KEYWORDS: Neural networks, Signal detection, Data centers, Signal processing, Receivers, Optoelectronics, Numerical analysis, Data communications, Computer architecture
The substantial increase in communication throughput driven by the ever-growing machine-to-machine communication within a data center and between data centers is straining the short-reach communication links. To satisfy such demand - while still complying with the strict requirements in terms of energy consumption and latency - several directions are being investigated with a strong focus on equalization techniques for intensity- modulation/direct-detection (IM/DD) transmission. In particular, the key challenge equalizers need to address is the inter-symbol interference introduced by the fiber dispersion when making use of the low-loss transmission window at 1550 nm. Standard digital equalizers such as feed-forward equalizers (FFEs) and decision-feedback equalizers (DFEs) can provide only limited compensation. Therefore more complex approaches either relying on maximum likelihood sequence estimation (MLSE) or using machine-learning tools, such as neural network (NN) based equalizers, are being investigated. Among the different NN architectures, the most promising approaches are based on NNs with memory such as time-delay feedforward NN (TD-FNN), recurrent NN (RNN), and reservoir computing (RC). In this work, we review our recent numerical results on comparing TD-FNN and RC equalizers, and benchmark their performance for 32-GBd on-off keying (OOK) transmission. A special focus will be dedicated to analyzing the memory properties of the reservoir and its impact on the full system performance. Experimental validation of the numerical findings is also provided together with reviewing our recent proposal for a new receiver architecture relying on hybrid optoelectronic processing. By spectrally slicing the received signal, independently detecting the slices and jointly processing them with an NN-based equalizer (wither TD-FNN or RC), significant extension reach is shown both numerically and experimentally.
In this Paper we investigate the optimization of constellations for polarization-division multiplexed transmission
in standard single-mode fiber systems and for space-division multiplexing in a multi-mode fiber system.
KEYWORDS: Forward error correction, Analog electronics, Digital signal processing, Tolerancing, Distortion, Interference (communication), Receivers, Signal processing, Picosecond phenomena, Dispersion
On the following pages we address some actual topics of analog and digital electronic equalisation for 10 and 40 Gbit/s transmission. Analog circuits currently overcome the bandwidth limits for 40Gb/s operation. With first 5 tap feed forward equaliser (FFE) SiGe chips CD and PMD tolerance enhancement can be demonstrated. According to numerical simulations, the combination of the FFE with a one-stage decision feedback equaliser (DFE) should be a good choice to mitigate signal degradation caused by various effects. At 10 Gb/s more complex signal processing based on digitalsignal-processing (DSP) is already implemented in first products as maximum likelihood sequence estimator (MLSE, also referred to as Viterbi equaliser VE) or in a lab prototype of soft error correction (soft FEC). Detailed numerical
studies on the performance of the VE reveal that opposite to the analog FFE+DFE, low electrical bandwidth cannot be handled by the standard MLSE scheme and an extension of the processing algorithm is needed. Though the combination of an MLSE based equaliser and soft-FEC has the potential to lead to further improvement, numerical analysis of turbo equalisers combining distortion mitigation and error correction in an iterative way indicate that with a FEC overhead in the range of 7% only moderate equalisation improvement seems to be possible.
Nonlinear optical fiber loop mirrors (NOLM) are attractive devices for multiplexing optical signals with bitrates of up to 100 Gbit/s and beyond. So far, less attention has been paid to the relation between design of the demultiplexer and the resulting system performance characterized by parameters like input power penalty and wavelength tolerance. Based on our own measurements of a polarization-independent 40 Gbit/s demultiplexer and in conjunction with a new theoretical approach modeling NOLM transmission and worst-case input power penalty of the demultiplexer, a device capable of 100 Gbit/s operation is analyzed. The survey illustrates the potential and trade-offs of the NOLM concept assuming a limited cw power of the switching wave. E.g., the calculations show that a 14.5 dBm booster EDFA together with a loop fiber length between 1 and 2 km allow low penalty drop rates (< 6 dB worst-case penalty) of the demultiplexed channel between 1.3 and 8.8 Gbit/s. Then 100 Gbit/s operation should be possible within a wavelength range of at least 5 nm.
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