The invention relates to an optical OFDMA communication network, comprising a plurality of optical network units (ONUs), wherein each one of the ONUs is configured to generate a ranging sequence for upstream synchronization of the ONUs, a receiving unit for receiving signals of the ONUs, the receiving unit being also configured to receive the ranging sequences of the ONUs. The receiving unit comprises a correlator unit in which a plurality of ranging sequences is stored. The receiving unit is configured to perform a ranging detection operation including performing a correlation operation with a plurality of the received ranging sequences using at least one of the stored ranging sequences and to transmit a ranging response to the ONUs depending on the result of the correlation operation.