In a mobile communication system, if a building is built in the vicinity of a base station, or if the installation status of a base station in the vicinity of the base station changes, the radio communication environment according to the base station changes. Therefore, conventionally, the operators make use of a measurement vehicle in which measurement equipment is loaded, and perform a drive test by measuring a state of a signal received from the network (base station), and then collecting the measurement data.
Such a measurement and collection process can, for example, contribute to the optimization of coverage of the base station, but faces the problem of too many man-hours and high cost. Thus, according to 3GPP (3rd Generation Partnership Project), i.e., a standardization project for mobile communication systems, a radio terminal belonging to the user is used to proceed with the specification planning of MDT (Minimization of Drive Test) for automation of the measurement and collection.
A type of MDT is the immediate report type MDT (referred to as “Immediate MDT”). According to Immediate MDT, a radio terminal in the connected state measures a reception signal state, and then reports the measurement results and location information during measurement to the network. The connected state refers to the state when the radio terminal is performing communications.
Furthermore, it is stipulated in the 3GPP standards that when a radio terminal in the connected state detects a connection failure with the network, a radio link failure (RLF) report about the connection failure is sent from the radio terminal to the network (see 3GPP TS 36.300 v10.4.0). A connection failure refers to RLF or a handover failure. Furthermore, it is stipulated that in Immediate MDT, the RLF report is extended, and the location information for identifying the location where the connection failure occurred is included in the RLF report (see 3GPP TS 37.320 v10.1.0).
Based on the Immediate MDT report and the RLF report received from the radio terminal, the network performs parameter adjustment (settings change) for coverage optimization.