An apparatus randomly assigns downlink sub-frame time slots for transmitting user data in an NB-TDD CDMA (Code Division Multiple Access) communication system. In the apparatus, a multiplexer creates a user data part by multiplexing user data for UEs (User Equipments), a TFCI (Transport Format Combination Indicator) symbol for the user data, and a TPC (Transmission Power Control command) symbol for controlling transmission power of a downlink channel, and a controller randomly assigns time slots for transmitting the user data part in the sub-frames, based on a time slot number initially assigned for the user data part, a sub-frame number at a transmission point of the user data part, and the number of assigned uplink time slots in the corresponding sub-frame.