A Step-Size Controlled Method for Fast Convergent Adaptive FD-LMS Algorithm in Few-Mode Fiber Communication Systems

Space-division multiplexing using few-mode fibers (FMF) has emerged as a promising technology to overcome the next capacity crunch. The key challenges of FMF systems are inter-modal crosstalk due to random mode coupling and large differential mode group delay (DMGD). Adaptive frequency domain least mean square (FD-LMS) algorithm has been proposed as the most hardware efficient multi-input multi-output equalization method for compensating large DMGD. Except for hardware complexity, the convergence speed of the adaptive FD-LMS algorithm is another important consideration. In this paper, we propose a noise power spectral density (PSD) directed adaptive FD-LMS algorithm, which adopts variable step size to render the posterior error of each frequency bin convergent to the background noise in an additive white Gaussian noise (AWGN) channel. In a 3000 km six-mode transmission system with 35 ps/km DMGD, compared with signal PSD dependent FD-LMS method and conventional FD-LMS method, our new algorithm could improve the convergence speed by 36.1% and 48.3%, but their hardware complexity will only increase 10.7% and 17.2%, respectively.