A mobile terminal is operated in a cellular communications system that comprises a base station operating in a time division duplex mode. The mobile terminal detects a downlink transmission from the base station and determines therefrom a future moment in time at which an end part of a downlink transmission just prior to a random access window will become detectable at the mobile terminal. An uplink random access transmission is begun at a moment in time that is before the future moment in time. The moment in time that is before the future moment in time can be a fixed amount of time before the future moment in time. If the mobile terminal is not near the edge of the cell, the base station may consequently receive only a partial uplink random access transmission, but this does not detrimentally affect performance.