Radio-over-fiber Link Research Articles

A Full-Duplex Radio-Over-Fiber Link Based on a Dual-Polarization Mach–Zehnder Modulator

A simple and low-cost full-duplex radio-over-fiber (RoF) link implementing wavelength reuse for short-range applications is proposed and demonstrated based on a dual-polarization Mach–Zehnder modulator (DPol-MZM). In the center unit, a continuous-wave optical carrier is sent to a DPol-MZM to generate an optical signal with data modulation along one polarization direction and an unmodulated optical carrier along the other orthogonal polarization direction. In the remote antenna unit, a polarization beam splitter is used to separate the modulated signal for downstream service and the unmodulated optical carrier for upstream signal remodulation. A proof-of-concept experiment is carried out. Performance of the established full-duplex RoF link operated at 18 GHz is investigated. The measured bit error rate and the error vector magnitude confirm that the proposed architecture is a promising candidate for future high-speed and low-cost short-range applications.

On the capacity of radio-over-fiber links at the W-band

In this work we discuss on the channel capacity of mm-wave Radio-over-Fiber (RoF) experimental transmissions at the W-band (75–110 GHz) over distances up to 300 m and bandwidth up to 20 GHz. In addition to an updated state-of-the-art on RoF demonstrations at such a promising frequency range, we provide a set of trade-off maps in terms of crucial resources on the design of W-band RoF links. The proposed framework offers a unified view for answering how fundamental spectrum resources can be optimally utilized, and how far we are from overcoming the challenge of offering seamless convergence between optical-fibers and wireless links.

Performance Comparison of Analog and Digitized RoF Systems With Nonlinear Channel Condition

We have compared the performance of analog and digitized radio-over-fiber (RoF) systems with a nonlinear channel condition to investigate which one is more suitable for in-building wireless backhauling. For the comparison, we demonstrated optical multicasting of Wi-Fi compatible signals over both RoF links with nonlinear wavelength conversion, and measured error vector magnitude (EVM) performance of the transmitted wireless signals. In the demonstration, the digitized RoF system showed constant performance with EVM (3.75%), while the performance of analog counterpart is highly dependent on optical link conditions.

Experimental verification of long-term evolution radio transmissions over dual-polarization combined fiber and free-space optics optical infrastructures.

This paper describes the experimental verification of the utilization of long-term evolution radio over fiber (RoF) and radio over free space optics (RoFSO) systems using dual-polarization signals for cloud radio access network applications determining the specific utilization limits. A number of free space optics configurations are proposed and investigated under different atmospheric turbulence regimes in order to recommend the best setup configuration. We show that the performance of the proposed link, based on the combination of RoF and RoFSO for 64 QAM at 2.6 GHz, is more affected by the turbulence based on the measured difference error vector magnitude value of 5.5%. It is further demonstrated the proposed systems can offer higher noise immunity under particular scenarios with the signal-to-noise ratio reliability limit of 5 dB in the radio frequency domain for RoF and 19.3 dB in the optical domain for a combination of RoF and RoFSO links.

Development of hybrid envelop memory polynomial based predistorter for RoF system

Radio over Fiber (RoF), an empowering innovation for dissemination of wireless broadband service signals through simple optical connections experiences non-linear distortion. We have designed and implemented a novel predistorter model to reduce nonlinear distortion using hybrid envelop memory polynomial (HEMPM) for the RoF system. The link gives satisfactory performance in the range of GHz signal. The designed algorithm is superior to popular approaches like memory polynomial model (MPM) and envelop memory polynomial model (EMPM) in terms of order of the memory and degree of nonlinearity. As per author's the best knowledge, this is exclusively first time result obtained for HEMPM technique for RoF link and also, for MPM, EMPM based predistorter for RoF link using MATLAB – OptiSystem co-simulation. The results shows that adjacent channel power (ACP) is suppressed by ∼0.45dBm, ∼28.4dBm and ∼7.65dBm of MPM & EMPM and HEMPM model for K=5 &Q=2 whereas for K=7 &Q=3 the value of suppressed power is ∼36.3dBm, ∼47.7dB and ∼05.8dBm, respectively as compared to input signal. NMSEs for (K=7) and memory length (Q=3) reduced by ∼−5.003dBm for HEMPM model and increased by ∼−4.13dBm for MPM model. The EVM is increased from ∼1.56% to ∼1.96% for HEMPM model and increased from ∼4.38% to ∼14.73% for MPM model for K=7 &Q=3.

Dither-free low-bias controller for deeply modulated Mach-Zehnder modulators

Stable bias control of lithium niobate Mach-Zehnder modulator (MZM) is crucial for radio-over-fiber (ROF) links, since the bias drift causes deviation from the optimum operating point. In this paper, we propose and experimentally demonstrate a novel dither-free low-bias control technique, especially for deeply modulated, low-biased analog photonic links. In our scheme, a second lightwave passes through the MZM, while the output power is locked. Since the reverse modulation efficiency is very low when the input radio frequency (RF) carrier is as high as GHz, the RF power shows little impact on the optical power even when the bias angle is very low, so that the bias stabilization is achieved. Experiment results show that bias locking at a low-bias position (about $$-81^{\circ }$$-81ź) with phase deviations less than $$\pm 2.6^{\circ }$$±2.6ź is achieved even when the input RF voltage is as high as $$1.3 \,V_{\pi }$$1.3Vź. Our proposal needs no low-frequency dither, which is promising for ROF links.

Millimeter-Wave Radio Over Fiber Optical Upconversion Techniques Relying on Link Nonlinearity

A study of advanced upconversion techniques used in radio over fiber (ROF) is provided. With the huge increase in both the number of wireless communication subscribers and the bandwidth required per customer, migrating to higher frequencies, i.e., from lower radio frequency to millimeter-wave carriers, is an essential solution. However, due to the short propagation range of millimeter waves, a large number of radio access points are required for providing reliable coverage, which would increase the infrastructure costs. Hence, the transmission of RF signals between the central (or control) points and radio access points (or remote antenna units) using optical fibers is one of the major access network solutions that have been proposed for future high-bandwidth wireless communication systems. In this paper, we introduce the basics of ROF communication, including optical modulation, the optical channel, and the optical detection techniques. Then we survey the family of advanced optical upconversion techniques that exploit the nonlinearity of the ROF link. Specifically, we describe how optical upconversion can be achieved by exploiting the Mach–Zehnder modulator's nonlinearity, wavelength conversion techniques, or the photodetector's nonlinearity. The wavelength conversion techniques rely on the nonlinearities present in the fiber, in the optical amplifier, or in the electroabsorption modulator.

2 × 2 MIMO-OFDM-RoF generation and transmission of double V-Band signals using a microring resonator system

Multi-input multi-output (MIMO) signals have been all-optically generated via orthogonal frequency division multiplexing, and transmitted over a 50 km single mode fibre (SMF) optical link and also a 5 m 2 × 2 MIMO wireless link. The series of constituent single and multi-carrier optical soliton pulses, at frequencies of f1 = 192.6988, f2 = 192.7588, f3 = 192.79 and f4 = 192.84 THz that are applicable for radio over fibre (RoF) systems, were created using a microring resonator system. Sixty four multi-carriers with specific free spectral range of 109.37 MHz across a selected range of 192.7553–192.7623 THz were produced for the purpose of establishing two MIMO V-Band RF signals, and single carriers operating at f3 and f4 optically transported two separate MIMO signals over an RoF link. The optical single sideband with carrier modulation technique has been implemented so as to overcome RF power degradation. The resultant bit error rate (BER) calculation of overall system performance is discussed, and power penalties of 1.7 and 4.7 dB in 25 and 50 km SMF transmissions at the BER threshold respectively have been identified. The results of this research show that transmission of MIMO signals is feasible for up to a 50 km SMF path length and wireless distance of 3 m.

Three dispersion compensation methods for radio-over-fiber system

This paper introduces three different dispersion compensation methods based on superstructure fiber Bragg grating (SSFBG) and an injection distributed feedback (DFB) laser. First, an SSFBG with a nonlinearity group delay spectrum was designed to achieve tunable dispersion compensation. Second, an approach is pro- posed for realizing single-sideband modulation with an optimum optical carrier to sideband ratio for maximizing the transmission performance of a radio-over-fiber (RoF) system based on a strong optical injection-locked DFB laser. Finally, a broadband chromatic dispersion compensation scheme using an optical phase conjugator based on a DFB semiconductor laser is proposed and experimentally demonstrated in RoF links. © 2015 Society of Photo-

Radio-on-radio-over-fiber: efficient fronthauling for small cells and moving cells

A combination of fiber and radio links would be attractive for efficient fronthauling to small cells and moving cells. However, achieving connectivity by using wired-wireless media converters significantly increases system complexity, power consumption, and latency. A seamless combination of fiber and radio links in which photonics-based technologies are used to generate and transmit radiowave signals is more suitable. In this article, we present an overview of fiber-radio link combination for mobile signal transmission. We propose an efficient solution for future mobile fronthauling based on the analog transmission of mobile signals over a bidirectional seamless system. We discuss the potential as well as the challenges and perspectives of the systems applied for mobile fronthauling in small-cell- and movingcell- based networks. We also present a proof-ofconcept demonstration of the transmission of mobile signals over the system. Satisfactory performance is obtained for different high-speed advanced signals. The transmission range over fiber links and the estimated range of the radio links are sufficiently long to facilitate the practical implementation of the system. The system is scalable to include new deployments of small cells and wireless services using wavelength-division and subcarrier multiplexing techniques. It can provide a high-speed, low-latency, and flexible solution for fronthauling in new emerging wireless systems such as 5G networks.

Direct and External Intensity Modulation in OFDM RoF Links

Radio-over-fiber (RoF) systems comprise light modulation and transmission of millimeter-wave signals over fiber links. The aim of this study is to investigate the performance of external and direct intensity modulation in RoF links and to analyze the drawbacks induced by different components of the optical system. In external modulation, the Mach-Zehnder modulator (MZM) is used, whereas the vertical-cavity surface-emitting laser (VCSEL) is utilized in direct modulation. Both modulation schemes are tested for a vector modulation format, i.e., the quadrature amplitude modulation (QAM), where an orthogonal frequency-division multiplexing (OFDM) scheme is used to generate signal subcarriers. The simulations are carried out with the same values of common global parameters for both schemes of intensity modulation. Although VCSEL is a promising device for future RoF systems, the external modulation shows a more robust performance compared with that of VCSEL when implemented with the OFDM modulation technique.

Multi-service RoF links with colorless upstream transmission based on orthogonal phase-correlated modulation.

We propose and experimentally demonstrate a full-duplex radio-over-fiber (RoF) system with colorless upstream transmission based on orthogonal phase-correlated modulation (OPM). This new OPM scheme, which realized by a polarization rotator (PR) and a single-driver Mach-Zahnder modulator (MZM) at the central office (CO), achieves polarization-orthogonality between the optical carrier (OC) and subcarriers generated by radio frequency (RF) signals. By adjusting a polarization controller (PC) in the remote access units (RAU), different modulation schemes can be flexibly implemented, e.g. double-sideband (DSB) modulation for low RF service and optical carrier suppression (OCS) modulation for millimeter-wave (mm-wave) service. In the meantime, the OC can be reused for the upstream transmission without any filtering and additional PC. A proof-of-concept experiment is conducted to demonstrate the feasibility of proposed scheme, where downstream 800-Mb/s orthogonal frequency division multiplexing (OFDM) signal at 58 GHz as an mm-wave service and 800-Mb/s OFDM signal at 0.3 GHz as a low frequency wireless service, as well as an upstream 1-Gb/s on-off keying (OOK) are simultaneously delivered in a shared architecture. By providing heterogeneous services and colorless upstream transmission, the proposed architecture can be seamlessly integrated in wavelength division multiplexing passive optical network (WDM-PON).

Flexible compensation of dispersion-induced power fading for multi-service RoF links based on a phase-coherent orthogonal lightwave generator.

A novel technique to simultaneously compensate dispersion-induced power fading for multi-service radio-over-fiber (RoF) links is proposed. At the central office (CO), a phase-coherent orthogonal lightwave generator (POLG) consisting of a polarization rotator (PR) and a single-driver Mach-Zehnder modulator (MZM) is used. By adjusting a polarization controller (PC) in the base station (BS), the phase difference between the orthogonal polarization carrier and two sidebands can be controlled, and the frequency response can be flexibly shifted for individual radio frequency (RF) service. We experimentally shift the destructive interference to the constructive interference at various frequencies for 25 km and 30 km fiber links. The performances of two services carrying 1 Gb/s on-off keying (OOK) data at 9 GHz and 16.6 GHz over a 30 km fiber in the proposed system show receiver sensitivity improvement of ∼12 dB at the bit error rate (BER) of 10(-3) compared with the conventional double sideband (DSB) modulation case.

Direct and external modulation of IF over fiber systems for 60 GHz wireless applications

Wireless domestic applications involving high data rates are required to work on millimeter wave band. Signal propagation at this frequency range is affected by walls and oxygen absorption which limits communication distances to few meters in one room. Radio coverage can be extended to other rooms by optical links. Performances of such photonic systems are dependent on optoelectronic devices, electrical driving, and receiver circuits. In this paper, radio-over-fiber (RoF) links based on the intensity modulation and direct detection technique are investigated for transmission of a broadband OFDM signal. Direct and external modulations are exploited to analyze system performances according to the ultra wideband (UWB) millimeter-band standard. To avoid component tolerances at high frequencies, an intermediate frequency modulation of the optical transducers is chosen. Optoelectronic and optical components of RoF links are modeled by equivalent electrical circuits with consideration of noise and nonlinearities. These models are validated in system simulation by error vector magnitude evaluation with a measurement setup according to the UWB centimeter-band standard.

Optimized Radio-over-Fiber-based cellular backhauling strategy using genetic algorithms and Pareto fronts

Radio-over-Fiber (RoF)-based cellular backhauling consitutes a promising solution to realize a cost-effective evolution of radio access networks towards higher radio-cells densification. In this paper, we recall the main characteristics of an innovative cellular backhauling all-optical network architecture called Generic Radio-over-Fiber Access Network (GeRoFAN) aiming to federate 4G system radio cells. However, the transport of multiple radio channels over an analog RoF link is subject to optical crosstalk and intermodulation distortions causing a decrease of the wireless system capacity. Supported by an analytical modeling of the relevant optical transmission limitations, we propose an impairment-aware radio cellular backhauling strategy for GeRoFAN able to maximize the radio cellular capacity while achieving an efficient use of optical resources. This optimization problem is solved through Pareto-based Genetic Optimization (PaGeO), a multi-objective meta-heuristic based on evolutionary algorithms and exploiting the concept of Pareto front. Numerical results indicate that PaGeO, applied to LTE cellular system, outperforms alternative backhauling strategies by achieving an excellent compromise between preserving the radio capacity of the system and the number of required optical channels. We also highlight how the optimal backhauling strategy calculated using PaGeO enables GeRoFAN to take full benefit from overlaying several radio channels per cell site.

Full-duplex fiber-wireless link for alternative wired and 40-GHz band wireless access based on differential quaternary phase-shift optical single sideband millimeter-wave signal

A full-duplex fiber-wireless link with a uniform single sideband differential quaternary phase-shift keying optical millimeter-wave signal is proposed to provide wired or 40-GHz band wireless access alternatively. The uniform optical millimeter-wave signal that supports services for wired or wireless users is produced via an LiNbO3 Mach-Zehnder modulator. After being transmitted to the hybrid optical network unit (HONU), it can be demodulated in different patterns on the demand of the user terminals for wired or wireless access. Simultaneously, part of the blank optical carrier abstracted from it is reused as the uplink optical carrier, so the HONU is free from the laser source, and thus, the complexity and cost of the system are reduced. Moreover, since the two tones of the dual-tone optical millimeter wave come from the same source, they maintain high coherency even after being transmitted over fiber. Additionally, the downlink data are carried by one tone of the dual-tone optical millimeter wave, so the downlink optical millimeter-wave signal suffers little from the fiber chromatic dispersion and laser phase noise. The theoretical analysis and simulation results show that our proposed full-duplex link for alternative wired and wireless access maintains good performance even when the transmission link with standard single mode fiber is extended to 30 km.

Bidirectional Radio-Over-Fiber Systems Using Double-Clad Fibers for Optically Powered Remote Antenna Units

This paper shows and experimentally demonstrates bidirectional radio over fiber (RoF) using a double-clad fiber (DCF) for optically powered remote antenna units (RAUs). The DCF for an RoF link has a single mode (SM) core and a multimode inner cladding; the SM core is used for simultaneous downlink and uplink transmissions of the optical RoF data signals, whereas the inner cladding is used for optical power delivery to the RAU. The aim of this approach is to optically power the RAU in such a way that external electrical power supplies, such as batteries or public power lines, are not required. The feasibility of the technique is demonstrated by bidirectional RoF transmission over a 100-m DCF optically feeding with 4.0 W. We successfully achieved high downlink and uplink transmission performance in terms of error vector magnitude measurements, which are based on the IEEE 802.11g wireless local area network (WLAN) standard at a carrier frequency of 2.45 GHz.

Noise performance of high-speed radio over fiber links employing vertical-cavity surface-emitting lasers

This study investigates the intensity noise in high-speed vertical-cavity surface-emitting lasers (VCSELs) and its contribution to the noise performance of radio over fiber (RoF) links. We evaluate the sinusoidal modulation of VCSELs in terms of the second-order harmonic distortion (2HD) and third-order intermodulation distortion (IMD3) in additions to the relative intensity noise (RIN). The spurious-free dynamic range of the proposed VCSEL is estimated. The noise performance of the RoF link is assessed by the noise figure. The modulation characteristics of the VCSEL and the gain and noise factor (NF) of the fiber link are compared under conventional and high-speed modulations of VCSELs. Also, we present comparison of the NF between short (300 m) and relatively long (2 km) fibers.

Compensation of chromatic dispersion for full-duplex ROF link with vector signal transmission using an optical phase shifter

We demonstrate the theory of chromatic dispersion (CD)-induced constellation rotation (CR) in a radio-over-fiber (ROF) link and a method for compensation. We also propose a 60 GHz full-duplex ROF system with vector signal transmission including no CD effect. The evaluation of 5 Gb/s 16 quadrature amplitude modulation signal transmission shows that the CD-induced CR can be entirely overcome due to the proposed method which is simply implemented through an optical phase shifter. The proposed ROF schedule is not only applicable for V-band (57-64 GHz) but also fits for W-band (75-110 GHz), or any other bandwidth.

Clip-and-Filter-Based Crest Factor Reduction and Digital Predistortion for WLAN-Over-Fiber Links

In this letter, we investigate the combined use of crest factor reduction and digital predistortion (DPD) as an effective method for increasing the performance in radio-over-fiber (RoF)-based downlinks. The experimental results show that combining the two techniques further improves the performance as compared with the case solely using DPD in terms of error vector magnitude (EVM) and adjacent channel power ratio (ACPR) for both directly modulated and externally modulated RoF links. Given a threshold in EVM or ACPR, the RF power transmit efficiency is also further enhanced.