First we have to check before the investigating of the CSFB problems –> is to understand CSFB performance counters and when do they count during the CSFB procedures:
Understanding of CSFB Counters.
L.CSFB.PrepAtt counter:
is incremented by 1 each time the eNB receives the IE “CS Fallback Indicator” from the MME contained in an INITIAL CONTEXT SETUP REQUEST message or in a UE CONTEXT MODIFICATION REQUEST message.
L.CSFB.PrepSucc counter:
is incremented by 1 each time the eNB successfully sends an INITIAL CONTEXT SETUP RESPONSE message or a UE CONTEXT MODIFICATION RESPONSE to the MME.
L.CSFB.E2W / L.RRCRedirection.E2W.CSFB counter:
The L.CSFB.E2W and L.RRCRedirection.E2W.CSFB counters are incremented by 1 each time the eNodeB sends an RRC Connection Release message to the UE and informs the UE to perform a CSFB procedure to WCDMA network. In this case, if the CSFB procedure is initiated by a UE in idle mode (when the UE initiates a CSFB, the initial state is determined by the eNodeB), the L.CSFB.E2W.Idle counter is incremented by 1.
L.CSFB.E2G/ L.RRCRedirection.E2G.CSFB
L.CSFB.E2G and L.RRCRedirection.E2G.CSFB counters are incremented by 1 each time the eNodeB sends an RRC Connection Release message to the UE and informs the UE to perform a CSFB procedure to GERAN.
CSFB Failure Scenarios.
Below are the most common scenarios where CSFB failure scenarios can occur:
Scenario 1:
CSFB Execution Failure (L.CSFB.E2W/ L.CSFB.E2G=0), i.e. CSFB is completed in the preparation phase but no redirection to UTRAN/GERAN.
CSFB Preparation: L.CSFB.PrepAtt = L.CSFB.PrepSucc
Redirection: L.CSFB.PrepSucc > L.CSFB.E2W+ L.CSFB.E2G
Possible Causes:
Ø No UTRANNCELL configuration on LTE cell
Ø No UTRANNFREQ configuration on LTE cell
Ø No GERANNFREQGROUPARFCN configuration on LTE cell
Ø No GERANNCELL configuration on LTE cell
Ø Lacking/missing UTRAN/GERAN neighbor configuration
Ø UTRANNCELL information with BlindHPriority = 0
Ø GERANNCELL information with BlindHPriority = 0
Scenario 2:
CSFB Execution Failure (L.CSFB.E2W/ L.CSFB.E2G >0), i.e. CSFB is completed in the preparation phase but low success redirection to UTRAN/GERAN (Low CSFB Execution success Rate)
Possible Causes:
Ø Missing CO-Sector Utran/Geran Neighbors
Ø Configure High BlindHOPriority to Non co-site Neighbors
Ø Wrong External Configuration
Scenario 3:
CSFB Preparation not triggered (L.CSFB.PrepAtt=0),
Possible Causes:
Ø UtranCsfbSwitch not turned ON
Ø GeranCsfbSwitch not turned ON
Ø Missing CSFB License on LTE Site
*Scenario: CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to interaction with another procedure
CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc
Possible Causes:
Ø Interaction or conflict with simultaneous procedure such as S1, X2, or IRAT handover procedure.
Scenario 4:
CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to RR procedure failure
CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc
Possible Causes:
Radio resource procedure failed which could be due to RF related issues. The following RR procedures can be affected when CSFB is triggered (Context Setup Request from MME to eNB):
Ø Security Mode
Ø RRC Reconfiguration
Ø UE Capability
Ø Measurement
Scenario 5:
CSFB Extended Service Request initiated but succeeding procedures not completed
CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc
Possible Causes:
Ø RF related reasons.
Ø UE Initiates a procedure with one site and have succeeding procedures on another site.
*Scenario: CSFB triggered (Context request from MME to eNB) but not completed in the preparation phase due to other reasons
CSFB Preparation: L.CSFB.PrepAtt > L.CSFB.PrepSucc
Possible Causes:
Ø UE doesn’t have IRAT capabilities.
Ø Timer expiry that could be RF related reason if on the RR layer or other reasons if timer expiry on the S1 interface