The invention discloses a method (1100) for a cellular wireless access TDD system (100) with a base stations (120, 220) for respective cells (110, 210) in the system, where each cell can accommodate at least a first user terminal (130, 230). The user terminals can receive traffic from their base stations during a down link period, and can transmit traffic to their base stations (120, 220) during an up link period (UL). There is a first guard period (TDu) at the transition from down link to up link, and a second guard period (TUD) at the transition from up link to down link. The method comprises measuring the interference level in a second plurality (110, 210) of cells during part of the time between two consecutive down link periods, and varying the duration of said first and/or second guard period in said second plurality of cells (110, 210) according to the interference level.