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1Introductione requirements for a h generation mobile com-munications system (5G) have been dened as enhanced mobile broadband, ultra-low latency, and massive Machine Type Communications. e goals for 5G communication performance are much higher than those of conventional mobile communications systems, including a maximum transmission speed of 10 Gbps, a connection density of 1 million devices per square kilometer, and latency of around 1 millisecond[1]. Many new applications requiring 5G communication performance are expected to emerge in the coming age of the Internet of ings (IoT). Examples of these include on-demand transmission/viewing of high-denition video, self-driving assistance for motor vehicles and real-time remote control of robots, and the collection of sensor data required for daily life. While 5G is a mobile communications system, it is likely to become the technol-ogy that enables the aforementioned applications to be brought to fruition, as the need for communications in social infrastructure grows in the future.Developing a single wireless system that satises all of the requirements of 5G communication performance will be dicult, so 5G systems will basically be composed of heterogeneous wireless networks that adaptively combine wireless systems with dierent communication capabilities. e approach centered on extensive access areas that has been the concept underpinning conventional mobile com-munications systems cannot be used to deploy access areas that satisfy dierent service requirements. As such, it is necessary to deploy microcells meticulously tailored to the location and conditions using low-power base stations with dierent capabilities. A single mobile operator is unlikely to be able to undertake the development of this kind of network alone, so it will be necessary to establish the new 5G concept of multiple mobile operators cooperating or-ganically based on uniform rules.ose with the best understanding of the functions and capabilities required by individual services are the manag-ers of those facilities and service providers. Accordingly, an eective approach to the use of microcells is to ensure that both microcells installed by mobile operators in the con-ventional way and microcells installed at the wishes of those facility managers/service providers are integrated as infrastructure in a way that guarantees a certain degree of reliability in respect of mobile communications system performance. In that situation, the ongoing operation of existing mobile communications systems by mobile opera-tors should be guaranteed, so it would be desirable to keep interface expansion to the minimum necessary level, without impacting on the design concept of mobile com-munications systems, as far as possible.Frequency allocations for mobile communications systems are already tight, so allocating frequencies for the operation of a large number of microcells would be prob-lematic. At the ITU’s World Radiocommunication Conference (WRC) in November 2015, participants agreed to include an item (Item 1.13) on the agenda for WRC-19 2 Terrestrial Communication Technology Research and Development2-1 Next Generation Mobile Communications System to Realize Flexible Architecture and Spectrum SharingKentaro ISHIZU, Homare MURAKAMI, Kazuo IBUKA, and Fumihide KOJIMAThe fifth mobile communication system is expected to be utilized as a communication infrastructure for emerging various applications. It will be operated under coordination of cellular operators who deploy access area universally and micro cell operators who deploy flexible access area in detail with specific performance for use scenarios. In this paper, its concept and necessary system architecture are proposed as well as functional verification using a prototype system. Also, issues on international roaming of mobile communications system are summarized.32 Terrestrial Communication Technology Research and Development