Joint time delay and frequency offset estimation scheme for multiple ONUs’ activation in point-to-multipoint coherent PON system

For the next-generation access network standard, the coherent passive optical network (PON) using digital subcarrier multiplexing (DSCM) is a competitive candidate. The architecture employs time and frequency division multiple access (TFDMA) to improve network flexibility and access efficiency. Initial access to PON requires the activation of multiple optical network units (ONUs) to complete upstream synchronization. To meet the stringent latency requirements, the activation process of ONU should not cause significant delay and degradation for the transmission of upstream data. Therefore, we propose what we believe to be a novel ONU activation method for TFDMA coherent PON to jointly estimate time delay and frequency offset. The registration signal used for the ONU activation is designed as a combination of Zadoff-Chu sequence and Gold sequence, and the registration signal can be transmitted together with the data signal without the need for a quiet window. At the receiver, the time delay and frequency offset are estimated by calculating correlation values. Furthermore, successive interference cancellation is introduced to support simultaneous registration of multiple ONUs. The proposed activation scheme is validated in an coherent PON system with 6 × 10 GHz subcarriers. The experimental results show that the scheme can achieve a time delay estimation accuracy of 1 ns and frequency offset estimation accuracy of 1 MHz. Moreover, it does not cause any delay for upstream signal transmission, and the resulting SNR penalty is less than 0.2 dB.