Optical Fronthaul Research Articles

Power over fiber in radio access networks: 5G and beyond

This paper introduces the concept of power over fiber (PoF) and potential applications envisioned in radio access networks with optical fronthauling using different types of optical fibers. It is an open discussion on exploring PoF technology with current experiments integrating analog radio over fiber using 5G-NR signals in compliance with 3GPP, PoF, and monitoring techniques and general requirements in future deployments along with its potential to be part of the energy efficiency strategy in beyond 5G networks.

DenseNet-Transformer: A deep learning method for spatial–temporal traffic prediction in optical fronthaul network

The rapid evolution of 6G Radio Access Networks (RAN) towards virtualization and intelligence is driven by the widespread adoption of high-bandwidth services and the proliferation of mobile communications. Active Antenna Units (AAU) interface with Distributed Units (DU) over Passive Optical Networks (PON), serving as crucial bearers that transmit extensive data through the optical fronthaul network to the core network. However, the escalating demand for high-bandwidth services, coupled with the uneven spatial–temporal traffic patterns influenced by factors such as geographical location and urban functionalities across various base stations, poses significant challenges to the optical fronthaul network in terms of operational costs and resource allocation efficiency. To tackle these challenges, we introduce the DenseNet-Transformer spatial–temporal traffic prediction algorithm tailored specifically for the traffic characteristics of optical fronthaul networks. In DenseNet-Transformer, DenseNet captures spatial feature correlations among nearby traffic in adjacent regions, facilitating enhanced learning of traffic characteristics across distant areas through dense connections. The Transformer component learns both long and short-term temporal dependencies, enhancing the algorithm’s temporal prediction capabilities using multi-head attention mechanisms and positional encoding. We validate the effectiveness of DenseNet-Transformer through a series of ablation experiments and comparative tests against other algorithms under identical conditions. Experimental results on real datasets demonstrate that, in most scenarios, DenseNet-Transformer outperforms existing algorithms for time traffic prediction in both wireless and optical communication domains, as well as spatial–temporal prediction algorithms.

Power Over Fiber Pooling as Part of 6G Optical Fronthaul

Power Over Fiber Pooling as Part of 6G Optical Fronthaul

Data-driven decision-making framework for optical fronthaul slice resizing in 6G networks

The third-generation partnership introduced three main types of slices: enhanced mobile broadband, massive machine-type communication, and ultrareliable low-latency communication. To accommodate these services, the next generation of mobile networks will require architecture with distinct requirements and network slices. To implement these services on an optical fronthaul, the slices will be hosted using lightpaths. Such lightpaths will have to accommodate latency and bandwidth constraints to keep radio units (RUs) and baseband units (BBUs) synchronized. However, the traffic in a slice may vary, and the resources allocated to a long-established lightpath could be out of date, leading to waste or lack of resources. For example, a lack of bandwidth can cause desynchronization between BBUs and RUs. Therefore, the slice must be resized regularly to meet the variable demands. This work proposes a data-driven decision-making (DDDM) framework to resize the fronthaul slices while mitigating the consequences of a lack of bandwidth. The framework uses long short-term memory to implement its analytical stage and integer linear programming (ILP) to reconfigure the entire network when it is required. The results show that the DDDM-based framework outperforms the state-of-the-art ILP-based heuristic by up to 15% in terms of radio blocking mitigation.

5G/6G optical fronthaul modeling: cost and energy consumption assessment

In fifth generatin (5G) and the future beyond 5G (6G) radio access networks (RANs), the cost of fronthaul deployment is a main challenge for mobile network operators. Depending on different constraints, there are various solutions to deploy an efficient fronthaul. Fiber-optic-based fronthaul offers long-term support with regard to a rapid increase in capacity demands. When fiber connections, either point-to-point (P2P) or point-to-multipoint [i.e., passive optical networks (PONs)], are not available due to economic or geographical constraints, new optical fronthaul solutions such as free space optics (FSO) can be applied. Before deploying any optical fronthaul architecture, mobile operators must assess its impact on the total cost of ownership (TCO) of the network (i.e., capital and operational expenditures). To assist operators in choosing the most cost-effective fronthaul architecture, in this paper, we show how to evaluate the TCO of 5G and beyond RANs while taking various fronthaul architectures (P2P, PON, and hybrid PON-FSO) into consideration. Furthermore, this paper answers the question of how much energy is needed to run a network using each of the considered optical fronthaul architectures. To do so, we propose a holistic framework based on an integer linear program that minimizes the TCO of the network. Furthermore, we propose a heuristic algorithm to solve large-sized problems. We run simulations to compare different fronthaul architectures for two deployment areas: dense and sparse.

Optimized MSA DPD method for improving 5G multiband optical fronthaul performance

AbstractA modified optimized magnitude‐selective affine (OMSA) model‐based digital predistortion (DPD) is presented that introduces the weighting function into our earlier proposed magnitude‐selective affine (MSA) method with an aim to further reduce the complexity overheads without affecting performance compared to the MSA method. This model utilizes a power‐reliant weighted function rather than the summation of MSA quantities for improving the multiband 5G new radio (NR) analog radio over fiber system performance. The OMSA‐DPD method is tested using 5G NR signals which are transmitted over a 10‐km fiber length. The performance of the OMSA‐DPD method is assessed in comparison to MSA and generalized memory polynomial (GMP) methods in terms of adjacent channel power ratio, error vector magnitude, and complexity. The experimental results show that the OMSA‐DPD method achieves better performance with lower complexity compared to the MSA and GMP models, meeting the 3GPP limits.

A Future Proof Reconfigurable Wireless and Fixed Converged Optical Fronthaul Network Using Silicon Photonic Switching Strategies

Broadband access services in the era of 5G and beyond are driving the evolution of optical access networks from a fiber-to-the-home/ fiber-to-the-building infrastructure to a more common access platform, compatible with both wired and wireless broadband services. Accordingly, a flexible optical access network plays a pivotal role in catering to various service requirements. This work proposes a reconfigurable converged optical fronthaul network for wireless and wired services using a 4◊4 microring resonator (MRR) based silicon photonic (SiP) switch fabric. Reconfigurable service selection is demonstrated for wavelength selective, multicast and space switching scenarios. The reconfiguration of digital data and digital radio-over-fiber (DRoF) services for cloud radio access networks (C-RAN) is demonstrated.

Single-Carrier, Single-λ Full-Duplex Analog Radio Feed Over a Single-Port RRH Transceiver

The densification of radio access and the massive deployment of radio heads calls for efficient optical fronthaul technologies. The adoption of analogue radio-over-fiber schemes promises greatly simplified equipment that can be distributed at the antenna sites for the purpose of radio signal conditioning and electro-optic conversion. Towards this direction, we propose and experimentally evaluate a full-duplex interface at the intersection between the optical and the radio frequency layer, aiming at bidirectional radio signal transmission over a single wavelength (1577 nm) and a single carrier frequency (5.375 GHz). Analogue coherent optical reception is performed through an electro-absorption modulated laser, which is employed as multi-functional element that accomplishes wavelength re-use for full-duplex radio-over-fiber transmission. The directional split is shifted to the electrical domain through adoption of a crosstalk-cancelling circulation stage, ensuring compatibility with a high dynamic power range for the simultaneous transmission and reception of up- and downlink radio signals, respectively, without the need for further duplexing methods subject to frequency translation or time slotting. We prove that margins of >2% in terms of error vector magnitude can be accomplished for an unpaired spectral configuration, where down- and uplink radio signals share the same spectrum in the optical and electrical domains.

Demonstration of M-QAM OFDM bidirectional 60/25 GHz transmission over 10 km Fiber, 100 m FSO and 2 m radio seamless heterogeneous fronthaul link

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 millimetre 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.

Power-over-Fiber with Simultaneous Transmission of Optical Carrier for a High Frequency Analog Signal over Standard Single-Mode Fiber

Efficient simultaneous transmission of light with a power of more than 2 W at a wavelength of 976 nm and an optical carrier for transmitting a high-frequency analog signal at a wavelength of 1550 nm over a distance of 1 km over a standard single-mode fiber was experimentally demonstrated. Electrical power up to 350 mW (5 V, 70 mA) was obtained from a multi-junction silicon photocell, resulting in the optical transmission efficiency of about 70% and a photocell efficiency of 25%. The power transmission did not affect the transmission of the high frequency analog signal. Key broadband analog transmission characteristics such as noise figure (NF < 25 dB) and spurious-free dynamic range (SFDR3 > 117 dB/Hz2/3) were achieved and were close to the fundamental shot noise limit. This approach is promising for powering a remote antenna unit in optical fronthaul architecture.

Double‐RSOA colorless WDM‐PON for 5G fronthaul applications

AbstractThe introduction of 5G mobile systems poses significant challenges to the design of the fronthaul structure. The dense coverage, comprising several small cells operating in the millimeter‐wave range, with an enormous number of antennas, makes conventional digital‐radio‐over‐fiber unfeasible. On the other hand, the centralized architecture of the optical fronthaul is well‐suited for the passive optical network (PON) topology, with WDM‐PON as a strong contender due to its capability to establish logical point‐to‐point optical links to each cell site. However, the large‐scale deployment of WDM‐PON requires the availability of colorless transmitters, where the same hardware is able to generate any wavelength needed by the network. In this context, this paper investigates an analog fronthaul configuration over a colorless WDM‐PON for 5G and beyond, employing the techniques of self‐seeding in the downlink and carrier reuse in the uplink. We numerically evaluated the EVM when transmitting 1.25 Gbps at 3.5 GHz and 10 Gbps at 26 GHz in a 20 km optical link. The results are within the 3GPP requirements, demonstrating the feasibility of the proposed fronthaul.

Dynamic Joint Functional Split and Resource Allocation Optimization in Elastic Optical Fronthaul

Dynamic reconfigurability in optical and mobile networks can facilitate heterogeneous service provisioning while utilizing minimal resources. This allows cost-efficient service delivery resulting in high revenues for network operators. Deployment of elastic mobile and optical networks is a key driver for enabling reconfigurability in modern networks. Elastic optical networks can be exploited as the fronthaul portion of new generation of mobile networks. Such elastic optical fronthaul networks facilitate joint reconfiguration of flexible radio and optical elements and provide considerable performance improvements. In this paper, we focus on the joint dynamic selection of functional splits and configuration of optical transponders and illustrate that designing a converged network with optical and radio elements improves network power efficiency. A time-averaged stochastic optimization problem is formulated and its solution is derived using a modified version of the Lyapunov drift technique. Simulation results demonstrate that the proposed scheme can reduce the average power consumption by up to 70% compared to a cloud radio access network with a traditional optical fronthaul. Further, the results show that the modified Lyapunov technique can afford stringent fronthaul delays below 250 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> . We also discuss how future technology upgrades such as increasing the number of radio antenna ports and decreasing the granularity of fiber spectrum grid may influence the results.

Design of Cost-Efficient Optical Fronthaul for 5G/6G Networks: An Optimization Perspective

Currently, 5G and the forthcoming 6G mobile communication systems are the most promising cellular generations expected to beat the growing hunger for bandwidth and enable the fully connected world presented by the Internet of Everything (IoE). The cloud radio access network (CRAN) has been proposed as a promising architecture for meeting the needs and goals of 5G/6G (5G and beyond) networks. Nevertheless, the provisioning of cost-efficient connections between a large number of remote radio heads (RRHs) in the cell sites and the baseband unit (BBU) pool in the central location, known as the fronthaul, has emerged as a new challenge. Many wired and wireless solutions have been proposed to address this bottleneck. Specifically, optical technologies presented by passive optical networks (PONs) are introduced as the best suitable solution for 5G and beyond network fronthaul due to their properties of providing high capacity and low latency connections. We considered time and wavelength division multiplexed passive optical networks (TWDM-PONs) as a fronthaul for 5G and beyond. Taking that into consideration, in this paper, we propose an integer linear program (ILP) that results in the optimal optical fronthaul deployment while minimizing the total cost of 5G and beyond instances. However, for larger network instances, solving the ILP problem becomes unscalable and time-consuming. To address that, we developed two heuristic-based algorithms (the K-means clustering algorithm and the one based on the genetic algorithm-GA). We evaluated the suitability of our proposed ILP and heuristic algorithms in simulations by utilizing them to plan different network instances (dense and sparse).

Analysis of an active and passive optical fronthaul with dense wavelength division multiplexing for coexisting 4G/5G cloud radio access network architectures based on software defined radio

We experimentally investigate the performance of an active and passive optical fronthaul to be employed in a cloud/centralized radio access network (C-RAN) architecture based on software-defined radio (SDR) for 4G systems, and we discuss the viability to integrate these fronthaul configurations in coexisting 4G/5G systems based on SDR by considering the reconfigurable characteristics of SDRs and the capacity and latency characteristics of the optical fronthaul. The active optical fronthaul consists of two transponders employing dense wavelength division multiplexing (DWDM), whereas the passive optical fronthaul consists of a point-to-point scheme based on DWDM only. To perform a comparative study between the active and passive optical fronthaul in the C-RAN architecture, we have measured the throughput (bit rate) generated via iPerf® for uplink and downlink transmissions at different DWDM channels, and we analyze the losses and latency presented in both optical fronthaul schemes. We found that for different optical fronthaul lengths up to 21.76 km, the bit rate is practically the same for the passive and active fronthaul despite the higher optical losses present in the passive fronthaul in comparison to the active fronthaul. In addition, the latency among active and passive fronthaul are almost similar with an estimated increment of 20 μs in the active fronthaul. These results are independently of the number of remote radio heads (RRHs) and user equipment (UEs) considered in the C-RAN architecture. Our findings put forward the proposed DWDM passive fronthaul as a viable, less complex, and cost-sensitive solution for C-RAN systems with fronthaul lengths up to 21.76 km. These results and all the experiences reported on the C-RAN implementation provide valuable information to design and develop 4G and 5G C-RAN architectures based on SDR with the capability to operate in a DWDM optical fronthaul infrastructure.

Bidirectional Gigabits Per Second Spatial Diversity Link Using POF for Passive Optical Front-Ends

This paper presents a novel concept for spatial diversity using plastic optical fibre (POF)-based luminaire transmission for in-home networks. We show the feasibility of an optical transmission system using POFs, acting as optical front-haul, for realizing a high capacity wireless bidirectional link aided by a spatial diversity scheme allowing user mobility around the access points. The light coming out of the 1 mm core diameter step-index POF is highly divergent and, therefore, for each POF-end an optical lens is placed to adjust the size of wireless cells, in order to allow user's mobility or to adjust the number of users served by the access points. Thus, the access points are completely passive, hence no electrical powering is needed, resulting in low maintenance. In addition, active components such as light sources and detectors can be located remotely from the access points and they are connected to each other by POFs. Futhermore, we performed optical wireless experiments with an eye-safe visible light laser and therefore our wireless link is limited to 1.2 m and 45 cm diameter of coverage areas. We obtained multi-Gbps for downlink and uplink and compared the link performance using discrete multitone, pulse amplitude modulation, and Nyquist sub-carrier modulation.

Photonic Delay Processing With Centralized Etalon Cascade for Radio Beamsteering in Simplified 2 × 5 Phased-Array Radio Heads

The adoption of beam-centric radio communication schemes calls for efficient methods of beamforming with multi-element antennas. We present a delay generation method that employs a shared and centralized cascade of Gires-Tournois etalons for element-by-element radio frequency beamsteering at remote radio heads, thus adding no complexity at the antenna site. We experimentally demonstrate beamsteering of up to 32° in combination with a 2 × 5 phased-array antenna operated at a radio carrier frequency of 3.5 GHz and transmitting 64-ary quadrature amplitude modulated, orthogonal frequency division modulated radio signals. The radio transmission performance over mobile optical fronthaul and free-space radio propagation is evaluated through offline error vector magnitude estimation and real-time transmission of high-definition video traffic. We further show carrier phase switching at 2 kHz using direct emission frequency modulation of the involved optical light sources, proving the proposed concept to support the fast tracking of mobile users.

Coupled-Theoretical-Model-Based on-Demand Quantum Secured Future Fronthaul Architecture Over Hybrid Core Fibers

In this paper, we propose a coupled theoretical model used for noise processing at nodes of hybrid core fiber with single core fiber (SMF)-multicore fiber (MCF), and intra-core noises and inter-core noises are considered in the model. Then, we propose an on-demand quantum secured future fronthaul architecture and a channel allocation algorithm over hybrid core fibers, and the proposed coupled theoretical model is used to calculate the noises in architecture. For fronthaul optical signals, wavelength modes and the number of transceiver modules in the distributed unit (DU) can be dynamically adjusted according to requirements, and the function of load aggregation is adopted. For quantum key distribution (QKD), quantum signals are transmitted in a dedicated core, and wavelength-time multiplexing is adopted to increase the number of quantum secured base station (BSs) while improving wavelength utilization. Finally, simulation results show that proposed channel allocation algorithm can suppress noise about 60% compared to First-Fit. The architecture can save up to 83.3% modules in 7-core feeder fiber, and it is feasible to support 128 BSs. The experimental and simulation results show that the proposed coupled theoretical model matches well with the experimental results, and the quantum bit error rates in experiment are both lower than 2.5%. The results show that the architecture has the advantages of allocating time-space-frequency resources on-demand, saving power consumption, and expanding the number of users. Therefore, it provides a feasible solution for future optical fronthaul networks secured by QKD.

Effect of erbium-doped fiber amplifier loss compensation on 5G new radio millimeter-wave seamless transmission over analog fiber and free space optical fronthaul at 60 GHz

We demonstrate a fifth generation (5G) new radio (NR) signal transmitted by an analog optical and seamless antenna wireless connection at the frequency band of 60 GHz to exploit a high-frequency unlicensed frequency range. An optical frequency doubling technique, using two Mach–Zehnder modulators operating in a carrier suppressed and linear regime, respectively, was adopted to obtain the desired millimeter wave frequency at the photodetector’s output. The proposed system was tested with the 5G NR signals with a maximum bandwidth of 200 MHz and 64 quadrature amplitude modulation format. It was shown that the signal transmitted through the optical fiber and free space optical link with 1 m long seamless antenna transmission at 62 GHz was capable of meeting the signal quality requirements in terms of error vector magnitude. Moreover, the system phase noise performance showed an almost negligible difference between the various system configurations.

Wavelength &amp; mm-Wave Flexible Converged Optical Fronthaul With a Low Noise Si-Based Integrated Dual Laser Source

A widely tunable hybrid InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> dual laser module (DLM) with low linewidth and relative intensity noise (RIN) is used to develop a wavelength and frequency flexible optical fronthaul system for high-capacity wireless networks. The wavelength flexible (over C-band) delivery of 5G signals over an analog radio-over-fiber (A-RoF) system, incorporating wireless transmission at 60 GHz is demonstrated. A phase noise (PN) and frequency offset (FO) cancelling receiver is used to overcome phase/frequency noise from the DLM due to thermal fluctuations, and this reduces the received (error vector magnitude) EVM to as low as 5%. Using the same DLM we demonstrate the mm-wave frequency flexible (over sub-V-band) delivery of 5G and WiGig signals over a simplified converged fronthaul link. The experimental results show excellent performance with bit error ratio (BER) values as low as 8.46×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> and 1.11×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , and lowest EVMs of 2.4% and 6.1%, achieved for sub-V-band 5G and WiGig signals, respectively. In addition to highlighting the high flexibility of the DLM, this work also presents an approach to deploy this source in future optical-wireless access networks paving way for beyond 5G and 6G technologies.

Experimental Evaluation of Hybrid Fibre–Wireless System for 5G Networks

This article describes a novel experimental study considering a multiband fibre–wireless system for constructing the transport network for fifth-generation (5G) networks. This study describes the development and testing of a 5G new radio (NR) multi-input multi-output (MIMO) hybrid fibre–wireless (FiWi) system for enhanced mobile broadband (eMBB) using digital pre-distortion (DPD). Analog radio over fibre (A-RoF) technology was used to create the optical fronthaul (OFH) that includes a 3 GHz supercell in a long-range scenario as well as a femtocell scenario using the 20 GHz band. As a proof of concept, a Mach Zehnder modulator with two independent radio frequency waveforms modifies a 1310 nm optical carrier using a distributed feedback laser across 10 km of conventional standard single-mode fibre. It may be inferred that a hybrid FiWi-based MIMO-enabled 5G NR system based on OFH could be a strong competitor for future mobile haul applications. Moreover, a convolutional neural network (CNN)-based DPD is used to improve the performance of the link. The error vector magnitude (EVM) performance for 5G NR bands is predicted to fulfil the Third Generation Partnership Project’s (3GPP) Release 17 standards.