Portable stations with a single receiver can perform communication using a plurality of slots in one frame in time division multiple access (TDMA) transmission. When communication is performed between one of the portable stations and a cell station with data of two 32 kbps data into which an exchange converts one 64 kbps data, the portable station occupies two slots in one TDMA frame. A controller of the cell station assigns these two slots as being separated from each other by at least one slot. Alternatively, the controller assigns these two slots as being adjacent to each other and assigns same carrier frequency to these slots. Thus, communication with a transmission rate of 63 kbps can be performed by using two slots or two channels of 32 kbps.