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3.3Prototype production and evaluation of its functionsFigure 5 shows the prototype produced on the basis of the system architecture in Fig. 4. In this prototype system, we constructed a simulated network with one cellular mobile operator and one microcell mobile operator.On the PCs, we ran core network functions, including CBC and MOM, for the respective mobile operator net-works. We assumed that the CBE is implemented outside the cellular operators as shown in Fig. 4 because this ar-rangement does not aect system evaluations. However, in reality, we implemented the CBE on the PC where the core network functions of the cellular operator are running. One base station was operating on each mobile operator net-work. Each base station was allocated a non-overlapping frequency band in the 2.6 GHz band. e devices were commercially available LTE modules compatible with the aforementioned frequency band as laptop computers, which we connected to laptop computers by USB. We used RF cables to connect the base stations with the LTE mod-ules.Using this prototype apparatus, we conrmed what happened when we ran the system, following the procedure below. e macrocell is the cellular mobile operator’s cell, while the microcell is the microcell mobile operator’s cell.(1) e device connects to the macrocell(2) e microcell begins operating, and operational information is broadcast from the macrocell and received by the device(3) e device connects to the microcellWhen we shut o the microcell’s operations, the device disconnected from the microcell and reconnected auto-matically to the macrocell.As a result of this function test, we were able to conrm that the microcell mobile operator’s operational informa-tion was transmitted by the cellular mobile operator to the device via the CBS and that this was the catalyst for the device’s successful connection to the microcell.4Challenges posed to mobile communications systems by international roaminge frequencies used for mobile communications sys-tems dier from one country’s system to another. Under existing mobile communications systems, devices are compatible with all possible frequency bands, so they can connect to wireless systems operating on other frequencies, even when in other countries, and are therefore capable of using roaming services, in many cases. However, in the 5G era, a very wide range of frequency bands — including the millimeter wave range — will become candidates for use and there is a possibility that wireless systems based on dierent standards will be put into operation, to fulll the functional requirements of enhanced mobile broadband, low latency, and massive Machine Type Communications. FiF6　Challenges posed to mobile communications systems by roamingMacrocell(Operator A)Macrocell(Operator B)Microcell(Operator B)Macrocell(Operator C)Street Cell(Operator C)Core Network(Operator B)Core Network(Operator A)Core Network(Operator C)User from Country X (Operator D) uses services while roaming in Country YPolicy serverDevice functions are only compatible with the following cells・Operator A’s macrocells・Operator B’s microcells・Operator C’s street cellsCountry YCountry XInter‐operator interconnection networkCore Network(Operator D)Core network function92-1 Next Generation Mobile Communications System to Realize Flexible Architecture and Spectrum Sharing