12 Determination of the OACS for AMR-NB
28.0623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
In case of inconsistencies between the TFO decision C-Code in Annex E and this clause the C-Code shall take precedence.
12.1 Principles
The determination of the OACS shall be done considering the available information (see 11.2.2).
The common MACS is defined as the minimum value of the local and distant MACS.
The determination of the OACS shall depend on the local and distant optimisation mode (LOM / DOM).
12.2 Algorithm for OACS Determination
12.2.1 Case 1: No side supports ACS change
If neither the local side nor the distant side supports an ACS change, the OACS is equal to the CACS if it fulfils the contiguity rule. Otherwise, the rules for contiguous subset selection are applied to the CACS in order to obtain the OACS.
Figure 12.2.1-1: OACS Determination when No side supports ACS Change
12.2.2 Case 2: Only one side supports ACS change
If only one side supports an ACS change, the CSCS is built with the common modes of the SCS of the flexible side and the unchangeable ACS.
If the CSCS doesn’t fulfil the contiguity rule or the common MACS is lower than the number of modes in the CSCS, the OACS is obtained by applying the rules for contiguous subset selection. Otherwise, the OACS is equal to the CSCS.
Figure 12.2.2-1: OACS Determination when only one side supports ACS Change
12.2.3 Case 3: Both sides support ACS change
If both sides support ACS change, the CSCS is built with the common modes of both SCS.
The Optimised Active Codec Set (OACS) is equal to the Common Supported Codec Set (CSCS) if the number of modes in the CSCS is equal or lower than the common MACS.
If the number of modes in the CSCS is higher than the common MACS, the OACS shall be defined as a subset of the CSCS using the OACS selection rules.
If the CSCS is not empty, then a Optimised Active Codec Set (OACS) exists.
The existence of an OACS doesn’t mean the OACS is acceptable. To check this, the acceptability rules for the OACS have to be applied.
Figure 12.2.3-1: OACS Determination when both sides support ACS Change
12.3 OACS Selection Rules
If both radio legs support ACS change and if the number of modes contained in the CSCS is greater than the common MACS, the OACS is determined by the following rules. These rules are skipped as soon as an OACS containing CMACS modes is found.
The reference C-Code also implements the OACS rules (see Annex E). In case of inconsistencies between this clause and the C-code, the C-code takes precedence.
12.3.1 Case 1: No Half Rate Channel is involved
Case MACS == 1
1) Select mode according to preference list {6,7, 7,4, 5,9, 5,15, 4,75, 7,95, 10,2, 12,2}.
Case MACS == 2
1) If mode 10,2 is supported, do not include mode 12,2.
2) Select highest mode.
3) If mode 12,2 or mode 10,2 is selected, select mode according to preference list {6,7, 7,4, 5,9, 5,15, 4,75, 7,95, 10,2, 12,2}.
4) Select lowest mode.
Case MACS > 2
1) If mode 10,2 is supported, do not include mode 12,2.
2) If mode 4,75 is supported, do not include mode 5,15.
3) If mode 5,15 is supported, do not include mode 5,9.
4) If mode 5,9 is supported and mode 4,75 is not supported, do not include mode 6,7.
5) If mode (12,2 or 10,2) and 7,4 is supported, do not include mode 7,95.
6) If mode 7,95 is supported, do not include 7,4.
7) Select lowest mode.
8) Select highest mode.
9) Select mode 6,7.
10) Select mode 5,9.
12.3.2 Case 2: A Half Rate Channel is involved
Case MACS == 1
1) Select mode according to preference list {5,9, 5,15, 4,75, 6,7, 7,4, 7,95}.
Case MACS == 2
1) Select highest mode.
2) Select lowest mode.
Case MACS > 2
The same rules apply as in clause 12.3.1 for the case MACS>2.
12.4 Rules for Contiguous Subset Selection
The rules for contiguous subset selection are necessary if one or both radio legs don’t support ACS change. If TFO should be established in these cases, the resulting OACS must fulfil the contiguity rule considering the fixed ACS.
If the CSCS doesn’t fulfil the contiguity rule, a contiguous subset with a maximum number of modes shall be selected as the new CSCS. This subset must contain the lowest mode of the fixed ACS, otherwise there is no OACS.
If the common MACS is lower than the number of modes in the CSCS, the highest modes shall be removed from the CSCS until the number of modes in the CSCS is equal to the common MACS. This new codec set defines the OACS.
12.5 Acceptability Rule for the OACS
An optimised ACS (OACS) is acceptable for AMR-NB TFO if
1) the Highest-Mode-Rule is fulfilled and
2) the Lowest-Mode-Rule is fulfilled.
High Mode Rule (don’t give up tandem with high quality modes)
The highest mode in the OACS is not lower than one mode below the minimum of the highest modes of both ACS.
Low Mode Rule (tandem AMR with robust low modes performs better)
Either the lowest mode of the OACS is not higher than a specific maximum mode or both ACS don’t contain lower modes than the lowest mode in the OACS. The specific maximum mode is 5,9 for TFO connections involving a half rate channel and 7,4 otherwise.
12.6 FR – HR – Matching
A common ACS (CACS) is acceptable for immediate TFO establishment without consideration of the OACS if all of the following conditions are fulfilled:
– the one radio leg uses FR_AMR or UMTS_AMR_2 or HR_AMR, the other uses HR_AMR [for AMR-NB TFO];
– the CACS is contiguous;
– the CACS fulfils the acceptability rule.
12.7 Contiguity Rule
The Contiguity Rule states that the codec modes of the CACS must be contiguous modes in the local ACS (LACS) and the distant ACS (DACS). Additionally, the CACS must contain the lowest mode of both ACS. The Contiguity Rule is used to enable TFO establishment on a CACS different from the ACS. In a GSM system this is necessary because link adaptation is only possible using maximum rate control with adjacent modes of the ACS.
Example A: LACS: 12,2 10,2 7,95 5,9
DACS: 10,2 7,95 5,9
CACS: 10,2 7,95 5,9 Contiguity Rule is fulfilled
Example B: LACS: 12,2 10,2 4,75
DACS: 10,2 7,4 4,75
CACS: 10,2 4,75 Contiguity Rule is not fulfilled for the DACS
12.8 Examples of OACS Computation
12.8.1 TFO between a full rate channel and a half rate channel
|
SCS |
ACS |
CACS |
OACS |
CSCS |
ACS |
SCS |
|
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12,2 |
x |
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10,2 |
x |
x |
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7,95 |
x |
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7,4 |
x |
x |
x |
x |
x |
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6,7 |
x |
x |
x |
x |
x |
x |
x |
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5,9 |
x |
x |
x |
x |
x |
x |
x |
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5,15 |
x |
x |
x |
||||
|
4,75 |
x |
x |
x |
x |
x |
x |
x |
This is an example for FR – HR – Matching.
Immediate TFO is possible using the CACS.
Afterwards, a codec mode optimisation is performed without interrupting the ongoing TFO connection.
12.8.2 TFO between two full rate channels with different ACS
|
SCS |
ACS |
CACS |
OACS |
CSCS |
ACS |
SCS |
|
|
12,2 |
x |
x |
x |
x |
|||
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10,2 |
x |
x |
x |
x |
x |
||
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7,95 |
x |
x |
x |
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7,4 |
x |
x |
x |
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6,7 |
x |
x |
x |
x |
x |
x |
x |
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5,9 |
x |
x |
x |
x |
x |
x |
x |
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5,15 |
x |
x |
x |
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4,75 |
x |
x |
x |
x |
x |
x |
x |
The CACS is a contiguous subset if the OACS.
Immediate TFO and subsequent codec mode optimisation without interrupting TFO is performed.
12.8.3 Full Rate Channel with restricted capabilities
|
SCS |
ACS |
CACS |
OACS |
CSCS |
ACS |
SCS |
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12,2 |
x |
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10,2 |
x |
x |
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7,95 |
x |
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7,4 |
x |
x |
x |
x |
x |
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6,7 |
x |
x |
x |
x |
x |
x |
x |
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5,9 |
x |
x |
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5,15 |
x |
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|
4,75 |
x |
x |
x |
x |
x |
x |
x |
Immediate TFO is not possible because the CACS is not contiguous.
TFO on the OACS is acceptable since a tandem connection would not provide a better speech quality.
The OACS is acceptable since both the High Mode Rule and the Low Mode Rule are fulfilled.
12.8.4 Scenario: Full Rate Channel with MACS == 2
|
SCS |
ACS |
CACS |
OACS |
CSCS |
ACS |
SCS |
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12,2 |
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10,2 |
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7,95 |
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7,4 |
x |
x |
x |
x |
x |
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6,7 |
x |
x |
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5,9 |
x |
x |
x |
x |
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5,15 |
x |
x |
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|
4,75 |
x |
x |
The OACS is acceptable for a TFO connection. A tandem connection would not provide better speech quality. Both High Mode Rule and Low Mode Rule are fulfilled. For good radio channels a tandem between 7,4 and 6,7 is worse than a 7,4 TFO connection. For poor radio channels a 5,9 TFO connection is considered to be robust enough.
12.8.5 Scenario: AMR codec type with only one supported mode
|
SCS |
ACS |
CACS |
OACS |
CSCS |
ACS |
SCS |
|
|
12,2 |
x |
x |
x |
x |
x |
||
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10,2 |
x |
x |
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|
7,95 |
x |
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|
7,4 |
x |
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6,7 |
x |
x |
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5,9 |
x |
x |
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5,15 |
x |
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|
4,75 |
x |
x |
One side offers an FR_AMR codec type with only the 12,2 mode in the supported codec set.
The OACS is not acceptable, TFO should not be established. A tandem connection would provide better overall speech quality. If the only supported mode is lower or equal to the 7,4 mode, TFO shall be established on this single mode. The 7,4 mode is considered to be robust enough in the case of poor radio channels. On the other hand, a tandem connection between 7,4 and 12,2 would be worse than a 7,4 TFO connection for good radio channels.
Annex A (normative):
In-band Signalling Protocol: Generic Structure