Abstract
A joint compensation scheme based on pilot symbols aiding adaptive Kalman filter (AKF) for phase noise and polarization cross-talk is proposed and investigated via numerical simulation and experimental demonstration. In the proposed scheme, the optimizing parameter Q is adaptively adjusted according to signal parameters or channel conditions. This improvement avoids the drawback of conventional extended Kalman filter (EKF), and its performance is strongly dependent on Q. Another improvement is that the convergence speed of AKF is improved. Pilot quadrature phase shift keyin (QPSK) symbols are inserted into 16 quadrature amplitude modulation (QAM) signals periodically. Besides accelerating convergence speed, the employment of pilot symbols also improves the tracking capability of AKF. The format ratio between pilot symbols and payload symbols is suggested under different system environments, for instance, optical signal-to-noise rate, polarization rotation frequency drift rate, and laser linewidth. With the proposed scheme, it has excellent tolerance to initial parameter Q and dramatically improves the performances in convergence speed, polarization rotation frequency drift rate tracking, and carrier phase recovery. Both the numerical simulation and experimental demonstration achieve convergence improvement for around 40 times than the original AKF. Additionally, the improvements in tracking ability are also demonstrated.
Highlights
The recent evolution in information technology, such as 5G, Internet of Things, and big data [1,2,3], is promoting the growing and dynamic demands on bandwidth and capacity of optical data transmission
The power ratio between the quadrature phase shift keyin (QPSK) and 16QAM is set as 6.5 dB, which can maximize the power efficiency [16]
The sampled data is processed by an offline digital signal processor (DSP), including timing recovery and frequency offset estimation based on maximum fast Fourier transform
Summary
The recent evolution in information technology, such as 5G, Internet of Things, and big data [1,2,3], is promoting the growing and dynamic demands on bandwidth and capacity of optical data transmission. With the limited bandwidth provided by electrical devices, exploiting polarization multiplexing and higher-order modulation formats, for instance, 16QAM and 64QAM can increase the spectrum efficiency of transmission systems [4,5,6]. With higher-order modulation formats the system performance has less tolerance against carrier phase estimation error, and it is difficult to achieve fast convergence during compensating polarization misalignment. The conventional algorithm for carrier phase noise is the blind phase search (BPS) algorithm, which has the disadvantage of having high complexity [7,8].
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