Performance of CS Fallback for Long Term Evolution Mobile Network

When a mobile operator migrates its network from the third generation (3G) system to Long Term Evolution (LTE), both 3G and LTE will coexist for a period of time. Since the 3G circuit-switched (CS) voice mechanism is more mature and available than that for the LTE Voice over Internet Protocol (VoIP), the operator may consider CS fallback (CSFB) as a solution for providing reliable voice calls. According to the 3G Partnership Project (3GPP) CSFB procedure, when a mobile user in the LTE network has an incoming or an outgoing call, the user equipment (UE) falls back from LTE to Universal Mobile Telecommunications System (UMTS). When the call is complete and released, the UE immediately returns to LTE. If the next activity for the UE is another voice call, immediately switching from UMTS to LTE may not be efficient. In this case, the UE has to perform another CSFB. To resolve this issue, we suggest delaying the returns to avoid unnecessary CSFBs, which is called delayed return (DR). Based on the measurements from the real UMTS and LTE networks, we develop an analytic model for investigating the performance of the CSFB with DR. This paper indicates that the DR scheme can effectively reduce the CSFB costs by up to 60%.