A wireless communication base station including a transceiver coupled to a controller configured to cause the base station to serve a plurality of user terminals in a series of downlink radio frames each having a first temporal region and a second temporal region. The controller is configured to assign the plurality of user terminals to first and second groups, wherein the user terminals in the first group are served in the first temporal region of the downlink radio frame and the user terminals in the second group are served in the second temporal region of the downlink radio frame. The controller is also configured to change a duration of either of the first and second temporal regions of at least one downlink radio frame in the series of downlink radio frames, wherein a downlink radio frame contains an indicator signaling a change of duration of either of the first or second temporal regions of at least one downlink radio frame in the series of downlink radio frames.