FPGA implementation of a 10 GS/s variable-length FFT for OFDM-based optical communication systems

[EN] The transmission rate in current passive optical networks can be increased by employing Orthogonal Frequency Division Multiplexing (OFDM) modulation. The computational kernel of this modulation is the fast Fourier transform (FFT) operator, which has to achieve a very high throughput in order to be used in optical networks. This paper presents the implementation in an FPGA device of a variable-length FFT that can be configured in run-time to compute different FFT lengths between 16 and 1024 points. The FFT reaches a throughput of 10 GS/s in a Virtex-7 485T-3 FPGA device and was used to implement a 20 Gb/s optical OFDM receiver. (C) 2018 Elsevier B.V. All rights reserved.This work was supported by the Spanish Ministerio de Economia y Competitividad under project TEC2015-70858-C2-2-R with FEDER funds.Bruno, JS.; Almenar Terre, V.; Valls Coquillat, J. (2019). FPGA implementation of a 10 GS/s variable-length FFT for OFDM-based optical communication systems. Microprocessors and Microsystems. 64:195-204. https://doi.org/10.1016/j.micpro.2018.12.002S1952046

Hybrid POF/VLC Links Based on a Single LED for Indoor Communications

[EN] A hybrid fiber/wireless link based on a single visible LED and free of opto-electronic intermediate conversion stages has been demonstrated for indoor communications. This paper shows the main guidelines for proper coupling in fiber/air/detector interfaces. Experimental demonstration has validated the design results with very good agreement between geometrical optics simulation and received optical power measurements. Different signal bandwidths and modulation formats, i.e., QPSK, 16-QAM, and 64-QAM, have been transmitted over 1.5 m polymer optical fiber (POF) and 1.5 m free-space optics (FSO). Throughputs up to 294 Mb/s using a 64-QAM signal have been demonstrated using a commercial LED, which paves the way for massive deployment in industrial applicationsThis work was supported by the Spanish Ministerio de Ciencia, Innovacion y Universidades RTI2018-101658-B-I00 FOCAL project.Apolo-Gonzaga, JA.; Ortega Tamarit, B.; Almenar Terre, V. (2021). Hybrid POF/VLC Links Based on a Single LED for Indoor Communications. Photonics. 8(7):1-12. https://doi.org/10.3390/photonics80702541128

A Computational Efficient Nyquist Shaping Approach for Short-Reach Optical Communications

[EN] Recently, Half-Cycle Nyquist Subcarrier Modulation (HC-SCM) was proposed to achieve high spectral efficiency in intensity-modulator direct-detection optical links. This paper shows that the HC-SCM scheme has a high computational load and proposes the rational Oversampled Subcarrier Modulation (OVS-SCM) as a computational efficient alternative that, furthermore, improves the spectral efficiency. The presented experimental results show that our 256-QAM proposal allows to transmit below the hard-decision forward error correction, with a throughput of 17.8 Gb/s in a 2.5 GHz bandwidth, and a spectral efficiency of 7.2 b/s/Hz, through 20 km of single-mode optical fiber.This work was supported by the Spanish Ministerio de Economia y Competitividad and FEDER under the Grant TEC2015-70858-C2-2-R and Grant RTI2018-101658-B-I00.Pérez Pascual, MA.; Bruno, JS.; Almenar Terre, V.; Valls Coquillat, J. (2020). A Computational Efficient Nyquist Shaping Approach for Short-Reach Optical Communications. Journal of Lightwave Technology (Online). 38(7):1651-1658. https://doi.org/10.1109/JLT.2019.2961506S1651165838

Hardware Architecture of a QAM Receiver for Short-Range Optical Communications

[EN] Short-reach optical fiber communications systems aim to achieve high throughput, in the order of tens of Gbps. The implementation of these high-speed systems requires parallel processing, which makes low-complexity designs of their subsystems a key to the successful large-scale deployment of this technology. Half-Cycle Nyquist Subcarrier Modulation (HC-SCM) was originally suggested for these systems with the goal of using as much bandwidth as possible and, therefore, achieving high communication rates. Recently, Oversampled Subcarrier Modulation (OVS-SCM) was proposed as an alternative more computational efficient than HC-SCM and also with a better spectral efficiency. This paper proposes a hardware-efficient architecture for an OVS-SCM receiver, which takes into account the inherent parallel processing of these systems. This receiver takes 16 samples in parallel from a 5 GSa/s analog-to-digital converter with a 3.2 GHz 3 dB bandwidth. Design solutions for the frame detection block, the mixer, the resampler, the fractional interpolator, the matched filter and the timing estimator are presented. Our results show that, compared to the HC-SCM receiver, this proposal reduces the computational load of the downconverter stages by 90%. FPGA implementation results are given to demonstrate that our proposal can be implemented in state-of-the-art devices.This work was supported in part by MCIN/AEI/10.13039/501100011033 under Grants RTI2018-101658-B100 and PID2021-126514OB-I00, and in part by the European Union through "ERDF Away of making Europe."Valls Coquillat, J.; Torres Carot, V.; Pérez Pascual, MA.; Almenar Terre, V. (2023). Hardware Architecture of a QAM Receiver for Short-Range Optical Communications. Journal of Lightwave Technology. 41(2):451-461. https://doi.org/10.1109/JLT.2022.321735745146141

Usability of a 5G fronthaul based on a DML and external modulation for M-QAM transmission over photonically generated 40 GHz

[EN] In this paper, we numerically and experimentally present the bandwidth constraints of a cost-effective 5G mobile fronthaul based on a directly-modulated laser for data modulation and a Mach-Zehnder modulator-based optical double sideband with carrier suppression scheme for optical millimeter wave (mmW) signal generation. The effect of chirp, fiber dispersion and a combination of both on different bandwidth M-Quadrature Amplitude Modulation (M-QAM) signals, i.e. M = 4, 16 and 64, at 40 GHz has also been investigated. Simulation results are firrst carried out to evaluate the impact of higher chirp of the directly-modulated laser on the link performance as a function of modulation format and signal bandwidth. We then experimentally demonstrate the same scheme transmitting M-QAM signals with bandwidths ranging from 50 to 1000 MHz over a 10 km long single mode fiber. Both experimental and simulation results show that larger signal bandwidths lead to higher optical power penalties due to the combined effect with the error vector magnitudes (EVMs), however still satisfying the required limits of 3GPP standard for allQAMsignals. Experimental measurements also showthe feasibility of including free space optics links in the optical distribution network with no further signi cant penalties. Finally, a multiband signal (three-band) transmission is demonstrated leading to an increase of the total bitrate with the measured EVMs are well below the EVM requirement.This work was supported in part by the Generalitat Valenciana under Grant PROMETEO 2017/103, in part by the Ministerio de Ciencia, Innovacion y Universidades under Grant FOCAL RTI2018-101658-B-I00, in part by the Ministerstvo Prumyslu a Obchodu under Grant FV40089, and in part by the European Cooperation in Science and Technology under Grant CA16220.Vallejo-Castro, L.; Ortega Tamarit, B.; Nguyen, D.; Bohata, J.; Almenar Terre, V.; Zvanovec, S. (2020). Usability of a 5G fronthaul based on a DML and external modulation for M-QAM transmission over photonically generated 40 GHz. IEEE Access. 8:223730-223742. https://doi.org/10.1109/ACCESS.2020.3042756S223730223742

Linear Response Modeling of High Luminous Flux Phosphor-Coated White LEDs for VLC

[EN] The widespread deployment of LEDs for illumination purposes has open the door to the use of these devices for visible light communications (VLC). Most lighting fixtures are mounted with phosphor-based white LEDs, and a driver connected to the LED is also required for VLC. This paper shows that the parasitic effects introduced by this setup change the frequency response of the intrinsic LED. A linear model to characterize the whole setup is proposed, as well as a methodology to extract its parameters. This methodology allows the designer to characterize the frequency response of LEDs without the additional difficulty of knowing the specific parasitic components introduced by the setup. The proposed model offers an accurate estimation of the slope of the LED frequency response in order to broaden the frequency range in which the model is useful to characterize and simulate VLC links. This was corroborated with the characterization of three commercial white LEDs whose measured and modeled frequency responses matched perfectly.Salvador-Llàcer, P.; Valls Coquillat, J.; Corral, JL.; Almenar Terre, V.; Canet Subiela, MJ. (2022). Linear Response Modeling of High Luminous Flux Phosphor-Coated White LEDs for VLC. Journal of Lightwave Technology. 40(12):3761-3767. https://doi.org/10.1109/JLT.2022.315090737613767401

Impact of Thermal-Induced Turbulent Distribution Along FSO Link on Transmission of Photonically Generated mmW Signals in the Frequency Range 26¿40 GHz

[EN] Microwave photonics is a promising solution to transmit millimeter wave (mmW) signals for the 5th generation (5G) mobile communications as part of a centralized radio access network (C-RAN). In this paper, we experimentally evaluate the impact of turbulent free space optics links on photonically generated mmW signals in the frequency range of 26¿40 GHz . We analyze the remote generation of mmW signals over hybrid links based on free-space optics (FSO) and standard single mode optical fiber (SSMF) with ¿39.97dBm received electrical power and phase noise level at 100kHz as low as ¿95.92dBcHz at 26GHz . Different thermal distributions along the FSO link have been implemented and Gamma-Gamma model has been employed to estimate the thermally induced turbulence. The results show high electrical power decrease and fluctuation of the generated mmW signal according to the particular level of the turbulence in terms of refractive index structure parameter and thermal distribution along the FSO link. 8Gb/s 16-quadrature amplitude modulation (QAM) data transmission at 42GHz has been demonstrated over the hybrid link with minimal error vector magnitude (EVM) value of 5% whereas turbulent FSO link introduced up to 5dB power penalty.This work was supported in part by the Research Excellence Award Programme GVA under Grant PROMETEO 2017/103 and in part by the Spanish Ministerio de Ciencia, Innovacion y Universidades, under RTI2018-101658-B-I00 FOCAL Project and MEYS Project LTC18008 within COST CA16220.Vallejo-Castro, L.; Komanec, M.; Ortega Tamarit, B.; Bohata, J.; Nguyen, D.; Zvanovec, S.; Almenar Terre, V. (2020). Impact of Thermal-Induced Turbulent Distribution Along FSO Link on Transmission of Photonically Generated mmW Signals in the Frequency Range 26¿40 GHz. IEEE Photonics Journal. 12(1):1-9. https://doi.org/10.1109/JPHOT.2019.2959227S1912

On the 40 GHz Remote Versus Local Photonic Generation for DML-Based C-RAN Optical Fronthaul

[EN] Local and remote photonic millimeter wave (mmW) signal generation schemes are theoretically and experimentally evaluated in order to compare both approaches for practical deployment in a cloud radio access network (C-RAN) fronthaul network. The paper presents a full comprehensive formulation of the frequency response of a system based on a directly modulated laser transmitting data over 40 GHz signal which is generated by external carrier suppressed modulation and optical frequency multiplication. Theoretical and experimental characterization of the system response at baseband and mmW band for local and remote generation setups show very good agreement. The remote configuration leads to a higher electrical output power (i.e., 15 dB higher in 25 km fiber links) than the local generation setup in the mmW band due to the combined effect of chirp and fiber dispersion, although intermodulation distortion is higher in the former case. Transmission experiments using quadrature phase-shift keying (QPSK) signals with 250 MHz bandwidth centered at 0.5 GHz over 10 and 25 km fiber links also confirm the superior performance of the remote setup, whereas the local setup leads to similar results to optical back-to-back (OB2B) measurements, which is also validated with data signals centered at different frequencies within the laser bandwidth frequency range. Finally, experimental results show the quality of the recovered signals in terms of error vector magnitude (EVM) as a function of the received electrical power and demonstrate that no further penalties are introduced by photonic mmW signal generation with respect to electrical back-to-back (EB2B) levels.This work was supported in part by Generalitat Valenciana through PROMETEO2017/103, in part by Ministerio de Ciencia, Innovacion y Universidades through FOCAL RTI2018-101658-B-I00, in part by MEYES under Grant LTC18008, in part by the Ministry of Industry and Trade in Czech Republic under Grant FV40089, and in part by European Cooperation in Science and Technology under Grants CA16220 and CA19111.Vallejo-Castro, L.; Mora Almerich, J.; Nguyen, D.; Bohata, J.; Almenar Terre, V.; Zvanovec, S.; Ortega Tamarit, B. (2021). On the 40 GHz Remote Versus Local Photonic Generation for DML-Based C-RAN Optical Fronthaul. Journal of Lightwave Technology. 39(21):6712-6723. https://doi.org/10.1109/JLT.2021.3102818S67126723392

Full-duplex transmission of multi-Gb/s subcarrier multiplexing and 5G NR signals in 39 GHz band over fiber and space

[EN] We propose a stable full-duplex transmission of millimeter-wave signals over a hybrid single-mode fiber (SMF) and free-space optics (FSO) link for the fifth-generation (5G) radio access networks to accelerate the Industry 4.0 transformation. For the downlink (DL), we transmit 39 GHz subcarrier multiplexing (SCM) signals using variable quadrature amplitude modulation (QAM) allocations for multi-user services. As a proof of operation, we experimentally demonstrate the transmission of 3 Gb/s SCM signals (1 Gb/s per user) over a hybrid system consisting of a 10 km SMF and 1.2 m FSO link. For the uplink (UL), satisfactory performance for the transmission of 2.4 Gb/s 5G new radio (NR) signal at 37 GHz over the hybrid system is experimentally confirmed for the first time, to the best of our knowledge. The measured error vector magnitudes for both DL and UL signals using 4/16/64-QAM formats are well below the third generation partnership project (3GPP) requirements. We also further evaluate by simulation the full-duplex transmission over the system in terms of received optical and RF powers and bit error rate performance. A wireless radio distance of approximately 200 m, which is sufficient for 5G small-cell networks, is estimated for both DL and UL direction under the heavy rain condition, based on the available data from Spain. Furthermore, simulation for the DL direction is conducted to verify the superior performance of the system using variable QAM allocation over uniform QAM allocation. Using a variable modulation allocation, up to five users (2 Gb/s per user) can be transmitted over a hybrid 10 km SMF and 150 m FSO link.Ceske Vysoke Uceni Technicke v Praze (SGS20/166/OHK3/3T/13); European Cooperation in Science and Technology (CA19111 NEWFOCUS).Nguyen, D.; Vallejo-Castro, L.; Almenar Terre, V.; Ortega Tamarit, B.; Dat, PT.; Le, ST.; Bohata, J.... (2022). Full-duplex transmission of multi-Gb/s subcarrier multiplexing and 5G NR signals in 39 GHz band over fiber and space. Applied Optics. 61(5):1183-1193. https://doi.org/10.1364/AO.4475291183119361

Photonic multiple millimeter wave signal generation and distribution overreconfigurable hybrid SSMF/FSO links

[EN] Microwave photonics provides attractive solutions for millimeter wave (mmW) signal generation. In this paper, we demonstrate photonically generated multiple mmW signals transmission over a wavelength division multiplexed (WDM) hybrid optical network based on optical fiber and free-space optics (FSO) links. The experimental results demonstrate the generation and reconfigurable signal distribution from a central office to base stations in the frequency range 14 40 GHz with phase noise levels below 87 dBc/Hz. Moreover, 10 Gb/s data transmission has been demonstrated over photonically generated 40 GHz mmW signal. We show that FSO technology provides a possible solution for mmW fronthaul in 5th generation networks to extend the optical access network providing increased wireless accessibility and maintaining transmission capacity.This work has been funded by the Research Excellence Award Programme GVA PROMETEO 2017/103 Future Microwave Photonics Technologies and Applications and MEYS INTER-COST project LTC18008 (within COST action CA 16220).Vallejo-Castro, L.; Ortega Tamarit, B.; Bohata, J.; Zvanovec, S.; Almenar Terre, V. (2020). Photonic multiple millimeter wave signal generation and distribution overreconfigurable hybrid SSMF/FSO links. Optical Fiber Technology. 54:1-7. https://doi.org/10.1016/j.yofte.2019.102085S1754Waterhouse, R., & Novack, D. (2015). Realizing 5G: Microwave Photonics for 5G Mobile Wireless Systems. IEEE Microwave Magazine, 16(8), 84-92. doi:10.1109/mmm.2015.2441593Hirata, A., Takahashi, H., Yamaguchi, R., Kosugi, T., Murata, K., Nagatsuma, T., … Kado, Y. (2008). 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Chang, Dual pump brillouin laser for RoF millimeterwave carrier generation with tunable resolution, TENCON 2015 – 2015 IEEE Region 10 Conference, pp. 1–6, Nov. 2015.Zhang, L., Zhu, M., Ye, C., Fan, S.-H., Liu, C., Hu, X., … Chang, G.-K. (2013). Generation and transmission of multiband and multi-gigabit 60-GHz MMW signals in an RoF system with frequency quintupling technique. Optics Express, 21(8), 9899. doi:10.1364/oe.21.009899Zhang, J., Wang, J., Xu, Y., Xu, M., Lu, F., Cheng, L., … Chang, G. (2016). Fiber–wireless integrated mobile backhaul network based on a hybrid millimeter-wave and free-space-optics architecture with an adaptive diversity combining technique. Optics Letters, 41(9), 1909. doi:10.1364/ol.41.001909Zhang, R., Lu, F., Xu, M., Liu, S., Peng, P.-C., Shen, S., … Chang, G.-K. (2018). An Ultra-Reliable MMW/FSO A-RoF System Based on Coordinated Mapping and Combining Technique for 5G and Beyond Mobile Fronthaul. 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