SDN-enabled THz Wireless X-Haul for B5G

— With the explosive data growth of user traffic in wireless communications, Terahertz (THz) frequency band is envisioned as a promising candidate to support ultra-broadband communications for beyond fifth generation (5G) networks. Software-based networking is being adopted in mobile communications to improve efficiency and reduce operational costs. This paper presents the design of a comprehensive SDN management architecture for joint optimization of radio and network resources. The proposed architecture obtains the most added value out of the use of THz technology integrated with software managed networking for mobile network beyond 5G. In this paper, leveraging optical concepts and photonic integration techniques for an ultra-broadband and ultra-wideband wireless system is presented.


I. INTRODUCTION
While the rollout of 5G worldwide is just in its infancy, and the joint development/standardization process by 3GPP/ITU-T is still ongoing, researchers from industry and academia are already looking ahead to what will come next.A clear picture of what Beyond-5G (B5G) and 6G networks will be like is premature now, but experts are outlining the envisioned requirements, use cases and enabling technologies for the nextgeneration network landscape a decade from now.It is expected that the main trends emerging and guiding the 5G development will further grow and evolve to establish the foundation of B5G/6G [1].The need for huge capacity at very low latencies is shifting attention to higher frequencies of the electromagnetic spectrum, where much larger bandwidths are available.This trend has already started in 5G, with the opening of mmWave frequencies (above 24 GHz) for the new 5G-NR air interface, complementing the traditional sub-6 GHz band shared with 4G and legacy access technologies.For B5G/6G the trend will continue up to the THz band (252-322 GHz) and infrared/visible light (optical wireless communication).The downside is that at such frequencies, the coverage of each antenna reduces drastically, due to the different radio propagation conditions.Thus, the number of cell sites must be increased accordingly, to ensure full coverage and uniform spatial capacity.This trend has already started too, with the concept of heterogeneous networks, composed by macro, micro, pico, femtocells and multi radio access technologies.The concept will be extended in B5G/6G to three-dimensional coverage employing non-terrestrial access points (drones, highaltitude platforms, satellites) complementing traditional terrestrial infrastructures [2].Aerial access networks show good potential as a complement of traditional terrestrial solutions in many scenarios of relevance for network operators.Ubiquitous connectivity in very low-density scenarios, or quick and ad-hoc provision of mobile connectivity to reinforce or replace terrestrial network infrastructure in temporal events, emergency or disaster recovery scenarios, are some examples, typically addressed nowadays with temporal infrastructure.Adaptability to a varying target coverage area, flexibility to deploy, high probability of LOS and combination with other parallel applications as imaging, surveillance, etc., are some of the main benefits that aerial moving nodes may unlock.Many of the applications, linked to the access capacity will demand in parallel the availability of high throughput transport connectivity between moving nodes and ground stations, aside from technologies to handle mobility, as beam steering.
Throughput needs in backhaul networks are expected to increase progressively towards the tens to hundreds of Gbps as the access networks and services evolve and extra capacity layers are implemented through network densification.As the radio access network gets denser, wideband point-to-point radio links become a necessary complement to optical fiber, especially in the last-mile section of the backhaul networks.The main reason is to enable a more flexible deployment both in terms of investment prioritization and lead time, as not all the sites can be connected via fiber cost-effectively or in a time frame that may be acceptable for the launch of new services.Because of the Gbps-level capacity needed for 5G backhauling links, and the short inter-site distances which are typical in high-density high-capacity macro or small cell deployments, mmWave and THz bands represent the viable solution, towards B5G/6G.Moreover, inter-band aggregation also benefits applicability of these technologies to other scenarios less demanding in terms of throughput, finding compromises between the latter, availability and range.While the establishment of different services in 5G and B5G networks will come in a progressive way, backhaul networks need to get prepared in advance.Penetration of existing mmWave technologies in operator networks, like E-band radios, is already showing consistent Year on Year (YoY) growth as enabler of multi-gigabit transport capabilities, backing up the need for further research and development in the field.Furthermore, mmWave and THz bands are also indispensable to enable the concept of X-Haul transport, in which base stations get separated between low-complexity Distributed Units (DU) located at the cell/antenna site and highcomplexity Centralized Units (CU), grouped together with other CUs into a few metro sites.Radio access network architecture has been typically distributed, with all the radio processing done at site, close to the antenna.
However, the 5G architecture allows for virtualization and centralization of all or part of the baseband functionality of the radio nodes.This has direct benefits (ultimately dependent on the functional split that defines what is kept distributed and what is centralized) in terms of efficiency, scalability, latency of some radio protocols as well as simplification of the sites themselves, which eases the operation, installation and management of the radio access networks.Simplification of radio nodes also becomes a key in aerial access network scenarios, where the energy consumption, form factor and weight are decisive relevant applicability aspects as autonomy or regulation constraints for the operation.According to the specific split option [4], the traffic between DUs and CUs exhibits very different features and requirements in terms of capacity and latency, from the conventional Backhaul (in which no split is employed and the CU is co-located with the DU at the cell/antenna site), up to full Fronthaul (in which the CU retains almost all the processing logic of the base station and the DU is simply a front-end for transmission/reception at the air interface).Mobile aggregation/transport networks will support a heterogeneous mix of such traffic types, denoted as X-Haul or Mid-Haul.Splits closer to fronthaul impose too stringent requirements in terms of throughput to current mmWave transport, and generally are only feasible with fiber connectivity, so new equipment operating in higher bands will also mean extra flexibility to implement distributed approaches via radio transport.
In general, the networks will become more complex, so harder to control and manage.The shift toward architectures based on Software-Defined Networking (SDN) and Network Function Virtualization (NFV) is therefore paramount to endow networks with "intelligence", such that they can be autonomous, dynamic, modularizable, resilient and cost-efficient.Centralizing the control plane enables global optimized routing decisions and makes the network flow programmable to fit specific requirements and it also helps simplifying operation of multi-vendor and multi-technology networks through appropriate architectures and standard information and data models and interfaces.SDN can enable then a programmable transport network, which is able to create multiple and isolated transport slices, where transport resources may then be allocated dynamically, interconnecting physical and virtualized network functions distributed geographically.Therefore, operators are transforming their transport networks moving to SDN-enabled architectures.Multi-operator initiatives are also in place to align vision, architectures and use cases, generating traction and development in the industry [3].SDN implementation in live networks has already started in many networks, and stringent requirements in terms of integration in the full SDN ecosystem and support of standard models and interfaces are being derived for any transport solution under consideration for deployment.Not only because of all the technical benefits, but also due to the relevance that the SDN architecture has as an enabler of slicing, which is a key to develop new service and business models linked to differentiation of service performance and quality targets.
Motivated by all these trends, this paper presents an SDNenabled, end-to-end architecture for managing fixed network infrastructure integrated with ultra-wideband X-Haul network based on THz radio links.This system is designed as part of the TERAWAY H2020 project [5].The main use case is the 5G coverage of outdoor events using moving nodes that consist of heavy-duty drones carrying gNBs or their radio parts (remote radio heads).Because of the diverse technical challenges, the project will face, we expect to gain valuable insights into the capabilities and limitations of such technologies for B5G/6G networks.This paper is structured as follows.Section II introduces the current state of the art in 5G networks which consists of the Service-Based Architecture (SBA).Section III presents a Software-Defined Network (SDN) architecture for managing the mobile network infrastructure.Section IV describes the design of THz based X-haul radio infrastructure to be integrated as part of the transport for 5G and B5G mobile networks.Section V introduces the SDN-based end-to-end TERAWAY architecture for efficient management of fixed and THz X-haul transport.Conclusions and final remarks are in Section VI.

II. 5G SERVICE-BASED ARCHITECTURE
3GPP, in its Release 15, already introduced a new architecture in the specification of the 5G Core Network, in TS 23.501 [3GPP23.501].This architecture of the 5G Core (5GC) follows several principles that are mainly targeted for reaching higher flexibility, supporting many different use cases.This includes the introduction of service-based principles, where network functions provide services to each other.A clean control plane/user plane split allows independent scaling of control plane and user plane functions and supports flexible deployments in terms of where the user plane can run (this principle was, in fact, already introduced in EPC in Release 14).The architecture allows for different network configurations in different network slices.The 5GC control plane is based on the Service-Based Architecture (SBA) depicted in Figure 1.The network functions communicate with each other via a logical communication bus they can provide services to each other.A network function instance is registered to a Network Repository Function (NRF).Using the NRF, a network function instance can find other network function instances providing a certain service.The goal of such architecture is to get a higher flexibility in the overall system, and to make it easier to introduce new services.The SBA based 5GC facilitates adding new Network Functions (NF) that implement new functionalities.Thus, in case of introducing new transport technologies, the SBA allows to design a new NF that will handle the specific management of the new network and radio technologies to deliver end-to-end transport.Those NF could benefit from SDN technologies for a more effective management of network and radio resources as described in next sections.

III. SOFTWARE-BASED NETWORKING ARCHITECTURE
SDN has been widely deployed and applied in the areas from data centers to enterprise networks and WANs.Then, it has started expanding into the field of transport network, at first in IP/Optical multi-layer networks, then, in the last few years, in the microwave transport.The success of SDN in fixed networks has motivated network operators to promote a similar approach for managing the microwave radio equipment and links.However, these kinds of networks have peculiar characteristics that make them differ radically from their fixed counterparts.A non-exhaustive list can be the following.
• They suffer from unpredictable impairments at the air interface, such as frequency interference, fading, and multipath.The countermeasures include adaptive modulation and coding (ACM).A change in the weather brings dynamic adjustment to air interface modulation and accordingly results in dynamic bandwidth adjustment.• They are sensitive to bandwidth capacity, so they typically feature the technique of physical link aggregation, such that the insufficient capacity at the air interface can be addressed by binding several links together.• They have tree or chain network topologies in most cases and ring topologies in few cases, the latter resulting in few redundant paths.For 5G backhaul, such as small cell backhaul or dense site deployment, it is expected to be more mesh networks.
• They have a control channel with limited bandwidth and unreliable, and the real-time requirement is also difficult to meet.
These criticalities are the major reasons why mobile network operators are locked in single-vendor equipment and network operation, configuration and maintenance activities are performed manually, thus difficult to change on run time.They are also tied to use Network Management Systems (NMS) directly provided by equipment vendors, with specific proprietary interfaces, and independent from fixed network NMS.Furthermore, this vendor specific NMS prevents the use of advanced applications that could provide more sophisticated features such as network slicing, dynamic power management or multi-layer coordination, among others.The SDN paradigm is thus still at its infancy for radio transport networks, but its potential is very promising.With the features of centralized control and openness, a network becomes simpler and its O&M is more efficient.The network is no longer "passive" but can have service awareness.According to service features, the network is automatically adjusted to improve its quality and ensure efficient service configuration.Open and standardized interfaces are a prerequisite for efficient management of equipment from multiple vendors, integrated in a single system/controller.

IV. X-HAUL FOR 5G AND BEYOND
The TERAWAY project is developing THz transceivers that leverage optical concepts and photonic integration techniques.This allows for the implementation of high-capacity radio links operating in an ultra-wide range of carrier frequency bands covering the W (92-114.5 GHz), D (130-174.8GHz) and THz band (252-322 GHz), using multichannel multibeam transceivers with increased directivity due to use of optical beamforming.To that end, the project is investigating the possibility to organize the spectral resources of a network within these bands into a common pool of radio resources that can be flexibly coordinated and used for given deployment and application scenarios.In specific terms, the target of the project is to develop transceivers that implement a 2-or a 4-channel radio module.The radio features are: operation from 92 up to 322 GHz, data rate per channel up to 108 Gb/s, transmission reach of more than 400 m in the THz band, and possibility for the formation of wireless beams that can be independently steered in order to establish backhaul and fronthaul connections between a set of fixed and moving nodes.TERAWAY's radio system data-plane consists of the baseband and the optoelectronic units (Figure 2).Each of these units is associated with, and interfaces to a different management processor which runs Linux.Based on this operating system, appropriate drivers will be developed to enable low-level operations with the hardware.This choice offers flexibility and increased modularity since different combinations of baseband units and analog front ends can be tested without creating impediments to the system management.These radio modules will include a NETCONF agent whose purpose is to map highlevel operations coming from the SDN Controller into low-level API commands.This agent will serve requests and responses in the form of standardized ONF TR-532 YANG model, encoded in XML.The radio controller, which initiates NETCONF sessions towards the different agents, will retrieve alarms, statistics, and performance metrics, configure radio parameters and run radio resource optimization algorithms.NETCONF and OpenFlow controllers are required to support different vendor switches.Finally, the radio controller itself will provide appropriate APIs to the transport network controller for joint optimization and coordination between switch and radio element controllers.The SDN controller requests periodically performance metrics such as the SINR and RSSI values, evaluates link quality [6] and runs specific algorithms in order to detect interference.In case interference is detected, an alarm occurs, and the operator input is needed on whether to allocate new licensed or unlicensed spectrum.
• In case of licensed spectrum allocation, the SDN controller runs the radio planning-based frequency selection algorithm and is in charge of triggering frequency change within the licensed spectrum block, in order to minimize interference.• In case of unlicensed spectrum assignment, the SDN controller initiates frequency scanning and based on the results from the elements, runs a frequency selection algorithm and triggers a frequency change in order to minimize interference.Other countermeasures to transient or permanent interference except for frequency change include: in digital/algorithm domain, re-configuration of beam-forming antennas (beamnulling techniques); in space domain, adapting traffic distribution; in service domain, adapting some parameters (e.g., CIR and PIR rates for specific flows) to the new conditions.
The TERAWAY project is developing a new software defined networking (SDN) controller and an extended control hierarchy that will perform the management of the network resources (SDN switches) and the radio resources in a homogeneous way.These management features provide potential benefits in terms of network performance and energy efficiency enhancements, as well as the possibilities for the provision of network slices to support heterogeneous services.There is a plethora of applications and use cases of SDN in the context of mmWave/THz communications and one of the most predominant ones, related to the radio subsystem, is interference detection and frequency allocation [7].The road to mainstream adoption of Terahertz communications is still a long way ahead and the licensing schemes have not been defined yet, thus both licensed and unlicensed spectrum policies should be taken into account.Flexible and innovative licensing strategies combined with SDN capabilities can significantly simplify the way operators control the underlying network.To add to this, the ultra-wide range of carrier frequencies that TERAWAY's transceivers can operate on opens up new opportunities while creating new challenges to the network operations management.Figure 3 shows the sequence diagram between the radio controller and radio elements, describing an interference detection and frequency allocation use case.V. TERAWAY ARCHITECTURE TERAWAY is designing and developing an innovative SDN controller that will perform the management of the network and radio resources in a homogeneous way.The SDN controller should be an integral part of the 5G and beyond architecture to provide the required benefits for network performance, energy efficiency and slicing-enabled capabilities.Such SDN functionality becomes part of a new network function (NF), named Mobile Backhaul Orchestrator (MBO), and compliant with the Third Generation Partnership Project (3GPP) specifications as part of the Service Based Architecture (SBA).
The SDN controller is needed to manage the whole TERAWAY communication system in a centralized way, by receiving management requests (typically, provisioning, monitoring, fault reporting) at the network level, through the North-Bound Interface (NBI).These requests are elaborated and transformed into element-level management commands (typically, configuration, monitoring, subscription to notification events), that are sent to the managed network elements through the South-Bound Interface (SBI) of the controller.Similarly, in the opposite direction, all responses and notifications generated by the network elements are received from the SBI and processed to be transformed to network-level messages that are sent to the NBI.It is worth noting that in general, the controller exposes several different NBIs and SBIs.In the north-bound direction, there are as many NBIs as the number of higher-level controllers or orchestrators managing the overall system.Similarly, in the south-bound direction, there are different SBIs according to the different kinds of network elements that must be managed.Figure 4 depicts the logical SDN management network architecture, superimposed on the TERAWAY physical network.The figure highlights the relations and connections among SDN entities present in both the controller and the network devices (also known as Management Plane).However, it is important to note that this is a logical topology, where the actual SDN flows are transported on the same physical connections used for the Data Plane.In other words, the entire management traffic is in-band.The separation of management traffic from payload traffic in terms of bandwidth and Quality of Service (QoS) is performed by the network slicing techniques through the reservation of a "Slice 0" for this kind of traffic.For safety reasons, a fallback communication mechanism will be developed for the drone management traffic, employing a separate, out-of-band wireless transmission system, typically provided by the drone manufacturer.In the TERAWAY system, three kind of network elements are managed, each paired with a different subsystem: • Switch Element Controller: The SDN subsystem in charge of controlling the SDN Ethernet switches present in both the Fixed Node (FN) and the Moving Node (MN), by means of the OpenFlow protocol.
• Radio Element Controller: The SDN subsystem in charge of controlling the TERAWAY radio modules present in both the FN and the MN, by means of the NETCONF protocol.
• Drone System Controller: The Drone System Controller is the SDN subsystem in charge of controlling all the nontelecom systems needed for the operation of the drones hosting the MNs.
Furthermore, the Transport Network Controller sits on top of those three controllers to expose one or more NBIs to higher layer SDN controllers and provides a complete network-level view of the underlying system.This makes it possible to perform all the operations related to the overall optimization of transport network resources, including, for instance, dynamic traffic re-routing and network load optimization.

A. Switch Element Controller
The switch element controller is the SDN module that handles the management of fixed switches shown in Figure 5.The switch element will allocate the required network resources in the physical switches using OpenFlow.The MBO will instruct the Switch and Radio elements to reserve the required network and radio resources to support multiple network slices.
The management traffic will utilize the "Slice 0" with higher priority while other traffic will be routed through additional slices.TERAWAY is used for the transport of x-haul (i.e., a mix of backhaul and fronthaul) and the 5G Quality Indicators (5QI) should be extended with TERAWAY specific QoS parameters to deliver the high reliability and low latency slices.An example of mapping between TERAWAY slice parameters and 5G QoS classes is given in Table I.The switch element controller will interact with network switches using either OpenFlow or NETCONF to configure the different VLANs and their priority, such that the traffic from T0-T3 slices will be scheduled with higher priority than traffic from 5QI coming from mobile users.The integration of different applications into the TERAWAY system will follow 3GPP standard mechanism, where the application consumes NEF services to register itself and request the specific network slice.

B. Radio Element Controller
The There are currently two open radio models that are readily available for the management of point-to-point microwave network elements: TR-532, defined by the ONF and RFC-8432, defined by the IETF.These open standardized interfaces allow connecting of multi-vendor devices to an open source SDN Controller enabling the development of independent third-party applications.Both models exhibit an extensive set of parameters that can be configured and retrieved for the management of a microwave radio link.Specifically, the ONF TR-532 has been chosen as the basis for the development of the TERAWAY SDN architecture.In case some TERAWAY features cannot be covered by the last version of TR-532, a specific extension will be developed and implemented.

C. Drone System Controller
The Drone System Controller is the SDN subsystem in charge of controlling all the non-telecom systems needed for the operation of the drones hosting the MNs.In this case, the kind of information that must be carried by the SBI is quite diversified, because it has to deal with the configuration and monitoring of parameters relative to different drone subsystems, like the navigation and positioning, the power management, the status of vital parameters and related alarms, and possibly others.The SBI for drone control most probably will consist a combination of proprietary protocols that must be adapted to be transported on the TERAWAY radio link.The Transport Network controller part of the Mobile Backhaul Orchestrator will allow the Drone System Controller to request additional high reliability slice dedicated to the drone control operations.A fallback mechanism shall be provided for safety reasons, allowing drone management traffic to be transported over the proprietary wireless communication system provided by the drone manufacturer.

CONCLUSIONS
This paper has presented the TERAWAY architecture for the management of an ultra-wideband THz radio network for the transport of 5G and B5G x-haul.The proposed architecture is based on SDN, which fully enables the optimal management of end-to-end network slicing and allows an effective integration with the 5G SBA architecture defined by current 3GPP standards.The proposed TERAWAY SDN controller includes different modules for managing the network element, using OpenFlow for fixed Ethernet switches and NETCONF for THz radio modules, employing an open radio model based on the ONF TR-532.In this controller design, an overlay Transport Network Controller has been designed to provide a single interface for managing both fixed Ethernet switches and radio modules and provide a single coherent interface to north-bound modules.The initial mapping of network slices and priorities has been defined as part of the controller design to allocate both network and radio resources.The resulting end-to-end x-haul management architecture will be validated after completing and integrating the radio modules with a commercial 5G SA implementation.

TABLE I .
MAPPING BETWEEN TERAWAY SLICE PARAMETERS AND 5G QOS CLASSES.
Radio Element Controller is the SDN subsystem in charge of controlling the TERAWAY radio modules.The software implementation of the SDN Controller will be based on OpenDaylight[8]which is used as a base component for several commercial SDN controllers as well as integrated into other open-source SDN/NFV orchestration solutions such as OpenStack, Kubernetes, OPNFV and ONAP projects.OpenDaylight features a fully modular and extensible architecture, compliant to the Java component model defined in the OSGi specification[9].As OSGi container, it uses the Apache Karaf environment that allows loading different modules, denoted as "bundles", dynamically at runtime.The Radio Element Controllers will be integrated into OpenDaylight as a set of internal applications which implement the business and algorithmic logic on top of the provided SBI plugins.For the radio part, the NETCONF protocol will be employed, which defines configuration datastores and a set of Create, Read, Update, and Delete (CRUD) operations.They allow manipulation and queries on the management database exposed by the element in the form of a well-defined data model, which is specified by the YANG modeling language.