Fast-Convergence Digital Signal Processing for Coherent PON Using Digital SCM

It is foreseeable that the 100 Gb/s/$\lambda$ and beyond passive optical network (PON) will be required in future optical access networks to meet the explosive growth of data traffic. The coherent optical systems could be a promising solution for the future beyond 100 G PON. Coherent PON using digital subcarrier multiplexing (DSCM) can provide flexible bandwidth allocation to a large number of access subscribers by dividing subcarriers of the DSCM signal into time slots for time-and-frequency division multiple access. When the optical network unit is allocated a new subcarrier, digital signal processing (DSP) should converge fast in the allocated time slot to ensure a low handoff latency for real-time bandwidth allocation. However, the traditional coherent DSP is hard to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific training sequence (TS) structure and propose data-aided DSP to achieve fast convergence for coherent PON. The feasibility of the proposed scheme is experimentally verified in an 8 Gbaud/SC×8 SCs 400 Gb/s-net-rate coherent PON using DSCM with 16 quadrature amplitude modulation. The experimental results show that fast convergence is jointly realized by the proposed TS structure and data-aided DSP using a 416-symbol TS with a 52 ns duration. The receiver sensitivity at the 20% soft-decision forward error correction limit is approximately $\mathbf{-27}$ dBm and an optical power budget of about 35.5 dB is achieved with a booster amplifier.