A wireless transmit/receive unit (WTRU) includes circuitry configured to receive information from a base station indicating an assignment of periodic time slots to transmit sounding signals and circuitry configured to transmit sounding signals to the base station in the assigned periodic time slots in response to the received information, wherein a timing of transmission of the sounding signals in response to the received information differs between a time division duplex mode and a frequency division duplex mode, and wherein the transmitted sounding signals are distinguishable from sounding signals from other WTRUs.