Radio-over-fiber Link Research Articles

Multitone Upconverted Phase-Modulated Signal RoF Link for 6G & Next-Generation Networks

The Next-Generation Networks offer a green environment with demanded data rates with ultra-low latency. Future efforts will be oriented in more degrees of freedom and system complexity with incorporating global optimization in terms of non-linearities, cost-effective, and reduced environmental impacts. The multitone upconverted radio over fibre (RoF) link with an optical modulator as phase modulator as desired alternatives with respect to optimization features and quantum photonic integration. In this paper, a framework has been developed to characterise the upconverted phase modulated signal model based on next-generation networks, in which an analytical model is investigated for single-tone upconverted, two-tone upconverted, and three-tone upconverted. Numerical simulations have been carried out to verify the novel outcomes using OptSIM software. The developed analytical models have a major contribution to the implementation of next-generation networks such as 5G, 6G, WiMAX, Quantum Communication, etc. The fundamentals of harmonics and the intermodulation product terms of third and fifth order via optical link are also evaluated for multitone Phase modulated RoF link with upconversion method. As per the finding outcomes, the proposed link is adaptable with frequency upconversion and long fibre distances.

All-optical, filter-free, image-reject, and fundamental/sub-harmonic downconverted RoF link

An all-optical, image-reject, and fundamental/sub-harmonic downconverted radio-over-fiber (RoF) link using a polarization-division multiplexing Mach–Zehnder modulator (PDM-MZM) and fiber-dispersion effect is proposed. At the remote antenna unit, the receiving radio frequency (RF) signal and the local oscillator (LO) signal are used to drive two sub-MZMs of the PDM-MZM. After a length of optical fiber transmission, at the central office, by using the fiber-dispersion effect and controlling the polarization controller to introduce an appropriate phase shift between the RF and LO sidebands, image rejection is achieved and power fading is avoided simultaneously. Because the scheme is a simple and all-optical system without electrical frequency-dependent devices or filters, it has a good image-reject capability over a large working frequency range. Besides, the downconverter in sub-harmonic mode can decrease frequency requirement of the LO signal. Experimental results indicate that in the fundamental and sub-harmonic downconverters, for a single-tone signal, IRRs are over 47 dB. Image rejection for the 100-Mbaud 16-QAM signal and demodulated desired IF signal with a good EVM of less than 7% are realized. The proposed structure can be applied in RoF systems and microwave and millimeter-wave receivers.

Analysis of hybrid integrated-alternate mark inversion (I-AMI) modulation and symmetrical-symmetrical-post (SSP) dispersion compensation technique in single-tone radio over fiber (RoF) system

Abstract This paper presents the high capacity single-tone Radio over Fiber (RoF) system utilizing advanced modulation technique in conjunction with optimized dispersion compensation scheme to support the high data rate transmission upto 120 Gbps. The proposed system incorporates the Integrated-Alternate Mark Inversion (I-AMI) modulation and Symmetrical-Symmetrical-Post (SSP) compensation technique to enhance the spectral efficiency as well as to reduce the chromatic dispersion effects in long-haul RoF link. The proposed system i.e., I-AMI with SSP has been examined at 60, 80, 100 and 120 Gbps bit rate for various transmission distance. The simulation results obtained in terms of Q-factor, Bit Error Rate (BER) and eye diagram reveal that the proposed system provides wider eye-opening, low BER and high Q-factor as compared to the existing system. The proposed I-AMI with SSP provides the Q-factor of 30.47 whereas the I-AMI with PSP and AMI with SSP provides the Q-factor of 18.77 and 18.40 respectively at 80 Gbps bit rate over a transmission distance of 800 km.

Eliminating Dispersion Power Fading in Radio over Fiber Communication through Unbalanced Sideband Modulation

In Radio Over Fiber (RoF) systems, a commonly used approach to reduce the Dispersion Power Fading (DPF) effects is to convert the spectrum of modulated optical signals into Optical Single Sideband (OSSB). This process can be done by modulating the optical carrier using two RF signals that differ in phase by 90. The optical modulator used is the Dual-Drive Mach Zehnder Modulator (DD-MZM). However, implementing this modulation scheme requires a complex transmitter setup. In this study, a novel technique is proposed to address DPF, called as Unbalanced Sideband Modulation (USM). USM is generated by varying the optical signal carrier according to the RF signal using a Single-Drive Mach Zehnder Modulator (SD-MZM). The output of the SD-MZM is subsequently combined with an optical signal of a frequency equivalent to the sum of the optical carrier frequency and the Radio Frequency (RF) signal frequency. The result outcomes indicate that at a modulation index (m) of 1, the DPF level in an RoF link employing conventional SDMZM as an Electro-Optic (E/O) converter is approximately 8.2. In contrast, when using the OSSB produced by the DDMZM, the DPF level of a RoF link is around 0.9. Remarkably, the DPF level significantly drops to 0.2 when the USM scheme is adopted. This result shows the high efficacy of the USM scheme in eliminating DPF compared to both the conventional SD-MZM modulation scheme and the OSSB scheme generated by DD-MZM. Both model calculations and numerical simulations conclusively demonstrate that the USM method effectively mitigates DPF, offering a promising solution in optical communication systems.

Analog Wavelength Locking in an Optical Single-Sideband Transmitter of a Millimeter-Wave Radio-Over-Fiber Link Featuring a Micro-Ring Resonator and a Heat-Pump-Controlled Laser

This paper presents a novel approach to addressing the issue of temperature-induced instability in an optical, single-sideband transmitter based on a micro-ring resonator (MRR) suitable for millimeter-wave (mmW) radio-over-fiber (RoF) communications. We propose utilizing the drop port of the MRR to provide a feedback signal to the closed-loop control (CLC) system. The latter serves to maintain the optimal alignment between the laser’s carrier and the MRR’s resonant wavelength, thus mitigating the adverse effects of chromatic-dispersion-induced power fading at the receiving end. Since the feedback information is extracted from the otherwise-wasted resonant energy at the drop port, the control system does not compromise the delicate optical signal at the through port. A CLC was synthesized, designed, and prototyped to provide real-time wavelength tuning of the heat-pump-controlled laser based on the feedback signal. Experimental evaluations demonstrate that the wavelength of the laser could be successfully locked to the MRR’s resonance with a wavelength dither of less than 0.004 nm (~491 MHz). This allowed us to limit the power-penalty deterioration to less than 2 dB for a RoF link with a 2.5-km standard telecommunication single-mode fiber (SMF), a modulation frequency of 37.8 GHz, and a carrier wavelength of 1563.97 nm (~191.820 THz). The proposed solution offers an alternative approach for the carrier and the MRR’s resonant wavelength interlocking without the need for complex photonics like thermo-optic or electro-optic structures to control the temperature or phase velocity, respectively.

Integration of Sensing and Communication in a W-Band Fiber-Wireless Link Enabled by Electromagnetic Polarization Multiplexing

The coexistence needs of sensing and communication in millimeter-wave (mmW) bands have urgently driven the seamless integration of sensing and communication in the upcoming mmW era. However, the time-frequency competition between the two functions makes it difficult to accommodate both high sensing resolution and large communication capacity. In this paper, we have designed a W-band fiber-wireless link with the integrated sensing and communication functions enabled by electromagnetic polarization multiplexing. The ultra-wideband fiber-wireless link in W band is enabled by the asymmetrical single-sideband modulation along with the optical heterodyne up-conversion. The electromagnetic polarization multiplexing allocates the sensing and communication functions on two orthogonal electromagnetic polarizations, respectively. Thus, all time-frequency resources of the fiber-wireless link can simultaneously serve these two functions without any resource competition, contributing to an ultra-high spatial resolution and an ultra-large data capacity at the same time. Our experimental results show the spatial resolution of up to 15 mm and data rate as high as 92 Gbit/s were simultaneously realized in W band after delivering over a 10.8-m wireless distance. The overall improvement of both the sensing and communication performance, to the best of our knowledge, led to a record capacity-resolution quotient of 61.333 Gbit/s/cm. In addition, we have qualitatively investigated the integrated sensing and communication fiber-wireless link, in terms of the carrier frequency, system bandwidth, multi-mmW access, and electromagnetic polarization crosstalk.

Simple polarization-insensitive coherent RoF link with increased capacity.

A simple polarization-insensitive coherent radio-over-fiber (RoF) link with increased spectrum efficiency and transmission capacity is proposed and demonstrated. Instead of using two polarization splitters (PBSs), two 90° hybrids, and four pairs of balanced photodetectors (PDs) in a conventional polarization-diversity coherent receiver (PDCR), a simplified PDCR with only one PBS, one optical coupler (OC), and two PDs is employed in the coherent RoF link. At the simplified receiver, a novel, to the best of our knowledge, digital signal processing (DSP) algorithm is proposed to achieve polarization-insensitive detection and demultiplexing of two spectrally overlapping microwave vector signals as well as the elimination of the joint phase noise originating from the transmitter and the local oscillator (LO) laser sources. An experiment is performed. The transmission and detection of two independent 16QAM microwave vector signals at identical microwave carrier frequencies of 3 GHz with a symbol rate of 0.5 GSym/s over a 25-km single-mode fiber (SMF) is demonstrated. Thanks to the spectrum superposition of the two microwave vector signals, the spectral efficiency as well as the data transmission capacity is increased.

Design of a RoF fronthaul link based on optical generation of mm-waves for 5G C-RANs

In this paper, a cloud-radio access networks based (C-RANs) radio-over-fiber (RoF) system have been reported with the optical generation of 28 GHz and 57 GHz millimeter-wave using phase modulation and stimulated Brillouin scattering effect in optical fibers. As a result, a cost-effective system with improved performance is achieved by the proposed method. Based on the proposed scheme, a 28 GHz (licensed band) and 57 GHz (Un-licensed band) optical millimeter-waves are generated by simulation only using 14 GHz and 16 GHz radio frequency (RF) respectively. Because the performance of any optical transmission system is limited by the Q factor and the bit error rate (BER) and any input power of the laser into the fiber needs to be less than the Brillouin threshold in order to avoid the sharp degradation of the Q factor. Also, the Brillouin threshold for this study setup is determined which is around 6 dBm in the case of licensed band and 2 dBm in the case of unlicensed band. The simulation results show that the proposed scheme can provide 100 km single-mode fiber for 28 GHz and 25 km for 57 GHz with transmission rates of 10 Gbps for both of them. In addition, a stable millimeter-wave RoF link, high quality carriers, and a reduction in nonlinearity effects are achieved with the proposed scheme.

A Scalable 26 GHz RoF Relay System Using Optoelectronic Extender

This letter presents data transmission over a 26 GHz millimeter-wave (mmW) radio signal and its coverage extension in an indoor environment using the radio-over-fiber (RoF) technology. The proposed system consists of two RoF seamless wireless links to emulate a mobile fronthaul using a transparent RoF relay link to overcome higher attenuation of the mmW signal. The system is experimentally evaluated for up to 64-quadrature amplitude modulation (QAM), showing the possibility of signal transmission over up to 20 km of optical fiber in the first Section transmission scheme without the usage of an optical amplifier. For the system with 20 km and 30 m long RoF links as well as 4 m and 1 m long relays with no optical amplification, the minimal experimentally measured error vector magnitude is as low as 5% for 64-QAM. Further, a self-developed integrable optoelectronic extender is used for the relay RoF system to simplify the antenna site and keep its footprint small. Moreover, the proposed setup is extended using an optical amplifier toward a scalable scheme with up to 75 km of optical fiber and 5 m long free space optical link.

Coherent-detection radio-over-fiber link with high spectral efficiency based on digital phase noise cancellation.

A radio-over-fiber (RoF) link with high spectral efficiency using digital phase cancellation is proposed and experimentally verified. The link can be utilized to transmit four microwave vector signals. The four microwave vector signals are modulated on optical signals by using a dual-polarization dual-parallel Mach-Zehnder modulator at the transmitter. The optical signals subsequently pass through a spool of single-mode fiber (SMF) and are sent into a coherent receiver. A digital signal processing (DSP) algorithm is proposed to eliminate the phase noise and frequency difference caused by two free-running lasers at transmitter and receiver. Two 16-quadrature amplitude modulation (16-QAM) vector signals at 6GHz and another two 16-QAM vector signals at 7GHz transmit over a 10-km SMF and are successfully recovered after a DSP algorithm. In order to evaluate the transmission performance of the RoF link, error vector magnitudes and bit error rates are also estimated.

Improved Nonlinear Model Implementation for VCSEL Behavioral Modeling in Radio-Over-Fiber Links

This paper proposes an improved model for the nonlinear behavior of the components of Radio-over-Fiber (RoF) links focusing here on the laser source. In a previous work, it was shown with a flexible new opto-microwave design approach a first behavioral model of RoF links that exploited the hyperbolic tangent function (the so-called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tanh</i> model) to model the high-power nonlinearities. Since this was not sufficient to simulate adequately all the nonlinearities, in this work, for the first time in the authors’ knowledge, the improved nonlinear model based on Cann's model is developed, on an electro optic transducer. Then the amplitude-to-amplitude conversion of the link is better taken into account with the introducing of nonlinear properties of the link at high input power in the model. The new transfer equation includes several parameters that can be adjusted to adapt the model to any of the components, especially the photo-emitters under study, which are the main cause of this type of nonlinear behavior of the system. In particular, this improved model is applied on the state-of-the-art 850 nm GaAs Vertical Cavity Surface Emitting Laser (VCSEL), simulating several Figures of Merit such as the Error-Vector Magnitude (EVM) of a Long Term Evolution (LTE) signal with a Quadrature Phase Shift Keying (QPSK)and the output amplitude-modulated power versus input amplitude-modulated power, also called AM/AM curve. This new proposed nonlinear model outperforms other existing behavioral models and its non linear simulation better fits with the measurements. Finally, we conclude that the performance improvement highly depends on the nonlinearity sharpness parameter, depending on the frequency and the bias current.

Performance enhancement of 8 channel SCM–WDM based RoF link using different type of transmission fibers along with different type of channel spacing

Abstract A system is represented that uses 20 nm of channel spacing and unequal channel spacing along with four different types of transmission fibers on subcarrier multiplexing–wavelength division multiplexing (SCM/WDM) based radio over fiber (RoF) link. These five different kinds of fibers have been analyzed offering different propagation parameters. In case of 20 nm of channel spacing the performance of the link is improved by 35.32% with the use of True wave fiber. In this paper, 8 channel SCM–WDM based RoF based model is simulated and analyzed with different type of optical fibers covering distance of 10 km. A comparison of Q-factor at different optical power and different optical lengths has been made. Q-factor, bit error rate and eye diagrams have been examined for assessing the performance of the model.

High-performance dual-band radio-over-fiber link for future 5G radio access applications

For the fronthaul between the baseband unit and the remote radio unit (RRU), radio over fiber (RoF) is promising to replace the Common Public Radio Interface in beyond fifth-generation and the sixth-generation wireless communication systems. The weight, size, and power consumption of the RRU can be greatly reduced, along with easy support for the millimeter-wave band. In this paper, a dual-band RoF link is proposed, where flexible switching between high-frequency (28 GHz) and low-frequency (3.5 GHz) RF channels is realized through polarization control. Single-sideband modulation is performed on 28 GHz signals to avoid periodic power fading during long-distance fiber transmission. By adjusting the DC bias, the third-order intermodulation distortion component in the 3.5 GHz signal is suppressed, and the spur-free dynamic range (SFDR) is improved. Experimental results show a channel isolation of above 43 dB. The link gain of the 28 GHz signal channel is flat, and the SFDR of the 3.5 GHz signal channel can reach 118.6 d B ⋅ H z 4 / 5 . In the case of RF vector signal transmission, the adjacent channel power ratio of the 3.5 GHz signal channel is optimally up to 48 dB, and the error vector magnitude is 2.9%.

A Bidirectional RoF System for the Multi-Band Signal with Mitigation of the Nonlinear Effects

A bidirectional full duplex radio over fiber (RoF) system is proposed for multi-band signal generation and transmission. A multi-band signal is generated at the central station (CS), which includes 10 GHz baseband, 40, and 70 GHz millimeter-wave (mm-wave) by optical frequency multiplication technique of radio frequency (RF) local oscillator (LO) with frequency of 15 GHz. The simultaneous transmission of the 16-QAM OFDM-based multi-band downlink signal and 4-QAM uplink signal over a single fiber is demonstrated. The carrier for uplink transmission is extracted from the unmodulated downlink signal with tunable FBG which is modulated by OFDM-based 4-QAM uplink data obtained from the user end via an antenna. The frequency reuse from the downlink signal makes the base station (BS) free from the laser source. At the user end, the multi-band signals for multi-serve application are obtained by heterodyne beating of multiple bands. An increase in launch power induces fiber dispersion and non-linearities like self-phase modulation (SPM) and cross phase modulation (XPM), which depends on the nonlinear refractive index of the fiber. Different industrial standard fibers with common specifications and modulation format along with co-shared components are deployed, and system performance is analyzed. The stimulated Brillouin scattering (SBS) can be mitigated by optimizing the launch power within the SBS threshold. The nonlinear effects are minimized at the system level without the requirement of additional digital signal processor (DSP) . The theoretical and simulation analysis shows that the designed bidirectional RoF link exhibits an improved system performance for bidirectional transmission with negligible power penalty.

Practical Demonstration of 5G NR Transport Over-Fiber System with Convolutional Neural Network

This study describes an experimental realization using digital predistortion (DPD) for a fifth generation (5G) multiband new radio (NR) optical front haul (OFH) based Radio over Fiber (RoF) link. For the performance enhancement and complexity reduction of RoF links, a novel Convolutional Neural Network (CNN) based DPD technique is proposed, followed by comparisons with the generalised memory polynomial (GMP) based DPD method. To support enhanced mobile broad band scenario, the experimental testbed uses the 5G NR waveforms at 10 GHz with 20 MHz bandwidth and a flexible-waveform signal at 3 GHz with 20 MHz bandwidth. For 10 km of typical single mode fiber, a Mach Zehnder Modulator with two distinct radio frequency waveforms modulates a 1310 nm optical carrier utilizing distributed feedback laser. The error vector magnitude and number of estimated coefficients, and multiplications are all used to describe the experimental outcomes. The goal of the research is to see if CNN-based DPD improves performance while lowering complexity levels to meet 3GPP Release 17 criteria.

A linearized optical single-sideband modulation RoF link with tunable optical carrier-to-sideband ratio

In this paper, an optical single-sideband (OSSB) modulation radio over fiber (RoF) link with tunable optical carrier-to-side-band ratio (OCSR) and simultaneous third-order intermodulation distortion (IMD3) and power fading suppression is proposed. In the proposed link, a phase-modulated signal and an optical carrier are polarization multiplexed using a Sagnac loop. The periodic power fading caused by chromatic dispersion is avoided by filtering the lower sideband. By adjusting the polarization state, the IMD3 is suppressed and a tunable OCSR can be achieved. Compared with the traditional Mach–Zehnder modulator (MZM) based link, the spurious free dynamic range (SFDR) of the proposed link is improved by more than 18 dB with different RF frequencies. In the broadband characteristic test, the proposed link exhibits lower error vector amplitude (EVM) and higher adjacent channel power ratio (ACPR) in a larger input RF power range.

An MPSK Millimeter-Wave Point-to-Point Link With Radio Over Fiber Synchronous Baseband Receiver

This paper demonstrates a millimeter-wave point-to-point radio link of MPSK (QPSK, 8-PSK, and 16-PSK) modulation without using any analog to digital converter (ADC). The proposed point-to-point radio link, not only use the radio over fiber (RoF) concept to extend the transmit distance of the central unit and the remote radio unit, but also use the clock and data recovery (CDR) applied in the optical-electrical converter (OEC) to recover the MPSK intermediate frequency (IF) signals frequency offset. Compared with the traditional RoF link, the proposed point-to-point synchronous radio link eliminated the high-speed ADC requirement for the digital-intermediate-frequency-over-fiber (DIFoF) link, which can significantly reduce the complexity and the cost of the remote radio unit. Simultaneously, we have designed a simple symbol synchronization algorithm to realize the real-time point-to-point radio link on an Field Programmable Gate Array (FPGA). The proposed millimeter-wave radio link has successfully demonstrated 6 Gbps QPSK transmission and can tolerate a frequency offset up to 100 MHz.

3 × 3 MIMO Fiber–Wireless System in W-Band With WDM/PDM RoF Transmission Capability

We propose and demonstrate a 3 × 3 full multiple-input multiple-output (MIMO) fiber–wireless system in the W-band with wavelength-division multiplexing (WDM) and polarization-division multiplexing (PDM) radio-over-fiber (RoF) transmission capability. The system is realized using two highly stable RoF links based on the optical self-heterodyne method and PDM transmission in one of the RoF links. We experimentally demonstrate and confirm satisfactory performances for a 3 × 3 MIMO offset quadrature amplitude modulation-based filter-bank multicarrier signal. Total capacities of approximately 110 and 132 Gb/s, corresponding to spectral efficiencies of approximately 8.5 and 10.2 bits/s/Hz, respectively, are achieved for the system using antennas placed in the same and alternately different polarizations, respectively. The system is applicable to WDM and combined WDM/PDM RoF transmissions. It provides a scalable solution for facilitating large-scale MIMO signal transportation and can be a promising solution for future mobile transport and radio access networks in high-frequency bands.

Experimental Demonstration and Performance Enhancement of 5G NR Multiband Radio over Fiber System Using Optimized Digital Predistortion

This paper presents an experimental realization of multiband 5G new radio (NR) optical front haul (OFH) based radio over fiber (RoF) system using digital predistortion (DPD). A novel magnitude-selective affine (MSA) based DPD method is proposed for the complexity reduction and performance enhancement of RoF link followed by its comparison with the canonical piece wise linearization (CPWL), decomposed vector rotation method (DVR) and generalized memory polynomial (GMP) methods. Similarly, a detailed study is shown followed by the implementation proposal of novel neural network (NN) for DPD followed by its comparison with MSA, CPWL, DVR and GMP methods. In the experimental testbed, 5G NR standard at 20 GHz with 50 MHz bandwidth and flexible-waveform signal at 3 GHz with 20 MHz bandwidth is used to cover enhanced mobile broad band and small cells scenarios. A dual drive Mach Zehnder Modulator having two distinct radio frequency signals modulates a 1310 nm optical carrier using distributed feedback laser for 22 km of standard single mode fiber. The experimental results are presented in terms of adjacent channel power ratio (ACPR), error vector magnitude (EVM), number of estimated coefficients and multiplications. The study aims to identify those novel methods such as MSA DPD are a good candidate to deploy in real time scenarios for DPD in comparison to NN based DPD which have a slightly better performance as compared to the proposed MSA method but has a higher complexity levels. Both, proposed methods, MSA and NN are meeting the 3GPP Release 17 requirements.

Performance analysis of DP-MZM radio over fiber links against fiber impairments

Abstract In this research article, the radio over fiber (RoF) Architecture using dual-parallel Mach–Zehnder modulator (DP-MZM) is analyzed and simulated. The nonlinearity is the main issues which degrade the performance of the RoF links. Using DP-MZM modulators, the nonlinearity can be reducing in the significant amount by controlling the bias voltages. The DP-MZM consists of two MZM modulators in parallel form with the variations of extinction ratio (ER). The proposed RoF link operated with single tone frequency signal at 10 GHz and the optical signal is transmitted further on Single mode optical fiber. The electrical spectrum at the output of photodetector confirms the quality of received signal while having desired and undesired terms. The proposed structure is designed using Optical Simulator simulation Software to confirm and validate the proposed results. The simulation output shows that the RF power of DP-MZM based RoF link has been found a decrement of 6.95 dB corresponding to undesired components with respect to desired RF signal for the increment in fiber length from 10 to 30 km. The digital transmission RoF link model is proposed and optimized for fiber impairments for 2–30 km, using 10 Gbps digital data string. electrical spectrum analyzer shows that very good eye openings for different fiber impairments, its shoes that proposed RoF model is optimized and having good SNR.