CSFB Strategy: HO based or Redirection based

For redirection-based CSFB, UE initiates call setup from RACH. For handover-based CSFB, UE establish call in 4G LTE, and then performs handovers to 3G. Handover-based CSFB would deliver higher call setup reliability than redirection. The reason is that network provides higher priority to handover than initial call.

In contrast, redirection-based CSFB can be expected to deliver higher call setup reliability than handover-based CSFB, simply because redirection-based CSFB takes the IRAT measurement immediately before attempting access on the identified cell. Since the Release 9 SI Tunnelling and Release 8 Skip SIBs redirection-based CSFB methods have only slightly higher call setup times, their higher call setup reliability has guided the general industry trend to rely on redirection-based CSFB approaches. The reliability of redirection-based CSFB call setup has been tested by 3dB consult using device traces in field testing on live 3G networks, and no call setup failures were observed out of more than 160 calls, covering both good and bad RF conditions for redirection-based CSFB to UMTS.

Redirection based CSFB is more reliable but HO based CSFB is incredibly fast. Redirection based CSFB can me measurement based or blind, the measurement based redirection is more reliable but it increases CSFB time. Thus, the best option with moderate delay, better reliability and setup time is blind redirection with sib tunnelling.

HO based CSFB will guarantee a data service continuity and as resources are reserved beforehand this will help to enhance the CST.

Redirection CSFB is good in terms of reliability and setup time with some optimisation which should be done in LTE phase, in reselection 3G cell phase and in access the 3G network.

HO based CSFB should be better provided the HO relations are properly defined. It might be worse if there are missing neighbours or some misconfigured neighbours etc.

In the end, it all depends on networks capability and configuration. PSHO will need to setup Data bearer on target network first. Since Data Bearer usually has lower priority, it might not get a chance if network is busy, comparison to drop Data call and setup CS call. Ericsson products can assign the Data Bearer the same priority as CS when doing PSHO CSFB. After HO, setup CS call and then drop Data Bearer priority back to its original priority.

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