Abstract: A method for a time division duplex mobile communication system comprises that setting a minimum unit of transmission time interval and a system frame structure, wherein each radio sub frame is composed of a downlink pilot time slot, a switching guard period, a synchronization time slot and a plurality of service time slots, and the transmission time interval uses the length of time slot as a unit; a user obtaining downlink synchronization by receiving downlink pilot information of the downlink pilot time slot, and obtaining corresponding configuration information of cell by reading cell broadcasting information, and then transmitting uplink synchronization information in the synchronization time slot to fulfill a random access process, wherein the configuration information includes transmission time interval and system frame structure information; the network side/the user transmitting call information to an opposite end, and the opposite end processing call acknowledgement according to the transmission time