6 Receive side

06.313GPPDiscontinuous Transmission (DTX) for 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 Radio Subsystem continuously passes the received traffic frames to the RX DTX handler, individually marked by various pre‑processing functions with 3 flags. These are the BFI, the SID and the TAF flags described below, which serve to classify the traffic frame according to the list of terms defined in clause 3. This classification, summarized in table 1 below, in turn 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







Valid SID frame

Good speech frame


Invalid SID frame

Unusable frame

6.1.1 Functions of the RX Radio Subsystem

The binary BFI flag (Bad Frame Indication, see also GSM 05.05) indicates whether the traffic frame is considered to contain meaningful information bits (BFI=0) or not (BFI=1). In the context of this technical specification, a FACCH frame is considered not to contain meaningful bits and must also be marked with BFI=1. The BFI flag must fulfil the performance requirements of GSM 05.05.

The ternary SID flag is the output of a SID frame detector, which compares bit by bit the relevant bits of the received traffic frame (the SID field) with the SID code word defined in GSM 06.12. The flag is coded as follows, where n designates the number of bit deviation:

– SID=2 when n < 2;

– SID=1 when 2  n < 16;

– SID=0 when n  16.

The binary TAF flag (Time Alignment Flag) marks with TAF=1 those traffic frames that are aligned with the SACCH multiframe structure as described in the technical specifications referenced in clause 5.1.2.

6.1.2 Functions of the RX DTX handler

The RX DTX Handler is responsible for the overall DTX operation on the receive side, which 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 06.11 shall be applied;

‑ valid SID frames shall result in comfort noise generation, as defined in GSM 06.12, 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 Radio Subsystem;

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

NOTE: If the first SID frame after a speech burst (a series of good speech frames) is invalid, then the comfort noise parameters can be taken from the last valid SID frame or from the last received good speech frame which, because of the VAD hangover time (see GSM 06.32), may be supposed to contain noise only.

Annex A (informative):
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