Radio+Link+Timeout+and+AMR.ppt

Radio Link Timeout BackgroundThe effect of AMR on dropped calls
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Setting the RLT Value
Radio Link Timeout Definition
TS 05.08 states that Radio Link Failure (RLF) in the MS is determined by the success rate of decoding messages on the downlink SACCH.
The aim of determining RLF in the MS is to ensure that calls with unacceptable voice quality, which cannot be improved either by RF power control or handover, are either re-established or released in a defined manner.
The Radio Link Timeout (RLT) parameter prevents a forced release (drop) occurring until the call has degraded to a quality below that at which the majority of subscribers would have manually released it.
The RLF procedure is implemented in the RRM at the BSC and is as follows:
After the assignment of a dedicated channel a counter is initialized to RLT
When a SACCH message is unsuccessfully decoded the counter is decreased by 1
When a SACCH message is successfully decoded the counter is increased by 2
If the counter reaches 0 a RLF is declared Call is released
Dropping Experience - FER
Dropping Experience: how much time before dropping a call is the perceived user speech quality unacceptable.
Definition of unacceptable criteria: FER > 15 % MOS < 2 no audible speech.
Company Confidential
AMR Codecs
AMR introduces a new set of codecs that allow for varying amounts to channel coding to be applied. This implies that there is less coding available for voice.
The more robust codecs mean that the voice quality can be preserved in bad C/I conditions.
However, these codecs only apply to the speech frames, not to the SACCH frames.
The SACCH frames will continue to be affected by bad quality conditions just like during an EFR call.
This means that the voice quality can still be of an acceptable level for the user but a RLF is declared because of the bad quality of the SACCH.
Altering the RLT to keep the same performance degradation will allow more calls to benefit from the improved voice quality provided by the AMR codecs.
AMR vs. EFR
The RLT is based on SACCH erased frames, which are independent of speech frames.
The principle of the tests is to find RLT value producing the same speech degradation (FER > 15 % MOS < 2.0 no audible speech during 30 sec before dropping) in AMR as EFR would suffer with default RLT value for this traffic (i.e. 20).
The driving route started at a good coverage location and ended at a bad coverage area with the mobile locked to the cell. This is a fairly extreme test.
With RLT 20:
EFR quality is unacceptable for the last 30 s (at least)
AMR quality is unacceptable for the last 15 s
FER 15%
Unacceptable quality
With RLT 28:
FER 15%
With RLT 32:
FER 15%
With RLT 36:
AMR quality is unacceptable during the last 30 s
AMR dropping experience with RLT 36 is similar to EFR’s with RLT 20
FER 15%
AMR vs EFR
Dropped call experience: For how long does the terminal not receive any audible speech (MOS<2.0) before it drops?
During 30 seconds before dropping, FERAMR is lower than FEREFR
FEREFR > 15 % during last 30 seconds before dropping, for RLT = 20
Conclusion:
AMRRLT=36 has a similar dropped call experience to EFRRLT=20
AMR, FER > 15 %
EFR, FER > 15%
The RLT parameter will affect both EFR and AMR mobiles.
Because of this, the parameter must be set taking into account the proportion of AMR traffic present.
The tests, and practical implementations, show that values of RTL from 36 to 44 with AMR match the equivalent performance degradation of EFR with RLT of 20 or 24.
This values are valid for penetrations of AMR traffic of 70 – 80%.
Note that the penetration of AMR mobiles is a good first approximation, but it is the AMR traffic that is more important.
A value of 24 or 28 would be a good starting point for TEMM until the AMR traffic penetration can be determined.
At that point the RLT can be adjusted up or down.
Consequences of not Changing RLT
If the RLT is left at the current EFR value then the immediate effect is a worsening of the Dropped Call Rate.
This is not due to AMR itself but is a consequence of user behaviour.
Users have a pre-set time before ending a call that has poor voice quality. They prefer to start a new call some time later than struggle through a bad quality call. This would count as a normal release.
This time is normally related to the amount of bad quality or silence that they are experiencing.
With AMR, the speech frames now have much more robust coding and speech quality does not degrade as much.
But the call may end in RLF due to the SACCH erasures. This call would now count as a dropped call.
Users would also perceive worse network quality as the call has dropped for no apparent reason, as the AMR codecs are preserving voice quality.
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Radio+Link+Timeout+and+AMR.ppt

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