Embodiments of the present disclosure relate to devices, methods, apparatuses and computer readable storage media of ONU activation for OLT Equalizer training. The method includes determining, at OLT, configuration information indicating a bandwidth allocation dedicated for an ONU and a first target preamble sequence associated with the bandwidth allocation, the first target preamble sequence to be used by the ONU for a transmission from the ONU to the OLT on a first wavelength; transmitting the configuration information to the ONU; and receiving the transmission from the ONU from the ONU on the first wavelength, the transmission is performed by the ONU based on the first target preamble sequence.