Field Trial of a SDN-Controlled, Hybrid FiWi FSO/mmWave X-haul with Zero-Touch Handovers and Record-High Data Rate for 6G

1 Field Trial of a SDN-Controlled, Hybrid FiWi FSO/mmWave X-haul with Zero-Touch Handovers and Record-High Data Rate for 6G Maria Vargemidou, Chris Vagionas, Argyris Kokkinis, Argiris Ntanos, Aristeidis Stathis, Panagiotis Kourelias, Giannis Giannoulis, Hercules Avramopoulos, Kostas Siozios, Nikos Pleros, Amalia Miliou Abstract—In the emerging era of 5G+/6G, flexibility and autonomous operation of Radio Access Networks (RANs) without strict bandwidth limitations or energy-waste trade-offs are actively being pursued to satisfy the immense capacity demands and time-varying nature of mobile networks. The current work reports the first field trial of a fully automated, Software Defined Network (SDN) controlled hybrid Fiber Wireless (FiWi) Free Space Optics (FSO)/millimeter-wave (mmWave) X-haul link with a record-high data-rate up to 100Gb/s using real-time traffic. Specifically, the hybrid FiWi X-haul link relies on wavelengthtunable transmissions across an O-band FSO link or a Cband FiWi mmWave channel, being dynamically allocated and wavelength-routed by means of an all-passive O-band/C-band diplexer. A complete SDN algorithmic workflow that periodically polls the status of the wireless channel, triggering a fully automated handover to the optimum functional channel upon adverse weather conditions or channel outages is developed, relying exclusively on open-source and inter-operable SDN agent and network controller solutions. The hybrid link is initially evaluated at a cascaded transmission across a 1km fiber and a 25m-indoor wireless stages, achieving a periodic time-slotted operation with 25 µs beam-switching time, O/C-band crosstalk less than -30 dB and wide open eye diagrams with 6.8 dB Extinction Ratio (ER). Finally, the hybrid link is benchmarked in a 24-hour long, automated, zero-touch, outdoor setup with 1km fiber and 50m wireless distance, providing robust rooftopto-rooftop communication across two neighboring buildings with up to 100 Gb/s capacity, transporting real-time services and 4K-UHD video streaming with uninterrupted operation under both daylight and nighttime turbulent conditions with 5 Beaufort wind speed. The proposed real-time, automated hybrid FiWi FSO/mmWave link is the first to break the 10Gb/s barrier of conventional edge networks, shaping a promising hardwaresoftware X-haul solution towards survivable 5G+/6G RANs. Index Terms—Radio Access Networks, 6G, FSO, MmWave, Hybrid links, SDN control, Optical Switching I. INTRODUCTION At the dawning of the 5G+/6G era for mobile networks, an insatiable demand for ubiquitous broadband connectivity Received ...; This work was supported by Horizon Europe Projects OCTAPUS under Grant Agreement No. 101070009 and PARALIA under Grant Agreement No. 101093013. (Corresponding author: Maria Vargemidou.) Maria Vargemidou, Chris Vagionas, Nikos Pleros and Amalia Miliou are with the Department of Informatics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, 54636 Thessaloniki, Greece (e-mail: v[email protected]). Argyris Kokkinis and Kostas Siozios are with the Department of Physics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece Argiris Ntanos, Aristeidis Stathis, Panagiotis Kourelias, Giannis Giannoulis and Hercules Avramopoulos are with the School of Electrical & Computer Engineering, National Technical University of Athens, Greece. Color versions of one or more figures in this article are available at https://doi.org/.... Digital Object Identifier 10.... across various different applications (e.g. AR/VR, IoT, Industry 4.0, Immersive Video) has been driving a relentless growth of wireless traffic capacities circulating in the RAN [1]. Indicative Key Performance Indicators (KPIs) of the networks for the IMT2030 vision promote aggregate traffic capacities beyond 100Gb/s to the Remote Radio Unit, extreme 1Gb/s user rate anywhere in the RAN, resilient wireless links with >99.999% reliability and service availability using affordable mmWave or THz antennas, as well as zero-touch network operation with minimal or even no human intervention. This massive wireless inter-connectivity urgently calls for developing energy-efficient, broadband aerial links that can offer ’fiber equivalent’ capacities that can naturally and flexibly extend the fiber-optical connectivity over the challenging lastmile, while being also capable to adapt their operation on top of any outdoor RAN environment (e.g. rooftop-to-rooftop, across rivers, disaster areas etc.) and field-network conditions (e.g. under harsh environments, bad weather, disaster situations etc.), contributing towards autonomous, self-configured, outdoor RANs. Stemming from the high traffic demands and ’fiberequivalent’ wireless connectivity to the 6G Radio Units, FiWi transmission technologies relying on FSO and FiWimmWave/THz links have long validated their broadband credentials and high spectral efficiencies in transporting high datarates over directional Line of Sight (LOS) link-demonstrations, exhibiting capacities even beyond 100Gb/s. Yet, despite the high-capacities of FiWi transport links, their deployment and reliability in real-life, outdoor environments is susceptible to challenging propagation conditions, deep-fades and channel outages, as they are severely impacted by weather conditions, hindering wide deployment of truly flexible FiWi X-haul (fronthaul/midhaul/backhaul) links. Yet, a closer look and investigation of the dependence of the channel-response to weather conditions reveals that FSO and mmWave channels are prone to complementary atmospheric conditions. In particular, the effects of turbulence [2], fog [3] and snow [4] in static FSO links have been studied indicating signal attenuation by up to hundreds of dB per kilometer, distorting the optical wavefront and affecting the visibility range and thus the performance of the FSO link. On the other hand, mmWave frequencies are known to be impacted by rain, as rain-drops across the LOS path stimulate Rayleigh scattering of the mmWave Electromagnetic (EM) wave, thus adding additional loss up to tens of dB per kilometer, depending on the rain intensity and the mmWave frequency [5], [6]. This complementarity of the channel responses vividly outlines that both FSO and mmWave links should co-exist and operate in parallel, in order to bring 2 the much anticipated reliability in aerial links, with hybrid links being the primary candidate solution for weather-tolerant [7], [8], broadband RANs. Furthermore, the performance of a hard-switching based Hybrid FSO/RF system affected by turbulence can be further studied in [9]. Towards implementing such resilient 6G RANs, hybrid FSO/mmWave links have been recently gaining increased momentum, simultaneously aiming to unravel both high capacities, as well as intelligent, SDN controlled algorithms and handover mechanisms that can continuously monitor the link performance and ensure network availability. To this end, early profound hybrid FSO/mmWave link demonstrations usually targeted to demonstrate solely the first part, by separately optimizing each of the two FSO or FiWi-mmWave channels, operating in parallel and using discrete transmissionchains with offline, post Digital Signal Processing (DSP). Indicatively, simultaneous transmissions utilizing a shared transmitter have achieved capacities up to 1 Tb/s [18], while combining coherent FSO-transmissions with Analog Radio over Fiber (A-RoF) mmWave schemes have recently revealed capacities up to 400 Gb/s [26], [15], [16], yet not-operating in real-time and without any handover-schemes. At the same time, various detailed theoretical and numerical investigations on the use of a single RF-chain transmitting over both the FiWi FSO/mmWave links have revealed that increase RF gain exploiting channel diversity, yet in this case the data rates are strongly limited to the data-rates and bandwidths supported by the lower-performing channel of the two, hindering high data-rates, while challenging processing is still required for combining the beams of the received signal. Thus, such realtime hybrid operation schemes have only been demonstrated at channels of few MHz-wide [10]–[12]. At the same time, big telecom vendors, such as Ericsson [13], [27] and Huawei [7], have steered their research focus and efforts towards integrating FSO/mmWave hybrid links for flexible rooftop-to-rooftop communications, with real-time operations and handovers, still not exceeding beyond the conventional 10Gb/s line-rates of edge networks, while stimulating parallel developments towards inter-operable, intelligent SDN orchestration schemes to interface and adapt the underlying FiWi X-haul infrastructure. Moreover, SDN-control mechanisms of hybrid links have mostly relied on closed-proprietary algorithmic solutions or FGPA-implementations. A summary of the current state of the art hybrid FSO/mmWave links is presented in Table I, providing an overview of real-time operations and SDNcontrolled handovers at frequencies ranging from 0.07 GHz up to 140 GHz [12], [19]–[25]. The benchmarks shown in Table I pinpoint that real-time hybrid FiWi FSO/RF links have not surpassed the conventional 10 Gb/s bottlenecks in data rates of legacy edge/access networks for the primary FSO and <1.6 Gb/s for the secondary radio-link, that consequently fail to keep up with the increasing optical line-rates, as beamand frequency-handovers are mainly performed by energy-hungry, time-consuming electronic switching circuitry that underline the distinct absence of resilient broadband hybrid X-haul links for emerging 6G networks. We recently demonstrated a 10 Gb/s real-time hybrid indoor link using an open source SDN solution [28]. In this work, extending our prior 10 Gb/s demonstration, we experimentally demonstrate the first fully automated, SDNcontrolled hybrid FiWi FSO/mmWave X-haul link that can transport any incoming real-time Ethernet (ETH)-based traffic with capacities up to 100 Gb/s across the FSO link and 5 Gb/s across the mmWave link respectively. Selecting the optimum between the FSO or FiWi mmWave wireless channels is performed by means of wavelength tunable transmissions through an all-passive O-/C-band diplexer. A complete algorithmic handover workflow is developed, operating at the active XTABLE I: SUMMARY OF FSO-RF HYBRID COMMUNICATION SYSTEM Reference Year Hybrid Frequencies Technology Switch Type Indoor-Outdoor FSO Throughput RF Throughput Distance RealTime Univ. Victoria [10] 2014 FSO / mmWave Theoretical Soft Simulation - - - - Cal. Tech, Effat [11] 2015 FSO / RF Theoretical Soft Simulation - - - - US Naval [12] 2016 FSO / 70 MHz L2/L3 Switch Hard Simulation 20 Mb/s 5 Mb/s - - Ericsson Research [13] 2022 FSO / THz Theoretical Soft Simulation - - - - Huawei [7] 2024 FSO / 73/83 GHz Parallel links n/a Outdoor - - 800 m - Taipei Univ. [14] 2021 FSO / 28 GHz Optical Switch Hard Outdoor 10 Gb/s 1 Gb/s 600 m No Univ. Aveiro [15] 2022 FSO / 28 GHz Parallel links n/a Indoor 200 Gb/s 10 Gb/s 3 m No Univ. Aveiro [16] 2024 FSO / 28 GHz Parallel links n/a Indoor 400 Gb/s 25 Gb/s 1.2 m No NTUA [17] 2024 FSO / 60 GHz WSS Hard Indoor 40 Gb/s 20 Gb/s 10 m No Fudan Univ. [18] 2025 FSO / 300 GHz Shared Tx n/a Indoor 524.4 Gb/s 524.4 Gb/s 50 m No Ankara Univ. [19] 2003 FSO / 2.4 GHz Switch Hard Outdoor 155 Mbps 11 Mbps 2.9 km Yes Qatar Univ. [20] 2015 FSO / 60 GHz L2/L3 Switch Soft Outdoor 1 Gb/s 100 Mb/s 600 m Yes Georgia Tech [21] 2019 FSO / 60 GHz FPGA Hard Indoor 250 MHz 250 MHz 1 m Yes DLR [22] 2020 FSO / 2-4 GHz Switch Hard Outdoor 610 Mb/s 5.8 Mb/s 300 m Yes Chongqing Univ. [23] 2022 FSO / 13 GHz L2/L3 Switch Hard Outdoor 10 Gb/s 1 Gb/s 500 m Yes ZJU [24] 2023 FSO / 2.45 GHz L2/L3 Switch n/a Indoor 1 Gb/s 750 Mb/s n/a Yes SJTU [25] 2024 FSO / 140 GHz FPGA Soft Indoor 10 Gb/s 1.6 Gb/s 20 m Yes THIS WORK 2025 FSO / 60-80 GHz O-C band Hard Outdoor 100 Gb/s 5Gb/s 50 m Yes 3 Fig. 1:a) Schematic of the proposed SDN-controlled, FiWi X-haul network architecture, transporting ETH traffic with real-time services from the metro network towards the hybrid FSO/mmWave link with automated handovers between the O-band FSO and C-band FiWi mmWave channels induced by adverse weather conditions and instructed by a TeraFlow-based SDN controller, b) progress beyond state of the art real-time hybrid FiWi FSO/mmWave link demonstrations at frequencies ranging from 0.07 GHz radio, up to mmWave at 60/70 GHz and THz/Sub-THz at 140 GHz. haul interface that transmits the ETH traffic to the hybrid-link, utilizing exclusively standard, open-source software tools to monitor and controller the channel and to communicate its status to a centralized Network-controller. The open source SDN workflow periodically polls the status of the wireless link, and upon a link outage, e.g. due to fading by adverse weather conditions, triggers a change in the wavelength for the upcoming frames of the flow, as e.g. being depicted in Fig. 1(a) where the incoming grey ETH traffic flow is initially loaded on the blue-colored ETH flow and subsequently to red-colored ETH frames. The hybrid FiWi FSO/mmWave X-haul is evaluated in terms of traffic-throughput, latency metrics and beamswitching time for the two channels using an indoor setup with 1km-fiber and 25m wireless link-distances in a periodic time-slotted operation. Subsequently, the dynamic operation of the SDN-workflow is demonstrated using intentionally induced link outages via interleaving blocking objects. Finally, the Tx and Rx terminals of the hybrid link are transferred to outdoor locations at the rooftops of two neighboring buildings in the campus with 50m-link distance and benchmarked in a fieldtrial for over 24-hours under both daylight and night-time turbulent conditions of 5 Beaufort, corresponding to a windspeed of >30 km/h. Uninterrupted operation across the 50m outdoor link is demonstrated using real-time services, including 4K-UHD video streaming and iperf measurements with up to 100 Gb/s data rates, achieving an order of magnitude speed enhancement compared to current real-time hybrid links, as depicted in Fig. 1(b), thus shaping a clear technology path towards resilient wireless 6G RANs equipped with self-healing capabilities in all weather conditions. II. EXPERIMENTAL SETUP &DEVICES The proposed FiWi hybrid X-haul transport link leverages the broad-band, low-latency and high energy efficiency credentials of optical wireless links and FSO technologies with the broadband mmWave frequencies operating in parallel under an overarching, intelligent SDN Transport Network controller that supports constant aerial monitoring and link performance evaluation with automated optimal channel selection. An experimental setup was implemented for the Control and Data Plane in the downlink, shown in Fig. 2(a). In the data plane, two servers are equipped with CX6 NVIDIA Smart Network Interface Cards (NICs), exchanging real-time traffic via the hybrid FiWi link. NIC #1 is emulating the interface of the base-band unit (BBU) and NIC #2 is emulating the RAN equipment. Regarding the optical transmissions, the NICs are initially equipped with 10 Gb/s tunable C-band SFP+ transceivers with an emitted optical output power of 1.6 dBm. Following the description of the downlink communication direction, the stream is transmitted first through a 1-km-long single-mode fiber spool with 1 dB overall attenuation, including the fiber connectors, before being fed to the common input of a O-/C-band diplexer with insertion losses of 0.8 dB. After the fiber transmission stage, the diplexer outputs are interconnected to the wireless setup. In particular, the O-band output of the diplexer is in-turn fed to an FSO link of 25 meters in an indoor setup using 42.5 mm aperture fiber collimators, that is later-on extended to a 50 meter link in the outdoor one. On the other hand, the C-band output of the diplexer is fed to a mmWave antenna unit with the mmWave transmitter and receiver placed in parallel with the FSO lenses and thus places at exactly the same distance. FSO transmission is carried out using two air-spaced achromatic fiber collimators, mounted on two-adjuster kinematic mounts with ±3○angular adjustment range and 80 mm effective focal length to the fiber. The mmWave link employs directive digital RoF antennas designed for 5 Gb/s traffic, electrically upconverted at 60-80 GHz with adaptive coding-modulation between a 2.5 Gbaud QPSK and 1 GBaud 32-QAM. At the RAN side, the two outputs are multiplexed via a O-/C-band diplexer and received by NIC #2, to reconstruct the ETH frames and transmit to the Remote Radio Head (RRH). For the indoor experimental demonstration, portable FSO/mmWave terminals shown in Fig.2(b) were implemented, comprising the FSO and mmWave transmitter or receiver and the O-/C-band diplexer. The power consumption of the devices in the data plane are: 1.65W per tumnable SFP+ transceiver by FS, with minimal and almost negligible 0.05W compared to its conventional, fixed, static equivalent SFP transceiver, 50W for a mmWave antenna unit, and zero power for the diplexer or the FSO collimators. 4 The FiWi X-haul data plane is controlled via its SDN agent, as shown in the top insert of Fig. 2(a). The NETCONF protocol [29] is used for the interaction of the controller with the agent, via the creation of custom YANG data models that described the channel-band (Oor C-band) of the tunable SFPs and in-turn the FiWi FSO/mmWave X-haul topology. At this point, it is worth noting that any wavelength that were lower than 1538 nm would emerge at the O-band output port of the diplexer, owing to the sharp spectral-filtering response of the diplexer, and therefore would be routed to the FSO stage. The open source tools Sysrepo [30] and Netopeer2 [31] are used in the current work, aiming to avoid the use of any proprietary SDN solutions and ensure inter-operability with any standard networking equipment that supports SDN basic functionalities via linux/unix based terminals. In our solution, sysrepo serves as a datastore for the YANG configurations defined by NETCONF, while the agent Netopeer2 is a server managing NETCONF packets and storing them within Sysrepo. The developed SDN workflow maps the two wireless channels of the hybrid link to two channel numbers of NIC #1 via the YANG model (eg. NIC Tx id 1) to two channel numbers ‘1’ or ‘2’ that correspond to different wavelengths. Once sysrepo data store is created, it instructs the selection of the desired FSO or mmWave path via a custom SDN script selects the proper wavelength emission. This script has been developed to support two different types of transceiver solutions: a first one that has been developed to access and tune the EEPROM configuration of the tunable SFP+ transceiver modules via an i2c interface, mapping the SFP-channel number of the YANG model to the selected wireless path of the sysrepo data-store or a second one that selectively activates either a NIC-interface with an O-band QSP transceiver or a NIC-interface with a C-band SFP transceiver module. Subsequently, another wireless-channel tracking script is developed with periodic and infinite monitoring of the interface status, in order to automatically detect link failures, e.g. by adverse weather conditions. Upon a link failure after a certain number of consecutive pings, this script, triggers a switch of the hybrid FiWi link to the alternate the path. In this work, this SDN controlled threshold-delay was arbitrarily selected to be 30 s, as we intended to study the changes in weather conditions that typically do not occur abruptly, but rather evolve gradually over time, without misinterpreting temporary fluctuations as permanent link failures. As the threshold is not fixed or hardcoded, it can be programmed and adjusted depending on the adaptability of the system to different weather conditions or link-statistics, e.g. small delays could allow more frequent handover-activations. A high-level overview of the developed automated workflow in the Control Plane is shown in Fig. 3. The SDN workflow operation is separated in four stages. At time t1, Netopeer2 is initiated and the SDN controller continuously monitors the status of the interface. If the transmission link fails for more than 40 seconds, e.g. due to an outage of the FSO at t2, the SDN controller automatically issues a NETCONF command at time t3 to alter the routing table, updating the channel entries in the YANG model stored in Sysrepo. A modification to the YANG model’s description triggers an update to the configuration .xml file at time t4, which is automatically detected by the client device through a linux-daemon process based on inotify tools [17]. Upon detecting a change, a parser is automatically activated to identify the updated values in the YANG description of the .xml file and at time t6 it executes the command to tune the wavelength of the SFP+ modules, thereby properly changing the alternate wireless channel. The developed automated workflow is shown in Fig. 3. The SDN tools were developing targeting compatibility with the 3GPP Teraflow controller [32], while constantly communicating with the Netopeer2 agent on the NICs at port 830 via NETCONF protocol. Custom YANG data models have been defined for mapping the O-/C-band wavelength-bands and Sysrepo functions as a datastore of the wavelength-routing table for the YANG configurations. Additionally, another script continuously monitors the interface status, automatically detecting link failures, and switches the wavelength of the hybrid FiWi link to alternate the path when necessary. The workflow Fig. 2:(a) Schematic of the experimental setup of the indoor O-/C-band hybrid FiWi FSO/mmWave, including the SDN Control-Plane and Data-Plane and (b) Photo of assembled mobile transmitter and receiver terminals in indoor 25m-setup. 5 Fig. 3:SDN workflow for zero-touch handover using photonic-wavelength routing. is divided in 7-steps: At time t0, Netopeer2 is initiated to monitor the interface status. Upon a link failure for over 40 sec at t2, it issues a command at t3, modifying the routing table and entries in the YANG model, which in turn triggers an update of the .xml file at t4, while at t5a parser instructs a handover. At t6, the command is executed to adjust the wavelength between Oand C-band, effectively switching to the alternate wireless channel. While the current implementation focuses on a point-topoint scenario, the proposed SDN-based handover strategy can be extended to support multiple FiWi nodes in more complex SDN-controlled topologies such as mesh or ring-based 6G X-haul architectures, incorporating features such as link-state monitoring across nodes, and distributed control policies to enhance scalability and resilience. III. INDOOR DEMONSTRATIONS The first indoor experimental evaluation of the hybrid link is based on programmable 10G C-band SFP+ transceivers, supporting a tunable wavelength emission ranging from 1528.77 nm to 1563.86 nm and transmission distances up to 80 km. Here, as the O/Cband diplexer featured a band-edge in its transfer function at around 1540nm, all wavelength channel emissions at wavelengths lower than the 1538nm would emerge in the O-band output port of the diplexer and thus to the FSO. The receiver sensitivity for 10Gb/s transmission is measured at -15 dBm, resulting in a power budget of 16.5 dB. The 1x2 O-/C-band diplexer is characterized using a tunable laser source swept between 1530 nm-1565 nm and measuring the optical power at the two outputs of the diplexer. The insertion losses are 0.8 dB, as shown in the obtained spectrum of Fig. 4(a). The C-band channel of the diplexer features a flattop response centered at 1549 nm and a 3dB bandwidth of 18 Fig. 4:Static characterization of the all-passive 1x2 C-/O-band fiber-based diplexer including (a) Spectrum of the Diplexer at the C-band and SFP+ transmissions through the Diplexer (b) O-band and (c) C-band output at the selected channels. nm, as depicted with the red color curve. The transfer function of the O-band channel of the diplexer, as depicted with the blue color, features a complementary spectral response. The wavelength-edges and crossing points of the Oand C-band channel spectral transfer functions are measured to be at 1540 nm and 1557 nm, thus all wavelengths lower than 1538 nm emerge in the O-band output port of the diplexer, allowing to operate our hybrid links even with the current tunable SFP+ transceivers. Based on the static characterization of the diplexer, wavelengths λ1=1537 nm and λ2=1554 nm of the SFPs, each clearly appearing at the O-band and C-band outputs ports of the diplexer transfer function, are selected for the FSO and mmWave link respectively, allowing distinct FiWi channel separation with a crosstalk of -35 and -30 dB. The captured spectrum for transmissions at λ1and λ2through the common path of the diplexer, measured at the O-band output is shown in Fig. 4(b) and for the C-band output in Fig. 4(c), showing the separation of the two channels. The two FiWi links are first characterized while operating statically. For the FiWi mmWave link, the eye diagram is captured at the input of the transmitter antenna, featuring an ER of 6.88 dB and a Q-factor of 11.2 as shown in Fig. 5(a). For the FSO link, the eye diagram is captured after the FSO receiver, featuring an ER of 6.86 dB and a Q-factor of 9.68, as shown in Fig. 5(b). Both links exhibit error-free operation. Additional measurements are carried out using the Iperf3 tool to measure the data transmission rate on the two links. The achieved maximum data rates are 9.42 Gb/s capacity for the FiWi FSO link and 4.79 Gb/s for the FiWi mmWave link. The links are further characterized in terms of latency. Latency measurements are conducted using the ping command for the 6 Fig. 5:Static characterization of the wireless channels including: a) eye diagram of the transmitted signal at the input of the Tx mmWave antenna, b) eye diagram of the transmitted signal after the Rx FSO, c) latency measurements through the experimental setup and (d) histogram of the latency distribution for transmissions through the FSO and mmWave channels. communication between the two servers. At first, a short 30-meter fiber link is established between the SFPs and 1,000 consecutive latency measurements are conducted with an interval of 1 second with a packet size of 64 bytes. The average round-trip time is shown to be 135 µs. Then, a 7km fiber is added between the two NICs of the servers, and the same measurements are conducted again, resulting in an average 36 µs delay. The transmission is then carried out through the 7 km-fiber and the 25-meter FSO wireless link, revealing an average extra latency of 8 µs. Finally, the FiWi link through the 7-km fiber and the mmWave antennas is also characterized in terms of latency, revealing 60 µs latencypenalty, when passing via the mmWave due to the limitations of the RF signal-processing chain that requires electro-optic (e-o) and opto-electronic (o-e) conversions following by up- /down-conversion stages. Having completed the latency measurement traffic statics, the recorded minimum, average and maximum measured round-trip times for each transmission scenario are plotted in Fig. 5(c). All packets are successfully transmitted during these measurements without any packet drop between, while the comparative latency measurements for the two links indicated the FSO link as the one with the higher bandwidth, lower latency and lower power consumption due to the lack of optoelectronic conversions, therefore it is selected and promoted as the main link of choice under stable aerial channel conditions. To evaluate the packet error rate and jitter of our system we statically set the transmission over the FSO or the mmWave channel and performed 1,000 consecutive wireless transmissions using the ping tool with a packet size of 64 Bytes and captured the packet level statistics. The latency Fig. 6:Dynamic SDN-controlled operation including: (a) time-slotted traffic to the FSO/mmWave links, (b) capture of the slot rise-time and (c) real-time handover operation of the hybrid link. distributions of the two links are depicted in Fig. 5 (d), showing a slightly higher standard deviation of 0.025 ms for the mmWave link, compared to the 0.020 ms of the FSO link. Measurements on both links verified that all packets were transmitted successfully with no packet loss. The next step is the evaluation of the time-slotted operation of the hybrid link. Periodic pre-determined slots of 1s with wavelength routed FiWi downlink are used to evaluate a dynamic on-demand wireless path selection. This is performed by interleaving a 99% −1% coupler in the optical links before each wireless path to monitor the real-time traffic circulated between the servers using a real-time oscilloscope (RTO). Fig. 6(a) shows the captured traffic being circulated alternately between the two wireless links in time-slots of 1s, indicating a clear separation between the slots of the two FiWi channels. As shown in fig. 6(b), a close zoom in at the beginning of the traces of the time-slots exhibits a rise time of 25µs for the wavelength-routed operation, that defines and selects the wireless channel. Finally, the automated SDN process for the real-time handover between the FSO and mmWave links is tested and evaluated after manually inserting a blocking elements alternately in one of the two channels of the hybrid links, to induce deep signal-fading and channel-outage, that in-turn triggers an automated handover. The trace of this induced dynamic handover is measured and evaluated at the RTO, using a setting for large time-recording of the 10Gb/s signal traffic for a duration of 2s with an ultra-small sampling rate of only 5 Ms/s, that makes the trace appear a bit ‘blurry’ in the vertical y-axis of the amplitude shown in Fig. 6 (c). Specifically, the time trace is captured after the FSO receiver collimator, monitoring the FSO link. Since one of the two channels is monitored, two handover loops are depicted to demonstrate the complete handover operation from the main FSO link to the mmWave link and back to the FSO link again. The time-traces show the operation of the two hand-overs, which are also further described by inserting seven time indicators t1-t7 as pointers 7 for the different stages of the handover workflow of Fig. 3. As it can be seen, the FSO link is operating properly since time t1, is then blocked at time t2 and the wireless connection is lost, therefore marker t2 indicates the lower signal-power of the FiWi channel due to its outage. This outage triggered a wavelength-switched handover that routed the traffic to the mmWave link after 40 s. This is depicted as the first loop of the handover process. At a later time, the mmWave link is in turn blocked, triggering a subsequent handover back to the FSO link. This is shown in the captured time trace at time t5, where the traffic is passing through the FSO link again. It is worth noting that the test was performed in the FiWi downlink direction, including all SDN control processes at NIC #1, to reduce the complexity of the setup, while in real bidirectional operation, all respective interfaces at the RAN side of the network need to be automated. IV. OUTDOOR FIELD TRIAL DEMONSTRATIONS Additional experimental field trial demonstrations are performed in an outdoor setup to showcase the dynamic switching operation over a 24-hour period, aiming to increase the demonstrated capacity ten times, achieving 100 Gb/s operation through the FSO link. For this experimental demonstration, two-port NICs are utilized and are now equipped with a 100 GbE (4λx25 Gb/s) CWDM QSFP transceiver operating in the O-band with 5dBm output power and a power consumption of 3.5W and a 10Gb/s C-band SFP+ transceiver with 1.6dBm output power and a power consumption of 1.6W. In this case, the selection of the emission wavelength is performed via the script that activated of de-activated the respective interface of the NICs. The downlink streams are coupled together in a similar dual-band O-/C-band FiWi downlink transmission again and transmitted through the experimental setup with the diplexer, exactly as performed at the indoor setup and described in Fig. 2(a). At the receiver side, the Oband transmission through the FSO link is fed to the the port of the NIC where the 100 GbE transceiver is connected, and the C-band transmission to the mmWave antennas to the port with 10 GbE transceiver. The assembled FSO/mmWave terminals, mounted in portable racks, are placed on the rooftops of two neighboring buildings in the campus at a distance of 50 meters, as shown in Fig 7 (a) for the rooftop-communications link between the two buildings at daylight conditions, with the night-views of the link as seen from the Tx and Rx side in Fig. 7(b) and (c) respectively. Output power and spectrum measurements of the hybrid links are performed for a 24-hour long recording period, conducted here under both daylight and night-time turbulent conditions at a wind speed of at least 5 Beaufort according to local weather-monitoring log-files, that correspond to a wind speed greater than 30 km/h. The received signal after the FSO transmission, as the primary path, is recorded and evaluated in terms of signal quality with an indicative time trace for channel λ1 of the 4 QSFP channels for 8 ns being depicted in Fig. 8 (a), captured in the RTO. The eye diagrams of all four channels indicate clear data transmission, with a 40 second pulse width and a total data rate of 4×25 Gb/s per λ= 100 Gb/s. The spectrum of Fig. 7:Photos of the outdoor 50m experimental setup (a) in daylight, (b) in night-time from the Tx or (c) Rx point of view. the diplexed multi-band transceiver outputs at the Oand Cband is depicted in Fig. 8 (b), showing the four wavelength (4λ) transmission of the 100G transceiver at the O-band, and the emission wavelength of the 10G transceiver at the Cband. End-to-end throughput and packet loss were evaluated through the iperf tool for an optical dynamic range of >7dB power level for the FSO and 22dB for the mmWave, the latter being greater due to the o/e-regeneration-amplifications at the antenna. The traffic statistics shown in Fig. 8(c), reveal 93.7 Gb/s capacity. Latency measurements with the ping tool for 1million 65Kbyte-packets showed a normal latency distribution with 0.45ms mean latency, jitter-deviation of 30µs and a zero packet loss under still weather conditions. Finally, the resiliency of real-time ETH services and the dynamic FSO/mmWave handover were benchmarked against 24h-operation. The received optical power was continuously monitored as shown in Fig. 8(d), exhibiting fast, deep fading phenomena attributed to wind-induced mechanical instabilities or vibrations, causing temporal misalignments of the setup, adding peak-loss 13 dB and 9 dB in daylight and nighttime conditions. When the losses exceed the defined power threshold for the outage-induced handovers, as marked with a blue horizontal line in the power measurements of Fig 8(d), and the connection is considered then to be lost by the SDN controlled channel-monitoring agent, an automatic real-time handover process is initiated, switching the traffic flow to the FiWi mmWave path, according to the SDN workflow described in Fig. 3. In this case, instead of performing wavelength tuning through the tunable SFP+ transceiver, the different wavelength-band emission is executed by enabling and disabling the respective NIC interfaces of the NIC interfaces with the QSFP or SFP that are each routed to a different wireless link. To verify stable operation, real-time video streaming services were employed, emulating a lowlatency eMBB scenario, transferring an 8K-UHD to a client 8 Fig. 8:Experimental results of the outdoor 24-hour field trial demonstrations including: (a) time trace of 25 Gb/s per λtransmission and eye diagrams of the signal transmissions of the four λ(b) Spectrum of the diplexed optical downlink of the 100G (O-band) and 10G (C-band) transceivers, (c) iperfbandwidth of the FSO/mmWave downlink, (d) 24-hour Daylight/Nighttime optical power measurements and (e) Real-time 8K video transmission through the hybrid link. VLC media player, resulting in a smooth media delivery for 30 minutes, as shown at photo of a real-time streamed frame at the client end-point terminal in Fig. 8 (e). V. CONCLUSIONS A hybrid FiWi FSO/mmWave X-haul with recordhigh data rate of 100Gb/s and automated resiliency in real-time services with zero-touch handovers is presented, achieving an order of magnitude speed enhancement compared to previous realtime hybrid links, surpassing for the first time the conventional 10Gb/s barrier at the linerates of edge networks. Moreover, the use of open source SDN tools and the complete algorithmic workflow development allow for vast deployment of the proposed technology at various standard X-haul networking equipments, red that can be easily extended from a single hybrid link towards multiple real-time FiWi mmWave and FSO interfaces with Coordinated MultiPoint Transmission control [33], [34], FSO mesh networks [35] and resilient broadband wireless networks for 6G [36], thus shaping a promising holistic, hardware-and-software technology roadmap towards broadband, flexible, terrestrial 6G networks with self-healing features in all weather-conditions. 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