C.5 TFO_BSC

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

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

The role of the BSC in TFO depends on the speech Codec Type in use, and on the degree of flexibility desired.

For the GSM_FR, GSM_EFR and GSM_HR speech Codec Types the BSC may perform the Resolution of Codec Type Mismatch and Codec Type Optimization (see clause C.5.1).

For the AMR and AMR-WB Speech Codec Types the role of the BSC can be much more important (see clause C.5.2).

C.5.1 Resolution of Codec Type Mismatch and Codec Type Optimization

The BSC is in charge of solving the Codec Type Mismatches. The BSC receives from the TRAU or the BTS in case of AMR and AMR-WB the distant speech service configuration (e.g. the distant used codec and the distant codec list), or alternatively, the Optimal Codec Type and the Optimal Codec Configuration.

The BSC transmits to the TRAU the Local configuration, via the BTS for AMR and AMR-WB. It may have to refresh this information if the configuration changes along the time.

The BSC may implement the TFO decision algorithm provided in the Clause 11 and 12, or alternatively get the results from the BTS or TRAU. This TFO decision algorithm ensures that both BSCs obtain the same result. The BSC can then initiate an intra-cell Handover if a different Codec Type is required to ensure Tandem Free Operation.

C.5.2 Role of the BSC for AMR and AMR-WB TFO

AMR introduces a degree of complexity, due to its multi-rate nature, to its link adaptation and to the different options it allows. It is required that the AMR configurations of the two terminals and two BSS be aligned.

The ACS can vary and depends on the BTS generation, BTS manufacturer or on Operators’ preferences. The ACS can be tailored to cope with the environment of a given cell, e.g. a dense urban area or a flat rural area.

The MS may either support FR_AMR only or FR_AMR and HR_AMR and FR_AMR-WB. The BSS can support from one mode to all fourteen AMR modes (8 in FR_AMR and 6 in HR_AMR) and between between 3 and 11 modes for AMR-WB (3 in FR_AMR-WB, 3 in OHR_AMR-WB and 5 in OFR_AMR-WB). The ACS in GSM may include between 1 and 4 modes for AMR and AMR-WB.

In addition to resolving the Codec Type Mismatch as explained in clause C.5.1, the BSC can also be involved in the following TFO related tasks:

1. Determination and Establishment of the Optimal ACS.

2. Keep as far as possible the same ACS during Handovers.

C.5.2.1 Configuration of the AMR and AMR-WB speech service.

The MS is configured by the BSC at Call set-up and during handovers through Layer 3 signalling (see GSM 04.18 [14]). The BTS is configured through the CHANnel ACTIVation message (see 3GPP TS 48.058). The TRAU circuit pools are managed by the MSC on request of the BSC (see 3GPP TS 48.008 [10]).

The AMR configuration of the MS and BTS can be changed during the call by:

– Intra-Cell Handover (see 3GPP TS 44.018 and 3GPP TS 48.058 [12]),

– Mode-Modify (see 3GPP TS 44.018 and 3GPP TS 48.058 [12]),

– RATSCCH (see 3GPP TS 45.009 [9] and 3GPP TS 48.058 [12]).

These procedures are initiated by the BSC. The RATSCCH can in addition be delegated to the BTS by the BSC at the Channel Activation. This can modify the way TRAU handles TFO setup. (see clause C.5.2.2)

The RATSCCH is the most efficient technique from a speech quality point of view since it can be faster and can minimize the number of lost frames.

The Intra-Cell Handover is a synchronized handover and creates less speech frame losses than the typical Handovers.

The Mode Modify offers the advantage of keeping the same radio resource but can introduce long speech blanks.

C.5.2.2 Determination and Establishment of the Common ACS

The resolution of the AMR Codec Configuration Mismatch is based on similar principles as the Codec Type Mismatch. The corresponding TFO Decision algorithm is defined in Clause 12. When applied, it leads to a common optimal ACS at both ends of the TFO connection.

The resolution of Codec Configuration Mismatch depends on the Optimisation Mode, see table C.5.2.2-1.

Table C.5.2.2-1: Coding of the Optimisation Mode (OM)

OM Code	Optimisation Mode	Comment
0	No Change	Change of the ACS is not supported
1	Change	Change of the ACS is supported

The reporting of the Configuration parameters from the TRAU to the local BTS depends on the "Optimal or Distant Configuration (OD)" parameter, see table C.5.2.2-2.

Table C.5.2.2-2: Optimal or Distant Configuration (OD)

OD Code	Optimal or Distant Configuration	Comment
0	Distant	TRAU shall send Distant Configuration Parameters
1	Optimal	TRAU shall send Optimal Configuration Parameters

In case of OM = Change, the TRAU provides the BTS and further on the BSC (see 3GPP TS 48.058 clause 4.15) with the Distant Configuration (OD = Distant) or the Optimal Configuration (OD = Optimal).

OD is a configuration parameter set by the BTS (respectively the BSC) and send to the local TRAU.

The configuration is changed using one of the methods listed in the clause C.5.2.1.

C.5.2.3 Handovers and the AMR TFO

Handover in an ongoing AMR-TFO connection needs more attention. It can be handled more efficiently, if the BSC takes the configurations (the active local one in the serving, old BTS, the future local one in the new BTS and the distant one in the distant BTS) into account and informs the serving BTS a before performing the handover ("Pre-Handover Notification"). The sending of the Pre-Handover Notification should take into account the round-trip delay if it has been reported by the BTS (see clause C.4.5).

The BSC, as a central point of the BSS, manages the AMR Speech Service configuration along the communication.

The BSC has at any time control over the ongoing call, especially over all used resources. Some AMR specific adaptation procedures are, however, handled by lower layer inband signalling directly, e.g. time alignment, CMI/CMC phase alignment and Codec_Mode adaptation (Rate Control).