06.933GPPDiscontinuous Transmission (DTX) for Adaptive Multi-Rate speech traffic channelsTS

A block diagram of the receive side DTX functions is shown in figure 3.

Figure 3: 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 RX_TYPE as described in clause 6.1.1 and table 2, which serves to classify the traffic frame. This classification allows the RX DTX handler to determine in a simple way how the received frame is to be handled.

Table 2: RX_TYPE identifiers

6.1.1 Functions of the RX radio subsystem

The RX radio subsystem uses a combination of gross-bit markers, receiver measurements, and CRC checks to classify each received frame. The basic operation for each frame is outlined below:

– the receiver first searches for the RATSCCH, SID_UPDATE, SID_FIRST or ONSET gross bit markers.

– If the RATSCCH signalling is detected, then the RATSCCH frame (TCH/AFS) respectively the RATSCCH_MARKER and RATSCCH_DATA frames (TCH/AHS) shall be decoded and handled as described in GSM 05.09. They shall be passed to the RX DTX handler as a NO_DATA frame(s).

– If the SID_FIRST marker is detected the frame is passed to the RX DTX handler as a SID_FIRST frame.

– If the SID_UPDATE marker is detected, then the frame shall be decoded and passed to the RX DTX handler as a SID_UPDATE or a SID_BAD or a NO_DATA frame, depending on the CRC and the information bits, along with the comfort noise parameters, if applicable. A NO_DATA frame shall be passed on, if all information bits of a SID_UPDATE frame are set to "1" and the CRC is bad (see SID_FILLER in clause 5.1.2.3).

– If the ONSET marker is detected, then an ONSET frame shall be passed to the RX DTX handler.

– If neither SID_UPDATE nor SID_FIRST markers are detected, the frame shall be channel decoded assuming it to be a speech frame. Depending on the CRC for speech frame channel decoding along with other receiver measurements the frame shall then be passed to the RX DTX handler marked as either SPEECH_GOOD, SPEECH_ DEGRADED, SPEECH_BAD or NO_DATA frame.

6.1.2 Functions of the RX DTX handler

The RX DTX handler is responsible for the overall DTX operation on the RX side. It consists of two main modes: SPEECH and COMFORT_NOISE. The initial mode shall be SPEECH.

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

– the RX DTX handler shall enter mode SPEECH, when a frame classified as SPEECH_GOOD or SPEECH_DEGRADED is received. ONSET frames may be taken into account to identify the beginning of a speech burst;

– whenever a frame classified as SPEECH_GOOD is received the RX DTX handler shall pass it directly on to the speech decoder:

– if the RX DTX handler is in mode SPEECH, then frames classified as SPEECH_DEGRADED, SPEECH_BAD or NO_DATA shall be substituted and muted as defined in GSM 06.91. Frames classified as NO_DATA shall be handled like SPEECH_BAD frames without valid speech information;

– frames classified as SID_FIRST, SID_UPDATE or SID_BAD shall bring the RX DTX handler into mode COMFORT_NOISE and shall result in comfort noise generation, as defined in GSM 06.92. SID_BAD frames shall be substituted and muted as defined in GSM 06.91. In mode COMFORT_NOISE the RX DTX handler shall ignore all unusable frames (NO_DATA, SPEECH_BAD) delivered by the RSS; comfort noise generation shall continue, until timeout may apply (see GSM 06.91).

Annex A (informative):
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