A flexible radio frame structure for both FDD and TDD is proposed. Under the flexible frame structure, each radio frame consists of a plurality of slots, and each slot within a radio frame has a flexible slot type. As a basic scheduling unit, each slot can be configured by the base station via physical layer signaling. The slot type can be changed dynamically based on current system needs to support different DL/UL ratios and latency requirements. With the support of different slot types and asynchronous DL/UL HARQ operation, HARQ operation for DL/UL can share the same HARQ timing to simplify the system design and reduce implementation complexity.