Andrés Macho

On the Suitability of Multicore Fiber for LTE–Advanced MIMO Optical Fronthaul Systems

This paper proposes and evaluates analytically and experimentally the suitability of spatial multiplexing in multicore fiber (MCF) for multiple-input multiple-output (MIMO) LTE-Advanced (LTE-A) optical fronthaul systems including carrier aggregation. The experimental test-bed comprises a 150-m optical fronthaul of four-core homogeneous MCF, which can be configured with different bending radius for analysis. Performance of linear crosstalk in MCF media is evaluated by simulation and validated by the experimental work in the laboratory with the radio-over-fiber transmission of full-standard LTE-A wireless signals considering both SISO and MIMO configurations. The optical fronthaul analysis evaluates both same-propagation and counter-propagation spatial multiplexing, i.e., all cores propagating in the same direction and two versus two cores propagating in opposite directions, corresponding to a dual LTE-A roof-mounted system. The performance of the MCF optical fronthaul system is evaluated using a single 20-MHz LTE-A carrier and also carrier-aggregated signals with 16QAM and 64QAM subcarrier modulation. The experimental results indicate that intercore crosstalk (IC-XT) increases up to 9.6 dB when raising the bending radius from 35 to 67 cm, being the bending radius a key parameter in MCF fronthaul systems. The demonstration performed at 1550.12 nm reported an increase of IC-XT of 4.6 dB with the bending radius, which deteriorates the quality of the SISO LTE-A signal up to 8.1 dB error vector magnitude (EVM). This IC-XT impairment can be mitigated using 2×2 MIMO processing, which reduces the EVM impact to less than 1 dB when increasing the MCF bending radius from 35 to 67 cm. The experimental results indicate that 3GPP MIMO LTE-A algorithms already in-place in the wireless LTE-A standard can be used to compensate the IC-XT in the 150-m multicore optical fronthaul system with spatial multiplexing.

Optical fronthaul of LTE-advanced MIMO by spatial multiplexing in multicore fiber

Optical wireless fronthaul of dual Mx2×2 MIMO LTE-A systems by radio-over-fiber spatial multiplexing is demonstrated over 125m 4-core fiber using 16QAM and 64QAM subcarrier mapping. 3GPP LTE MIMO algorithms in-place successfully mitigate core-to-core impairments.

Experimental evaluation of nonlinear crosstalk in multi-core fiber

In this paper we evaluate experimentally and model theoretically the nonlinear crosstalk random process in multi-core fiber. The experimental results indicate that mode coupling in multi-core fibers is reduced in presence of fiber Kerr nonlinearities. An analytical study of the inter-core crosstalk probability density function in nonlinear regime is performed, where the theoretical distribution, derived from the nonlinear coupled-mode equation, is experimentally validated in homogeneous four-core fiber. The herein presented analysis includes the evaluation of the inter-core crosstalk probability density function, mean and variance evolution considering the optical power launched into the fiber.

Unified Model of Linear and Nonlinear Crosstalk in Multi-Core Fiber

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.

Next-Generation Optical Fronthaul Systems Using Multicore Fiber Media

This paper proposes and investigates the use of multicore fiber (MCF) media performing space-division multiplexed transmission for next-generation optical fronthaul systems. We report the experimental demonstration of combined radio-over-fiber (RoF) transmission of full-standard LTE-Advanced (LTE-A) and WiMAX signals providing fronthaul connectivity in 150 m of 4-core fiber (4CF), transmitting simultaneously fully independent wireless services. Operating in linear and nonlinear optical power regimes, the experimental evaluation verifies that the error vector magnitude (EVM) is not degraded when intercore and intracore Kerr nonlinearities are excited in MCF with high input power levels. As a result, nonlinear regime is proposed as a key factor to reduce the temporal EVM fluctuation induced by the random nature of the intercore crosstalk in MCF. In addition, MCF fronthaul applied to converged fiber-wireless polarization multiplexed passive optical networks is demonstrated to transmit LTE-A and WiMAX signals over two orthogonal optical polarizations. The polarization-multiplexed signal is transmitted in RoF over 25.2 km of standard single-mode fiber and then demultiplexed and injected in different cores of the 4CF to provide fronthaul connectivity. Finally, the extension of multicore optical fronthaul capacity is proposed using MIMO LTE-A signals. The tolerance of the MIMO LTE-A RoF transmissions to in-band crosstalk is reported and compared to single-input single-output (SISO) configuration. The experimental results indicate that MIMO configuration is more tolerant than SISO to in-band crosstalk considering both internal and external interferences. MIMO and SISO configurations are compared when transmitted in RoF over a 4CF operating in linear and nonlinear regimes and core interleaving nonlinear stimulation is proposed to reduce the temporal and spectral EVM fluctuation when the same wireless standard is propagated in each core.

Birefringence effects in multi-core fiber: coupled local-mode theory

In this paper, we evaluate experimentally and model theoretically the intra- and inter-core crosstalk between the polarized core modes in single-mode multi-core fiber media including temporal and longitudinal birefringent effects. Specifically, extensive experimental results on a four-core fiber indicate that the temporal fluctuation of fiber birefringence modifies the intra- and inter-core crosstalk behavior in both linear and nonlinear optical power regimes. To gain theoretical insight into the experimental results, we introduce an accurate multi-core fiber model based on local modes and perturbation theory, which is derived from the Maxwell equations including both longitudinal and temporal birefringent effects. Numerical calculations based on the developed theory are found to be in good agreement with the experimental data.

Deep optical access on multi-core and multi-mode fiber for integrated wireless applications

Deep integrated optical access networks target to provide great capillarity and multiple ONTs for cost- and energy-efficient pervasive connectivity seamless supporting integrated wireless. Several key optical technologies are herein reported supporting integrated deep optical access: Bundled radio-over-fiber transmission is proposed and demonstrated for the provision of quintuple-play services achieving 125 km SSMF optical reach. Bend-insensitive fiber in-building distribution is also proposed and demonstrated supporting joint legacy coaxial transmission. Multimode POF is also proposed and demonstrated suitable for joint in-building distribution of MATV and SMATV broadcasting signals. Optical comb technology us is also demonstrated suitable for mm-wave radio generation of multiband OFDM wireless signals. Finally, multicore fiber transmission is also proposed and demonstrated suitable for the transmission of LTE and WIMAX in wireless fronthaul applications in a minimized inter-core crosstalk penalty configuration.

Multicore fiber beamforming network for broadband satellite communications

Multi-core fiber (MCF) has been one of the main innovations in fiber optics in the last decade. Reported work on MCF has been focused on increasing the transmission capacity of optical communication links by exploiting space-division multiplexing. Additionally, MCF presents a strong potential in optical beamforming networks. The use of MCF can increase the compactness of the broadband antenna array controller. This is of utmost importance in platforms where size and weight are critical parameters such as communications satellites and airplanes. Here, an optical beamforming architecture that exploits the space-division capacity of MCF to implement compact optical beamforming networks is proposed, being a new application field for MCF. The experimental demonstration of this system using a 4-core MCF that controls a four-element antenna array is reported. An analysis of the impact of MCF on the performance of antenna arrays is presented. The analysis indicates that the main limitation comes from the relatively high insertion loss in the MCF fan-in and fan-out devices, which leads to angle dependent losses which can be mitigated by using fixed optical attenuators or a photonic lantern to reduce MCF insertion loss. The crosstalk requirements are also experimentally evaluated for the proposed MCF-based architecture. The potential signal impairment in the beamforming network is analytically evaluated, being of special importance when MCF with a large number of cores is considered. Finally, the optimization of the proposed MCF-based beamforming network is addressed targeting the scalability to large arrays.

Towards multidimensional multiplexing in multicore fiber optical data links

This paper describes our current research in optical transmission technology targeting to overcome the capacity limitations in high-bitrate optical data links. In particular, the enabling steps taken towards multidimensional multiplexing (M2) transmission schemes for transmission in multicore fiber are described. The development of an integrated device capable of performing mode conversion for modal division multiplexing (MDM) is reported in this work. This device enables the generation of a mode-division multiplexed signal to be transmitted through the different cores of a multi-core optical fiber (MCF) employing spatial division multiplexing (SDM). The transmission performance is limited by the crosstalk from each core in the MCF. A theoretical and experimental analysis of the inter-core crosstalk expected when several cores are excited simultaneously is herein presented.

Impact of inter-core crosstalk in radio-over-fiber transmission on multi-core optical media

Optical transmission in multi-core optical media has the potential of great capacity and scalability for current and future optical networks. Optical fronthaul networks are expected to employ relatively high optical intensity levels when a large number of cores are connected to a large number of antennas. In this paper, the crosstalk characteristics of multi-core fiber operating in non-linear regime are identified, indicating advantageous performance in optical fronthaul radio-overfiber transmission. The nonlinear coupled-mode and coupled-power theories are revisited to demonstrate theoretically that the underlying Kerr effect mismatches the phase constant of the core modes reducing the mean and variance of the crosstalk when nonlinear regime is employed. This theoretical analysis is validated experimentally in this work using a homogeneous 4-core optical fiber in radio-over-fiber transmission for LTE fronthaul applications. In addition, the impact of the linear and nonlinear inter-core crosstalk in the error vector magnitude (EVM) is evaluated with the optical transmission of fully-standard LTE-Advanced signals using MIMO and SISO configurations operating in both linear and nonlinear power regimes.