[EN] We demonstrate a convergent optical fronthaul link for enabling heterogeneous wireless access technologies in 6G future networks deployment based on a directly modulated laser and an external Mach¿Zehnder modulator biased at null point for optical frequency multiplying. Simultaneous transmission of two M-QAM data signals over baseband and millimeter wave (mmW) frequency band, i.e. 42 GHz, is demonstrated over a 10 km fiber link, showing the suitability for visible light communications and mmW radio access, respectively, to the final user. System intermodulation in mmW band is identified under high received optical power levels and overcome by inserting a dispersion compensation module., The research has been supported by the national project MCIN/AEI/ and ERDF A way of making Europe (PID2021-126514OB-I00 OPTIMISE), and the
regional project PROMETEO/2021/015 Disruptive Photonics Technologies for 2030 funded by Generalitat Valenciana, Spain.

[EN] This work experimentally demonstrates the convergence of visible light communications (VLC) and millimeter wave (mmW) wireless access technologies by using a unified optical fronthaul link that provides simultaneous signals to be transmitted over light emitting diode (LED) and seamless antennas-based links, respectively. Frequency multiplication is provided in the fiber link by using a carrier suppressed Mach-Zehnder modulator (CS-MZM) that offers a high-quality signal generation at both baseband (BB) and mmW frequencies. Moreover, this flexible and efficient photonic signal generation approach uses a directly modulated laser (DML) to take advantage of the relative gain given by the combination of the DML chirp and the fiber dispersion in the mmW band. Experimental trials of simultaneous VLC and mmW access have been done over distances up to 2.5 and 4 m, respectively; with maximum throughputs of around 70.3 and 393.1 Mbps for signal bandwidths of 30 and 100 MHz, respectively. Moreover, the effective transmission of different M-QAM OFDM modulation formats validates the approach as a promising enabler for future convergent heterogeneous 6G networks., This work was supported in part by OPTIMISE by MCIN/AEI/ 10.13039/501100011033 under Grant PID2021-126514OB-I00, in part by ERDF A way of making Europe , and in part by regional Project PROMETEO 2021/015 Disruptive Photonics Technologies for 2030 funded by (Generalitat Valenciana). (Corresponding author: M. Botella-Campos.)

[EN] In this paper, we demonstrate the experimental transmission of 16-and 64-quadrature amplitude modulation (QAM) Long Term Evolution -Orthogonal Frequency Division Multiplexing (LTE-OFDM) signals by using mil-limetre wave frequencies at 60 GHz and 25 GHz for downlink and uplink, respectively, over a heterogeneous optical fronthaul infrastructure. A directly modulated laser was employed for both links, which enables the cost-effective full-duplex system proposal. The bidirectional link consists of a 10 km of single mode fiber, a 100 m long free space optics channel and 2 m long wireless radio link, which brings flexibility for future wireless networks. The error vector magnitude (EVM) parameter is measured for a range of the received optical and electrical power as well as the signal-to-noise ratio. A comprehensive estimation of penalty factors in the different network segments is presented. The successful transmission over the whole proposed network with the EVM below the required limit of 9 % for 64-QAM with 20 MHz bandwidth is experimentally demonstrated for the received optical power of-2.7 dBm and-1 dBm for the downlink and uplink, respectively., The research has been supported by the CTU in Prague SGS20/166/OHK3/3T/13, project from Ministry of Industry and Trade in Czech Republic (FV40089) and within COST action CA19111 (NEWFOCUS) . It has also been funded by the regional project from Generalitat Valenciana PROMETEO 2021/015 and Grant RTI2018-101658-B-I00 FOCAL by MCIN/AEI/10.13039/501100011033 and ERDF "A way of making Europe".

[EN] We experimentally demonstrate a photonic full-duplex seamless millimeter wave fronthaul link, based on phase modulation, to address the challenges arising from the increased demands on the mobile fronthaul capacity and network densification toward 5G and beyond. The proposed system relies on phase modulation techniques, implemented at both the central office (CO) and remote radio head (RRH), to achieve optical frequency up-conversion of the downlink (DL) signal and optical modulation of the down-converted uplink (UL) signal. Furthermore, our approach includes the frequency down-conversion of the 40 GHz UL signal through an optically generated local oscillator (LO) signal in the RRH, while the laser employed for UL data transmission is situated at the CO, simplifying the remote site's equipment. An optical waveshaper serves here as a programmable optical filter to provide signals for DL frequency up-conversion, LO generation, and also the optical carrier for UL transmission. In our experimental validation, we have tested our proposed system using 64-quadrature amplitude modulation (64-QAM) for the DL at a frequency of 41 GHz and quadrature phase shift keying (QPSK) for the UL at a frequency of 40 GHz. Notably, our results demonstrate the successful transmission of up to 200 MHz bandwidth for both digital modulation schemes, all while maintaining the error vector magnitude (EVM) well below the specified threshold. Additionally, when employing 5G new radio (NR) orthogonal frequency-division multiplexing (OFDM) signals with the same modulation formats for both links in full-duplex communication, we achieved EVM values as low as 5.2% for the DL and 6.9% for the UL, further highlighting the efficacy and robustness of our proposed solution., Generalitat Valenciana (PROMETEO 2021/015); European Regional Development Fund (PID2021-126514OB-I00); Technology Agency of the Czech Republic (FW07010026).

[EN] In this letter, we demonstrate a novel and cost-effective scheme for upconverting data to radiofrequency (RF) in a millimeter wave (mmW) photonic fronthaul based on optical frequency multiplication for frequency up-conversion. The approach is based on an electrical mixer and a Dual-Drive Mach-Zehnder modulator (DD-MZM) where carrier suppressed double-sideband (CS-DSB) modulation is held for mmW signal generation after photodetection. The viability of the approach is demonstrated by transmitting 5G orthogonal frequency division multiplexing (OFDM) signals with quadrature phase shift keying (QPSK) and 64-quadrature amplitude modulation (QAM) across a 3 m long radio link as well as 10 km of standard single-mode fiber (SSMF) link in the 40 GHz mmW band. We assessed the system performance by evaluating the error-vector-magnitude (EVM), also focusing on the received optical and electrical power (RoP and ReP, respectively), with minimum EVM of 2.8 % for maximum RoP, while the full link leads to 3.4 % in this condition. Moreover, the maximum transmission throughput of the system for full link scenario reached 692 Mb/s and 1800 Mb/s for 5G OFDM and single carrier signals, respectively. The experimental measurements confirm the robustness and simplicity of the proposed approach to be employed for future mobile mmW communication networks deployment without using precoding techniques and optical filtering., This work was supported in part by OPTIMISE under Grant PID2021-126514OBI00 and Grant MCIN/AEI/10.13039/501100011033, and in part by ERDF "A way of making Europe", through Project PROMETEO 2021/015 (Generalitat Valenciana).

[EN] In recent years, there has been extensive research in carrier-free phase and amplitude (CAP) modulation for optical communications, especially in the field of visible light communications (VLC). While the majority of the published works have been focused on characterizing its performance in optical channels, both theoretically and experimentally, only a limited number of publications have presented practical solutions to solve signal synchronization and demodulation, probably because experiments are usually conducted with externally synchronized equipment. The interest in CAP modulation comes from its lower complexity compared to quadrature amplitude modulation (QAM), but this advantage comes at the expense of a worse performance with time jitter. This paper presents a low-complexity, low-jitter, fully-digital CAP receiver with synchronization that improves CAP demodulation performance. Its enhanced behavior with time jitter is achieved by preprocessing the input of the timing error detector (TED) without changing the rest of the stages of standard CAP processing. That is, the receiver preserves the main advantage of CAP: it does not require frequency synchronization as neither the transmitter nor the receiver uses oscillators for passband up and down conversion. The proposed receiver operates with 4 samples per symbol and comprises only three stages: a matched filter, a time synchronizer, and a blind equalizer., This work was supported in part by MCIN/AEI/10.13039/501100011033 and European Union by "ERDF A way of making Europe" under Grant PID2021-126514OB-I00, and in part by the Ministerio de Ciencia, Innovacion y Universidades para la Formacion de Profesorado Universitario under Grant FPU19/04648.

[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."

[EN] This paper presents a theoretical and experimental analysis of the millimeter wave (mmW) signal gain observed in photonically generated signals by using carrier suppressed (CS) external modulation in a centralized network architecture while the data is transmitted over a directly modulated laser (DML) operating under different signal regimes. Measurements of the frequency response under small and large signal regimes are presented for the sake of comparison. Moreover, the mmW signal gain has been measured for different standard single mode fiber (SSMF) lengths in order to identify the conditions and impact of large signal regime operation. The results are also validated by the experimental transmission of a 25 MHz QPSK signal transmitted over 40 GHz since the error-vector-magnitude (EVM) is measured for different input electrical power values as well as several received optical power (RoP) levels. Moreover, the spurious-free dynamic range (SFDR) ratio is measured to compare the third-order intermodulation distortion (IMD3) between small and large signal regime. The quantitative results demonstrate that proper system conditions allow the exploitation of the mmW signal gain towards the future deployment of high energy efficient networks., This work was supported in part by MCIN/AEI/10.13039/501100011033 and ERDF A way of making Europe under Grant PID2021-126514OB-I00 OPTIMISE, and in part by Generalitat Valenciana through Project PROMETEO 2021/015.

[EN] To address the unprecedented technical challenges arising from ultra-dense cellular network deployment for
applications in densely populated urban areas envisioned for 6G, this paper proposes and experimentally
demonstrates a novel, to our knowledge, centralized bidirectional heterogeneous access network with advanced
baseband unit (BBU) pooling and cost-effective remote radio head (RRH) designs free from both lasers and
digital signal processing (DSP). The network supports flexible deployments of fiber, free space optical (FSO),
and millimeter wave (mmWave) segments, thus ensuring ubiquitous network connectivity. More importantly,
it also seamlessly converges various network segments (fiber, FSO, and mmWave) and enables their uplink (UL)
and downlink (DL) signals to concurrently and continuously flow between the BBU and user equipment (UE)
without requiring optical-electrical/electrical-optical conversions and/or DSPs at any intermediate nodes. In
the proposed network, DL mmWave signals are generated and detected using a free-running laser and a passive
envelope detection. For the UL case, conventional electrical local oscillators and mixers are used for mmWave
up-conversion and down-conversion. The performances of the proposed networks, including UL/DL channel
interferences and achievable throughputs, are experimentally evaluated over a fiber-FSO-mmWave setup with
10 km fiber, 1.8 m FSO, and 3 m mmWave links (39 GHz/0.4 Gbit/s for DL, 36.5 GHz/0.2 Gbit/s for UL). The
experimental results show robust bidirectional transmissions with negligible UL/DL interferences and minimal
impacts from Rayleigh and Brillouin backscattering., UK Research and Innovation, Engineering and Physical Sciences Research Council (TITAN, [EP/Y037243/1], HASC,
[EP/Y037197/1/EP/X040569/1]); Welsh Government and the UK Government (North Wales Growth Deal through Ambition North Wales); Ministerio de Ciencia, Innovación y Universidades, MCIN/AEI/10.13039/501100011033, ERDF A way of making Europe, by grant PID2021- 126514OB-I00 OPTIMISE; Science and Technology Commission of Shanghai Municipality (SKLSFO2021-02, SKLSFO2022-04); National Natural Science Foundation of China
(62475144); International Science and Technology Cooperation Program of Sichuan Province (2023YFH0067).

[EN] In this work, we propose and demonstrate the remote delivery of a photonically generated local oscillator (LO) signal for uplink down-conversion in photonically generated millimeter-wave (mmW) signals over duplex centralized radio access network fronthaul. The technique, based on carrier-suppressed external modulation in a single Mach-Zehnder modulator, is employed for generating the mmW signals, which are distributed over hybrid optical links composed of fiber and free-space optics (FSO) prior to the radio wireless link at 42 GHz, and also for the uplink (UL) LO generation enabling the 38 GHz UL signal down-conversion. A 5G new radio (NR) quadrature phase-shift keying (QPSK) and 64-quadrature amplitude modulation (QAM) signal transmission is successfully demonstrated using low-frequency directly modulated lasers for both the downlink and the UL mmW transmission, and thus a low-cost and simple intermediate frequency over fiber and FSO UL deployment are presented. The lowest achieved error vector magnitudes with the 5G NR signals at 100 MHz bandwidth in the proposed mmW UL schemes, including 10 km of optical fiber and a 1.1 m long FSO link, are 5.8% and 6% for QPSK and 64-QAM, respectively., Generalitat Valenciana (PROMETEO 2021/015); Ministerio de Ciencia, Innovacion y Universidades (PID2021-126514OB-I00 OPTIMISE, ERDF "A way of making Europe," MCIN/AEI/10.13039/501100011033); Grantova Agentura Ceske Republiky (GA22-32180S).

[EN] This paper proposes and presents the first experimental demonstration of a high-precision indoor 2D and 3D visible light positioning (VLP) system using an imaging multiple-input multiple-output (MIMO) configuration with supervised artificial neural network. The proposed system utilises four distributed transmitters and receiver with four photodiodes and an imaging optics. The experiments are conducted in a typical indoor environment with transmitter separations of 300 mm and a link distance of 1400 mm. The experimental results show 2D and 3D positioning accuracies of 3.7 and 51 mm, respectively. A simulation model is also developed for the VLP system to validate the experimental results. Further optimisation of the VLP system in the simulation platform leads to improved 2D and 3D positioning accuracies of 2 and 14.7 mm, respectively. The proposed system can be seamlessly integrated with existing lighting infrastructures and is also compatible with the MIMO visible light communication system, indicating the potential for practical implementation in integrated communications and positioning applications., This work has been supported by COST action CA19111 (NEWFOCUS). It has also been funded by Grant PID2021-126514OB-I00 optimise by MCIN/AEI/10.13039/501100011033 and ERDF 'A way of making Europe'. SR and JT acknowledge support by the UK Engineering and Physical Sciences Research Council under Grant EP/Y037243/1 (TITAN Extension).

[EN] This letter proposes a full-duplex analog fronthaul seamless transmission system employing wavelength reuse for simultaneous downlink (DL) and uplink (UL) signal data streaming at 25 GHz frequency band. The proposed architecture integrates centralized optical sources at a central office (CO), an optical distribution network (ODN) comprising 10 km of standard single-mode fiber (SSMF) and a 1.9 m long free-space optical (FSO) link, and a remote radio head (RRH) utilizing 1.9 m long wireless radio links. Intensity modulation is used for DL, while phase modulation combined with optical filtering enables efficient carrier reuse and data modulation for UL. The system's performance is assessed using orthogonal frequency-division multiplexing (OFDM) signals with 64-quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) modulation for DL and UL. Results demonstrate low error vector magnitude (EVM) across various input and received optical power levels, with minimal penalties from fiber dispersion and bidirectional reflections. The study also evaluates the impact of nonlinearities, phase noise, and intermodulation distortions, showcasing the system's robustness. This scalable, efficient solution advances full-duplex optical fronthaul networks and supports millimeter-wave (mmW) applications beyond 25 GHz., This work was supported in part by the Technology Agency of Czech Republic, TREND, FW07010026 in part by OPTIMISE through Ministerio de Ciencia, Innovacion y Universidades/Agencia Estatal de Investigacion (MCIN/AEI)/10.13039/501100011033 and ERDF "A way of making Europe" under Grant PID2021-126514OB-I00; and in part by the COST Action under Grant CA22104 (BEiNG-WISE).

[EN] We propose and experimentally demonstrate the use of a directly modulated laser (DML) in an optical phase modulated (PM)-assisted millimeter-wave (mmW) signal generation system. A comprehensive analytical model is provided to accurately describe the system frequency response, which is also validated by the experimental measurements. For the purpose of the system characterization, the transmission performance is analyzed using a QPSK signal at different operating mmW frequencies over large received optical power (RoP) and electrical data power ranges in a 10 km standard single mode fiber (SSMF) link. Both the impact of the noise and the DML nonlinearities lead to identify the optimal conditions for the system operation where PM-based mmW signal photonic generation demonstrates total transparency with no further signal degradation. Final comparisonwith IM technique for mmW signal generation leads to assess the robustness of PM solution which makes it promising for the deployment of future communication networks., This work was supported in part by OPTIMISE through MCIN/AEI/
10.13039/501100011033 under Grant PID2021-126514OB-I00, in part by
ERDF A way of making Europe, and in part by Generalitat Valenciana under
Project PROMETEO 2021/015.

[EN] Phosphor-coated white LEDs have become the predominant lighting technology thanks to their energy efficiency. In recent years, interest in visible light communications (VLC) technology has grown to take advantage of the massive deployment of these lighting LEDs for data transmission. In this context, these LEDs have two major limitations: limited modulation bandwidth and nonlinear behavior. Extending signal modulation beyond these limitations is crucial for achieving the transmission rates necessary to make this technology competitive. To support the further development of these systems, this paper introduces an integrated model that merges dynamic nonlinear and linear system approaches to simulate accurately data transmission with LEDs. It combines a dynamic nonlinear model, which captures complex behaviors under high-speed modulation, with a linear model for high luminous flux LEDs. Experimental validation confirms the precision of the model, with less than 5% error in predicting power spectrum and throughput using OFDM signaling, highlighting its utility for designing VLC systems that utilize LEDs for both illumination and data transmission., This work was supported in part by MCIN/AEI/10.13039/501100011033 andby "ERDF A way of making Europe", through the "European Union", underGrant PID2021-126514OB-I00,andinpartby Ministeriode Ciencia, Innovaciony Universidades para la Formacion de Profesorado Universitario, under Grant FPU19/04648.

[EN] LED lighting has become the standard solution for illumination purposes thanks to its energy efficiency. Nowadays, there is growing interest in the use of LEDs for data transmission to develop future-generation communication systems. The low cost and widespread deployment of phosphor-based white LEDs make them the best candidate for visible light communications (VLC), although they have a limited modulation bandwidth. This paper presents a simulation model of a VLC link based on phosphor-based white LEDs and a method to characterize the VLC setup used to perform the data transmission experiments. Specifically, the simulation model incorporates the frequency response of the LED, the noise levels coming from the lighting source and the acquisition electronics, and the attenuation due to both the propagation channel and the angular misalignment between the lighting source and the photoreceiver. In order to validate the suitability of the model for VLC, carrierless amplitude phase (CAP) and orthogonal frequency division multiplexing (OFDM) modulation signals were employed for data transmission, and simulations with the proposed model and measurements over the equivalent scenario show high agreement., This research was funded by MCIN/AEI/10.13039/501100011033; by the European Union's "ERDF A way of making Europe" grant numbers PID2021-126514OB-I00 and RTI2018-101296-B-I00; and by the Ministerio de Ciencia, Innovacion y Universidades para la Formacion de Profesorado Universitario, grant number FPU19/04648.

[EN] This paper presents a comprehensive experimental analysis of a cost-effective and spectrally efficient approach for sub-terahertz (sub-THz) signal generation and transmission, consisting of a directly modulated laser (DML) combined with carrier-suppressed Mach-Zehnder modulator (CS-MZM) in a radio-over-fiber (RoF) architecture. The use of optical and electrical filtering by means of a fiber Bragg grating (FBG) and an electrical bandpass filter (EBPF), respectively, permits to mitigate undesirable optical and electrical signal components and reduce fiber dispersion effects. For the proof-of-concept demonstration, a standard-compliant 5G new radio 64-QAM OFDM signal at different intermediate frequencies was successfully transmitted over a fiber-wireless system in the W band (75-110 GHz). The system characterization under different hardware configurations was comprehensively analyzed to give insights for the system design under different operation conditions. The results demonstrate improved sideband rejection, minimized intermodulation distortion, and high spectral purity, making this technique a promising solution for future 6G fronthaul deployments., This work was supported by OPTIMISE through MCIN/AEI/10.13039/501100011033 under Grant PID2021-126514OB-I00 and under Grant PID2024-161624OB-I00.