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— to be held in 2019 — concerning the identication of frequency bands for the future development of international mobile telecommunications (IMT) in the 24.25–86 GHz range. As such, it is likely that some of the new frequencies for operating microcells will be in the millimeter wave range. e key feature of microcells is their small cell radius, so in this situation, cell placement is not expected to be as comprehensive as with conventional macrocells. From the perspective of radio wave propaga-tion, there is considerable free space propagation loss in the millimeter wave range, so buildings and similar struc-tures are likely to cause substantial attenuation. Consequently, as this reduces concerns about mutual inter-ference due to sharing the millimeter wave range, there is a strong possibility that the need for stringent management of frequency allocations for base stations will disappear. However, it is not practical to manage frequency allocations individually for the plethora of microcells likely to be de-ployed. Given that it would therefore be preferable for microcells to share some frequency bands, research and development of the technological components required for this is necessary.NICT has, for many years, been conducting research and development focused on control algorithms and system architecture for the basic technology of cognitive wireless networks, aiming to bring to fruition heterogeneous wire-less networks that allow cooperation among mobile opera-tors. Some technologies were demonstrated to be eective through real-world verication on a municipal scale[2] and support provided for areas aected by the Great East Japan Earthquake[3]–[5], among others, and were put to practical use in the form of a cognitive wireless router[6]. In addi-tion, this work made a substantial contribution in such areas as the formulation of IEEE 1900.4[7][8], the world’s rst international standard for the fundamental architec-ture of cognitive wireless networks. Given that it was predicated on cooperation between separately managed wireless networks, it is likely that this research and develop-ment work by NICT was tremendously signicant as re-search and development focused on basic technologies and, more recently, their practical application, as it corre-sponded with the aforementioned concept of 5G. However, these deliberations were conducted independently of the standards formulated by the ird Generation Partnership Project (3GPP), which focuses primarily on mobile com-munications systems. As such, when looking ahead to full-scale commercialization in the future, further delib-erations will be required to map the technologies required against the functions prescribed in the 3GPP standards, in accordance with the common interface. Accordingly, in light of the medium to long-term plan that began in April 2016, NICT is conducting research and development fo-cused on technology for making the aforementioned 5G concept a reality, taking into account 3GPP’s deliberations on a 5G standard.At the same time, in the realm of spectrum sharing, NICT has also been engaged in research and development of television white space technology (TVWS technology)[9]. It is technically dicult to control interference between television broadcasts, which involve high-power, one-way transmissions, and low-power, two-way wireless communi-cations. e basis of TVWS technology is a central control system that uses a database to avoid interference, of the kind seen in regulations in many countries. However, further research and development is required, as a simpler, more eective system could likely be devised with a view to spectrum sharing by low-power microcells in 5G sys-tems.As well as introducing the concept of private areas in mobile communications systems as a means of making the aforementioned 5G concept a reality, this article proposes system architecture based on 3GPP that would facilitate the exible installation of microcells in private areas and presents the results of performance tests using a prototype system. In addition, it summarizes the challenges posed to mobile communications systems by international roaming.2Microcell operation in private areas2.1Introduction of private areas and positioning of microcell mobile operatorsFigure 1 shows the concept of cell deployment and device topology envisaged in future mobile communica-tions systems. Cellular mobile operators are mobile opera-tors that carry out subscriber management and deploy and operate macrocells — which have a wide communication range — themselves, as in the case of conventional mobile communications services. In addition to conventional macrocells, which have a communication range of up to 10 km or so, cellular mobile operators also deploy numer-ous microcells, which use new frequency bands such as millimeter wave bands and mostly have a communication range of no more than 100 m. Cellular mobile operators operate these microcells and macrocells in an integrated way.In contrast, microcell mobile operators (mobile opera-2 Terrestrial Communication Technology Research and Development4　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017)