A Low-Complexity ONU Activation Scheme for TFDMA Coherent PON Systems

The emergence and development of new communication services have prompted optical access systems to evolve toward higher speed rates. Coherent passive optical network (PON) based on digital subcarrier multiplexing (DSCM) has become one of the key technologies driving the evolution of optical access networks, owing to its flexible spectrum resource allocation and support for time-frequency division multiple access (TFDMA). In PON systems, a new joining optical network unit (ONU) must achieve upstream synchronization through an activation process. The activation scheme should meet the requirements of future communication services for high capacity and low latency, while imposing minimal impact on the data signal. For the TFDMA-based coherent PON architecture, we propose an innovative low-complexity and low-latency ONU activation scheme without a quiet window, which supports both ranging and frequency offset calibration. To validate the proposed scheme, experiments are conducted in a TFDMA coherent PON system with $6\times 10$ GHz subcarriers. The experimental results show that the scheme can achieve a time delay estimation accuracy of 2.5 ns, while the impact of the registration signal on upstream data transmission performance is less than 0.2 dB.