The present disclosure is directed to a user equipment and a base station which use a dynamic time division duplex (TDD) configuration mechanism. The present disclosure proposes implementing the dynamic TDD mechanism by imposing a restriction to the dynamic TDD configurations to adhere to a set of allowable dynamic TDD configurations. The uplink and downlink HARQ timing reference configurations could be derived from the set of allowable dynamic TDD configurations. The set of allowable TDD configurations, the uplink HARQ timing reference configuration and the downlink HARQ timing reference configuration could be derived based on specific rules. The maximum number of downlink HARQ processes for TDD would follow the downlink HARQ timing reference configuration.