Outage Probability Due to Crosstalk from Multiple Interfering Cores in PAM4 Inter-Datacenter Connections

Abstract

In this work, we propose to use four-level pulse amplitude modulation (PAM4) and multi-core fibers (MCFs) to support very high capacity datacenter interconnect (DCI) links. The limitations imposed by inter-core crosstalk (ICXT) on the performance of 112 Gb/s up to 80 km-long optically amplified PAM4 inter-DCI links with intensity-modulation and direct-detection and full chromatic dispersion compensation in the optical domain are analyzed through numerical simulation for high and low skew-symbol rate product (SSRP). With only one interfering core, we show that those PAM4 inter-DCI links achieve an outage probability (OP) of 10−4 with a maximum ICXT level of −13.9 dB for high SSRP and require an ICXT level reduction of about 8.1 dB to achieve the same OP for low SSRP. Due to using full dispersion compensation, for an OP of 10−4, the maximum acceptable ICXT level shows only a 1.4 dB variation with the MCF length increase from 10 km to 80 km. When considering the ICXT induced by several interfering cores, the maximum ICXT level per interfering core for an OP of 10−4 decreases around 3 dB when doubling the number of interfering cores. This conclusion holds for high and low SSRP regimes. For two interfering cores, we show that a single interfering core with low SSRP is enough to induce a severe reduction of the maximum acceptable ICXT level.

Deep learning for BER prediction in optical connections impaired by inter-core crosstalk

“…The impact of ICXT on the performance of optically amplified PAM4 links for inter-datacenter connections has been analyzed in [2] by resorting to the dual polarization-discrete changes model (DP-DCM) that describes with reasonable accuracy the ICXT effect in homogeneous WC-MCFs [10]. In this work, as in [2], only two cores, the interfering core m and the interfered core n are considered.…”

“…The impact of ICXT on the performance of optically amplified PAM4 links for inter-datacenter connections has been analyzed in [2] by resorting to the dual polarization-discrete changes model (DP-DCM) that describes with reasonable accuracy the ICXT effect in homogeneous WC-MCFs [10]. In this work, as in [2], only two cores, the interfering core m and the interfered core n are considered. Two transmitters, one for each core, generate the signal transmitted along core m, which is the interfering PAM4 signal, and the signal transmitted along core n corresponding to the interfered PAM4 signal.…”

“…The PAM4 signal, with the extinction ratio given by the inverse of the ITU-T definition [2], travels along core n of the MCF, where linear propagation is assumed, since non-linear effects are usually insignificant in inter-datacenters distances [1], [11]. Fiber loss is also assumed the same in both cores.…”

“…However, it is expected that such fibers will not fulfill the future capacity demand growth, as transmission in SC-SMFs is approaching its limit of 200 Gbit/s per lane with IM-DD [1]. Multicore fibers (MCFs) have been proposed in order to overcome this issue and to reduce fiber per area density in datacenters [2]. In particular, homogeneous weakly-coupled multicore fibers (WC-MCFs) ensure simplicity, low cost and power consumption as required by datacenter links.…”

“…This crosstalk in MCFs arises from power coupling between cores and is particularly relevant to the transmission of signals with the same wavelength in neighboring cores [5], [6]. The ICXT has been experimentally shown to have a stochastic time evolution, which can result in high levels of ICXT in short time periods and cause large bit error rate (BER) fluctuations, that can lead to system outage periods [2], [4], [7]. Hence, ICXT monitoring in such datacenter connections is crucial to predict and maintain acceptable BERs.…”