The present invention relates to methods and apparatus for providing a more flexible configuration of sounding reference signal, SRS, transmission. This is achieved by dynamically triggering a mobile terminal to transmit SRS. Thus, embodiments of the invention include a method implemented by a base station in a communications network that semi-statically configures some uplink symbols for periodic SRS transmission and other uplink symbols for aperiodic SRS transmission. The method includes dynamically determining whether an uplink symbol that has been semi-statically configured for aperiodic SRS transmission is to be used by a given mobile terminal at a given time for SRS transmission. The method then further includes signaling a terminal-specific instruction that indicates said determination to the given mobile terminal.