Non-orthogonal basis expansion based laser phase noise suppression in CO-OFDM systems

A laser phase noise suppression approach, based on non-orthogonal basis expansion (NOBE), is proposed for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. In the NOBE approach, the time-varying laser phase noise is (sub-) optimally reconstructed using a bank of non-orthogonal expansion bases. Theoretical derivation of the non-orthogonal basis expansion matrices using the minimum mean-square error (MMSE) criteria is presented analytically. Compared with the conventional inverse discrete Fourier transform (IDFT)-based orthogonal basis expansion (OBE) method, the proposed NOBE approach, with the same expansion order L, offers higher accuracy in carrier phase estimation. Numerical analysis shows that the system's tolerance to laser phase noise is significantly enhanced by the new method thanks to the lower modeling error. For example, the laser linewidth-symbol period product in an ∼ 100-Gbps 16QAM CO-OFDM system at an optical signal-to-noise-ratio (OSNR) of 22 dB achieves 3.87×10−2 with the NOBE (L=3), which is approximately twice that with the OBE. The impact of residual carrier frequency offset (CFO) on the NOBE is also semi-analytically investigated, which shows that the NOBE has robust performance for the residual CFO up to 2 MHz under a subcarrier spacing of 31.25 MHz. By taking advantage of the elaborately-designed comb-type pilot subcarriers pattern, the computational complexity of NOBE is also lower than that of the OBE. Since the NOBE with a sub-optimal basis expansion matrix does not require a priori knowledge of the product of laser linewidth and symbol period, it is well-suited for practical implementations.