Load balancing in hybrid VLC and RF networks based on blind interference alignment

Visible light communications (VLC) are proposed for increasing the spectral efficiency and the number of devices served in indoor environments, while providing illumination through light emitting diodes (LED). For VLC, each optical access point (AP) provides a small and confined area of coverage. Since several sources of light are usually deployed in overlapping fashion in order to provide satisfactory illumination, VLC are limited by inter-cell interference. Moreover, transmission from a specific optical AP can be blocked by the elements of the scenario. On the other hand, radio-frequency (RF) systems such as WiFi are usually available in most of the indoor scenarios. In this work, we first propose a dynamic cell formation method for grouping the optical APs in multiple optical cells that cover a footprint each minimizing the inter-cell interference. After that, we use transmission based on blind interference alignment (BIA) in each optical cell. Considering the coexistence with RF systems based on orthogonal frequency division multiplexing (OFDM), a load balancing algorithm is proposed for managing the resources of the resulting hybrid VLC/RF network and determining the user association to each system. However, the complexity of this optimization problem is excessively high for practical VLC/RF networks. In order to obtain a suboptimal but tractable solution, we propose a decentralized optimization method based on Lagrangian multipliers. Simulation results show that the proposed scheme outperforms other approaches for user grouping and managing the resources of hybrid VLC/RF networks.This work was supported in part by the Spanish National Project TERESA-ADA under Grant TEC2017-90093-C3-2-R and Grant MINECO/AEI/FEDER, UE, and in part by the research project GEOVEOLUZ-CM-UC3M funded by the call Programa de apoyo a la realización de proyectos interdisciplinares de I+D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020 under the frame of the Convenio Plurianual Comunidad de Madrid, Universidad Carlos III de Madri

The electronically steerable parasitic array radiator antenna for wireless communications : signal processing and emerging techniques

Smart antenna technology is expected to play an important role in future wireless communication networks in order to use the spectrum efficiently, improve the quality of service, reduce the costs of establishing new wireless paradigms and reduce the energy consumption in wireless networks. Generally, smart antennas exploit multiple widely spaced active elements, which are connected to separate radio frequency (RF) chains. Therefore, they are only applicable to base stations (BSs) and access points, by contrast with modern compact wireless terminals with constraints on size, power and complexity. This dissertation considers an alternative smart antenna system the electronically steerable parasitic array radiator (ESPAR) which uses only a single RF chain, coupled with multiple parasitic elements. The ESPAR antenna is of significant interest because of its flexibility in beamforming by tuning a number of easy-to-implement reactance loads connected to parasitic elements; however, parasitic elements require no expensive RF circuits. This work concentrates on the study of the ESPAR antenna for compact transceivers in order to achieve some emerging techniques in wireless communications. The work begins by presenting the work principle and modeling of the ESPAR antenna and describes the reactance-domain signal processing that is suited to the single active antenna array, which are fundamental factors throughout this thesis. The major contribution in this chapter is the adaptive beamforming method based on the ESPAR antenna. In order to achieve fast convergent beamforming for the ESPAR antenna, a modified minimum variance distortionless response (MVDR) beamfomer is proposed. With reactance-domain signal processing, the ESPAR array obtains a correlation matrix of receive signals as the input to the MVDR optimization problem. To design a set of feasible reactance loads for a desired beampattern, the MVDR optimization problem is reformulated as a convex optimization problem constraining an optimized weight vector close to a feasible solution. Finally, the necessary reactance loads are optimized by iterating the convex problem and a simple projector. In addition, the generic algorithm-based beamforming method has also studied for the ESPAR antenna. Blind interference alignment (BIA) is a promising technique for providing an optimal degree of freedom in a multi-user, multiple-inputsingle-output broadcast channel, without the requirements of channel state information at the transmitters. Its key is antenna mode switching at the receive antenna. The ESPAR antenna is able to provide a practical solution to beampattern switching (one kind of antenna mode switching) for the implementation of BIA. In this chapter, three beamforming methods are proposed for providing the required number of beampatterns that are exploited across one super symbol for creating the channel fluctuation patterns seen by receivers. These manually created channel fluctuation patterns are jointly combined with the designed spacetime precoding in order to align the inter-user interference. Furthermore, the directional beampatterns designed in the ESPAR antenna are demonstrated to improve the performance of BIA by alleviating the noise amplification. The ESPAR antenna is studied as the solution to interference mitigation in small cell networks. Specifically, ESPARs analog beamforming presented in the previous chapter is exploited to suppress inter-cell interference for the system scenario, scheduling only one user to be served by each small BS at a single time. In addition, the ESPAR-based BIA is employed to mitigate both inter-cell and intracell interference for the system scenario, scheduling a small number of users to be simultaneously served by each small BS for a single time. In the cognitive radio (CR) paradigm, the ESPAR antenna is employed for spatial spectrum sensing in order to utilize the new angle dimension in the spectrum space besides the conventional frequency, time and space dimensions. The twostage spatial spectrum sensing method is proposed based on the ESPAR antenna being targeted at identifying white spectrum space, including the new angle dimension. At the first stage, the occupancy of a specific frequency band is detected by conventional spectrum-sensing methods, including energy detector and eigenvalue-based methods implemented with the switched-beam ESPAR antenna. With the presence of primary users, their directions are estimated at the second stage, by high-resolution angle-of-arrival (AoA) estimation algorithms. Specifically, the compressive sensing technology has been studied for AoA detection with the ESPAR antenna, which is demonstrated to provide high-resolution estimation results and even to outperform the reactance-domain multiple signal classification

ISAC-Enabled Beam Alignment for Terahertz Networks: Scheme Design and Coverage Analysis

As a key pillar technology for the future 6G networks, terahertz (THz) communication can provide high-capacity transmissions, but suffers from severe propagation loss and line-of-sight (LoS) blockage that limits the network coverage. Narrow beams are required to compensate for the loss, but they in turn bring in beam misalignment challenge that degrades the THz network performance. The high sensing accuracy of THz signals enables integrated sensing and communication (ISAC) technology to assist the LoS blockage and user mobility-induced beam misalignment, enhancing THz network coverage. In line with the 5G beam management, we propose a joint synchronization signal block (SSB) and reference signal (RS)-based sensing (JSRS) scheme to predict the need for beam switches, and thus prevent beam misalignment. We further design an optimal sensing signal pattern that minimizes beam misalignment with fixed sensing resources, which reveals design insights into the time-to-frequency allocation. We derive expressions for the coverage probability and spatial throughput, which provide instructions on the ISAC-THz network deployment and further enable evaluations for the sensing benefit in THz networks. Numerical results show that the JSRS scheme is effective and highly compatible with the 5G air interface. Averaged in tested urban use cases, JSRS achieves near-ideal performance and reduces around 80% of beam misalignment, and enhances the coverage probability by about 75%, compared to the network with 5G-required positioning ability

State of the Art, Taxonomy, and Open Issues on Cognitive Radio Networks with NOMA

The explosive growth of mobile devices and the rapid increase of wideband wireless services call for advanced communication techniques that can achieve high spectral efficiency and meet the massive connectivity requirement. Cognitive radio (CR) and non-orthogonal multiple access (NOMA) are envisioned to be important solutions for the fifth generation wireless networks. Integrating NOMA techniques into CR networks (CRNs) has the tremendous potential to improve spectral efficiency and increase the system capacity. However, there are many technical challenges due to the severe interference caused by using NOMA. Many efforts have been made to facilitate the application of NOMA into CRNs and to investigate the performance of CRNs with NOMA. This article aims to survey the latest research results along this direction. A taxonomy is devised to categorize the literature based on operation paradigms, enabling techniques, design objectives and optimization characteristics. Moreover, the key challenges are outlined to provide guidelines for the domain researchers and designers to realize CRNs with NOMA. Finally, the open issues are discussed.Comment: This paper has been accepted by IEEE Wireless Communications Magazine. Pages 16, Figures

Investigation on Evolving Single-Carrier NOMA into Multi-Carrier NOMA in 5G

© 2013 IEEE. Non-orthogonal multiple access (NOMA) is one promising technology, which provides high system capacity, low latency, and massive connectivity, to address several challenges in the fifth-generation wireless systems. In this paper, we first reveal that the NOMA techniques have evolved from single-carrier NOMA (SC-NOMA) into multi-carrier NOMA (MC-NOMA). Then, we comprehensively investigated on the basic principles, enabling schemes and evaluations of the two most promising MC-NOMA techniques, namely sparse code multiple access (SCMA) and pattern division multiple access (PDMA). Meanwhile, we consider that the research challenges of SCMA and PDMA might be addressed with the stimulation of the advanced and matured progress in SC-NOMA. Finally, yet importantly, we investigate the emerging applications, and point out the future research trends of the MC-NOMA techniques, which could be straightforwardly inspired by the various deployments of SC-NOMA

Técnicas de gestão de feixe de onda para sistemas Massive MIMO nas redes 5G NR

The use of Millimeter wave (mmWave) spectrum frequencies is seen as a key enabler technology for the future wireless communication systems to overcome the bandwidth shortage of the sub 6GHz microwave spectrum band, enabling high speed data transmissions in the 5G/6G systems. Nevertheless, mmWave propagation characteristics are associated to significant free-path losses and many more attenuations that become even more harsher as the frequency increases, rendering the communication challenging at this frequencies. To overcome these distinct disadvantages, multiple antenna arrays are employed to allow beamforming techniques for the transmission of narrower concentrated beams in more precise directions and less interference levels between them, consequently improving the link budget. Thus, to constantly assure that the communication with each device is done using the beam pair that allows the best possible connectivity, a set of Beam Management control procedures is necessary to assure an efficient beamformed connection establishment and its continuous maintenance between the device and the network. This dissertation will address the description of the Initial Beam Establishment (IBE) BM procedure, focusing the selection of the most suitable transmit-receive beam pair available after completed beam sweeping techniques to measure the different power levels of the received signal. The main goal is to design a new 3GPP-standard compliant beam pair selection algorithm based on SSS angle estimation (BSAE), that makes use of multiple Synchronization Signal Blocks (SSBs) to maximize the Reference Signal Received Power (RSRP) value at the receiver, through the selected beam pair. This optimization is done using the Secondary Synchronization Signals (SSSs) present in each SSB to perform channel estimation in the digital domain (comprising the effects of the analog processing). Afterwards, the combination of those estimations were used to perform the equivalent channel propagation matrix estimation without the analog processing effects. Finally, through the channel propagation matrix, the angle that maximizes the RSRP was determined to compute the most suitable beam through the aggregated response vector. The obtained results show that the proposed algorithm achieves better performance levels compared to a conventional beam pair selection algorithm. Furthermore, a comparison with an optimal case is also done, i.e., the situation where the channel is known, and the optimal beam pair angle can be determined. Therefore, the similar performance results compared to the optimal case indicates that the proposed algorithm is interesting for practical 5G mmWave mMIMO implementations, according to 3GPP-compliant standards.O uso de frequências na banda das ondas milimétricas é visto como uma tecnologia chave para os futuros sistemas de comunicação móveis, tendo em vista a ultrapassar o problema da escassez de banda a sub-6 GHz, e por permitir as elevadas taxas de dados requeridas para sistemas 5G/6G. Contudo, a propagação deste tipo de ondas está associado a perdas acentuadas em espaço livre e várias atenuações que se tornam cada vez mais significativas com o aumento do valor da frequência, impondo obstáculos à comunicação. Para ultrapassar estas adversidades, agregados constituídos por múltiplos elementos de antena são implementados por forma a permitir técnicas de formação de feixe e possibilitar a transmissão de feixes mais estreitos e altamente direcionais, diminuindo os níveis de interferência e melhorando consequentemente o link budget. Deste modo, para assegurar constantemente que a comunicação efetuada em cada dispositivo ocorre utilizando o conjunto de feixes que proporciona o melhor nível de conectividade, é então necessário um conjunto de procedimentos de controlo de gestão de feixe, assegurando um estabelecimento eficiente da comunicação e a sua contínua manutenção entre um dispositivo e a rede. Esta dissertação descreve o procedimento de gestão de feixe conhecido como estabelecimento inicial de feixe, focando o processo de seleção do melhor par de feixe de transmissão-receção disponível após o uso de técnicas de varrimento de feixe por fim a efetuar medições dos diferentes níveis de potência do sinal recebido. O principal objetivo passa pela conceção de um novo algoritmo de estabelecimento de par de feixes baseado em estimações de ângulo (BSAE), que explora o uso de múltiplos SSBs definidos pelo 3GPP, por forma a maximizar o RSRP no recetor, através do feixe selecionado. Esta otimização é feita usando os sinais de sincronização secundários (SSSs) presentes em cada SSB para efetuar uma estimação de canal no domínio digital (que contém o efeito do processamento analógico). Depois, combinando essas estimações, foi feita uma estimação da matriz do canal de propagação, sem o efeito desse processamento analógico. Finalmente, através da matriz do canal de propagação, foi determinado o ângulo que maximiza o RSRP, e calculado o feixe através do vetor de resposta do agregado. Os resultados obtidos demonstram que o algoritmo proposto atinge melhor desempenho quando comparado com o algoritmo convencional de seleção de par de feixes. Foi feita ainda uma comparação com o caso ótimo, isto é, com o caso em que se conhece completamente o canal e se obtém um ângulo ótimo. Os resultados obtidos pelo algoritmo proposto foram muito próximos do caso ótimo, pelo que é bastante interessante para sistemas práticos 5G mmWave mMIMO, que estejam de acordo com o padrão 3GPP.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Analysis and Design of Non-Orthogonal Multiple Access (NOMA) Techniques for Next Generation Wireless Communication Systems

The current surge in wireless connectivity, anticipated to amplify significantly in future wireless technologies, brings a new wave of users. Given the impracticality of an endlessly expanding bandwidth, there’s a pressing need for communication techniques that efficiently serve this burgeoning user base with limited resources. Multiple Access (MA) techniques, notably Orthogonal Multiple Access (OMA), have long addressed bandwidth constraints. However, with escalating user numbers, OMA’s orthogonality becomes limiting for emerging wireless technologies. Non-Orthogonal Multiple Access (NOMA), employing superposition coding, serves more users within the same bandwidth as OMA by allocating different power levels to users whose signals can then be detected using the gap between them, thus offering superior spectral efficiency and massive connectivity. This thesis examines the integration of NOMA techniques with cooperative relaying, EXtrinsic Information Transfer (EXIT) chart analysis, and deep learning for enhancing 6G and beyond communication systems. The adopted methodology aims to optimize the systems’ performance, spanning from bit-error rate (BER) versus signal to noise ratio (SNR) to overall system efficiency and data rates. The primary focus of this thesis is the investigation of the integration of NOMA with cooperative relaying, EXIT chart analysis, and deep learning techniques. In the cooperative relaying context, NOMA notably improved diversity gains, thereby proving the superiority of combining NOMA with cooperative relaying over just NOMA. With EXIT chart analysis, NOMA achieved low BER at mid-range SNR as well as achieved optimal user fairness in the power allocation stage. Additionally, employing a trained neural network enhanced signal detection for NOMA in the deep learning scenario, thereby producing a simpler signal detection for NOMA which addresses NOMAs’ complex receiver problem