This thesis describes a real-time software-defined-radio implementation of the time-slotted round-trip carrier synchronization protocol in two-source and three-source communication systems. The techniques developed in this thesis can be used to synchronize the carriers of two or three single-antenna wireless transmitters with independent local oscillators so that their band-pass transmissions combine constructively at an intended receiver. Synchronization is achieved via the time-slotted transmission of (i) an unmodulated primary beacon from the destination to the sources and (ii) a series of secondary unmodulated beacons between the sources. Explicit channel state information is not exchanged between the sources and/or the destination. When synchronized, the single-antenna sources are able to cooperatively transmit as a distributed beamformer and achieve increased transmission range, reduced transmission energy, and/or increased security. The experimental results in this thesis confirm the theoretical predictions and also provide explicit guidelines for the real-time implementation of a carrier synchronization technique suitable for distributed transmit beamforming.

Os Pontos de Troca de Tráfego participam de maneira substancial e crítica no ecossistema da Internet, possibilitando conexões entre múltiplos Sistemas Autônomos (ASes, do inglês Autonomous Systems). O gerenciamento das redes de PTT possui como objetivos primários, o gerenciamento dos chamados fluxos elefante (do inglês, elephant flows). Fluxos elefante tendem a existir em número reduzido, porém correspondem à maioria do tráfego em uma infraestrutura de rede. O gerenciamento dos fluxos elefante envolve uma adequada identificação e quando necessário, um redirecionamento destes fluxos para caminhos mais apropriados, de forma a minimizar os possíveis impactos sobre os outros fluxos ativos na rede. Além disso, o gerenciamento de fluxos elefante tornou-se um importante objeto de discussão em PTTs baseados em redes SDN, principalmente porque estas redes dispõem de controladores que possuem uma visão consistente da rede subjacente, o que permite uma gerência destes fluxos de forma refinada. Nesta dissertação, será proposto, desenvolvido e avaliado um sistema de identificação dos fluxos elefante e seus respectivos caminhos de rede, em conjunto com um sistema de recomendação, que possui o objetivo de sugerir configurações alternativas para os fluxos elefante identificados anteriormente nas redes de PTTs baseadas em SDN. Neste sistema, o operador do PTT pode definir templates que em última instância definem como os caminhos dos fluxos elefante serão modificados para atender objetivos específicos. Por fim, será demonstrado que o sistema proposto pode auxiliar o operador do PTT a identificar, gerenciar e mitigar o impacto dos fluxos elefante da rede do PTT. / Internet Exchange Points (IXPs) play a key role in the current Internet architecture enabling cost-effective connections among multiple autonomous systems (ASes). Management of IXP networks is primarily concerned with the management of the so-called elephant flows. Such flows represent a small portion of the total flows of a IXP network but usually have high impact on the overall traffic. Managing elephant flows involves adequate identification and eventually rerouting of such flows to more appropriate locations to minimize the possible negative impact on the other (mice) flows active in the network. Elephant flow management becomes more important in SDN-based IXPs that require controllers to have a consistent view of the underlying network to allow fine-grained adjustment. In this master thesis, we propose, develop, and evaluate an identification system to identify elephant flows and their respectively paths, as well as a recommendation system to suggest alternative configurations to previously identified elephant flows in an SDN-based IXP network. In this solution, the IXP operator can define templates that ultimately define how elephant flows can be reconfigured to achieve a specific objective. We demonstrate that our system can help IXP operators to identify, handle and mitigate the impact of elephant flows in the IXP network.

O projeto de mecanismos flexíveis e eficientes para o posicionamento e encadeamento de funções virtualizadas de rede (VNFs) é essencial para o sucesso de Virtualização de Funções de Rede (Network Function Virtualization, NFV). A maioria das soluções existentes, no entanto, considera custos fixos (e imutáveis) de processamento de fluxos e de largura de banda ao posicionar as VNFs em Pontos de Presença da Rede (N-PoPs). Essa limitação torna-se crítica em redes NFV com fluxos cujos comportamentos são altamente dinâmicos e nas quais os requisitos de processamento e os recursos disponíveis nos NPoPs mudam constantemente. Para preencher essa lacuna, propõe-se o NFV-PEAR, uma plataforma para o posicionamento e encadeamento adaptativo de VNFs. O NFV-PEAR visa (re)organizar periodicamente os posicionamentos e encadeamentos de VNFs previamente determinados, objetivando-se manter um desempenho fim-a-fim aceitável mesmo durante flutuações nos custos de processamento e nos requisitos dos fluxos. Paralelamente, busca-se minimizar as mudanças na rede (por exemplo, a realocação de VNFs ou de fluxos) realizadas para cumprir esse objetivo. Os resultados obtidos, a partir de uma avaliação experimental, mostram que o NFV-PEAR tem potencial para reduzir significativamente o número de mudanças na rede necessárias para assegurar o desempenho fim-a-fim esperado para os fluxos, garantindo assim o funcionamento estável dos serviços. / The design of flexible and efficient mechanisms for proper placement and chaining of virtual network functions (VNFs) is key for the success of Network Function Virtualization (NFV). Most state-of-the-art solutions, however, consider fixed (and immutable) flow processing and bandwidth requirements when placing VNFs in the Network Points of Presence (N-PoPs). This limitation becomes critical in NFV-enabled networks having highly dynamic flow behavior, and in which flow processing requirements and available N-PoP resources change constantly. To bridge this gap, we present NFV-PEAR, a platform for adaptive VNF placement and chaining. In NFV-PEAR, network operators may periodically (re)arrange previously determined placement and chaining of VNFs, with the goal of maintaining acceptable end-to-end flow performance despite fluctuations of flow processing costs and requirements. In parallel, NFV-PEAR seeks to minimize network changes (e.g., reallocation of VNFs or network flows). The results obtained from an experimental evaluation provide evidence that NFV-PEAR has potential to deliver more stable operation of network services, while significantly reducing the number of network changes required to ensure end-to-end flow performance.

Redes definidas por software (SDN) são uma abordagem recente para o projeto e operação de redes de computadores. Este paradigma é empregado com sucesso em cenários de datacenter, no entanto redes de acesso definidas por software (SDAN) ainda são raras em operadoras de telefonia e provedores de Internet. Isso deve-se em grande parte à inexistência de uma arquitetura de referência, algo que diversos projetos tentaram suprir nos últimos anos. Independente da vertente escolhida, a opção por uma mudança tão disruptiva como o SDN precisa ser justificada. Modelos tecno-econômicos são comumente utilizados para estimar o impacto da mudança na rentabilidade da operação. O custo total de propriedade (TCO) é uma métrica utilizada para esse fim. O presente trabalho desenvolveu um modelo simplificado de comparação de arquiteturas de rede, que diferentemente dos demais, tem foco nos serviços oferecidos ao assinante e no custo de gerenciamento dos mesmos. A análise é realizada em duas dimensões. Na primeira o esforço de operar a rede é estimado através de uma análise qualitativa, de forma similar ao realizado por outros modelos. O segundo eixo é a quantidade de recursos exigida para provisionar e monitorar cada serviço. Essa estimativa é gerada através do uso de simulação de tempo discreto dos elementos e protocolos relevantes. A combinação das duas análises permite identificar as tendências de comportamento dos serviços geradas pela adoção de uma nova arquitetura de rede, e avaliar a conveniência dessa migração. O uso de simulação neste trabalho demandou a construção de modelos dos equipamentos e protocolos envolvidos. Elegeu-se o pacote NS-3 como ferramenta de simulação. Definir e implementar modelos para todos os serviços analisados poderia tornar-se tão oneroso quanto as técnicas que desejávamos suplantar, e por isso utilizou-se uma abordagem indireta. São simulados apenas os componentes essenciais, e os custos de cada serviço são inferidos a partir deles. O modelo de rede SDAN utilizado, denominado SDCN, é inspirado no SplitArchitecture, proposto pelo projeto SPARC. / Software defined networks are a novel approach to design and operation of computer networks. Although this paradigm is employed successfully in many data-centers and campus, software defined access networks (SDAN) are still in their infancy. Carriers and ISPs have not converged on a standard architecture to build their infrastructure upon, a problem many initiatives are trying to solve. Regardless of the chosen model, the option for such a disruptive change as SDN needs to be justified. Techno-economic models are commonly used to estimate the impact of a change in the operation. The Total Cost of Ownership (TCO) is a metric commonly used for this purpose. The present work developed a simplified evaluation model for network architectures, which focuses on the services offered to the subscriber and on the cost of managing them. The analysis is performed in two dimensions. In the first, the effort to operate the network is estimated through a qualitative analysis, similar to the one performed by other existing models. The second axis is the amount of resources required to provision and monitor each service. This estimation is produced through the use of discrete time simulation of selected elements and protocols. The combination of the analyzes allows us to identify the behavioral trends provoked by the adoption of a new network architecture, and to evaluate the convenience of such migration. The simulation in this work demanded models for the equipment and protocols involved. The package NS-3 was the chosen simulation tool. Defining and implementing models for all services analyzed could become as costly as the techniques we wish to overcome, so an indirect approach has been used. Only the essential components are simulated, and the costs of each service are inferred from them. The SDAN model evaluated in this work, called SDCN, is a simplification of the SplitArchitecture proposed by the SPARC project.

Energy conservation is a main concern in Wireless Sensor Networks (WSN). To reduce energy consumption it is important to know how it is spent and how much is available during the node and network operation. Several previous works have proposed energy consumption models focused on the communication module, while neglecting the processing and sensing activities. Other works presented more complex and complete models, but lacked experiments to demonstrate their accuracy in real deployments. The main objective of this work is to design and to evaluate an accurate energy consumption model for WSN, which considers the sensing, processing, and communication modules usage. This model was used to implement two energy consumption prediction mechanism. One mechanism is based in Markov chains and the other one is based in time series analysis. The metrics to evaluate the model and prediction mechanisms performance were: energy consumption estimation accuracy, energy consumption prediction accuracy, and node\'s communication and processing resources usage. The energy consumption prediction mechanisms performance was compared using two implementation schemes: running the prediction algorithm in the sensor node and running the prediction algorithm in a Software-Defined Networking controller. The implementation was conducted using IT-SDN, a Software-Defined Wireless Sensor Network framework. For the evaluation, simulation and emulation used COOJA, while testbed experiments used TelosB devices. Results showed that considering the sensing, processing, and communication energy consumption into the model, it is possible to obtain an accurate energy consumption estimation for Wireless Sensor Networks. Also, the use of a Software-Defined Networking controller for processing complex prediction algorithms can improve the prediction accuracy. / A conservação da energia é uma das principais preocupações nas Redes de Sensores Sem Fio (WSN, do inglês Wireless Sensor Networks). Para reduzir o consumo de energia, é importante saber como a energia é gasta e quanta energia há disponível durante o funcionamento da rede. Diversos trabalhos anteriores propuseram modelos de consumo de energia focados no módulo de comunicação, ignorando o consumo por tarefas de processamento e sensoriamento. Outros trabalhos apresentam modelos mais completos e complexos, mas carecem de experimentos que demonstrem a exatidão em dispositivos reais. O objetivo principal deste trabalho é projetar e avaliar um modelo de consumo de energia para WSN que considere o consumo por sensoriamento, processamento e comunicação. Este modelo foi utilizado para implementar dois mecanismos de previsão de consumo de energia, um deles baseado em cadeias de Markov e o outro baseado em séries temporais. As métricas para avaliar o desempenho do modelo e dos mecanismos de previsão de consumo de energia foram: exatidão da estimativa de consumo de energia, exatidão da previsão de consumo de energia e uso dos recursos de comunicação e processamento do nó. O desempenho dos mecanismos de previsão de consumo de energia foram comparados utilizando dois esquemas de implementação: rodando o algoritmo de previsão no nó sensor e rodando o algoritmo de previsão em um controlador de rede definida por software. A implementação foi conduzida utilizando IT-SDN, um arcabouço de desenvolvimento de redes de sensores sem fio definidas por software. A avaliação foi feita com simulações e emulações utilizando o simulador COOJA e ensaios com dispositivos reais utilizando o TelosB. Os resultados mostraram que considerando o consumo de energia por sensoriamento, processamento e communicação, é possivel fazer uma estimativa de consumo de energia em redes de sensores sem fio com uma boa exatidão. Ainda, o uso de um controlador de rede definida por software para processamento de algoritmos de previsão complexos pode aumentar a exatidão da previsão.

This thesis focuses on proposing a Software Defined Network (SDN) based programmable and capacity optimized backhaul and core network which is critical for 5G network design. Cloud Radio Access networks (CRAN) which is key enabler of 5G networks can address a number of challenges that mobile operators face while trying to support ever-growing end-users' needs towards 5th generation of mobile networks (5G). A novel layered and modular programmable CRAN architecture called Software Defined Virtualised Cloud Radio Access Network (SD-vCRAN) is introduced with Network Function Virtualization (NFV) and Software Defined Network (SDN) capabilities. The SDN-Base Band Unit (BBU) pool is shifted to the programmable core network site, where a centralised SDN controller manages the network servers and virtualised network function entities - Mobile Management Entity (MME), Serving/Packet Data Network Data plane (S/PGW-D), Serving/Packet Data Network Control plane (S/PGW-C), Software Network Defined Baseband Unit (SDN-BBU) and Local controllers (LC) via OpenFlow (OF) protocol. This approach simplifies network operations, improve traffic management, enable system-wide optimisation of Quality of Service (QoS) and network-aware application development. The control plane (excluding the preserved 3GPP standard interfaces: S1-MME, S6a, Gx) managed by the network servers provides load balancing, traffic management and optimisation tools for the data plane. The proposed work starts by reviewing the requirements of 5G networks, followed by discussion on 5G backhaul and core challenge. Then, an overview of CRAN, Evolved Programmable Core (EPC), SDN, NFV and related works. The simulation details of the proposed architecture are discussed along with the challenges faced by adopting SDN and NFV in mobile core. A thorough assessment of the interfaces and protocols that should be conserved or enhanced on both data and control plane is conducted. The result enables an architecture where the SDN-BBU pool shares a single cloud with the programmable EPC and the control plane is migrated from the network elements to a centralized controller, running on a virtual machine in the mobile core. The data and control plane separation removes overlaps and provides better signalling, as well as efficient network functioning to comply with latency demands. The proposed system performance is validated in terms of throughput, datagram loss, and packet delay variation under three scenarios: 1. single policy installation, 2. multiple policy installation and 3. load balancing. The load balancing performance of proposed system is validated comparing the performance of two different SDN controllers: Floodlight and OpenDaylight, where the later performs better in terms of throughput (no bandwidth restriction), packet loss (below 0.3%) and jitter (below 0.2ms). Furthermore, a detailed comparison of two SDN controller's - Floodlight and OpenDaylight performances is presented, which shows that OpenDaylight performs better only for less dense networks which needs less processing of messages without being blocked, and the Floodlight performs better in ultra-dense network. Some directions and preliminary thoughts for future work and necessary information to operators for building their roadmap to the upcoming technologies is presented.

In this thesis, components and parameters based power models (PMs) are produced to measure the power consumption (PC) of cloud radio access network (CRAN) architecture. In components PM, the power figure of each component within C-RAN is evaluated. After, this model is parametrised such that the computation complexity of each component is converted to a straightforward, but accurate method, called parameterised PM. This model compares cooling and total PC of traditional LTE architecture with C-RAN. This comparison considered different parameters such as, utilised bandwidth, number of antenna, base band units (BBUs) and remote radio heads (RRHs). This model draws about 33% reduction in power. Next, this PC model is updated to serve and exhibit the cost of integrating software defined networks (SDNs) with C-RAN. Alongside, modelling the power cost of the control plane units in the core network (CN), such as serving gateway (SGW), packet gateway (PGW) and mobility management entity (MME). Although there is power cost, the proposed model shows the directions to mitigate it. Consequently, a simplified PM is proposed for virtualisation based C-RAN. In this model, the power cost of server virtualisation by hosting several virtual machines (VMs) is shown, in a time and cost effective way. The total reduction in the PC was about 75%, due to short-cutting the number of active servers in the network. Alongside, the latency cost due to such technique is modelled. Finally, to enable efficient virtualisation technology, live migrating the VMs amongst the servers is vital. However, this advantageous situation is concurrent with VM's migration time and power cost. Therefore, a model is proposed to calculate the power cost of VM's live migration, and shows the effect of such decision upon the total PC of the network/C-RAN. The proposed work converts the complexity of other proposed PMs, to a simplified and costless method. Concurrently, the time cost is added to the imposed virtualisation's time cost to formulate the total delay expected prior to these techniques' execution.

Submitted by LÍGIA RODRIGUES PRETE (ligiaprete@gmail.com) on 2017-02-09T18:51:10Z No. of bitstreams: 1 ligia_rodrigues_prete.pdf: 7338300 bytes, checksum: 749589aef86e7cc89e556975ee3c0763 (MD5) / Approved for entry into archive by LUIZA DE MENEZES ROMANETTO (luizamenezes@reitoria.unesp.br) on 2017-02-14T17:42:37Z (GMT) No. of bitstreams: 1 prete_lr_dr_ilha.pdf: 7338300 bytes, checksum: 749589aef86e7cc89e556975ee3c0763 (MD5) / Made available in DSpace on 2017-02-14T17:42:37Z (GMT). No. of bitstreams: 1 prete_lr_dr_ilha.pdf: 7338300 bytes, checksum: 749589aef86e7cc89e556975ee3c0763 (MD5) Previous issue date: 2016-12-13 / O consumo de energia no setor de Tecnologia da Informação e Comunicação (TIC) tem crescido exponencialmente nos últimos anos, em virtude da quantidade crescente de equipamentos para armazenamento e processamento de dados. O paradigma de Redes Definidas por Software (do inglês, Software-Defined Networking - SDN) e a arquitetura OpenFlow estão permitindo uma nova gama de aplicações e serviços para redes. A presente tese apresenta um estudo que aplica tecnologias SDN em um ambiente virtualizado com a federação GENI (Global Environment for Network Innovation). Neste trabalho foi desenvolvido um módulo no controlador Floodlight intitulado como Módulo Economia de Energia que emprega um algoritmo denominado MiNet (Mínima Rede) para a construção da Árvore de Extensão Mínima (do inglês, Minimum Spanning Tree - MST) sobre os componentes de comutação em redes. Este estudo apresenta três simulações em duas topologias de rede Fat Tree, sendo, uma com dez (FatTree10) e outra com vinte switches (FatTree20). Na primeira simulação foi realizada sem o módulo com a configuração padrão do controlador Floodlight para servir de comparação com os resultados de desempenho obtidos nas outras duas simulações. Já a segunda, com o Módulo Economia de Energia incluído no controlador, foi avaliada quanto aos custos iniciais nas ligações entre os switches. Na terceira, os custos nas ligações dos switches foram alterados para evidenciar que o Módulo Economia de Energia é capaz de recalcular uma nova Árvore de Extensão Mínima sobre os custos fornecidos e assim adaptar-se à rede para uma nova situação de atualização. Por meio de simulações realizadas, considerando somente as ligações entre os switches, sendo, quarenta portas Ethernet para a topologia menor e oitenta portas Ethernet para a topologia ampla, de acordo com os resultados alcançados, o módulo incorporado no Floodlight reduziu o consumo de energia final em 35% para a topologia FatTree10 e 32,5% na topologia FatTree20. / Energy consumption in the Information and Communication Technology (ICT) sector has grown exponentially recently, due to the increasing amount of equipment for data storage and processing. The paradigm of Software-Defined Networking (SDN) and OpenFlow architecture are enabling a new range of applications and services for networks. This thesis presents a study that applies SDN technologies in a virtualized environment with the GENI federation (Global Environment for Network Innovation). This paper developed a module in Floodlight controller titled Energy Saving Module employing an algorithm called MiNet (Minimum Network) for the construction of the Minimum Spanning Tree (MST) on the switching components in networks. This study presents three simulations in two network topologies Fat Tree, as it follows, a ten one (FatTree10) and another with twenty switches (FatTree20). In the first simulation, it was performed without the module with the default configuration of Floodlight controller to serve as a comparison with the performance results in the other two simulations. The second, with Module Energy Saver included in the controller, it evaluated the initial costs on the links between switches. In the third, the costs in the connections of the switches were changed to high light that the Energy Savings Module is able to recalculate a new Minimum Spanning Tree on the provided costs and thus adapt the network to a new update situation. Through the performed simulations, considering only the links between switches, as it is, forty Ethernet ports for smaller topology and eighty Ethernet ports for wide topology, according to the achieved results, the embedded module Floodlight reduced the final energy consumption to 35% FatTree10 topology and 32.5% FatTree20 topology.

Detectar e analisar malwares é um processo essencial para aprimorar os sistemas de segurança. As soluções atuais apresentam limitações no processo de investigação e detecção de códigos maliciosos sofisticados. Mais do que utilizar técnicas para evadir sistemas de análise, malwares sofisticados requerem condições específicas no ambiente em que são executados para revelar seu comportamento malicioso. Com o surgimento das Redes Definidas por Software (SDN), notou-se uma oportunidade para aprimorar o processo de investigação de malware propondo uma arquitetura flexível apta a detectar variações comportamentais de maneira automática. Esta tese apresenta uma arquitetura especializada para analisar códigos maliciosos que permite controlar de maneira unificada o ambiente de análise, incluindo o sandbox e os elementos que o circundam. Dessa maneira, é possível gerenciar regras de contenção, configuração dinâmica de recursos, e manipular o tráfego de rede gerado pelos malwares. Para avaliar a arquitetura foi analisado um conjunto de malwares em dois cenários de avaliação. No primeiro cenário de avaliação, as funcionalidades descritas pela solução proposta revelaram novos eventos comportamentais em 100% dos malwares analisados. Já, no segundo cenários de avaliação, foi analisado um conjunto de malwares projetados para dispositivos IoT. Em consequência, foi possível bloquear ataques, monitorar a comunicação do malware com seu controlador de botnet, e manipular comandos de ataques. / Mechanisms to detect and analyze malicious software are essential to improve security systems. Current security mechanisms have limited success in detecting sophisticated malicious software. More than to evade analysis system, many malware require specific conditions to activate their actions in the target system. The flexibility of Software-Defined Networking (SDN) provides an opportunity to develop a malware analysis architecture that can detect behavioral deviations in an automated way. This thesis presents a specialized architecture to analyze malware by managing the analysis environment in a centralized way, including to control the sandbox and the elements that surrounds it. The proposed architecture enables to determine the network access policy, to handle the analysis environment resource configuration, and to manipulate the network connections performed by the malware. To evaluate our solution we have analyzed a set of malware in two evaluation scenarios. In the first evaluation scenario, we showed that the mechanisms proposed have increased the number of behavioral events in 100% of the malware analyzed. In the second evaluation scenario, we have analyzed malware designed for IoT devices. As a result, by using the MARS features, it was possible to block attacks, to manipulate attack commands, and to enable the malware communication with the respective botnet controller. The experimental results showed that our solution can improve the dynamic malware analysis process by providing this configuration flexibility to the analysis environment.

Emerging 5G mobile networks are envisioned to become multi-service environments, enabling the dynamic deployment of services with a diverse set of performance requirements, accommodating the needs of mobile network operators, verticals and over-the-top service providers. The Radio Access Network (RAN) part of mobile networks is expected to play a very significant role towards this evolution. Unfortunately, such a vision cannot be efficiently supported by the conventional RAN architecture, which adopts a fixed and rigid design. For the network to evolve, flexibility in the creation, management and control of the RAN components is of paramount importance. The key elements that can allow us to attain this flexibility are the programmability and the virtualization of the network functions. While in the case of the mobile core, these issues have been extensively studied due to the advent of technologies like Software-Defined Networking (SDN) and Network Functions Virtualization (NFV) and the similarities that the core shares with other wired networks like data centers, research in the domain of the RAN is still in its infancy. The contributions made in this thesis significantly advance the state of the art in the domain of RAN programmability and virtualization in three dimensions. First, we design and implement a software-defined RAN (SD-RAN) platform called FlexRAN, that provides a flexible control plane designed with support for real-time RAN control applications, flexibility to realize various degrees of coordination among RAN infrastructure entities, and programmability to adapt control over time and easier evolution to the future following SDN/NFV principles. Second, we leverage the capabilities of the FlexRAN platform to design and implement Orion, which is a novel RAN slicing system that enables the dynamic on-the-fly virtualization of base stations, the flexible customization of slices to meet their respective service needs and which can be used in an end-to-end network slicing setting. Third, we focus on the use case of multi-tenancy in a neutral-host indoors small-cell environment, where we design Iris, a system that builds on the capabilities of FlexRAN and Orion and introduces a dynamic pricing mechanism for the efficient and flexible allocation of shared spectrum to the tenants. A number of additional use cases that highlight the benefits of the developed systems are also presented. The lessons learned through this research are summarized and a discussion is made on interesting topics for future work in this domain. The prototype systems presented in this thesis have been made publicly available and are being used by various research groups worldwide in the context of 5G research.