Next-Generation Optical Fronthaul Systems Using Multicore Fiber Media

In this paper, the theoretical unification of linear and nonlinear inter-core crosstalk (IC-XT) in step-index single-mode multicore fiber (MCF) media is reported and validated experimentally. In order to estimate the IC-XT when operating in linear and nonlinear regimes, the coupled-mode theory (CMT) and the coupled-power theory (CPT) have been unified in both power regimes. The theoretical analysis of the CMT indicates that in coupled MCFs with reduced core-to-core distance (core pitch) the nonlinear self-coupling and cross-coupling effects should be considered when operating with high optical powers. However, considering a core pitch value higher than three times the core radius only the self-coupling effect should be taken into account for estimating the nonlinear IC-XT. Considering these results, the CPT is also extended to nonlinear regime including the dominant nonlinear coupling effect. Using both CMT and CPT, the statistical model of nonlinear IC-XT is completed with the closed-form expressions for estimating the cumulative distribution function, the probability density function and the crosstalk mean and variance as a function of the power level launched into a single-core of the MCF. The crosstalk model presented is additionally extended when multiple cores are simultaneously excited. Finally, the theoretical model is experimentally validated in a homogeneous four-core fiber considering different bending radius configurations.

Semi-analytical approach for calculation of intercore crosstalk in multicore fibers

The discrete changes model (DCM) of inter-core crosstalk (ICXT) in weakly-coupled homogeneous multi-core fibers (MCFs) is generalized to a dual polarization (DP) scheme. This model provides theoretical expressions for the two polarization fields of the ICXT at the MCF output. Therefore, it may be of particular interest in the design of direct-detection MCF systems, where the photodetected ICXT results mainly from the beating between the ICXT field at the MCF output and the carrier of the interfered core. The DP-DCM is validated by comparison of the mean ICXT power and ICXT field amplitude estimates with the ones obtained with a rigorous, yet much more computationally demanding, model based on the coupled local mode theory (CLMT). Good agreement between the mean ICXT power estimates obtained with the DP-DCM and CLMT is observed when the inter-core coupling coefficient variation along the MCF is small. Good agreement is also observed when comparing the probability density functions of the ICXT field amplitude.

With the advancement of mobile communications, technologies based on high-element-count antenna systems—such as massive Multiple Input Multiple Output (massive MIMO)—are playing an increasingly important role in enhancing network capacity. However, they introduce new challenges in the measurement and evaluation of electromagnetic field (EMF) exposure. This study presents a detailed, laboratory-based methodology for assessing EMF exposure in cellular systems using Single Input Single Output (SISO) and MIMO technologies. To address the limitations of traditional exposure assessment techniques—particularly under the conditions introduced by 5G and active antenna systems—a shielded test environment with directional antennas was developed and applied across lowband and midband frequency ranges (700–2100 MHz). Downlink electromagnetic power density was measured under standardized modulation, coding, and bandwidth settings for both SISO and MIMO configurations. The results show that MIMO technology does not lead to a significant increase in EMF exposure compared to SISO, with average differences remaining below 1 dB. Moreover, in lower-frequency bands, massive MIMO systems can ensure the required user capacity at significantly lower transmission power, resulting in more than 15 dB reductions in EMF exposure. These findings confirm the potential of massive MIMO to enhance network performance while reducing the level of electromagnetic exposure.

Wideband QAM-over-SMF/turbulent FSO downlinks in a PON architecture for ubiquitous connectivity

The paper shows how both spatial and multi-user (MU) diversity can be jointly exploited in a cellular system. By combining space-time block-coding with transmit antenna selection (AS), system performance in terms of channel capacity and for a low to moderate number of users can be substantially improved with respect to the single-input single-output (SISO) case. However, a SISO configuration with AS outperforms both configurations for the whole range of active users. In terms of throughput, however, part or all of the advantage exhibited by the SISO and SISO-AS configurations is lost in favor of those featuring STBC schemes. Closed-form expressions are derived analytically for the SNR densities along with an approximate lower bound for system throughput.

Control moment gyroscopes are widely used as actuators in some applications like satellites and spacecrafts attitude control, which involves a large operational range problem, thus posing additional challenges to the control design. A modeling process of a control moment gyroscope, model 750 from Educational Control Products, is presented and its multiple-input multiple-output (MIMO) nonlinear model is obtained, as well as one of its single-input single-output (SISO) configuration model. A feedback linearization, which is an approach to nonlinear control design, is proposed to control the single output of CMG SISO configuration. Both numerical simulation and practical experiment results are obtained to evaluate the designed nonlinear controller.

Crosstalk in multi-core fibers with α-index single-mode cores is investigated theoretically. It is clarified that the index parameter α of the core has a great effect on the crosstalk of multi-core fibers.

This article describes a novel experimental study considering a multiband fibre–wireless system for constructing the transport network for fifth-generation (5G) networks. This study describes the development and testing of a 5G new radio (NR) multi-input multi-output (MIMO) hybrid fibre–wireless (FiWi) system for enhanced mobile broadband (eMBB) using digital pre-distortion (DPD). Analog radio over fibre (A-RoF) technology was used to create the optical fronthaul (OFH) that includes a 3 GHz supercell in a long-range scenario as well as a femtocell scenario using the 20 GHz band. As a proof of concept, a Mach Zehnder modulator with two independent radio frequency waveforms modifies a 1310 nm optical carrier using a distributed feedback laser across 10 km of conventional standard single-mode fibre. It may be inferred that a hybrid FiWi-based MIMO-enabled 5G NR system based on OFH could be a strong competitor for future mobile haul applications. Moreover, a convolutional neural network (CNN)-based DPD is used to improve the performance of the link. The error vector magnitude (EVM) performance for 5G NR bands is predicted to fulfil the Third Generation Partnership Project’s (3GPP) Release 17 standards.

Radio over Fiber (RoF) fronthaul system linearization has become much more important due to the distortions produced by the optical link between the Baseband Unit (BBU) and the Remote Radio Head (RRH) in a Cloud Radio Access Network (C-RAN) architecture. Its main disadvantage is that in a real scenario the output signal may be far a few kilometers. Along the literature, digital predistortion (DPD) on RoF mobile fronthaul architectures considers an ideal feedback link, but in a real scenario the captured output signals at the BBU side are affected by the RoF feedback effects. In a first place, we evaluate the effects of this feedback loop on the downlink RoF predistorter. Then, we propose and experimentally demonstrate the use of a postdistorter on the feedback RoF in order to compensate its effects. This feedback RoF link can be accomplished in the same optical link by means of wavelength division-multiplexing (WDM). Experiments are carried out with a 20MHz-LTE downlink signal (16QAM) at 2.65 GHz within the Band 7 of the LTE standard. The DPD performance has been tested in an intensity modulation/direct detection (IM/DD) RoF mobile fronthaul system. Experimental results reveal that with a properly feedback RoF postdistortion it is possible to achieve Adjacent Channel Power Ratio (ACPR) and Error Vector Magnitude (EVM) values in the downlink RoF system close to the results with an ideal feedback link.

We developed an analog optical system-on-package (SoP) transmitter for a 60-GHz-band radio-over-fiber (RoF) link. The SoP transmitter consisted of an electroabsorption modulator, radio frequency amplifiers, and a bandpass filter. The 60-GHz RoF wireless link was prepared to measure the performance of the SoP transmitter. The transmission characteristics of 64-quadrature amplitude modulation (64-QAM) data of the 60-GHz RoF wireless link, including the SoP transmitter, were investigated by measuring the error vector magnitude (EVM) and signal-to-noise ratio (SNR) with a baseband frequency. The EVM of the 60-GHz RoF wireless link was between 2.25% and 2.80%, and the SNR was between 27.36 and 29.31 dB from 140 and 770 MHz, at input baseband power of -9 dBm. The noise figure had the minimum of 8.44 dB at 500 MHz. We successfully transmitted digital community antenna television (CATV) system signals through the 60-GHz RoF wireless link, including the SoP transmitter. Digital CATV signals of 86 channels could be transmitted through the 60-GHz RoF wireless link, and the total throughput was found to be 2.61 Gb/s.

The complete analytical solutions of the mode-coupling dynamics for seven-core multicore fibers (MCFs) with identical cores is proposed. All the coupling coefficients C(mn) of adjacent cores and nonadjacent cores as a function of the structural parameters are investigated. It is shown that the coupling coefficients can decrease by adjusting the structural parameters. In addition, the trench-assisted structure could be used for suppressing cross talk in MCF, and the effective area A(eff) can be enlarged without degrading the crosstalk properties. Simulations suggest that low cross talk and/or large A(eff) could be achieved by adjusting the trench parameters. Large mode area could be obtained by utilizing small trench (small trench width c, small refractive index difference Δ(trench)) in the trench-assisted MCF (TA-MCF), whereas low cross talk could be achieved by utilizing larger trench (large trench width c, large Δ(trench)) in the TA-MCF.

Analysis of crosstalk in multi-core fiber is using the trench-assisted technique. To reduce the crosstalk between the cores in the fiber, coupled-mode theory and coupled power theory are adopted for crosstalk estimation and considering different design parameters such as core pitch, bending radius, and wavelength to optimize the crosstalk performance. The homogeneous fiber, which works under single-mode operation, has been considered. The study of performance by varying the trench width is also analyzed. Crosstalk variation in outer cores and center cores of the fiber is studied. The numerical simulation results of crosstalk behavior over bending radius and wavelength are obtained.

We investigated and compared the performances of three different types of optical twin-single sideband (twin-SSB) modulation methods. Twin-SSB can transmit two different signals on lower side-band (LSB) and upper side-band (USB), and has been studied as a technique to improve spectral efficiency. Twin-SSB arranges the spectra side-by-side without any spectral spacing. Dispersion tolerance of twin-SSB is as high as that of conventional SSB schemes. The twin-SSB modulation can be performed by using a dual-polarization quadrature phase shift keying (DP-QPSK) modulator, a QPSK modulator, or a single Mach-Zehnder modulator (MZM). The twin-SSB with the three modulation methods need to be modulated at a limited linear domain of the optical field output function of the modulator. Therefore, if we apply a large electrical modulation signal to the modulators to decrease the optical loss, the nonlinear characteristics of the modulators degrade the performance of the twin-SSB signals. We investigated the modulation performance by changing the amplitude of the electrical modulation signal for the modulators, and investigated the optimal conditions of the twin-SSB modulation methods. We also performed numerical simulations of 50-km standard single mode fiber (SSMF) transmission of the twin-SSB signals with the optimal modulation conditions, and evaluated the error vector magnitude (EVM) performances of the transmitted signals. The results revealed that the EVM performances of the twin-SSB signals using a DP-QPSK modulator and a QPSK modulator were superior to that of a single MZM.

We demonstrate a half-duplex system for 2.6-GHz-band radio-over-fiber (RoF) signals. Twelve 156-Mbits/s differential phase-shift keying channels operating at the same frequency band as the WiMax standard are successfully distributed after being transmitted through 26.5 km of standard single-mode fiber. At the access point, we employ a 1.5 GHz limited bandwidth reflective semiconductor optical amplifier to remodulate the downlink signal with a 156 Mbits/s2.6 GHz uplink RoF signal. Both the downlink and uplink signals reach error-free operation after transmission, with a negligible power penalty with respect to the back-to-back operation.