In a unidirectional or broadcasting communication system using OFDM transmission from a base station to subscriber units, a device for achieving a bi-directional channel comprising: A. a transmitter in the subscriber units for transmitting signals that are orthogonal to signals transmitted from the base station and are also orthogonal to signals from other subscriber units; B. a receiver in the base station for receiving and processing together signals from a plurality of subscriber units; C. a controller for allocating to each subscriber unit several carriers that are separated from each other (not adjacent to each other).