06.313GPPDiscontinuous Transmission (DTX) for full rate speech traffic channelsTS
A block diagram of the transmit 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 a flag SP, to the Radio Subsystem. This binary flag is redundant to the SID code word labelling. SP=1 indicates a speech frame, SP=0 a SID frame.
The scheduling of the frames for transmission on the air interface is controlled by the radio subsystem alone, on the basis of the SP flag as described next.
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 have to be treated as if there would have been speech frames for an infinitely long time. Therefore, the first N frames after the reset are always marked with SP=1, even if VAD=0 (hangover period, see below).
The Voice Activity Detector must be operating all the time in order to assess whether the input signal contains speech or not. The output is a binary flag (VAD=1 or VAD=0, respectively) on a frame by frame basis (see GSM 06.32).
The VAD flag controls indirectly, via the TX DTX Handler operations described below, the overall DTX operation on the transmit side.
Whenever VAD=1, the speech encoder output frame shall be passed directly to the radio subsystem, marked with SP=1.
At the end of a speech burst (transition VAD=1 to VAD=0), it takes N+1 consecutive frames to make a new updated SID frame available (see GSM 06.12). Normally, the first N speech encoder output frames after the end of the speech burst shall therefore be passed directly to the radio subsystem, marked with SP=1 ("hangover period"). The first new SID frame is then passed to the RSS as frame N+1 after the end of the speech burst, marked with SP=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 (N+1 consecutive frames marked with VAD=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 (see also figure 3: Note that figure 3 shows as example the longest possible speech burst without hangover).
Figure 3: Handling of short speech bursts (Nelapsed <24) (Example)
Once the first SID frame after the end of a speech burst has been computed and passed to the Radio Subsystem, the TX DTX Handler shall continuously compute and pass updated SID frames to the Radio Subsystem, marked with SP=0 as long as VAD remains VAD=0.
Consequently, the speech encoder must be operating all the time.
5.1.2 Functions of the TX radio subsystem
The following traffic frames shall be scheduled for transmission:
1) all frames marked with SP=1;
2) the first one with SP = 0 after one or more frames with SP=1;
3) those marked with SP=0 and aligned with the SACCH multiframe structure as described in GSM 05.08.
This has the overall function, that the radio transmission is cut 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 receive side (and to improve the measurement of the link quality by the radio subsystem).
If a SID frame (SP=0), scheduled for transmission is stolen for signalling (FACCH) purposes, then the subsequent frame shall be scheduled for transmission instead.