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Intensity modulation and direct detection (IM/DD) together with pulse amplitude modulation (PAM) is a cost-efficient solution for intra-datacenter links. However, sophisticated digital signal processing (DSP) is inevitable to reach the demanded high data rates for these short-reach applications. In this paper, we compare different DSP schemes that enable rates of 200 Gb/s/ and beyond. Those schemes include Tomlinson-Harashima precoding, partial response signaling, Viterbi equalization and post-filtering together with Volterra nonlinear equalization. Extensive experimental measurements are shown in which the DSP approaches are compared at gross rates between 180 Gb/s and 300 Gb/s. The experiments are done in C-band for fiber links of up to 2 km length. As modulation formats, PAM4, PAM6 and PAM8 are considered. Net bit rates well over 200 Gb/s are shown for all formats at a range of 1 km single mode fiber using a transmission system consisting of currently available commercial components. The optimum DSP configuration as well as the best modulation format varies along the investigated data rates. The best solution is shown to change from partial response PAM4 for lower rates to PAM6 and PAM8 at high bit rates.

Theoretical investigations have been carried out to analyze and compare the link power budget and power dissipation of non-return-to-zero (NRZ), pulse amplitude modulation-4 (PAM-4), carrierless amplitude and phase modulation-16 (CAP-16) and 16-quadrature amplitude modulation-orthogonal frequency division multiplexing (16-QAM-OFDM) systems for data center interconnect scenarios. It is shown that for multimode fiber (MMF) links, NRZ modulation schemes with electronic equalization offer the best link power budget margins with the least power dissipation for short transmission distances up to 200 m; while OOFDM is the only scheme which can support a distance of 300 m albeit with power dissipation as high as 4 times that of NRZ. For short single mode fiber (SMF) links, all the modulation schemes offer similar link power budget margins for fiber lengths up to 15 km, but NRZ and PAM-4 are preferable due to their system simplicity and low power consumption. For lengths of up to 30 km, CAP-16 and OOFDM are required although the schemes consume 2 and 4 times as much power respectively compared to that of NRZ. OOFDM alone allows link operation up to 35 km distances.

Fiber-induced intra- and interchannel nonlinearities are experimentally tackled using blind nonlinear equalization (NLE) by unsupervised machine-learning-based clustering (MLC) in ~46-Gb/s single-channel and ~20-Gb/s (middle-channel) multichannel coherent multicarrier signals (orthogonal frequency-division multiplexing (OFDM) based). To that end, we introduce, for the first time, hierarchical and fuzzy-logic C-means (FLC)-based clustering in optical communications. It is shown that among the two proposed MLC algorithms, FLC reveals the highest performance at optimum launched optical powers (LOPs), while at very high LOPs, hierarchical can compensate more effectively nonlinearities only for low-level modulation formats. When employing binary phase-shift keying and quaternary phase-shift keying, FLC outperforms K-means, fast-Newton support vector machines, supervised artificial neural networks, and NLE with deterministic Volterra analysis. In particular, for the middle channel of a QPSK wavelength-division multiplexing coherent optical OFDM system at optimum -5 dBm of LOP and 3200 km of transmission, FLC outperforms Volterra-NLE by 2.5 dB in Q-factor. However, for a 16-QAM single-channel system at 2000 km, the performance benefit of FLC over inverse Volterra-series transfer function reduces to ~0.4 dB at a LOP of 2 dBm (optimum). Even when using novel sophisticated clustering designs in 16 clusters, no more than additional ~0.3-dB Q-factor enhancement is observed. Finally, in contrast to the deterministic Volterra-NLE, MLC algorithms can partially tackle the stochastic parametric noise amplification .

The transmission performance of multichannel adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) signals is investigated numerically, for the first time, in optical-amplification-free and chromatic-dispersion-compensation-free intensity-modulation and direct-detection systems directly incorporating modulated distributed feedback (DFB) lasers (DMLs). It is shown that AMOOFDM not only significantly reduces the nonlinear wavelength-division multiplexing (WDM) impairments induced by the effects of cross-phase modulation and four-wave mixing but also effectively compensates for the DML-induced frequency chirp effect. In comparison with conventional modulated optical orthogonal frequency-division multiplexing (OFDM), which uses an identical signal modulation format across all the subcarriers, AMOOFDM improves the maximum achievable signal transmission capacity of a central WDM channel by a factor of 1.3 and 3.6 for 40- and 80-km standard single-mode fibers, respectively, with the corresponding dynamic input optical power ranges being extended by approximately 5 dB. In addition, AMOOFDM also causes the occurrence of cross-channel complementary modulation format mapping among various WDM channels, leading to considerably improved transmission capacities for all individual WDM channels.

Emerging cloud applications such as virtual reality and connected car fleets demand guaranteed connections, as well as low and stable latency, to edge data centres. Currently, user–cloud communications rely on time-scheduled data frames through tree-topology fibre networks, which are incapable of providing guaranteed connections with low or stable latency and cannot be scaled to a larger number of users. Here we show that a frequency-referenced multiplexing method can provide guaranteed bandwidth and low latency for time-critical applications. We use clock and optical frequency synchronization, enabled by frequency comb and signal processing techniques, to provide each user with dedicated optical bandwidth, creating scalable user–cloud upstream communications. As a proof of concept, we demonstrate a frequency-division multiplexing system servicing up to 64 users with an aggregate bandwidth of 160 GHz, exhibiting a data rate of up to 4.3 Gbps per user (240.0 Gbps aggregated capacity considering a 200 GHz wavelength band) with a high receiver sensitivity of –35 dBm.