A.5 Transmit (TX) side

26.1933GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecRelease 17Source controlled rate operationSpeech codec speech processing functionsTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

A block diagram of the transmit side DTX functions is shown in figure 1.

Figure 5: Block diagram of the transmit side DTX functions

A.5.1 General operation

The TX DTX handler passes traffic frames, individually marked by TX_TYPE, to the Radio Subsystem (RSS). Each frame passed to the RSS consists of bit fields containing the information bits, the codec mode indication, and the TX_TYPE. TX_TYPE is used to specify the contents of the frame. The table below provides an overview of the different TX_TYPEs used and explains the required contents in the information bit and the mode indication bit fields. In case of ongoing Tandem Free Operation (see 3GPP TS 28.062) frames with errors may arrive in downlink in the BTS.

Table 3: TX TYPE identifiers

TX_TYPE Legend	Information Bits	Mode Indication
SPEECH_GOOD	speech frame, size 132..397 bits depending on codec mode; no errors known.	current code mode
SPEECH_DEGRADED (only in downlink in TFO)	Speech frame, size 132..397 bits, depending on codec mode; there might be errors in class 2 bits.	current codec mode
SPEECH_BAD (only in downlink in TFO)	Speech frame, size 132..397 bits, depending on codec mode; there are errors in class 1 bits.	current codec mode
SPEECH_LOST (only in downlink in TFO)	no useful information (Note: If implementation does not support the SPEECH_LOST, SPEECH_BAD shall be used instead)	no useful information
SID_FIRST	marks the end of a talkspurt, respectively the beginning of a speech pause; does not contain information bits.	the codec mode that would have been used if TX_TYPE had been SPEECH
SID_UPDATE	comfort noise, 35 bits; no errors known	the codec mode that would have been used if TX_TYPE had been SPEECH
SID_BAD (only in downlink in TFO)	comfort noise, 35 bits; errors detected, parameters unusable	the codec mode that would have been used if TX_TYPE had been SPEECH
ONSET (only in downlink in TFO)	announces the beginning of a speech burst; does not contain information bits	the codec mode of the following speech frame
NO_DATA	no useful information	no useful information

TX_TYPE = "SPEECH_LOST" indicates that the Information Bit and Codec Mode fields do not contain any useful data (but still should be transmitted the air interface). The purpose of this TX_TYPE is indicate that the frame was transmitted but lost on some previous phase. This TX_TYPEs may occur only in TFO and TrFO situations. Note, that it is possible to replace SPEECH_LOST with SPEECH_BAD but this may degrade the quality of the error concealment in the receiving end because concealment may try to use part of the received parameters from the frame which do not contain any useful information.

TX_TYPE = "NO_DATA" indicates that the Information Bit and Codec Mode fields do not contain any useful data (and shall not be transmitted over the air interface). The purpose of this TX_TYPE is to provide the option to save transmission between the transcoder and the radio base station if a packet oriented transmission is used.

The scheduling of the frames for transmission on the air interface is controlled by the TX DTX handler by the use of the TX_TYPE field.

A.5.1.1 Functions of the TX DTX handler

To allow an exact verification of the TX DTX handler functions, all frames before the reset of the system are treated as if there were speech frames of an infinitely long time. Therefore, and in order to ensure the correct estimation of comfort noise parameters at RX DTX side, the first 7 frames after the reset or after enabling the DTX operation shall always be marked with TX_TYPE= " SPEECH_GOOD ", even if VAD flag ="0" (hangover period, see figure 2).

The Voice Activity Detector (VAD) shall operate all the time in order to assess whether the input signal contains speech or not. The output is a binary flag (VAD flag ="1" or VAD flag ="0", respectively) on a frame by frame basis (see 3GPP TS 26.194).

The VAD flag controls indirectly, via the TX DTX handler operations described below, the overall DTX operation on the transmit side.

Whenever VAD flag ="1", the speech encoder output frame along with mode information shall be passed directly to the radio subsystem (RSS), marked with TX_TYPE =" SPEECH_GOOD "

At the end of a speech burst (transition VAD flag ="1" to VAD flag ="0"), it takes 8 consecutive frames to make a new updated SID analysis available at receiver side (see 3GPP TS 26.192). Normally, the first 7 speech encoder output frames after the end of the speech burst shall therefore be passed directly to the RSS, marked with TX_TYPE =" SPEECH_GOOD " ("hangover period"). The end of the speech is then indicated by passing frame 8 after the end of the speech burst to the RSS, marked with TX_TYPE = "SID_FIRST" (see figure 2).

Figure 6: Normal hangover procedure (Nelapsed > 23)

If, however, at the end of the speech burst, less than 24 frames have elapsed since the last SID_UPDATE frame was computed and passed to the RSS, then this last analysed SID_UPDATE frame shall repeatedly be passed to the RSS whenever a SID_UPDATE frame is to be produced, until a new updated SID analysis is available (8 consecutive frames marked with VAD flag ="0").This reduces the activity on the air in cases where short background noise spikes are taken for speech, by avoiding the "hangover" waiting for the SID frame computation.

Once the first SID analysis after the end of a speech burst has been computed and the SID_FIRST frame has been passed to the Radio Subsystem, the TX DTX handler shall at regular intervals compute and pass updated SID_UPDATE (Comfort Noise) frames to the Radio Subsystem (RSS) as long as VAD flag = "0". SID_UPDATE frames shall be generated every 8^th frame. The first SID_UPDATE shall be sent as the third frame after the SID_FIRST frame.

The speech encoder is operated in full speech modality if TX_TYPE = " SPEECH_GOOD " and otherwise in a simplified mode, because not all encoder functions are required for the evaluation of comfort noise parameters and because comfort noise parameters are only to be generated at certain times.

In order to ensure TX/RX DTX handler synchronisation at handover, the uplink TX DTX handler in the MS shall accept messages from TX RSS with control parameter NSYNC, resulting in the following operation during a period of the next NSYNC frames:

– The TX DTX handler shall send SID_UPDATE instead of NO_DATA frames to the TX RSS.

– If, during this period of NSYNC frames, VAD flag is equal to 1 at least for one speech frame, TX DTX handler shall send SPEECH frames for the rest of the period of the NSYNC frames. Note the TX DTX handler shall send SPEECH frames at least for the duration of the hangover period.

A.5.1.2 Functions of the TX Radio Subsystem

The TX Radio Subsystem has the following overall functionality. The radio transmission is cut after the transmission of a SID_FIRST frame when the speaker stops talking. During speech pauses the transmission is resumed at regular intervals for transmission of one SID_UPDATE frame, in order to update the generated comfort noise on the RX side (and to improve the measurement of the link quality by the RSS). Note that the transcoder knows what frames to send. In the case when nothing is to be transmitted it outputs frames marked with TX_TYPE = "NO_DATA".

Within the TX Radio Subsystem the TX_TYPE Monitoring unit controls the operation of the Channel Encoder (as specified in 3GPP TS 25.003) and the Transmission of the frame. Control input to the TX_TYPE Monitoring unit is the TX_TYPE. Control output and input to the Channel Encoder are indicators specifying the frame format. These frame format indicators are defined in 3GPP TS 25.003.

A.5.1.2.1 Functions of the TX Radio Subsystem for TCH/WFS

The TX Radio Subsystem operates in the following way regarding DTX (without TFO):

– all frames marked with TX_TYPE = " SPEECH_GOOD " are scheduled for normal channel coding and transmission. The frame format for CHE operation shall be SPEECH. If, however, the previous frame was not of TX_TYPE = "SPEECH_GOOD", an ONSET frame format followed by SPEECH_GOOD shall be signalled to the CHE;

– for frames marked with TX_TYPE = "SID_FIRST" a SID_FIRST frame format is signalled to the CHE;

– frames marked with TX_TYPE = "SID_UPDATE" are scheduled for SID_UPDATE frame channel coding and transmission. The frame format signalled to CHE is SID_UPDATE;

– for frames marked with TX_TYPE = "NO_DATA" no processing or transmission is carried out.

If a SID_FIRST frame or a SID_UPDATE frame is stolen for Fast Associated Control Channel (FACCH) signalling purposes, then the subsequent frame shall be scheduled for transmission of the SID_FIRST or SID_UPDATE frame (whichever applies) instead.

SPEECH frames shall override possible SID_FIRST or SID_UPDATE frames in exceptional cases.

At handover, TX/RX DTX handler synchronisation shall be initiated. At the time instant before the MS starts sending to the new base station, a message shall be sent to the uplink TX DTX handler with the parameter NSYNC = 12.

A.5.1.2.2 Functions of the Downlink TX Radio Subsystem for TFO

The TX Radio Subsystem in the BTS shall in addition operate in the following way regarding DTX, if TFO is ongoing (see 3GPP TS 28.062):

– Frames with TX_TYPE = SPEECH_GOOD, SID_FIRST and SID_UPDATE shall be handled as usual in DTX, regardless whether DTX in downlink is requested or not. Also NO_DATA shall be handled as usual, if DTX is requested.

– Frames with TX_TYPE = NO_DATA shall be replaced by SID_FILLER frames, if DTX in downlink is not requested. By this the radio transmission continues in downlink, although no parameters are transmitted in speech pauses on the Abis interface. The MS generates Comfort Noise in these speech pauses.

– Frames with TX_TYPE = SPEECH_DEGRADED shall be handled exactly like SPEECH_GOOD frames.

– For frame with TX_TYPE = SPEECH_BAD and SID_BAD the CHE shall perform its regular processing, but then shall invert CRC bits before convolutional encoding and transmitting the frames on the air interface. By this the error concealment mechanism in the MS is triggered to handle these corrupted frames.

– Frames with TX_TYPE = SPEECH_LOST shall be replaced by SPEECH_LOST_FILLER frames, if DTX in downlink is not requested. By this the radio transmission continues in downlink. The MS performs special lost frame handling operations for the frame.

– ONSET frames may be ignored by the TX Radio Subsystem and need not to be processed.

Definitions:
SID_FILLER frames are like SID_BAD frames, but with all information bits set to "1". The CRC bits shall artificially be inverted by the CHE before convolutional encoding and transmission.
SPEECH_LOST_FILLER frames are like SPEECH_BAD frames, but with all information bits set to "1". The CRC bits shall artificially be inverted by the CHE before convolutional encoding and transmission.

A.5.1.2.4 Functions of the TX Radio Subsystem for RATSCCH

During regular speech transmission (in the middle of a speech burst) RATSCCH replaces (steals) one speech frame (see 3GPP TS 25.009). Also in all non speech cases the RATSCCH shall be handled like speech. The respective RATSCCH frame formats shall be signalled to the CHE.

If RATSCCH has to be sent during a speech pause in DTX, then first an ONSET frame shall be signalled to the CHE, followed by the RATSCCH frame and finally by the respective SID_FIRST frame.

If a SID_UPDATE frame is affected by RATSCCH signalling, then the SID_UPDATE frame shall be re-scheduled for transmission immediately after the RATSCCH signalling.

FACCH should be handled in the same way as a RATSCCH, i.e. like a short speech burst.