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

We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Capacity Limits of Optical Fiber Networks

The idea of programmable networks has recently re-gained considerable momentum due to the emergence of the Software-Defined Networking (SDN) paradigm. SDN, often referred to as a ''radical new idea in networking'', promises to dramatically simplify network management and enable innovation through network programmability. This paper surveys the state-of-the-art in programmable networks with an emphasis on SDN. We provide a historic perspective of programmable networks from early ideas to recent developments. Then we present the SDN architecture and the OpenFlow standard in particular, discuss current alternatives for implementation and testing of SDN-based protocols and services, examine current and future SDN applications, and explore promising research directions based on the SDN paradigm.

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A Survey of Software-Defined Networking: Past, Present, and Future of Programmable Networks

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

Differential-phase-shift keying (DPSK) has recently been used to reach record distances in long-haul lightwave communication systems. This paper will review theoretical, as well as implementation, aspects of DPSK, and discuss experimental results.

Optical phase-shift-keyed transmission

We discuss coherent optical orthogonal frequency division multiplexing (CO-OFDM) as a suitable modulation technique for long-haul transmission systems. Several design and implementation aspects of a CO-OFDM system are reviewed, but we especially focus on phase noise compensation. As conventional CO-OFDM transmission systems are very sensitive to laser phase noise a novel method to compensate for phase noise is introduced. With the help of this phase noise compensation method we show continuously detectable OFDM transmission at 25.8 Gb/s data rate (20 Gb/s after coding) over 4160-km SSMF without dispersion compensation.

Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF

A novel soliton transmission scheme to suppress the accumulation of the Gordon-Haus jitter using periodic dispersion compensation and inline optical filters has been proposed. With 90% dispersion compensation rate and optimum optical filters, a Q/sup 2/ of 19 dB and a large power window of 2 dB were confirmed for a 20 Gbit/s, 9000 km transmission system by using a 1000 km-long recirculating loop.

Reduction of Gordon-Haus timing jitter by periodic dispersion compensation in soliton transmission

We discuss optical multi-band orthogonal frequency division multiplexing (OFDM) and show that by using multiple parallel OFDM bands, the required bandwidth of the digital-to-analogue/ analogue-to-digital converters and the required cyclic prefix can significantly be reduced. With the help of four OFDM bands and polarization division multiplexing (PDM) we report continuously detectable transmission of 10 times121.9-Gb/s (112.6-Gb/s without OFDM overhead) at 50-GHz channel spacing over 1,000-km standard single mode fiber (SSMF) without any inline dispersion compensation. In this experiment 8 QAM subcarrier modulation is used which confines the spectrum of the 121.9 Gb/s PDM-OFDM signal within a 22.8 GHz optical bandwidth. Moreover, we propose a digital signal processing method to reduce the matching requirements for the wideband transmitter IQ mixer structures required for PDM-OFDM.

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121.9-Gb/s PDM-OFDM Transmission With 2-b/s/Hz Spectral Efficiency Over 1000 km of SSMF

Focus Issue on Orthogonal-Frequency-Division Multiplexed Communications Systems and Networks We discuss the realization and performance of polarization-division-multiplexed orthogonal frequency division multiplexing (PDM-OFDM) for long-haul transmission systems. Polarization demultiplexing of the PDM signal at the receiver is realized by employing a multiple-input multiple-output (MIMO) detector. Using a recirculating loop a long-haul transmission experiment is reported of 52.5 Gbits/s PDM-OFDM (40 Gbits/s after coding) over 4160 km of standard single-mode fiber (SSMF). In this transmission experiment, 16 wavelength-division-multiplexed (WDM) channels are transmitted at 50 GHz channel spacing, and we show that MIMO processing in the receiver enables both polarization demultiplexing and a large PMD tolerance.

Long-haul transmission of16×52.5 Gbits/s polarization-division- multiplexed OFDM enabled by MIMO processing (Invited)

We introduce a novel method to compensate for local oscillator offset and phase-noise in coherent-OFDM systems and report continuously detectable transmission at 20-Gb/s data rate (25.8 Gb/s before coding) over 4,160-km SSMF without dispersion compensation.

Itsuro Morita

Papers

Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF

Reduction of Gordon-Haus timing jitter by periodic dispersion compensation in soliton transmission

121.9-Gb/s PDM-OFDM Transmission With 2-b/s/Hz Spectral Efficiency Over 1000 km of SSMF

Long-haul transmission of16×52.5 Gbits/s polarization-division- multiplexed OFDM enabled by MIMO processing (Invited)

20-Gb/s OFDM Transmission over 4,160-km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation