Massive MIMO for Next Generation Wireless Systems

Multi-user Multiple-Input Multiple-Output (MIMO) offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned with roughly equal numbers of service-antennas and terminals and frequency division duplex operation, is not a scalable technology. Massive MIMO (also known as "Large-Scale Antenna Systems", "Very Large MIMO", "Hyper MIMO", "Full-Dimension MIMO" & "ARGOS") makes a clean break with current practice through the use of a large excess of service-antennas over active terminals and time division duplex operation. Extra antennas help by focusing energy into ever-smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include the extensive use of inexpensive low-power components, reduced latency, simplification of the media access control (MAC) layer, and robustness to intentional jamming. The anticipated throughput depend on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly-joined terminals, the exploitation of extra degrees of freedom provided by the excess of service-antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This paper presents an overview of the massive MIMO concept and contemporary research.Comment: Final manuscript, to appear in IEEE Communications Magazin

Channel Reciprocity Attacks Using Intelligent Surfaces with Non-Diagonal Phase Shifts

While reconfigurable intelligent surface (RIS) technology has been shown to provide numerous benefits to wireless systems, in the hands of an adversary such technology can also be used to disrupt communication links. This paper describes and analyzes an RIS-based attack on multi-antenna wireless systems that operate in time-division duplex mode under the assumption of channel reciprocity. In particular, we show how an RIS with a non-diagonal (ND) phase shift matrix (referred to here as an ND-RIS) can be deployed to maliciously break the channel reciprocity and hence degrade the downlink network performance. Such an attack is entirely passive and difficult to detect and counteract. We provide a theoretical analysis of the degradation in the sum ergodic rate that results when an arbitrary malicious ND-RIS is deployed and design an approach based on the genetic algorithm for optimizing the ND structure under partial knowledge of the available channel state information. Our simulation results validate the analysis and demonstrate that an ND-RIS channel reciprocity attack can dramatically reduce the downlink throughput

PHYSICAL LAYER SECURITY OF LARGE ANTENNA ARRAYS OPERATING IN THE NEAR-FIELD. NEAR-FIELD BEAMFOCUSING AND THE STUDY OF IT’S PHYSICAL LAYER SECURITY CAPABILITIES

This thesis explores the security aspects of near-field communications in the context of the upcoming 6th Generation wireless technology. With each decade witnessing significant advancements in cellular wireless communications, the demand for faster, more accessible, and secure networks continues to grow. Tra- ditional security approaches relying on higher-layer cryptography techniques may not be sufficient for the future massively connected world. Therefore, this research focuses on leveraging physical layer security techniques, specifically in the near-field regime, to enhance the confidentiality and privacy of wireless communications. The study begins with an examination of the state of the art, encompassing previous telecommunication generations, Multiple-Input Multiple-Output sys- tems, Multiple-Input Multiple-Output with Extremely Large Antenna Arrays, and an overview of physical layer security concepts. Subsequently, a compre- hensive analysis of the near-field channel modeling components is presented, exploring the unique propagation characteristics of electromagnetic waves within this region. The thesis then delves into the exploration of physical layer security at the near- field, employing the established channel model to calculate the secrecy rate in the downlink and achievable capacity in the uplink. Various scenarios involv- ing a malicious user in the communication link are considered, highlighting the inherent security capabilities of near-field communications. In conclusion, this research contributes to the advancement of secure com- munication technologies by providing insights into the physical layer security aspects of near-field communications in the context of 6th Generation. The findings show the potential of utilizing the propagation features of large an- tenna arrays such as the limited-depth beamforming to enhance security and privacy. The practical implications and potential applications of the research outcomes are also discussed.Esta tese explora os aspetos de segurança das comunicações de campo próximo no contexto da próxima tecnologia sem fios 6G. A cada década, testemunha- mos avanços significativos nas comunicações sem fio, a demanda por redes mais rápidas, acessíveis e seguras continua a crescer. Abordagens tradicionais de segurança que dependem de técnicas criptográficas em camadas superiores podem não ser suficientes para o futuro mundo altamente conectado. Portanto, esta pesquisa concentra-se nas técnicas de seguraça a nivél físico, especifica- mente no regime de campo próximo, para melhorar a confidencialidade e privacidade das comunicações sem fio. O estudo começa com uma análise do estado da arte, abrangendo gerações ante- riores de telecomunicações, sistemas Multiple Input Multiple Output (MIMO), MIMO com Extremely Large Antenna Arrays (ELAA)s, e uma visão geral dos conceitos de seguraça a nivél físico. Posteriormente, é apresentada uma análise abrangente dos componentes de modelagem do canal no campo próximo, explorando as características únicas de propagação de ondas eletromagnéticas nesta região. A tese, em seguida, aprofunda a exploração de seguraça a nivél físico no campo próximo, empregando o modelo de canal estabelecido para calcular a "Secrecy Rate"no "Downlink"e a capacidade máxima atingivel no "Uplink". Vários cenários envolvendo um utilizador malicioso na ligação de comunica- ção são considerados, destacando as capacidades inerentes de segurança das comunicações no campo próximo. Em conclusão, esta pesquisa contribui para o avanço das tecnologias de co- municação segura, fornecendo informações sobre os aspetos de segurança da camada física das comunicações no campo próximo no contexto de 6G. Os resultados mostram o potencial de utilizar as características de propagação de grandes matrizes de antenas, como o feixe de profundidade limitada, para melhorar a segurança e a privacidade. As implicações práticas e potenciais aplicações dos resultados da pesquisa também são discutidas

A Survey of Physical Layer Security Techniques for 5G Wireless Networks and Challenges Ahead

Physical layer security which safeguards data confidentiality based on the information-theoretic approaches has received significant research interest recently. The key idea behind physical layer security is to utilize the intrinsic randomness of the transmission channel to guarantee the security in physical layer. The evolution towards 5G wireless communications poses new challenges for physical layer security research. This paper provides a latest survey of the physical layer security research on various promising 5G technologies, including physical layer security coding, massive multiple-input multiple-output, millimeter wave communications, heterogeneous networks, non-orthogonal multiple access, full duplex technology, etc. Technical challenges which remain unresolved at the time of writing are summarized and the future trends of physical layer security in 5G and beyond are discussed.Comment: To appear in IEEE Journal on Selected Areas in Communication

Physical-Layer Secret Key Generation via CQI-Mapped Spatial Modulation in Multi-Hop Wiretap Ad-Hoc Networks

Providing security guarantee is a critical concern in the ad-hoc networks relying on multi-hop channels, since their flexible topology is vulnerable to security attacks. To enhance the security of a spatial modulation (SM) assisted wireless network, various SM mapping patterns are activated by random channel quality indicator (CQI) patterns over the legitimate link, as a physical-layer secret key. The SM signals are encrypted by random mapping patterns to prevent eavesdroppers from correctly demapping their detections. This secret key is developed for multi-hop wiretap ad-hoc networks, where eavesdroppers might monitor all the transmitting nodes of a legitimate link. We substantially characterise the multi-hop wiretap model with receiver diversity techniques adopted by eavesdroppers. The security performance of the conceived scheme is evaluated in the scenarios where eavesdroppers attempt to detect their received signals using maximal-ratio combining or maximum-gain selection. The achievable data rates of both legitimate and wiretapper links are formulated with the objective of quantifying the secrecy rates for both Gaussian-distributed and finite-alphabet inputs. Illustrative numerical results are provided for the metrics of ergodic secrecy rate and secrecy outage probability, which substantiate the compelling benefits of the physical-layer secret key generation via CQI-mapped SM