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Radio Link Timeout and AMR

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The effect of AMR on dropped calls

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Contents

Radio Link Timeout Definition

Dropping Experience - FER

AMR Codecs

AMR vs EFR Drive tests

Setting the RLT Value

Consequences of not Changing RLT

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Radio Link Timeout Definition

TS 05.08 states that Radio Link Failure (RLF) in the MS isdetermined by the success rate of decoding messages on thedownlink SACCH.

The aim of determining RLF in the MS is to ensure that calls with

unacceptable voice quality, which cannot be improved either by RFpower control or handover, are either re-established or released in adefined manner.

The Radio Link Timeout (RLT) parameter prevents a forced releasedro occurrin until the call has de raded to a ualit below that at

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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 counteris 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

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Dropping Experience - FER

Dropping Experience: how much time before dropping a call is theperceived user speech quality unacceptable.

Definition of unacceptable criteria: FER > 15 % MOS < 2 no

audible speech.

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AMR Codecs

AMR introduces a new set of codecs that allow for varying amountsto channel coding to be applied. This implies that there is less codingavailable for voice.

The more robust codecs mean that the voice quality can bepreserved in bad C/I conditions.

However, these codecs only apply to the speech frames, not to theSACCH frames.

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The SACCH frames will continue to be affected by bad qualityconditions just like during an EFR call.

This means that the voice quality can still be of an acceptable levelfor the user but a RLF is declared because of the bad quality of theSACCH.

Altering the RLT to keep the same performance degradation will allowmore calls to benefit from the improved voice quality provided by theAMR codecs.