5 Transmit (TX) side

06.413GPPDiscontinuous Transmission (DTX) for half rate speech traffic channelsTS

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

Figure 1: Block diagram of the transmit side DTX functions

5.1 General Operation

The TX DTX handler continuously passes traffic frames, individually marked by the SP flag, to the RSS. This binary flag is redundant to the SIlence Descriptor (SID) code word labelling. SP flag = "1" indicates a speech frame, SP flag = "0" a SID frame.

The scheduling of the frames for transmission on the air interface is controlled by the RSS alone, on the basis of the SP flag as described in clause 5.1.1.

5.1.1 Functions of the TX DTX handler

To allow verification of the TX DTX handler functions, all frames before the reset of the system are treated as if they were speech frames of an infinitely long time. Therefore, the first 7 frames after the reset are marked with SP flag = "1", even if VAD flag = "0" (hangover period, see figure 2).

The 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 GSM 06.42 [9]).

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

Whenever the VAD flag = "1", the speech encoder output frame shall be passed directly to the RSS, marked with SP flag = "1".

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 frame available (see GSM 06.22 [8]). 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 SP flag = "1" ("hangover period"). The first new SID frame is then passed to the RSS as frame 8 after the end of the speech burst, marked with SP flag = "0" (see figure 2).

Figure 2: Normal hangover procedure (Nelapsed > 23)

If, however, at the end of the speech burst, less than 24 frames have elapsed since the last SID frame was computed and passed to the RSS, then this last SID frame shall repeatedly be passed to the RSS, until a new updated SID frame is available (8 consecutive frames marked with VAD flag = "0"). This reduces the activity on the air interface in cases where short background noise spikes are taken for speech, by avoiding the "hangover" waiting for the SID frame computation (see figure 3).

NOTE: Figure 3 shows an example of the longest possible speech burst without hangover.

Once the first SID frame after the end of a speech burst has been computed and passed to the RSS, the TX DTX Handler shall continuously compute and pass updated SID frames to the RSS, marked with SP flag = "0" as long as the VAD flag = "0".

The speech encoder is operated in full speech modality if SP flag = "1" and in a simplified mode if SP flag = "0", because not all encoder functions are required for the evaluation of comfort noise parameters (see GSM 06.22 [8]).

5.1.2 Functions of the TX radio subsystem

The following traffic frames shall be scheduled for transmission:

– all frames marked with SP flag = "1";

– the first one with SP flag = "0" after one or more frames with SP flag = "1";

– those marked with SP flag = "0" and aligned with the Slow Associated Control CHannel (SACCH) multiframe structure as described in GSM 05.08 [4].

This has the overall function, that the radio transmission is turned off after the transmission of a SID frame when the speaker stops talking. During speech pauses, the transmission is resumed at regular intervals for transmission of one SID 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).

If a SID frame (SP flag = "0"), scheduled for transmission is stolen for Fast Associated Control CHannel (FACCH) signalling purposes, then the subsequent frame shall be scheduled for transmission instead.

Figure 3: Handling of short speech bursts for Nelapsed < 24 (An example)