6 Receive (RX) side

06.813GPPDiscontinuous Transmission (DTX) for encanced full rate speech traffic channelsTS

A block diagram of the receive side DTX functions is shown in figure 4.

Figure 4: Block diagram of the receive side DTX functions

6.1 General operation

Whatever their context (speech, SID, FACCH or none), the RSS continuously passes the received traffic frames to the RX DTX handler, individually marked by various pre‑processing functions with 3 flags. These are the Bad Frame Indicator (BFI) flag, the Silence Descriptor (SID) flag and the Time Alignment Flag (TAF) described in clause 6.1.1 and table 1, which serve to classify the traffic frame according to the list of terms defined in clause 3.1. This classification, summarized in table 1, allows the RX DTX handler to determine in a simple way how the received frame is to be handled.

Table 1: Classification of traffic frames

SID

BFI

2

1

0

0

Valid SID frame

Good speech frame

1

Invalid SID frame

Unusable frame

6.1.1 Functions of the RX radio subsystem

The binary BFI flag (see GSM 05.05 [4]) indicates whether the traffic frame is considered to contain meaningful information bits (BFI flag ="0") or not (BFI flag ="1"). In the context of the present document, a FACCH frame is considered not to contain meaningful bits and shall be marked with BFI flag ="1". The BFI flag must fulfil the performance requirements of GSM 05.05 [4].

The SID frame detector compares bit by bit the relevant bits of the received traffic frame (the SID field) with the SID code word defined in GSM 06.62 [11] and gives back the ternary SID flag. The SID flag is coded as follows, where n designates the number of bit deviations:

SID = 2 when n < 2

SID = 1 when 2  n < 16

SID = 0 when n  16

The binary TAF flag marks with TAF="1" those traffic frames that are aligned with the SACCH multiframe structure as described in GSM 05.08 [5].

6.1.2 Functions of the RX DTX handler

The RX DTX handler is responsible for the overall DTX operation on the RX side.

The DTX operation on the RX side shall be as follows:

– whenever a good speech frame is detected, the DTX handler shall pass it directly on to the speech decoder;

– when lost speech or lost SID frames are detected, the substitution and muting procedure defined in GSM 06.61 [10] shall be applied;

– valid SID frames shall result in comfort noise generation, as defined in GSM 06.62 [11], until the next SID frame is expected (TAF ="1") or good speech frames are detected. During this period, the RX DTX handler shall ignore any unusable frames delivered by the RSS;

– an invalid SID frame shall be substituted by the last valid SID frame and the procedure for valid SID frames be applied.

Annex A (informative):
Change Request History

Change history

SMG No.

TDoc. No.

CR. No.

Clause affected

New version

Subject/Comments

SMG#22

4.0.1

ETSI Publication

SMG#20

5.1.2

Release 1996 version

SMG#27

6.0.0

Release 1997 version

SMG#29

7.0.0

Release 1998 version

SMG#30

631/99

A004

5.1.1

7.1.0

Fix of DTX Synchronization Poblem at Handover

SMG#31

8.0.0

Release 1999 version

8.0.1

Update to Version 8.0.1 for Publication