IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 63, NO. 8, OCTOBER 2014 3977 Performance of CS Fallback for Long Term Evolution Mobile Network Ren-Huang Liou, Yi-Bing Lin, Fellow, IEEE, Yingrong Coral Sung, Pang-Cheng Liu, and Christian Wietfeld, Senior Member, IEEE Abstract—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 avail- able 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 Telecommunica- tions 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%. Index Terms—Circuit-switched fallback (CSFB), delayed return (DR) scheme, Long Term Evolution (LTE). Manuscript received October 16, 2013; revised December 4, 2013; accepted January 18, 2014. Date of publication January 27, 2014; date of current version October 14, 2014. The work of R.-H. Liou was supported by the MediaTek Fellowship. The work of Y.-B. Lin was supported in part by the National Science Council of Taiwan under Grant NSC 102-2221-E-009-056 and Grant NSC 103-2218-E-009-010, by Academia Sinica under Grant AS- 102-TP-A06, by Chunghwa Telecom, by the National Chiao Tung University and Industrial Technology Research Institute (ITRI) Joint Research Center Project, by the Information and Communications Research Laboratories/ITRI Project, by the Department of Industrial Technology Academic Technology Development Program under Grant 102-EC-17-A-03-S1-193, and by the Aim- ing for the Top University and Elite Research Center Development Plan of the Ministry of Education of Taiwan. The work of Y. C. Sung was supported by Chunghwa Telecom. The work of C. Wietfeld was supported in part by Deutsche Forschungsgemeinschaft under the Collaborative Research Center SFB 876 “Providing Information by Resource-Constrained Analysis” project B4. The review of this paper was coordinated by Prof. N. Kato. R.-H. Liou and P.-C. Liu are with the Department of Computer Science, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail: rhliou@cs.nctu. edu.tw; pcliu@cs.nctu.edu.tw). Y.-B. Lin is with the Department of Computer Science, National Chiao Tung University, Hsinchu 300, Taiwan, and also with the Institute of Information Science and the Research Center for Information Technology Innovation, Academia Sinica, Taipei 115, Taiwan (e-mail: liny@cs.nctu.edu.tw). Y. C. Sung is with the Center for Information and Communications Technol- ogy, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail: ycsung@ cs.nctu.edu.tw). C. Wietfeld is with the Communications Networks Institute, Technische Uni- versität Dortmund, Dortmund 44227, Germany (e-mail: christian.wietfeld@ tu-dortmund.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TVT.2014.2302832 Fig. 1. EPS and UMTS architecture for CSFB (dashed lines: signaling; solid lines: signaling/data). I. I NTRODUCTION T HE Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) [1] defines an all IP network archi- tecture that evolves from the Universal Mobile Telecommunica- tions System (UMTS) [2] to provide high-speed data services. When a mobile operator migrates its network from the 3G system to LTE, both 3G and LTE will coexist for a period of time, and initially, the 3G coverage is more complete than the LTE coverage. Since LTE is a packet-switched (PS) network, the circuit-switched (CS) voice service [3] is not as supported as the 3G. Therefore, the LTE voice service is offered through the Voice over Internet Protocol (VoIP) technology. Since the 3G CS voice mechanism is more mature and available than that for LTE VoIP, in the deployment of LTE, many operators consider CS fallback (CSFB) [4] as the solution for providing reliable voice calls. The CSFB technique switches the user equipment (UE) or the mobile phone from the LTE network to the 3G legacy system when a voice call is attempted. To use the CS domain in the UMTS system when the UE resides in LTE, the LTE network needs to register the UE with both the LTE and the UMTS CS domain and delivers the CS paging message from UMTS to the UE. Fig. 1 shows a simplified architecture of UMTS and evolved packet system (EPS) for LTE. This architecture includes two parts: the UMTS network and the LTE network. A UE [see (1) in Fig. 1] accesses UMTS and LTE services through the radio interfaces. In the UMTS network [see (a) in Fig. 1], the UMTS Terrestrial Radio Access Network (UTRAN) consists of NodeBs [see (2) in Fig. 1] and radio network controllers (RNCs) [see (3) in Fig. 1]. A NodeB provides wideband code- division multiple-access radio connectivity between the UE and 0018-9545 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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