: "Digital cellular telecommunications system (Phase 2+); Half rate speech Part 1: Half rate speech processing functions".

[6]

GSM 06.20: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech transcoding".

[7]

GSM 06.21: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Substitution and muting of lost frames for half rate speech traffic channels".

[8]

GSM 06.22: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Comfort noise aspects for half rate speech traffic channels".

[9]

GSM 06.42: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Voice Activity Detector (VAD) for half rate speech traffic channels".

[10]

GSM 08.61: "Digital cellular telecommunications system (Phase 2+); Inband control of remote transcoders and rate adaptors for half rate traffic channels".

[11]

GSM 06.06: "Digital cellular telecommunications system (Phase 2+); Half rate speech Part 7: ANSI-C code for the GSM half rate speech codec".

3 Definitions, symbols and abbreviations p. 6

3.1 Definitions p. 6

For the purposes of the present document, the following terms and definitions apply:

accepted SID frame:

traffic frame which is flagged with SID= "1" or SID= "2" by the Radio Subsystem.

bad traffic frame:

traffic frame flagged BFI= "1" (Bad Frame Indication) or with UFI= "1" (Unreliable Frame Indication) by the Radio Subsystem.

frame:

time interval of 20 ms corresponding to the time segmentation of the half rate speech transcoder defined in GSM 06.20 (ETS 300 969) [6]), also used as a short term for a traffic frame.

good speech frame:

good traffic frame which is not an accepted SID frame.

good traffic frame:

traffic frame flagged BFI= "0" and UFI= "0" by the Radio Subsystem.

GS averaging period:

period in which the quantized energy tweak parameters GS are averaged: it corresponds to the hangover period.

hangover period:

period of 7 frames added at the end of a speech burst in which VAD flag = "0" and SP flag= "1".

invalid SID frame:

accepted SID frame which was not classified as valid SID frame. This frame is not valid for updating comfort noise parameters, but the frame conveys information that comfort noise generations should be started or continued.

lost SID frame:

unusable frame received when the RX DTX Handler is generating comfort noise and a SID frame is expected (Time Alignment Flag TAF= "1").

lost speech frame:

unusable frame received when the RX DTX Handler is passing on traffic frames directly to the speech decoder.

SID code word:

fixed bit pattern defined in GSM 06.22 [8], for labelling a traffic frame as a SID frame.

SID field:

bit positions defined in GSM 06.22 [8], of the SID codeword within a SID frame.

SID frame:

frame characterised by the SID (Silence Descriptor) code word. It conveys information on the acoustic background noise.

SP flag:

boolean flag, generated by the TX DTX handler, indicating the presence of a speech frame ("1") or the presence of a SID frame ("0").

speech frame:

traffic frame that cannot be classified as a SID frame.

traffic frame:

block of 112 information bits transmitted on the half rate speech traffic channel.

unusable frame:

bad traffic frame that is not an accepted SID frame.

VAD flag:

boolean flag, generated by the VAD algorithm defined in GSM 06.42 [9], indicating the presence ("1") or the absence ("0") of a speech frame.

valid SID frame:

good traffic frame flagged with SID= "2" by the Radio Subsystem. This frame is valid for updating comfort noise parameters at any time.

3.2 Symbols p. 7

For the purposes of the present document, the following symbols apply:

Energy tweak parameter

N_elapsed

Number of elapsed frames since the last updated SID frame

3.3 Abbreviations p. 7

For the purposes of the present document, the following abbreviations apply:

BFI

Bad Frame Indicator

BSC

Base Station Controller

BSS

Base Station System

DTX

Discontinuous Transmission

FACCH

Fast Associated Control CHannel

FEC

Forward Error Correction

GSM

Global System for Mobile communications

Mobile Station

RSS

Radio Sub System

Receive

SACCH

Slow Associated Control CHannel

SID

SIlence Descriptor

TAF

Time Alignment Flag

Transmit

UFI

Unreliable Frame Indicator

VAD

Voice Activity Detector

For abbreviations not given in this clause, see GSM 01.04 [1].

4 General p. 7

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 MS;
to reduce the overall interference level over the air interface.

DTX shall be in operation in GSM MS if commanded so by the network, see GSM 04.08 [2].

4.1 General Organisation p. 7

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 TX side, in order to TX characteristic parameters to the receive (RX) side;
generation on the RX side of a similar noise, called comfort noise, during periods where the radio transmission is switched off.
The VAD is defined in GSM 06.42 [9], the comfort noise functions in GSM 06.22 [8]. Both are based partly on the speech transcoder and its internal variables, defined in GSM 06.20 [6].
In addition to these functions, if the parameters arriving at the (RX) side are detected to be corrupted by errors, the speech or comfort noise shall be generated from substituted data in order to avoid sound defects for the listener. This function is defined in GSM 06.21 [7].
An overall description of the speech processing parts can be found in GSM 06.02 [5].