Abstract
This paper provides an overview of latest progress on the novel advanced digital signal processing (DSP) techniques for long-haul mode division multiplexing (MDM) systems with high capacity. Space-division multiplexing (SDM) techniques have been developed for a period to increase the capacity of optical communication system by at least one order of magnitude through MDM techniques using few-mode fibers (FMFs) or multi-core multiplexing (MCM) using multi-core fibers (MCFs). The signals in MDM links are mainly impaired by the linear and nonlinear effects in FMFs, making DSP techniques become necessary to undo these impairments. In this paper, we not only review the advanced multiple-input multiple-output (MIMO) DSP techniques for compensating linear impairments in FMFs, but also enclose the state of the art of novel DSP techniques to deal with nonlinear effects. Firstly, we introduce the MIMO schemes for equalizing modal crosstalk and modal dispersion. Then, we focus on the fast tracking of time-varying (TV) channels in FMF links through frequency-domain (FD) recursive least square (RLS) algorithm. Besides, we also cover the mainstream DSP solutions for mode-dependent loss (MDL) and several possible methods to compensate nonlinearity in FMF. Moreover, artificial intelligence (AI) technologies are also discussed for its high nonlinearity tolerance and may bring a revolution in MDM systems on the process of channel equalization, link monitoring, etc. In the end, a brief conclusion and perspective will be provided.
Highlights
Space-division multiplexing (SDM) has gained great attention over the last decades as a potential technology to accommodate the demand for future long-haul and power-efficient optical fiber communication systems with extremely high capacity [1,2]
multi-core fibers (MCFs) are easy to be expanded from current single mode fiber (SMF) schemes [4] since each core is used as an independent spatial channel where crosstalk from other cores is well suppressed [5]
While recursive least square (RLS) algorithm owns faster convergence speed compared with least mean squares (LMS) algorithm, less training sequences are required so that higher spectrum efficiency would be achieved, especially in the long-haul mode division multiplexing (MDM) systems
Summary
SDM has gained great attention over the last decades as a potential technology to accommodate the demand for future long-haul and power-efficient optical fiber communication systems with extremely high capacity [1,2]. Long-haul SDM systems are impaired by MDL arising from the imperfections or mode-dependent difference of inline components such as fibers, amplifiers and multiplexers, who are inducing modal loss/gain disparities [20,21,22,23]. It is a fundamental limitation for a desirable data-rate or transmission distance. Detection and interference cancellation (IC) as well as transmitter-side space-time coding (STC) techniques Another fundamental limitation for SDM systems is Kerr-effect-caused nonlinear signal distortions. A brief conclusion and some perspectives will be provided in the last section
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