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In that situation, it will be unlikely that devices will gener-ally have the ability to access all wireless systems.Figure 6 shows a typical example of a mobile com-munications system during roaming. Let us consider the case of a user subscribed to Operator D in Country X, who uses a communications system aer moving to Country Y. In Country Y, Operators A, B, and C deploy wireless com-munications services, but Operator A operates macrocells only, Operator B macrocells and microcells, and Operator C macrocells and street cells. Street cells are a series of contiguous microcells laid out in a straight line. In this case, the device is assumed to be compatible only with Operator A’s macrocells, Operator B’s microcells, and Operator C’s street cells. Under the current roaming system, a device can only connect to one of the operators (either Operator A, B, or C) at a time and cannot simultaneously use cells deployed by several dierent operators. However, the need might arise to connect to both a macrocell and a street cell, depending on the requirements of the applica-tions used at the same time.e following functions would be required to resolve this issue.・Core network and device functions that simultane-ously accept roaming connections to multiple opera-tors・Optimal operator and cell selection functions, based on the frequency of the cells operated by each op-erator and the wireless access technology・Functions enabling cells operated by dierent opera-tors to be integrated in the upper layer as needed for communicationWe have proposed a system that satises these func-tional requirements and are currently using prototype ap-paratus to examine how it runs[13].5Conclusionsis article explained the need to build infrastructure in mobile communications systems in the 5G era that will facilitate cooperation among not only cellular mobile op-erators with extensive area coverage, but also microcell mobile operators, which can exibly and meticulously deploy specialized coverage oering a particular level of communication performance tailored to specic usage scenarios. In addition, it used a prototype to evaluate the proposed system for this. Furthermore, it pointed out the problems faced in international roaming, given that use of a wider range of frequency bands is envisaged, and indi-cated the need for future research and development.Some of the proposals in this article have been submit-ted to 3GPP SA2 and we plan to continue activities aimed at bringing next-generation mobile communications sys-tems to fruition.Acknowledgmentse outcomes of this study are the product of two contract research and development projects undertaken for the Ministry of Internal Aairs and Communications.ReferenceR1Report from the Radio Policy 2020 Council, The ministry of Internal Affairs and Communications, July 15, 2016. 2Kentaro Ishizu, Homare Murakami, Hiroshi Harada, “Development of wide-area cognitive radio testbed for empirical evaluation of optimized radio selec-tion control on municipal scale,” IEICE Tech. Rep., vol.110, no.153, SR2010-30, pp.81–88, July 2010.3Kentaro Ishizu, Homare Murakami, Hiroshi Harada, “Cognitive Wireless Network Infrastructure and Restoration Activities for The Earthquake Disaster,” WPMC 2011, Oct. 2011.4Hiroshi Harada, Kentaro Ishizu, Homare Murakami, “Effective Use of Cognitive Wireless Router in Emergency Situation of Network Infrastructure,” Journal o IEICE, vol.95, no.3, pp.207–212, March 2012. 5Kentaro Ishizu, Homare Murakami, and Hiroshi Harada, “Lessons from The Earthquake through Restoration Activity of NICT for Network Infrastructure,” IEEE R10-HTC2013, Aug. 2013.6K. Ishizu, H. Murakami, H. Harada, “Cognitive Wireless Router system by distributed management of heterogeneous wireless networks,” IEICE Transactions on Communications, vol.E93-B, no.12, pp.3311–3322, Dec. 2010.7Kentaro Ishizu, Homare Murakami, Stanislav Filin, Hiroshi Harada, Goh Miyamoto, Ha Nguyen Tran, Shuzo Kato, “[Invited Talk] Introduction of IEEE P1900.4 and Activities of NICT,” IEICE Tech. Rep., vol.107, no.352, SR2007-58, pp.83–90, Nov. 2007.8Homare Murakami, Stanislav Filin, Kentaro Ishizu, Goh Miyamoto, Hiroshi Harada, “Standardization activities of cognitive radio technologies in ITU-R and IEEE P1900.4,” IEICE Tech. Rep., vol.109, no.442, SR2009-93, pp.15–21, March 2010.9Kentaro Ishizu, Homare Murakami, Zhou Lan, Ha Nguyen Tran, Hiroshi Harada, “[Technology Exhibit] Wireless Network System for TV White Space with Coordination of Database,” IEICE Tech. Rep., vol.112, no.55, SR2012-4, pp.23–30, May 2012.10Kentaro Ishizu, Homare Murakami, Kazuo Ibuka, Fumihide Kojima, “Coordinated Management of Multiple Wireless Networks and Spectrum Sharing toward The Fifth Generation Mobile Communication Systems,” IEICE Tech. Rep., vol.117, no.56, SR2017-7, pp.39–46, May 2017.113GPP TS29.168 V8.1.0, “Cell Broadcast Centre Interfaces with the Evolved Packet Core; Stage 3,” 2009.123GPP TS22.168 V8.1.0, “Earthquake and Tsunami Warning System (ETWS) requirements; Stage 1,” 2009.13Kazuo Ibuka, Homare Murakami, Kentaro Ishizu, Fumihide Kojima, “[Technology Exhibit] Prototype of USIM Sharing Terminal Enabling Simultaneous Use of Multiple Networks during International Roaming in Next-Generation Mobile Communication System,” IEICE Tech. Rep., vol.117, no.56, SR2017-11, pp.59–65, May 2017.2 Terrestrial Communication Technology Research and Development10　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017)