Performance of Relaying Protocols

In wireless systems, cooperative diversity and relaying can exploit the benefit of spatial diversity and combat heavy pathloss without requiring multiple antennas at the receivers and transmitters. For practical networks, the use of relays is motivated by the need for simple, inexpensive terminals with limited power and a single antenna. The motivation for this thesis is to study and propose practical relaying protocols that can reduce the power consumption and ameliorate the performance with minimum additional complexity. Based on a dual-hop communication model, we exploit two upper bounds for the end-to-end SNR. These bounds further inspire us to propose new relaying protocols for wireless communication systems. We examine the case of a single user and relay under Rayleigh and Nakagami-m fading conditions. Based on the general upper bound, a new protocol is introduced: Clipped gain. This protocol makes it possible to save the transmit power by stopping the transmission when the quality of the first hop leads to an outage. We consider also user selection and user scheduling for dual-hop communication with multiple users and relays over a Rayleigh fading channel. We introduce new scheduling protocols based on one-bit feedback information. To the best of our knowledge, most of the available literature uses full channel state information to perform user selection and user scheduling. Interestingly, our protocols based on one bit feedback greatly improve the system performance while adding less additional complexity. To carry out rigorous comparison, close-form expressions are derived and analytical results used to assess the outage probability performance

Relaying in the Internet of Things (IoT): A Survey

The deployment of relays between Internet of Things (IoT) end devices and gateways can improve link quality. In cellular-based IoT, relays have the potential to reduce base station overload. The energy expended in single-hop long-range communication can be reduced if relays listen to transmissions of end devices and forward these observations to gateways. However, incorporating relays into IoT networks faces some challenges. IoT end devices are designed primarily for uplink communication of small-sized observations toward the network; hence, opportunistically using end devices as relays needs a redesign of both the medium access control (MAC) layer protocol of such end devices and possible addition of new communication interfaces. Additionally, the wake-up time of IoT end devices needs to be synchronized with that of the relays. For cellular-based IoT, the possibility of using infrastructure relays exists, and noncellular IoT networks can leverage the presence of mobile devices for relaying, for example, in remote healthcare. However, the latter presents problems of incentivizing relay participation and managing the mobility of relays. Furthermore, although relays can increase the lifetime of IoT networks, deploying relays implies the need for additional batteries to power them. This can erode the energy efficiency gain that relays offer. Therefore, designing relay-assisted IoT networks that provide acceptable trade-offs is key, and this goes beyond adding an extra transmit RF chain to a relay-enabled IoT end device. There has been increasing research interest in IoT relaying, as demonstrated in the available literature. Works that consider these issues are surveyed in this paper to provide insight into the state of the art, provide design insights for network designers and motivate future research directions

Esquemas distribuídos para seleção de múltiplas antenas em redes com retransmissores do tipo amplifica-e-encaminha

Orientador: José Cândido Silveira Santos FilhoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: A seleção de antena na transmissão tem sido apresentada como uma estratégia promissora para explorar os benefícios do uso de múltiplas antenas em sistemas de comunicações com retransmissores. No entanto, essa abordagem pode exigir um montante considerável de estimações de canal, transmissões de realimentação e atraso, dado que a sua implementação ótima e centralizada requer o monitoramento do estado do canal de todos os enlaces. Para aliviar essas deficiências, este trabalho propõe e analisa um conjunto de esquemas subótimos de seleção de antena na transmissão para sistemas com retransmissores do tipo amplifica-e-encaminha, os quais podem ser implementados de uma forma distribuída. Nos esquemas propostos, a antena é selecionada com base na informação local do estado de canal que está disponível na fonte, requerendo, portanto, um atraso e uma carga de realimentação pequenos e constantes. Tal abordagem é considerada em uso conjunto com diferentes técnicas, incluindo métodos de combinação de diversidade (combinação por máxima razão e combinação por seleção) no destino, protocolos de ganho fixo ou variável no relay, e transceptores com múltiplas antenas no relay. Além disso, para o caso particular em que o retransmissor tem ganho fixo e uma única antena, considera-se também o uso de um mecanismo de seleção de enlace na fonte. Para cada caso, o desempenho do sistema é avaliado em termos de probabilidade de outage, eficiência espectral e/ou vazão. O foco principal é direcionado à probabilidade de outage, para a qual são deduzidas expressões exatas e limitantes de desempenho. Uma análise assintótica é também efetuada para enriquecer a compreensão do comportamento do sistema quando operando sob alta relação sinal-ruído. Finalmente, como contribuição isolada, uma estratégia subótima e simples de alocação de potência é elaborada para um sistema com múltiplos retransmissores do tipo decodifica-e-encaminha, considerando-se enlaces com erros e codificação de fonte distribuídaAbstract: Transmit-antenna selection has been presented as a promising strategy for exploiting the benefits of multiple antennas in relaying communication systems. However, this approach may demand a considerable amount of channel estimations, feedback transmissions, and delay, since its optimal centralized implementation requires monitoring the channel state of all links. To alleviate those impairments, this work proposes and analyzes a set of suboptimal transmit-antenna selection schemes for amplify-and-forward relaying systems, which can be implemented in a distributed manner. In the proposed schemes, the antenna is selected based on the local channel-state information that is available at the source, thus requiring a low and constant delay/feedback overhead. Such an approach is considered along with different techniques, including diversity combining methods (maximal-ratio combining and selection combining) at the destination, fixed- and variable-gain protocols at the relay, and multi-antenna transceivers at the relay. A link-selection mechanism at the source is also considered for the special case of a single-antenna fixed-gain relay. For each case, the system performance is assessed in terms of outage probability, spectral efficiency, and/or throughput. The main focus is placed on the outage probability, for which exact or bound expressions are derived. An asymptotic analysis is also performed to provide further insights into the system behavior at high signal-to-noise ratio. Finally, as an isolated contribution, a simple suboptimal power allocation strategy is designed for a decode-and-forward multi-relay system with lossy intra-links and distributed source codingDoutoradoTelecomunicações e TelemáticaDoutora em Engenharia ElétricaCAPE

DESIGN AND OPTIMIZATION OF SIMULTANEOUS WIRELESS INFORMATION AND POWER TRANSFER SYSTEMS

The recent trends in the domain of wireless communications indicate severe upcoming challenges, both in terms of infrastructure as well as design of novel techniques. On the other hand, the world population keeps witnessing or hearing about new generations of mobile/wireless technologies within every half to one decade. It is certain the wireless communication systems have enabled the exchange of information without any physical cable(s), however, the dependence of the mobile devices on the power cables still persist. Each passing year unveils several critical challenges related to the increasing capacity and performance needs, power optimization at complex hardware circuitries, mobility of the users, and demand for even better energy efficiency algorithms at the wireless devices. Moreover, an additional issue is raised in the form of continuous battery drainage at these limited-power devices for sufficing their assertive demands. In this regard, optimal performance at any device is heavily constrained by either wired, or an inductive based wireless recharging of the equipment on a continuous basis. This process is very inconvenient and such a problem is foreseen to persist in future, irrespective of the wireless communication method used. Recently, a promising idea for simultaneous wireless radio-frequency (RF) transmission of information and energy came into spotlight during the last decade. This technique does not only guarantee a more flexible recharging alternative, but also ensures its co-existence with any of the existing (RF-based) or alternatively proposed methods of wireless communications, such as visible light communications (VLC) (e.g., Light Fidelity (Li-Fi)), optical communications (e.g., LASER-equipped communication systems), and far-envisioned quantum-based communication systems. In addition, this scheme is expected to cater to the needs of many current and future technologies like wearable devices, sensors used in hazardous areas, 5G and beyond, etc. This Thesis presents a detailed investigation of several interesting scenarios in this direction, specifically concerning design and optimization of such RF-based power transfer systems. The first chapter of this Thesis provides a detailed overview of the considered topic, which serves as the foundation step. The details include the highlights about its main contributions, discussion about the adopted mathematical (optimization) tools, and further refined minutiae about its organization. Following this, a detailed survey on the wireless power transmission (WPT) techniques is provided, which includes the discussion about historical developments of WPT comprising its present forms, consideration of WPT with wireless communications, and its compatibility with the existing techniques. Moreover, a review on various types of RF energy harvesting (EH) modules is incorporated, along with a brief and general overview on the system modeling, the modeling assumptions, and recent industrial considerations. Furthermore, this Thesis work has been divided into three main research topics, as follows. Firstly, the notion of simultaneous wireless information and power transmission (SWIPT) is investigated in conjunction with the cooperative systems framework consisting of single source, multiple relays and multiple users. In this context, several interesting aspects like relay selection, multi-carrier, and resource allocation are considered, along with problem formulations dealing with either maximization of throughput, maximization of harvested energy, or both. Secondly, this Thesis builds up on the idea of transmit precoder design for wireless multigroup multicasting systems in conjunction with SWIPT. Herein, the advantages of adopting separate multicasting and energy precoder designs are illustrated, where we investigate the benefits of multiple antenna transmitters by exploiting the similarities between broadcasting information and wirelessly transferring power. The proposed design does not only facilitates the SWIPT mechanism, but may also serve as a potential candidate to complement the separate waveform designing mechanism with exclusive RF signals meant for information and power transmissions, respectively. Lastly, a novel mechanism is developed to establish a relationship between the SWIPT and cache-enabled cooperative systems. In this direction, benefits of adopting the SWIPT-caching framework are illustrated, with special emphasis on an enhanced rate-energy (R-E) trade-off in contrast to the traditional SWIPT systems. The common notion in the context of SWIPT revolves around the transmission of information, and storage of power. In this vein, the proposed work investigates the system wherein both information and power can be transmitted and stored. The Thesis finally concludes with insights on the future directions and open research challenges associated with the considered framework

A Comprehensive Study of Multiple Access Techniques in 6G Networks

With the proliferation of numerous burgeoning services such as ultra-reliable low-latency communication (URLLC), massive machine type communications (mMTC), enhanced mobile broadband (eMBB), among others, wireless communication systems are expected to face daunting challenges. In order to satisfy these ever-increasing traffic demands, diverse quality-of-services (QoS) requirements, and the massive connectivity accompanied by these new applications, various innovative and promising technologies, and architectures need to be developed. Novel multiple-access techniques are currently being explored in both academia and industry in order to accommodate such unprecedented requirements. Non-orthogonal multiple access (NOMA) has been deemed as one of the vital enabling multiple access techniques for the upcoming six-generation (6G) networks. This is due to its ability to enhance network spectral efficiency (NSE) and support a massive number of connected devices. Owing to its potential benefits, NOMA is recognized as a prominent member of next-generation multiple access (NGMA). Several emerging techniques such as full-duplex (FD) communication, device-to-device (D2D) communications, reconfigurable intelligent surface (RIS), coordinated multipoint (CoMP), cloud radio access networks, are being gradually developed to address fundamental problems in future wireless networks. In this thesis, and with the goal of converging toward NGMA, we investigate the synergistic integration between NOMA and other evolving physical layer technologies. Specifically, we analyze this integration aiming at improving the performance of cell-edge users (CEUs), mitigating the detrimental effect of inter-cell interference (ICI), designing energy-efficient multiple access toward ``green’’ wireless networks, guarantying reliable communication between NOMA UEs and base stations (BSs)/remote radio heads (RRHs), and maintaining the required QoS in terms of the minimum achievable data rate, especially at CEUs. Regarding the ICI mitigation in multi-cell NOMA networks and tackling the connectivity issue in traditional CoMP-based OMA networks, we first investigate the integration between location-aware CoMP transmission and NOMA in downlink heterogeneous C-RAN. In doing so, we design a novel analytical framework using tools from stochastic geometry to analyze the system performance in terms of the average achievable data rate per NOMA UE. Our results reveal that CoMP NOMA can provide a significant gain in terms of network spectral efficiency compared to the traditional CoMP OMA scheme. In addition, with the goal of further improving the performance of CEUs and user fairness, cooperative transmission with the aid of D2D communication and FD or half-duplex (HD) transmission, has been introduced to NOMA, which is commonly known as cooperative NOMA (C-NOMA). As a result, we extend our study to also investigate the potential gains of investigating CoMP and C-NOMA. In such a framework, we exploit the cooperation between the RRHs/BSs and the successive decoding strategy at NOMA UEs that are near the RRHs/BSs. Specifically, we investigate both performance analysis and resource management optimization (power control and user pairing). Our results show that the transmit power at the BS, the transmit power at the relay user, and the self-interference (SI) value at the relay user determine which multiple access technique, CoMP NOMA, CoMP HD C-NOMA, and CoMP FD C-NOMA, should be adopted at the BSs. Now, to assist in designing energy-efficient multiple access techniques and guarantying reliable communication for NOMA UEs, this thesis explores the interplay between FD/HD C-NOMA and RIS. We show that the proposed model has the best performance in terms of network power consumption compared to other multiple access techniques in the literature, which leads to ``green'' future wireless networks. Moreover, our results show that the network power consumption can be significantly reduced by increasing the number of RIS elements. A more significant finding is that the location of the RIS depends on the adopted multiple access techniques. For example, it is not recommended to deploy the RIS besides the BS if the adopted multiple access is HD C-NOMA. Another insight that has been unveiled is the FD C-NOMA with the assistance of RIS has more resistance to the residual SI effect, due to the FD transmission, and can tolerate high SI values compared to the same scheme without RIS. Although much work has been conducted to improve the network spectral efficiency of multi-cell NOMA cellular networks, the required QoS by the upcoming 6G applications, in terms of the minimum achievable rate, may not be guaranteed at CEUs. This is due to their distant locations from their serving BSs, and thus, they experience severe path-loss attenuation and high ICI. This thesis addresses this research gap by studying the synergistic integration between RIS, NOMA, and CoMP in a multi-user multi-cell scenario. Unlike the developed high-complexity optimal solutions or the low-complexity sub-optimal solutions in the literature for the power allocation problem, we derive a low-complexity optimal solution in a such challenging scenario. We also consider the interdependency between the user clustering policies in different coordinated cells, which has been ignored in the literature. Finally, we prove that this integration between RIS, NOMA, and CoMP can attain a high achievable rate for CEUs, ameliorate spectral efficiency compared to existing literature, and can form a novel paradigm for NGMA