Method, apparatus, and systems for flexible aperiodic reference signal (RS) transmissions are provided. For example, a method implemented in a wireless transmit/receive unit (WTRU) for wireless communications comprises receiving configuration information of one or more sounding reference signal (SRS) resource sets, each SRS resource set of the one or more SRS resource sets is associated with a slot offset and a set of slot offset deltas; receiving downlink control information (DCI) indicating an SRS request, the SRS request indicates an SRS resource set of the one or more SRS resource sets; determining an SRS configuration from a set of SRS configurations for SRS transmissions; determining a slot for transmitting an SRS based on the determined SRS configuration; and transmitting, in the determined slot, the SRS using resources of the indicated SRS resource set.