06.413GPPDiscontinuous Transmission (DTX) for half rate speech traffic channelsTS
A block diagram of the RX 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 RSS continuously passes the received traffic frames to the RX DTX handler, individually marked by various pre-processing functions with 4 flags. These are the Bad Frame Indicator (BFI) flag, the Unreliable Frame Indicator (UFI) flag, the SIlence Descriptor (SID) flag and the Time Alignment Flag (TAF) described in clause 6.1.1 and table 1, which serve to classify the traffic frame according to the list of terms defined in clause 3.1. This classification, summarised in table 1, allows the RX DTX handler to determine how the received frame is to be handled.
Table 1 Classification of traffic frames
BFI "OR" UFI*
Valid SID frame
Good speech frame
Invalid SID frame
* BFI "OR" UFI indicates a logical OR combination of the BFI and UFI flags.
NOTE: The SID computation is left open to manufacturers with the constraint to meet the performance requirements given in the GSM 05.05 . An example solution is given in the C-code (GSM 06.06 ).
6.1.1 Functions of the RX radio subsystem
The binary BFI flag (see GSM 05.05 ) indicates whether the traffic frame is considered to contain meaningful information bits (BFI flag = "0") or not (BFI flag = "1"). In the context of the present document, a FACCH frame is considered not to contain meaningful bits and shall be marked with BFI flag = "1". The BFI flag shall fulfil the performance requirements of GSM 05.05 .
NOTE: That the BFI flag,which is generated by the channel error correction scheme, will in some cases be modified by the SID frame detection unit.
The UFI flag is the output of the Forward Error Correction (FEC) procedure and give indications about the channel performance. The UFI flag shall fulfil the performance requirements of GSM 05.05 .
The SID frame detector compares bit by bit the relevant bits of the received traffic frame (the SID field) with the SID code word defined in GSM 06.22  and gives back the ternary SID flag. The SID flag shall fulfil the performance requirements of GSM 05.05 .The binary TAF flag marks with TAF = "1" those traffic frames that are aligned with the SACCH multiframe structure as described in GSM 05.08 .
6.1.2 Functions of the RX DTX handler
The RX DTX handler shall be responsible for the overall DTX operation on the RX side.
The DTX operation on the RX side 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 06.21  shall be applied;
– valid SID frames shall result in comfort noise generation, as defined in GSM 06.22 , 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 RSS;
– 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, if the series of good speech frames received was longer than 30 (23+7), from the last received good speech frame which, because of the VAD hangover time (see GSM 06.42 ), may be supposed to contain noise only.
Annex A (informative):
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