C.3.2 Tx_TRAU Process

28.0623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

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

The Tx_TRAU Process builds autonomously the relevant Downlink TRAU Frames and sends them in the correct phase relation onto the Abis/Ater-Interface as commanded by the time alignment from the BTS.

Tx_TRAU has two major States: TFOdl == OFF (start-up default state) and TFOdl == ON (see Figure C.3.2-1).

TFO_Protocol Protocol controls the transitions between these states using the Accept_TFO (AT) and Ignore_TFO (IT) commands.

Figure C.3.2-1: States of the Tx_TRAU Process

During TFOdl == OFF Tx_TRAU performs all actions of a conventional downlink TRAU (see 3GPP TS 48.060 [3] respectively 3GPP TS 48.061 [4]): On command from Rx_TRAU it performs necessary downlink time alignments and starts or stops sending TRAU Frames. It samples one frame of speech samples in the correct phase position and calls the Speech Encoder. In case of AMR-WB the received PCM samples are decoded and upsampled before the WB speech encoder is called. The resulting speech parameters are then transmitted downlink on the Abis/Ater interface.

In case of AMR or AMR-WB Tx_TRAU furthermore modifies the CMI/CMR phase alignment when requested by TFO_BTS via the Rx_TRAU. The Tx_TRAU sends on command by TFO_Protocol the Distant or Optimal TFO configuration parameters by a Config Frame downlink to the BTS. This Tx_TRAU indicates in addition by TFO_Soon that TFO will be established soon, or by TFO_Off that a mismatch has been detected by the TRAU and TFO has been terminated.

During TFOdl == ON, in case of the GSM_FR, GSM_EFR and GSM_HR Codec_Types, the Tx_TRAU performs Bad Frame Handling and Comfort Noise Parameter Handling on parameter level on the received TFO Frames, if necessary. The resulting speech parameters and control bits are buffered until they are passed as Downlink TRAU Frames in correct phase position to the BTS.

During TFOdl == ON, in case of the AMR or AMR-WB Codec_Types, no Bad Frame Handling or Comfort Noise Parameter Handling are performed in the Tx_TRAU. The speech parameters and control bits extracted from the TFO Frames are passed as Downlink TRAU Frames with least possible delay down to the BTS.

In case of AMR or AMR-WB the Tx_TRAU sends on command by TFO_Protocol the distant TFO configuration parameters and/or the Optimal Codec Type and Optimal Configuration via a Config Frames downlink to the BTS. Tx_TRAU indicates in addition by TFO_On that TFO is established.

In case of AMR or AMR-WB the transition from TFOdl == OFF to TFOdl == ON and vice versa causes in general a phase shift of the downlink TRAU frames. Tx_TRAU shall in these cases always complete the transmission of the ongoing TRAU frame and shall then insert the necessary number (0 to 159) of "1" bits (TRAU_8k) or "11" pairs (TRAU_16k) or "1111" quadruples (TRAU_32k) on the Abis/Ater interface before the next TRAU frame is sent.

C.3.2.1 Downlink Speech Transmission and DTX handling if TFO is ON

There are four possible cases regarding DTX in a Mobile-to-Mobile communication, as reflected in table C.3.2.1-1.

Table C.3.2.1-1: DTX configurations in Mobile-To-Mobile communications

Case	Local TRAU: Downlink	Distant TRAU: Uplink
0	No-DTX	No-DTX
1	No-DTX	DTX
2	DTX	DTX
3	DTX	No-DTX

C.3.2.1.1 GSM_FR, GSM_EFR and GSM_HR cases

If neither Distant Uplink nor Local Downlink DTX are active (case 0 in Table C.3.2.1-1), the Tx_TFO Process receives TFO Frames from the A Interface with SID == "0". It synchronises to them, i.e. checks correct synchronization and content. It extracts the data bits and calls, if appropriate (e.g. if BFI == "1" or if the TFO Frame is not-valid, see clause C.6.2), a Bad Frame Handler to derive suitable parameters for Downlink TRAU Frames. This Bad Frame Handler on parameter level is subject to manufacturer dependent future improvements and is not part of this recommendation.

If Distant Uplink DTX is active, but not Local Downlink DTX (case 1 in Table C.3.2.1-1), then the Tx_TFO Process receives TFO Frames containing speech parameters (SID == "0": handling as in case 0, see above), but also TFO Frames containing SID parameters (SID == "1" or "2") and TFO Frames marked with BFI == "1" during speech inactivity. Tx_TFO then calls a Comfort Noise Generator to derive suitable speech parameters for Downlink TRAU Frames. The SP flag shall always be set to SP = "1". The Downlink TRAU Frames shall not contain the SID codeword, but parameters that allow a direct decoding. Also this Comfort Noise Generator on parameter level is subject to manufacturer dependent future improvements and is not part of this recommendation.

If Distant Uplink DTX and Local Downlink DTX are active (case 2 in Table C.3.2.1-1), then the Tx_TFO Process receives TFO Frames containing either Speech parameters (SID == "0, handling see clause C.7.1) or SID parameters (SID == "1" or "2") or TFO Frames marked with BFI == "1" during speech inactivity due to transmission errors.

If a TFO Frame marked as a valid SID frame (SID == "2", BFI == "0") is received, it shall be stored in Tx_TRAU and its parameters shall be sent directly as Downlink TRAU SID Frame with correct timing. The DL_TRAU SID Frames produced from the valid stored frame are output repeatedly to the Abis/Ater interface whilst invalid SID frames (SID == "1") or frames marked as bad (BFI == "1") are received. These Downlink TRAU SID Frames shall be marked with the SP flag = "0" and shall all contain the SID codeword.

The stored SID Frame shall be considered as being valid for SID frame generation purposes until the receipt of the second instance of TAF == "1" (in a TFO Frame) following its initial storage. On expiry of the stored SID frame a suitable Bad Frame Handler for SID Frames shall be invoked to mute the Comfort Noise. Also, this Bad Frame Handler for SID Frames on parameter level is subject to manufacturer dependent future improvements and is not part of this recommendation.

If distant Uplink DTX is not active, but local downlink DTX is on (case 3 in Table C.3.2.1-1), i.e. only TFO Frames containing speech parameters are received , then one of the following alternative methods shall be used. The implementation of any of these alternatives is manufacturer dependent.

Alternative 1: The speech Frames are passed as DL_TRAU Frames to the BTS. This is virtually identical to case 0 in Table C.3.2.1-1, with no speech pauses detected, and handled like described above.

Alternative 2: A voice activity detector makes the decision as to whether the frame contains speech or not based on the PCM samples received from the A interface. During periods decided as "Active Speech" the speech Frames are passed as DL TRAU Frames to the BTS as described above. During periods of "Speech Pause" Comfort Noise Parameters are calculated. These operations in alternative 2 are manufacturer dependent and not detailed here.

Alternative 3: The received Speech Frames may be decoded and the resulting PCM samples used for normal downlink VAD and DTX functions.

C.3.2.1.2 AMR and AMR-WB cases

The Tx_TRAU receives TFO Frames from the Rx_TFO and converts them in DL TRAU frames. No Error concealment and Comfort Noise Generation is performed by the Tx_TRAU. This is instead handled within the BTS and the Mobile Station. Since some of the control bits may change from TFO to TRAU frames it might be necessary to re-compute the relevant CRCs.

C.3.2.2 Synchronisation and Bit Errors in Received TFO Frames

C.3.2.2.1 GSM_FR, GSM_EFR and GSM_HR cases

If Rx_TFO detects an error in the received TFO Frame synchronization or control bits or if a CRC error is detected, and the error is detected prior to beginning the output of the same frame (as a Downlink TRAU Frame), then Tx_TRAU shall either substitute parameters from the last good TFO Frame, or shall encode the received PCM samples for the duration of this frame.

If Rx_TFO detects an error in the received TFO Frame synchronization or control bits or if a CRC error is detected, and the error is detected after beginning of the output of the same frame (as a Downlink TRAU Frame), then Tx_TRAU shall deliberately corrupt the remaining, still unsent synchronization bits by setting them all to "0" and deliberately shall corrupt the remaining CRC bits. This will result in the BTS discarding this TRAU Frame, and transmitting a Layer 2 Fill frame or CRC-Inverted frame to the Mobile station (see 3GPP TS 48.060 and 3GPP TS 48.061). The effect of the frame error will subsequently be masked by the Mobile station’s handling of bad frames.

C.3.2.2.2 AMR and AMR-WB cases

C.3.2.2.2.1 No format conversion

When TFO and TRAU frames have the same format i.e. TFO_16k and TRAU_16k for FR_AMR and FR_AMR-WB or TFO_32k and TRAU_32k for FR_AMR-WB or AMR_TFO_8+8k and AMR_TRAU_8+8k for HR_AMR, then the received TFO frame shall be relayed as a DL TRAU frame toward the BTS. The Tx_TRAU shall not perform any Error Correction.

C.3.2.2.2.2 With format conversion

If the BTS does not support the optional TRAU_8+8k Frame Format, then TFO and TRAU frames may have different formats, e.g. AMR_TFO_8+8k and TRAU_16k. Then the received TFO frame format is converted into a DL TRAU frame format toward the BTS. The Tx_TRAU shall not perform any Error Correction, but rather relay the received parameters unaltered through. It might be necessary to re-compute the relevant CRCs.

If a CRC error is detected in the TFO Frame, the corresponding CRC, if any, shall be inverted in the DL TRAU frame. If there is no corresponding CRC, the remaining synchronization bits shall be inverted.

If a synchronization error is detected, the remaining synchronization bits shall be inverted in the DL TRAU frame as well.

C.3.2.3 Maximum Rate Control

In case of the non_AMR Codec Types (GSM_FR, GSM_HR, GSM_EFR) no rate control is applied.

In case of AMR and AMR-WB Rate Control shall be performed for both directions. This Rate Control shall be independent of the TFO States in TRAU and BTS. In case the TFO_Protocol alters the Max_Rate parameter this shall be taken into account to the earliest possible point in time for all following frames in both directions. During the TFO negotiation the Max_Rate can be set to the TFO Setup Mode. While in Tandem Free it can be set to Handover Mode before a handover occurs.

TFO Setup Mode: AMR mode to be used when switching to Tandem Free Operation. During the TFO negotiation the CACS to be used in TFO is determined (see clause 12). The corresponding TSM is derived in a similar way as the ICM (see [9]). Prior to switching to TFO the AMR modes are steered to the TSM.

Handover Mode: It is determined before the handover based on the new CACS after handover according to the rules for the new default ICM available in [9].

NOTE 1: It is recommended that the operator uses the default rule of ICM definition rather than setting it to an arbitrary value. Otherwise the Handover Mode won’t be identical to the ICM of the new cell.

Maximum Rate Control for the downlink direction: Tx_TRAU shall switch the AMR codec mode for the downlink direction (encoding) according to the UL CMC (Rate Control) received from the Rx_TRAU and the local "Max_Rate" parameter by taking the minimum of both.

Maximum Rate Control for the uplink direction: Tx_TRAU shall take the minimum of the local "Max_Rate" parameter and the received Rate Control parameter (CMR) from Rx_TFO and shall send this result downlink to the BTS within the CMR field. If no CMR is received from Rx_TFO, because TFO is not ongoing, then this CMR shall be assumed to be at maximum (7).