C.4 TFO_BTS

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

The following clauses apply only when an AMR or AMR-WB Codec Type is the Used_Codec_Type and when TFO is enabled.

C.4.1 TFO_States and Transitions

The BTS needs to know the status of the TFO connection for best operation of the AMR Link adaptation and Optimal Handover procedure.

The TFO_BTS state machine is made of five states:

– TFO_DIS: No Tandem Free Operation is allowed or ongoing;

– TFO_NO: Tandem Free Operation is enabled, but is neither ongoing nor under establishment;

– TFO_MAYBE: Tandem Free Operation is under establishment, but is still not ongoing;

– TFO_YES: Tandem Free Operation is ongoing.

– TFO_TERM: Tandem Free Operation is still ongoing, but will terminate soon.

The following TFO_State diagram (Figure C.4.1-1) shows the five States and the most important transitions.

Figure C.4.1-1: Main TFO _State Diagram within the BTS

At resource allocation the BTS enters either TFO_DIS or TFO_NO, depending on the Configuration Message from the BSC (see clause C.5.2.1). The transition from one state to another one is triggered by the reception of a message, either from the BSC or the TRAU. According to the TFO_State the BTS shall initiate different actions.

In TFO_DIS and TFO_NO the BTS may perform Time and Phase Alignment. In all other States (TFO_MAYBE, TFO_YES, TFO_TERM which are often gathered under the expression "TFO ongoing", the BTS should not send Time or Phase Alignment Messages to the TRAU, since the TRAU shall not obey them. In State TFO_YES the BTS may perform Phase Alignment on the air interface, see 3GPP TS 45.009 [9].

TFO_Enable and TFO_Disable are not messages per se, but are included in Configuration Message from the BSC (see clause C.5.1) by setting or resetting the TFO_Enable bit. In any case the local configuration parameters shall be sent to the TRAU immediately.

TFO_Soon, TFO_On and TFO_Off are sent from the TRAU, either with or without configuration parameters and rate Control commands from the distant side.

TFO_Enable at resource allocation brings the BTS into TFO_NO. TFO_Enable is relayed to the TRAU by the BTS (TFOE bit in TRAU frames). The TRAU shall then start TFO_Negotiation with a potential TFO_Partner.

TFO_Enable in State TFO_DISABLED or TFO_TERMINATING starts the same procedure and brings the BTS also into State TFO_NO. In any other State the TFO_Enable has no effect on the ongoing procedures.

TFO_Disable at resource allocation brings the BTS into TFO_DISABLED. The TRAU shall not initiate nor respond to any TFO_Negotiation. It shall terminate TFO operation or Negotiation.

TFO_Disable in TFO_YES brings the BTS into State TFO_TERMINATING. TFO_Disable in any other State brings the BTS immediately into TFO_DISABLED.

If TFO is enabled the TRAU will get the knowledge about the distant side by the first received TFO_REQ or TFO_ACK Message or by Con_Req or Con_Ack Messages. As soon as the TRAU gets knowledge that a TFO_Partner exists, it informs the BTS in downlink about the Distant configuration, see clauses C.6.1 and C.6.2). If TFO is possible, the TRAU sends a TFO_Soon Message to the BTS. If TFO is not possible, the BSS may then perform Mismatch Handling. Alternatively the TRAU sends only the Optimal Codec Type and Optimal Codec Configuration to the BTS and/or further to the BSC.

TFO_Soon in State TFO_NO brings the BTS into State TFO_MAYBE. The BTS has to discontinue Time and Phase Alignment with the TRAU and instead has to buffer the received TRAU frames for downlink transmission.

TFO_On reports that finally TFO is ongoing, i.e. TFO Frames are exchanged in both directions. The BTS enters State TFO_YES and enables the AMR Adaptation, now considering both radio legs for the selection of the optimal Codec_Mode. In TFO handover situations a Con_Ack instead of a TFO_On will bring the BTS into State TFO_YES.

TFO_Off brings the BTS immediately into the State TFO_NO. The BSC should be informed.

C.4.2 Handling of downlink DTX in TFO

If TFO is ongoing and the BTS receives downlink TRAU frames classified with "SID_First or "SID_Update", it shall use one of the following options:

– Option 1) The BTS performs normal DTX operation in downlink if DTX DL is enabled.

– Option 2) The BTS shall send the SID_First, SID_Update frames as in normal DTX, but shall send SID_Filler frames between SID frames when DTX DL is disabled.

See 3GPP TS 26.093 for the definition of the SID_Filler frames.

Note : In all cases ONSET frames may be ignored, see 3GPP TS 45.009 [9], but may be used to ensure proper synchronisation.

C.4.3 Handling of Errors in Configuration Parameters

The BTS shall check the consistency of the configuration data sent by the TRAU. If inconsistent they shall be ignored, i.e. no report is made to the BSC, no change of the MS-BTS ACS is attempted, no acknowledgement is sent back to the TRAU. The missing Acknowledgement will trigger a repetition of the configuration data.

C.4.4 Procedures for Round Trip Delay Measurements

In case of AMR and AMR-WB, the link adaptation may need information on the round trip delay between the local BTS and the local TRAU or – when TFO is ongoing – with the distant BTS. Therefore, the BTS shall count the number of elapsed TRAU frames between the sending of a "Con_Req" (see clause C.6.2) message and the receipt of the corresponding acknowledgement. This number, multiplied by 20 ms, gives an estimate of the round trip delay between the BTS and its partner. The type of acknowledgement (DL_Ack or Con_Ack) indicates the type of partner, i.e. whether the local TRAU or the distant BTS has answered.

This procedure may be repeated whenever the status of the connection changes. The round trip delay measurement is triggered by the transition into State TFO_YES. But there are other cases, where a new delay measurement is required, although the State TFO_YES has not changed. This is e.g. the case when a distant handover occurred. The BTS where the handover takes place shall send the "Handover_Complete" Notification within the Time Alignment field of a Con_Req frame to the other BTS. This then shall repeat the Delay Measurement on its side.
The Handover_Complete Notification shall be re-sent in every Con_Req frame until a Con_Ack was received.

The BTS may report the round trip delay measurement result to the BSC by sending a round Trip Delay Report (see 3GPP TS 48.058). Any substantial change (more than 60 ms difference) in the round trip delay may be reported, too.