characteristics 54 of a band pass filter provided for the channel i 1 fm signal commonly used in a mobile communications system has a spectrum which theoretically extends to infinite therefore there exists signal spectrum of the channel i which pass through the band pass filter of the channel i 1 electric power leaked into an adjacent channel through a band pass filter produces an amount of interference this amount of interference depends on the following parameters a channel interval df spectrum of an incoming fm signal filtering characteristics of a band pass filter provided for each channel and a signal level of an interfering wave it is understood that interchannel interference is low when a spectrum width of an incoming fm signal is narrow b a channel interval is large c a pass range of a bass pass filter is narrow and cutoff characteristics of the filter is sharp and e an interfering signal level is low the parameters referred to in the above mentioned items a and b are determined depending on what kind of modulation is adopted in a communications system so that the values of these two parameters are determined at the system design according to this invention the parameters of channel interval and interfering signal level are considered it should be noted that the interfering level itself is not significant but a ratio of an interfered viz desired signal level and the aforesaid amount of interference is important the signals transmitted from a plurality of mobile units and received by a land site are different in their levels and it is often the case that a ratio of maximum and minimum signal levels reaches more than 70 db consequently if.