Content for TS 46.061 Word version: 18.0.0

1 Scope p. 5

The present document defines a frame substitution and muting procedure which shall be used by the Receive (RX) Discontinuous Transmission (DTX) handler when one or more lost speech or Silence Descriptor (SID) frames are received from the Radio Sub System (RSS).

The requirements of the present document are mandatory for implementation in all GSM Base Station Systems (BSS)s and Mobile Stations (MS)s capable of supporting the enhanced Full Rate speech traffic channel. It is not mandatory to follow the bit exact implementation outlined in the present document and the corresponding C source code.

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 05.03: "Digital cellular telecommunications system (Phase 2+); Channel coding".

[2]

GSM 06.60: "Digital cellular telecommunications system (Phase 2+); Enhanced Full Rate (EFR) speech transcoding".

[3]

GSM 06.81: "Digital cellular telecommunications system (Phase 2+); Discontinuous transmission (DTX) for Enhanced Full Rate (EFR) speech traffic channels".

[4]

GSM 08.60: "Digital cellular telecommunications system (Phase 2+); Inband control of remote transcoders and rate adaptors for Enhanced Full Rate (EFR) and full rate traffic channels".

3 Definitions and abbreviations p. 5

3.1 Definitions p. 5

For the purposes of the present document, the following term and definition applies:

5-point median operation: consists of sorting the 5 elements belonging to the set for which the median operation is to be performed in an ascending order according to their values, and selecting the third largest value of the sorted set as the median value.

Further definitions of terms used in the present document can be found in GSM 06.60 [2], GSM 06.81 [3], GSM 05.03 [1] and GSM 08.60 [4].

3.2 Abbreviations p. 5

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

BFI

Bad Frame Indication from Radio Sub System

BSI_Abis

Bad Sub-block Indication obtained from A-bis CRC checks

CCU

Channel Coding Unit

CRC

Cyclic Redundancy Check

DTX

Discontinuous transmission

median5

5-point median operation

PrevBFI

Bad Frame Indication of Previous frame

RSS

Radio Sub System

Receive

SID

SIlence Descriptor frame

TRAU

Transcoding Rate Adaptation Unit

4 General p. 6

The purpose of frame substitution is to conceal the effect of lost frames. The purpose of muting the output in the case of several lost frames is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from frame substitution procedure.

The RSS indicates lost speech or SID frames by setting its Bad Frame Indication flag (BFI) based on its 3 bit and 8-bit CRCs and possibly other error detection mechanisms. The TRAU calculates from the CRCs inserted by the CCU in the TRAU frames one BSI_Abis flag for every sub-block of speech parameters. If either one or more of these flags is set, the speech decoder shall either perform frame substitution or subframe substitution.

The example solution provided in clause 6 applies only for bad frame handling on a complete speech frame basis. However some parts could be modified for substitution of bad sub-blocks.