Fronthaul Network Research Articles

Impact of Channel Aging on Zero-Forcing Precoding in Cell-Free Massive MIMO Systems

In the context of cell-free massive multi-input multi-output (mMIMO), zero-forcing precoding (ZFP) requires the exchange of instantaneous channel state information and precoded data symbols via a fronthaul network. It causes considerable propagation and processing delays, which degrade performance. This letter analyzes the impact of channel aging on the performance of ZFP in cell-free mMIMO. The aging effects of not only user mobility but also phase noise are considered. Numerical results in terms of per-user spectral efficiency are illustrated.

Experimental comparison of DSB and CS-DSB mmW formats over a hybrid fiber and FSO fronthaul network for 5G.

The telecommunication world is experiencing the 5th generation (5G) networks deployment including the use of millimeter wave (mmW) frequency bands to satisfy capacity demands. This leads to the extensive use of optical communications, especially the optical fiber connectivity at the last mile access and the edge networks. In this paper we outline fiber and free space optics (FSO) technologies for use as part of the 5G optical fronthaul network. We investigate two different mmW transmission schemes based on (i) the conventional analog radio over fiber transmission using one Mach-Zehnder modulator (MZM) with double sideband (DSB) optical modulation, and (ii) an optical-based frequency doubling with one MZM biased at the null point to introduce carrier suppression DSB (CS DSB) transmission and second MZM used for data modulation. Both systems are assessed in terms of the error vector magnitude, signal-to-noise ratio, dynamic range and phase noise. We consider a configuration for the fronthaul network in the frequency range 2 (FR2) at 27 and 39 GHz with the scale of bandwidth up to 400 MHz with M-quadrature amplitude modulation and quadrature phase shift keying. Results are also shown for FR1 at 3.5 GHz. Moreover, we investigate for the first time the 5G new radio signal transmission under strong turbulence conditions and show the turbulence-induced FSO link impairment. We finally demonstrate the CS DSB scheme performs well under chromatic dispersion-induced fading for the frequency up to 40 GHz and single mode fiber length of 30 km, whereas the DSB format seems more appropriate for an antenna seamless transmission.

End-to-End Demonstration of Fiber-Wireless Fronthaul Networks Using a Hybrid Multi-IF-Over-Fiber and Radio-Over-Fiber System

This paper reports an end-to-end demonstration of a fiber-wireless mobile fronthaul (MFH) network based on hybrid intermediate-frequency-over-fiber (IFoF) and radio-over-fiber (RoF) links using a 28-GHz millimeter-wave (MMW) band. We successfully transmitted 24 64-QAM OFDM signals with a total bandwidth of 9.12 GHz over 20-km broadband IFoF, 1-km narrowband IFoF, 500-m RoF, and 10-m free-space links with an aggregate capacity of up to 34.2 Gbps, which complies with the required peak data capacity of IMT-2020. A digital demultiplexer was exploited to flexibly perform channel selection and frequency conversion. The obtained results show the feasibility of implementing fiber-wireless systems with the hybrid IFoF/RoF links for future mobile fronthaul networks.

AI-assisted intent-based traffic grooming in a dynamically shared 5g optical fronthaul network.

The extremely high number of services with large bandwidth requirements and the increasingly dynamic traffic patterns of cell sites pose major challenges to optical fronthaul networks, rendering them incapable of coping with the extensive, uneven, and real-time traffic that will be generated in the future. In this paper, we first present the design of an adaptive graph convolutional network with gated recurrent unit (AGCN-GRU) network to learn the temporal and spatial dependencies of traffic patterns of cell sites to provide accurate traffic predictions, in which the AGCN model can capture potential spatial relations according to the similarity of network traffic patterns in different areas. Then, we innovatively consider how to deal with the unpredicted burst traffic and propose an AI-assisted intent-based traffic grooming scheme to realise automatic and intelligent cell sites clustering and traffic grooming. Finally, a software-defined testbed for 5G optical fronthaul network was established, on which the proposed schemes were deployed and evaluated by considering traffic datasets of existing optical networks. The experimental results showed that the proposed scheme can optimize network resource allocation, increase the average efficient resource utilization and reduce the average delay and the rejection ratio.

Optical access network interfaces for 5G and beyond [Invited

Point to point (PtP), wavelength division multiplexing (WDM) and time division multiplexing (TDM) optical interfaces are discussed as solutions for backhaul, midhaul, and fronthaul networks. The evolution of radio access networks (RANs) for 5G and beyond is introduced and PtP is identified as the most deployed solution, with many transceiver technologies available to cover the different needs for each RAN configuration. WDM and TDM interfaces remain of interest when a lack of fiber occurs. WDM technologies are being adapted to answer to this RAN market with the appearance of medium-WDM (MWDM) and autotunable dense-WDM (DWDM) transceivers. TDM technologies are trying to evolve towards higher bit rates and lower latency to cope with RAN backhaul specifications. A gap in the transceiver technologies is identified for each of those interface types and also for bit rates above 25 Gbit/s that will impose more complex optics, electronics, and integration.

Performance analysis of a 1024-port Hipoλaos OPS in DCN, HPC, and 5G fronthauling Ethernet applications

The explosive traffic growth of emerging cloud, augmented/virtual reality, artificial intelligence, and 5G applications along with the inherent need for high-bandwidth transport of big data has fueled the expansion of the Ethernet switch market for data centers (DCs), high-performance computers (HPCs), and 5G fronthaul networks. As such, next-generation switches have to be capable of conforming to the requirements of a versatile traffic environment, expanding along DC, HPC, and 5G fronthauling infrastructures while meeting the performance needs of these different application sectors. Within this frame, we experimentally validate for the first time, to the best of our knowledge, the performance of the recently reported Hipoλaos optical packet switch (OPS) within Ethernet-based traffic exchange in DC and 5G fronthaul testbeds, highlighting the credentials of the 1024-port and 10.24 Tb/s capacity OPS to successfully support real-world DC and 5G applications. Error-free unicast and dual-output multicast Ethernet packet transmission at 10 Gb/s are successfully validated for different output ports of the switch, followed by successful server-to-server high-definition video transmission, both when the OPS was employed in the DC as well as in the 5G fronthauling testbed. Network performance and stability over time were confirmed through several measurements carried out through the iperf application suite, revealing submicrosecond end-to-end (Layer 7) latency performance. Finally, an OMNeT++ simulation analysis for an Ethernet-switched Hipoλaos network utilizing real-world application traces collected from the MareNostrum HPC system revealed up to 89% lower latency performance compared to the actual system.

A QoS-Aware Dynamic Bandwidth Allocation Algorithm for Passive Optical Networks with Non-Zero Laser Tuning Time

The deployment of new 5G services and future demands for 6G make it necessary to increase the performance of access networks. This challenge has prompted the development of new standardization proposals for Passive Optical access Networks (PONs) that offer greater bandwidth, greater reach and a higher rate of aggregation of users per fiber, being Time- and Wavelength-Division Multiplexing (TWDM) a promising technological solution for increasing the capacity by up to 40 Gbps by using several wavelengths. This solution introduces tunable transceivers into the Optical Network Units (ONUs) for switching from one wavelength to the other, thus addressing the ever-increasing bandwidth demands in residential broadband and mobile fronthaul networks based on Fiber to the Home (FTTH) technology. This adds complexity and sources of inefficiency, such as the laser tuning time (LTT) delay, which is often ignored when evaluating the performance of Dynamic Bandwidth Allocation (DBA) mechanisms. We present a novel DBA algorithm that dynamically handles the allocation of bandwidth and switches the ONUs’ lasers from one wavelength to the other while taking LTT into consideration. To optimize the packet delay, we introduce a scheduling mechanism that follows the Longest Processing Time first (LPT) scheduling discipline, which is implemented over the Interleaved Polling with Adaptive Cycle Time (IPACT) DBA. We also provide quality of service (QoS) differentiation by introducing the Max-Min Weighted Fair Share Queuing principle (WFQ) into the algorithm. The performance of our algorithm is evaluated through simulations against the original IPACT algorithm, which we have extended to support multi-wavelengths. With the introduction of LPT, we obtain an improved performance of up to 73% reduction in queue delay over IPACT while achieving QoS differentiation with WFQ.

Backhaul-Aware Intelligent Positioning of UAVs and Association of Terrestrial Base Stations for Fronthaul Connectivity

The mushroom growth of cellular users requires novel advancements in the existing cellular infrastructure. One way to handle such a tremendous increase is to densely deploy terrestrial small-cell base stations (TSBSs) with careful management of smart backhaul/fronthaul networks. Nevertheless, terrestrial backhaul hubs significantly suffer from the dense fading environment and are difficult to install in a typical urban environment. Therefore, this paper considers the idea of replacing terrestrial backhaul network with an aerial network consisting of unmanned aerial vehicles (UAVs) to provide the fronthaul connectivity between the TSBSs and the ground core-network (GCN). To this end, we focus on the joint positioning of UAVs and the association of TSBSs such that the sum-rate of the overall system is maximized. In particular, the association problem of TSBSs with UAVs is formulated under communication-related constraints, i.e., bandwidth, number of connections to a UAV, power limit, interference threshold, UAV heights, and backhaul data rate. To meet this joint objective, we take advantage of the genetic algorithm (GA) due to the offline nature of our optimization problem. The performance of the proposed approach is evaluated using the unsupervised learning-based k-means clustering algorithm. We observe that the proposed approach is highly effective to satisfy the requirements of smart fronthaul networks.

Mitigation of the XPM in 5G fronthaul wavelengths by using orthogonal polarization method

In the dense wavelength-division multiplexing (DWDM)-based 5G fronthaul network, cross-phase modulation (XPM) would become more obvious to degrade the signal when the interval between the channels becomes narrower. Meanwhile, theoretically, the relative polarization angle between the adjacent channels tends to be more consistent in the same case, that it appears effective on the suppression of XPM by using the orthogonal polarization method In this paper, the orthogonal polarization method has been analyzed and applied on multi-channel transmission for suppressing the XPM effect at a 5G fronthaul system which is illustrated on both simulation and experimental results. By the simulation in the case of 25 Gbit/s per channel, ∼0.6/∼0.8 dB sensitivity improvement can be achieved in orthogonal-polarization-state after 10 km/15 km transmission, comparing with the same-polarization-state. In the real experiment, ∼0.7 dB/∼0.9 dB sensitivity improvement has been realized under the same condition as the simulation. This is relatively-more effective comparing with the published results in using one single method to suppress XPM in the WDM systems.

Intelligent Media Forensics and Traffic Handling Scheme in 5G Edge Networks

The 5th generation (5G) communications evolved with heterogeneous user terminals and applications. A convergence of Mobile Edge Computing (MEC) and Software-Defined Networks (SDN) delivers gigantic challenges and opportunities for enhancing computing resources and user Quality of Service (QoS) in fronthaul and backhaul networks. Due to the precipitous expansion of user media in the 5G epoch, efficient media forensics methods are mandatory for specifying and offering effective safety handling based on individual application requirements. According to the exponential increment of Heterogeneous Internet of Things (HetIoT) devices, gigantic traffic will generate through bottleneck 5G fronthaul gateways. 5G fronthaul network environments consist of inadequate resources to surmount the enormous user traffic and communications, QoS will be reduced when the existence of traffic congestion occurs. To confront the aforementioned issues, this paper proposed intelligent media forensics and traffic handling scheme for controlling the Uplink (UL) transmission according to the Downlink (DL) statuses. Support Vector Machine (SVM) algorithm was applied to conduct the media forensics and MEC server integrated into fronthaul gateways, in which gateways resources are divided into UL and DL. Caching technology will be a part of 5G environments, and DL will be utilized for traffic caching. So, it is compulsory to adjust the communication traffic according to UL/DL resource utilization and control the forwarding traffic which relies on resource availability. The experiment was conducted by using computer software, and the proposed scheme illustrated a noteworthy outperformance over the conventional method in terms of diverse significant QoS factors including reliability, latency, and communication throughput.

Cloud radio access network fronthaul solution using optimized dynamic bandwidth allocation algorithm

In order to address the challenges that have come with the exploding demand for higher speed, traffic growth and mobile wireless devices, Mobile network operators have decided to move to the notion of small cells based on cloud radio access network. The merits of cloud based RAN includes the ease of infrastructure deployment and network management as well as the fact that its performance are optimized and it is cost effective the merits of cloud based RAN includes the ease of infrastructure deployment and network management as well as the fact that its performance are optimized and it is cost effective. Notwithstanding, cloud radio access network comes with so many strict requirements to be fulfilled for its fronthaul network. In this paper, we have presented these requirements for a 5G fronthaul network. Particular interest on the time division multiplex passive optical network’s challenge of latency was treated by proposing an optimized version of the round robin dynamic bandwidth allocation algorithm. Results obtained show an improvement in the latency of the original algorithm which meets the fronthaul requirement. Other test parameters like jitter and BER were also improved by our proposed optimized algorithm.

Beyond the 100 Gbaud directly modulated laser for short reach applications

It is very attractive to apply a directly modulated laser (DML)-based intensity-modulation and direct-detection (IM/DD) system in future data centers and 5G fronthaul networks due to the advantages of low cost, low system complexity, and high energy efficiency, which perfectly match the application scenarios of the data centers and 5G fronthaul networks, in which a large number of high-speed optical interconnections are needed. However, as the data traffic in the data centers and 5G fronthaul networks continues to grow exponentially, the future requirements for data rates beyond 100 Gbaud are challenging the existing DML-based IM/DD system, and the main bottleneck is the modulation bandwidth of the DML. In this paper, the data rate demands and technical standards of the data centers and 5G fronthaul networks are reviewed in detail. With the modulation bandwidth requirements, the technical routes and achievements of recent DMLs are reviewed and discussed. In this way, the prospects, challenges, and future development of DMLs in the applications of future data centers and 5G fronthaul networks are comprehensively explored.

A low cost PON-FSO based fronthaul solution for 5G CRAN architecture

The continuous growth in mobile network traffic driven by the extensive range of new user applications, for instance, interactive e-learning, social networking sites, remote medical service and machine-to-machine communication has pushed the mobile network operators to enhance their network capacity. The main solution to deal with this problem is to deploy thousands of low power small cell base stations through Centralized Radio Access Network (C-RAN) architecture. The C-RAN offers a green solution for the deployment of small cells with advanced wireless features such as coordinated multipoint, joint scheduling, and multi-radio access technology. However, the provision of fronthaul connections to these small cells at a reasonable cost is a big concern for mobile operators especially when fronthaul link requires high capacity. Therefore, in this work, we present the hybrid fronthaul approach for C-RAN architecture based on Passive Optical Network (PON) and Free Space Optics (FSO) to enhance flexibility and minimize deployment costs of fronthaul network. The paper aims to examine the Total Cost of Ownership (TCO) of fronthaul networks taking into account both capital expenditure and operational expenditure costs, moreover, the efficacy of our proposed model is also analyzed by using optisystem simulator. The proposed model, in terms of TCO, is compared with the standalone PON based fronthaul system in green field scenarios. The numerical results revealed that the hybrid PON-FSO based fronthaul solution can save up to 42.89% TCO associated with small cell fronthaul architecture.

Experimental Demonstration of Extended 5G Digital Fronthaul Over a Partially-Disaggregated WDM/SDM Network

We experimentally demonstrate a 5G digital fronthaul network that relies on multi-adaptive bandwidth/bitrate variable transceivers (BVTs) and an autonomic software-defined networking (SDN) control system for partially-disaggregated wavelength division multiplexing (WDM)/space division multiplexing (SDM). Transmission of 256-QAM 760.32 MHz orthogonal frequency-division multiplexing (OFDM) radio signal is performed, with a total radio transmission capacity of 5.667 Gb/s. Digitized signal samples are carried as a 22.25 Gb/s digitized radio-over-fiber (DRoF) data stream and transmitted over a WDM/SDM infrastructure including 40-wavelength 100-GHz arrayed waveguide gratings (AWGs) and 19-core fiber. The autonomic SDN controller deploys a control loop for the multi-adaptive OFDM-based BVTs that monitors the per-subcarrier signal to noise ratio (SNR) and assigns the optimal constellation based on the actual signal degradation. An error vector magnitude (EVM) below the targeted 2.1% is achieved while setting up connections in less than 5 s.

Wavelength Division Multiplexed Radio Over Fiber Links for 5G Fronthaul Networks

The ever-increasing demand for data rates by mobile users can be fulfilled by radio over fiber (RoF) combined with spectrally-efficient modulation techniques. Thus, RoF is critical to the design of the fronthaul for 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> generation (5G) mobile communication networks. We propose and experimentally demonstrate a low-cost directly modulated laser (DML)-based wavelength division multiplexing (WDM)-RoF transmission system for use in next-generation 5G networks. The proposed architecture provides flexible resource allocation as well, under low and high traffic loads at the remote radio heads (RRHs). We also present robust analytical models of the proposed DML-based WDM-RoF network with spectrally-efficient M-ary-quadrature amplitude modulation ( M-QAM), beneficial for system designers. In OptiSystem software, we evaluate the error vector magnitudes (EVM) as a function of the optical link lengths, input optical power, and the received optical power with 4/16/64-QAM over the optical link. The EVMs of the link for 4/16/64-QAM are below the 3GPP standards for 5G. We find good agreement between the simulations and the analytical results. We also experimentally demonstrate a small scale model of this network with resource allocation. Such a flexible architecture is suitable for meeting the demands of femtocells in 5G fronthaul networks.

Performance and evaluation of Volterra Nonlinear Equalizer for coherent RoF mobile front-haul network: a case study of QOSTBC-OFDM and QOSTBC-GFDM waveforms

In this paper, we propose and investigate a multi-Gigabits-per-second baseband signal over Standard Single-Mode Fiber (SSMF) link using a 16-Quadrature Amplitude Modulation (16-QAM). This architecture can be deployed within the context of fifth-generation (5G) Cloud Radio Access Network (C-RAN). It consists of 4 × 4 Quasi-Orthogonal Space–Time Block Code Generalized Frequency Division Multiplexing (QOSTBC-GFDM) and Orthogonal Frequency Division Multiplexing (QOSTBC-OFDM) waveforms. It is based on the implementation of a third order Volterra Nonlinear Equalizer (VNLE) in the Remote Radio Unit (RRU). The VNLE has been efficiently employed to reduce the impact of the harmful effect of the photonic devices. A Zero Forcing (ZF) equalizer has been used at the User Equipment (UE) to overcome the wireless channel impairments. Our research shows that VNLE can be a promised solution to compensate signal distortions and non-linearity effects. Furthermore, this study shows that QOSTBC-GFDM is more efficient than QOSTBC-OFDM in terms of Out-Of-Band (OOB) emission and Spectral Efficiency (SE). Moreover, a comparable performance between the two waveforms in terms of Bit Error Rate (BER) and Error Vector Magnitude (EVM) is attained.

A New Technique for Incorporating RLL Properties into 5G LDPC Codes Without Additional Redundancy

In C-RAN research, different functional splits are considered for 5G mobile crosshaul networks. The typical fronthaul network based on optical fibers will be affected by the functional split. In some cases, it can be advantageous that the sequences of digital bits transmitted in fronthaul channels are runlength length limited (RLL). Therefore, in this paper a technique is presented which allows us to implement RLL properties into 5G LDPC codes specified in ETSI TS 138 212 without adding any additional redundancy and without changing the basic encoding and decoding algorithms. When compared with ordinary scrambling the new technique offers the advantage of guaranteeing the maximal runlength value.

Effects of optical local oscillator power on the network bit error rate in optical heterodyning of comb lines from a gain-switched VCSEL comb

The photonic technique is a feasible solution for generation of high-frequency RF carrier signals, attracting considerable attention in the past decades. In the laser gain-switching technique for photonic generation of high-frequency RF carrier signals, two optical lines, separated by the desired RF carrier frequency are filtered from the generated optical frequency comb. One of the filtered comb lines is externally modulated with an optical modulator, while the other acts as an optical local oscillator signal. In this paper, we report on the effects of optical local oscillator signal power on the RF carrier signal and the baseband signal of a fronthaul network using a sub-60 GHz RF carrier signal generated from a gain-switched, low-power, VCSEL-based optical frequency comb source. A 56-GHz RF carrier signal was generated by filtering out from the generated optical frequency comb, two optical lines with wavelength difference corresponding to the desired RF carrier frequency. The resultant RF carrier signal was photonically amplitude-modulated with 16-Gbps data before transmitted over 21-km of SSMF. In this study, we investigated the effects of attenuating the optical local oscillator signal power below the modulated optical carrier signal power. It was found that the RF carrier power is reduced when attenuating the optical local oscillator power below the power of the modulated optical carrier signal. However, the performance of the baseband signal is improved. It was found that by optimally establishing a power ratio between the two filtered optical signals, a power gain of up to 18-dB can be achieved to reach an error-free BER of 10−9 for the baseband signal. For the RF signal at the beating frequency, it was found that reducing the power of the modulated optical carrier signal below the power of the optical local oscillator signal could results in power improvements.

Multi-IF-over-fiber transmission using a commercial TOSA for analog fronthaul networks aiming beyond 5G.

An IF-over-fiber (IFoF)-based analog transport technology for mobile fronthaul applications has recently attracted significant attention. However, most previous studies have employed discrete optical components. For the analog transport technology to be a more cost-effective and power-efficient solution, it is necessary to utilize existing integrated optical transceivers. In this paper, we demonstrate IFoF transmission using a commercial off-the-shelf transmitter optical sub-assembly (TOSA). Although the TOSA was developed for a digital system employing non-return-to-zero (NRZ) signals, we show that it is also possible for the TOSA to support high-capacity analog transmission. As a demonstration, by using the TOSA, we could successfully transmit 64- and 256-ary quadrature-amplitude-modulated (64/256QAM) orthogonal-frequency-division-multiplexed (OFDM) signals with net bit rates of 54.74 and 36.49 Gbps per wavelength, respectively. Since the TOSA has four wavelength channels, the total capacities are 218.94 and 145.98 Gbps, respectively. To the best of our knowledge, these rates are the highest among all the demonstrations using analog transport technology.

16-channel tunable and 25-Gb/s EAM-integrated DBR-LD for WDM-based mobile front-haul networks.

We report a tunable distributed Bragg reflector-laser diode (DBR-LD) integrated with an electro-absorption-modulator (EAM) at an operating wavelength of 1.3 µm. This LD consists of gain, phase control (PC), DBR, and EAM sections, realized by using a butt-coupling technique in monolithically integrating the multiple quantum wells (MQWs) with the passive core and by applying an etched-mesa buried hetero-structure (EMBH) to the resonance cavity (i.e., gain to DBR section) and a deep-ridge type to the EAM section in fabricating the waveguide structure. Wavelength tuning of the LD is achieved by both applying a voltage to the heater metal of DBR section (coarse tuning) and injecting a current to the ohmic metal of PC section (fine tuning). From the work, the fabricated chips show a threshold current of about 13 mA, a side mode suppression ratio (SMSR) of more than 35 dB, and a tuning range of 15 nm within a heater voltage of 2 V. Dynamic tests for the EAM-integrated LD show the 3 dB bandwidth of more than 20 GHz and clear 25 Gb/s eye openings with a dynamic extinction ratio (DER) of over 7 dB for 16 channels spaced at the wavelength interval of 0.55 nm. Based on these results, we conclude that the EAM-integrated DBR-LD is capable of providing 16 channel operation at a data rate of 25 Gb/s and can be used as an effective light source for WDM-based mobile front-haul networks.