This Review summarizes the simultaneous transmission of several independent spatial channels of light along optical fibres to expand the data-carrying capacity of optical communications. Recent results achieved in both multicore and multimode optical fibres are documented.

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Space-division multiplexing in optical fibres

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

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Survey on Free Space Optical Communication: A Communication Theory Perspective

Digital coherent receivers have caused a revolution in the design of optical transmission systems, due to the subsystems and algorithms embedded within such a receiver. After giving a high-level overview of the subsystems, the optical front end, the analog-to-digital converter (ADC) and the digital signal processing (DSP) algorithms, which relax the tolerances on these subsystems are discussed. Attention is then turned to the compensation of transmission impairments, both static and dynamic. The discussion of dynamic-channel equalization, which forms a significant part of the paper, includes a theoretical analysis of the dual-polarization constant modulus algorithm, where the control surfaces several different equalizer algorithms are derived, including the constant modulus, decision-directed, trained, and the radially directed equalizer for both polarization division multiplexed quadriphase shift keyed (PDM-QPSK) and 16 level quadrature amplitude modulation (PDM-16-QAM). Synchronization algorithms employed to recover the timing and carrier phase information are then examined, after which the data may be recovered. The paper concludes with a discussion of the challenges for future coherent optical transmission systems.

Digital Coherent Optical Receivers: Algorithms and Subsystems

Orthogonal Frequency Division Multiplexing (OFDM) has recently been proposed as a modulation technique for optical networks, because of its good spectral efficiency, flexibility, and tolerance to impairments. We consider the planning problem of an OFDM optical network, where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers and choosing an appropriate modulation level, taking into account the transmission distance. We introduce the Routing, Modulation Level and Spectrum Allocation (RMLSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, prove that is also NP-complete and present various algorithms to solve it. We start by presenting an optimal ILP RMLSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RMLSA into its two substituent subproblems, namely 1) routing and modulation level and 2) spectrum allocation (RML+SA), and solves them sequentially. We also propose a heuristic algorithm that serves connections one-by-one and use it to solve the planning problem by sequentially serving all the connections in the traffic matrix. In the sequential algorithm, we investigate two policies for defining the order in which connections are considered. We also use a simulated annealing meta-heuristic to obtain even better orderings. We examine the performance of the proposed algorithms through simulation experiments and evaluate the spectrum utilization benefits that can be obtained by utilizing OFDM elastic bandwidth allocation, when compared to a traditional WDM network.

https://www.ceid.upatras.gr/webpages/faculty/manos/kchristodou/journals/ofdm_jlt_revision_2.pdf

Elastic Bandwidth Allocation in Flexible OFDM-Based Optical Networks

A received optical signal (S H , S v ) is coherently demodulated and converted into orthogonal x-polarisation samples (r x (n)), and y-polarisation samples (r y (n)). These samples are converted into signal x-samples z x (n) and signal y-samples z y (n) by an FIR butterfly filter (8). Correction values are calculated in an error calculating circuit (12) of a control unit (11) and added to filter transfer functions derived by a standard algorithm to determine corrected filter coefficients. Degenerate convergences calculating the transfer functions are avoided.

Method and arrangement for blind demultiplexing a polarisation diversity multiplex signal

Few-mode fiber transmission systems are typically impaired by mode-dependent loss (MDL). In an MDL-impaired link, maximum-likelihood (ML) detection yields a significant advantage in system performance compared to linear equalizers, such as zero-forcing and minimum-mean square error equalizers. However, the computational effort of the ML detection increases exponentially with the number of modes and the cardinality of the constellation. We present two methods that allow for near-ML performance without being afflicted with the enormous computational complexity of ML detection: improved reduced-search ML detection and sphere decoding. Both algorithms are tested regarding their performance and computational complexity in simulations of three and six spatial modes with QPSK and 16QAM constellations.

Near-ML detection for MDL-impaired few-mode fiber transmission.

We show that OFDM requires the lowest equalizer complexity for crosstalk compensation in a mode-division-multiplexing receiver. For a 2000-km transmission distance and less than 10% OFDM-specific overhead, the modal dispersion must be below 12 ps/km.

Equalizer complexity of mode division multiplexed coherent receivers

An adaptive digital pre-distortion method to enable single carrier 400G DP-128QAM transmission over 328 km fiber with 30% FEC is presented. Experimental comparison in reach and spectral efficiency is made with 400G DP-64QAM.

400G single carrier transmission in 50 GHz grid enabled by adaptive digital pre-distortion

We report on the nonlinear transmission limits of various super-channel configurations in a flex-grid network upgrade scenario. In particular, we consider flexible data-rates ranging from 180Gb/s to 1.2Tb/s, employing either single-carrier, dual-carrier, or penta-carrier polarization multiplexed m-state quadrature amplitude modulation (PM-8QAM/PM-16QAM) –termed as super-channels, and establish transmission performance margins for each configuration, both with and without super-channel fiber nonlinearity compensation. Our results show that the benefit of intra super-channel nonlinearity mitigation (nonlinear compensation addressing full super-channel bandwidth) reduces with increasing sub-carrier count within the super-channel, and that single-carrier super-channel achieves the maximum improvement from nonlinearity mitigation (up to ~4.5dB, in Q-factor), better than dual-carrier (up to ~3.5dB) and penta-carrier (up to ~2dB) configurations. Moreover, the maximum reach improvement, compared to linear compensation only, is found to be ~170% (180Gb/s, PM-8QAM), ~150% (240Gb/s, PM-16QAM), ~100% (360Gb/s, PM-8QAM), ~100% (480Gb/s, PM-16QAM), and ~65% (1.2Tb/s, PM-16QAM).

https://pure.tue.nl/ws/files/3993735/373155687694338.pdf

Bernhard Spinnler

Papers

Method and arrangement for blind demultiplexing a polarisation diversity multiplex signal

Near-ML detection for MDL-impaired few-mode fiber transmission.

Equalizer complexity of mode division multiplexed coherent receivers

400G single carrier transmission in 50 GHz grid enabled by adaptive digital pre-distortion

Intra super-channel fiber nonlinearity compensation in flex-grid optical networks