dcd grtn rfrgn5tg

1. RADIO LINK FAILURE FOR
AMR/NON AMR & UL/DL

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RADIO LINK FAILURE
o The objective of prematurely terminating the call by the radio access network is to remove
calls experiencing such a bad radio link quality that retaining the connection is useless.
o Excessive errors on the air interface not only cause unacceptable speech quality but they also
may completely compromise the transmission of signaling messages required to maintain the
connection in a mobile environment.
o In GSM, the RLF criterion has been define as a main trigger mechanism to drop speech calls
suffering from excessive bad quality.
o The aim of detecting an RLF in both the MS and BTS was to ensure that connections with
unacceptable speech quality, which cannot be improved either by RF power control or HO, are
either released or re-established in a define manner.
o The RLF criterion is based on the radio link counter S. If the MS/BTS is unable to decode a SACCH
message, S is decreased by one. In the case of a successful reception of a SACCH message, S is
increased by two.
o In any case, S shall not exceed the value define by the parameter RADIO LINK TIMEOUT set in
the data base, for example at 24, and broadcast via SYS INFO messages. If S reaches zero, an RLF
due to radio link timeout is declared and the call is dropped.

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AMR V/S Non AMR RADIO LINK TIMEOUT
o Given that AMR calls are more robust than non AMR calls and provide acceptable speech
quality at much lower C/I ratios, the specification of a common RLT value for AMR and non AMR
calls may cause the AMR calls to be unnecessarily dropped.
o For this reason, separate RLT thresholds for AMR and non-AMR calls are generally used in
networks.
o This necessitates that the RLT threshold for AMR should be at higher value than for non AMR.
o Studies in a real network with AMR penetration of 70% have shown that when the RLT
threshold is increased by more than 50% (e.g. from 20 for non AMR to 36 for AMR), the overall
CDR was reduced by 25%, while keeping the speech quality at the acceptable level

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Case Study-AMR V/S Non AMR RADIO LINK TIMEOUT
*-Source GSM/EDGE Evolution & Performance By Mikko Saily & Dr. Eddie Riddington

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UL/DL AMR RLT THRESHOLDS
o Since in RF environment UL is weak link, It is always a good practice to keep the UL more
protected as compared to DL due to relatively low MS powers and better BTS sensitivity.
o In other networks as well, UL RLT is usually kept at least 4 SACCH Multi-frames greater than the
DL RLT, just like we keep a cushion for UL in all Handover and power control algorithms.
o So it is highly recommended to get the UL SACCH Multi-frame value changed from 28 to 32 for
the cells where applicable.

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CURRENT & PROPOSED RLT VALUES
Pre Values Current Values Huawei Propsoed Values
AFR SACCH Multi-Frames (UL RLT AMR FR) 28 28 32
AHR SACCH Multi-Frames (UL RLT AMR HR) 28 28 32
AFR Radio Link Timeout (DL RLT AMR FR) 24 28 28
AHR Radio Link Timeout (DL RLT AMR HR) 24 28 28

7. Page 7
Thank You!