376 research outputs found

    Full-duplex wireless communications: challenges, solutions and future research directions

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    The family of conventional half-duplex (HD) wireless systems relied on transmitting and receiving in different time-slots or frequency sub-bands. Hence the wireless research community aspires to conceive full-duplex (FD) operation for supporting concurrent transmission and reception in a single time/frequency channel, which would improve the attainable spectral efficiency by a factor of two. The main challenge encountered in implementing an FD wireless device is the large power difference between the self-interference (SI) imposed by the device’s own transmissions and the signal of interest received from a remote source. In this survey, we present a comprehensive list of the potential FD techniques and highlight their pros and cons. We classify the SI cancellation techniques into three categories, namely passive suppression, analog cancellation and digital cancellation, with the advantages and disadvantages of each technique compared. Specifically, we analyse the main impairments (e.g. phase noise, power amplifier nonlinearity as well as in-phase and quadrature-phase (I/Q) imbalance, etc.) that degrading the SI cancellation. We then discuss the FD based Media Access Control (MAC)-layer protocol design for the sake of addressing some of the critical issues, such as the problem of hidden terminals, the resultant end-to-end delay and the high packet loss ratio (PLR) due to network congestion. After elaborating on a variety of physical/MAC-layer techniques, we discuss potential solutions conceived for meeting the challenges imposed by the aforementioned techniques. Furthermore, we also discuss a range of critical issues related to the implementation, performance enhancement and optimization of FD systems, including important topics such as hybrid FD/HD scheme, optimal relay selection and optimal power allocation, etc. Finally, a variety of new directions and open problems associated with FD technology are pointed out. Our hope is that this treatise will stimulate future research efforts in the emerging field of FD communication

    Esquemas de retransmissão baseados no protocolo decodifica-e-encaminha em redes cognitivas do tipo underlay

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    Orientador: José Cândido Silveira Santos FilhoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: O conceito de compartilhamento de espectro do tipo underlay tem sido proposto como uma técnica promissora para contornar o problema da escassez e da subutilização do espectro, permitindo que usuários não licenciados, chamados de usuários secundários, possam acessar simultaneamente uma banda licenciada, alocada aos usuários primários, desde que o nível de interferência sobre os mesmos seja mantido abaixo de um dado limiar aceitável. Entretanto, isso implica que a potência de transmissão na rede secundária deve ser restringida, comprometendo assim a confiabilidade e a cobertura da comunicação. A fim de contornar esse problema, técnicas de retransmissão cooperativa, as quais proveem um novo tipo de diversidade espacial, podem ser exploradas como um meio eficaz para melhorar o desempenho da rede secundária. De fato, a utilização conjunta de ambas as técnicas ¿ compartilhamento de espectro do tipo underlay e retransmissão cooperativa ¿ em redes cooperativas e cognitivas tem recebido especial atenção, já que a eficiência espectral do sistema e o desempenho da rede secundária podem ser melhorados significativamente. Esta tese apresenta quatro contribuições principais na referida área. Em particular, estuda-se o efeito conjunto de duas restrições de potência importantes sobre o desempenho de outage de redes cooperativas e cognitivas, especificamente, o nível máximo de potência interferente tolerada pelo receptor primário e o valor máximo de potência transmitida nos usuários secundários. Foca-se em esquemas de retransmissão baseados no protocolo decodifica-e-encaminha, abordando cenários em que o enlace direto entre fonte e destino está disponível para transmitir informação útil. Como uma primeira contribuição, analisa-se o desempenho de dois esquemas baseados no protocolo de retransmissão half-duplex incremental, os quais exploram a diversidade espacial dos enlaces diretos em redes cooperativas e cognitivas multiusuário. A segunda contribuição investiga o impacto de estimativas desatualizadas do canal sobre o mecanismo de seleção de destino para um desses esquemas anteriores, focando-se apenas no aspecto cooperativo da rede, ou seja, desconsiderando-se o uso de compartilhamento espectral. A terceira contribuição estuda o desempenho de redes cooperativas e cognitivas baseadas no modo full-duplex. Para esse cenário, avalia-se o impacto tanto da autointerferência residual, que é inerente ao modo full-duplex, bem como das restrições de potência que caracterizam o compartilhamento de espectro do tipo underlay. Como contribuição final, motivada pelo compromisso entre a perda de eficiência espectral e o problema da autointerferência residual, próprios dos modos half-duplex e full-duplex, respectivamente, propõe-se e analisa-se um esquema de transmissão adaptativo para redes cooperativas e cognitivas, através do qual, antes de cada processo de comunicação, um dos seguintes modos de transmissão é selecionado: retransmissão half-duplex, retransmissão full-duplex ou transmissão direta. Para todos os cenários considerados, expressões analíticas exatas para a probabilidade de outage são obtidas. Adicionalmente, uma análise assintótica é realizada a fim de caracterizar a ordem de diversidade e o comportamento de outage da rede secundária no regime assintótico de alta relação sinal-ruído. Simulações de Monte Carlo validam os resultados analíticos apresentadosAbstract: Underlay spectrum sharing has been proposed as a promising technique to alleviate the problem of spectrum scarcity and underutilization, by enabling secondary (unlicensed) users to concurrently access a licensed band, provided that the resulting interference on the primary (licensed) users remains below a given acceptable level. However, such a technique implies that the transmit power at the secondary network must be constrained, thereby compromising the communication reliability and coverage. To counteract this, cooperative relaying techniques, which provide a new form of spatial diversity, can be exploited as an effective means to boost the performance of the secondary network. Indeed, the joint use of both techniques¿underlay spectrum sharing and cooperative relaying¿in cognitive relaying networks has drawn special attention, since the overall spectral efficiency and the secondary-network performance can be significantly improved. This dissertation comprises four main contributions in this field. In particular, we examine the combined effect of two crucial power constraints on the outage performance of cognitive relaying networks, namely, the maximum tolerable interference power at the primary receiver and the maximum transmit power at the secondary users. We focus on relaying schemes operating under the decode-and-forward protocol, for scenarios in which the direct link between source and destination is available to convey useful information. As a first contribution, we analyze the performance of two incremental half-duplex relaying schemes, which exploit the spatial diversity of the direct links in a multiuser scenario. Our second contribution investigates the impact of outdated channel estimates on the destination-scheduling mechanism of one of those incremental schemes, from the perspective of a cooperative network only, that is, in the absence of spectrum sharing. The third contribution addresses cognitive full-duplex relaying networks. More specifically, we assess the system performance as a function of both the residual self-interference, which is inherent to the full-duplex relaying mode, and the underlay spectrum-sharing power constraints. As a final contribution, driven by the tradeoff between the spectral-efficiency loss and the residual self-interference problem, intrinsic to the half- and full-duplex relaying modes, respectively, we propose and analyze an adaptive transmission scheme whereby, before each communication process, one out of the following transmission modes is selected: half-duplex relaying, full-duplex relaying, or direct transmission. For all the considered scenarios, exact analytical expressions for the outage probability are derived. In addition, an asymptotic analysis is performed to obtain further insights on the diversity order and outage behavior of the secondary network at the high signal-to-noise ratio regime. Monte Carlo simulations corroborate the accuracy of the presented mathematical analysisDoutoradoTelecomunicações e TelemáticaDoutor em Engenharia ElétricaCAPE

    Wireless-Powered Communication Assisted by Two-Way Relay with Interference Alignment Underlaying Cognitive Radio Network

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    This study investigates the outage performance of an under-laying wireless-powered secondary system that reuses the primary users (PU) spectrum in a multiple-input multiple-output (MIMO) cognitive radio (CR) network. Each secondary user (SU) harvests energy and receives information simultaneously by applying power splitting (PS) protocol. The communication between SUs is aided by a two-way (TW) decode and forward (DF) relay. We formulate a problem to design the PS ratios at SUs, the power control factor at the secondary relay, and beamforming matrices at all nodes to minimize the secondary network's outage probability. To address this problem, we propose a two-step solution. The first step establishes closedform expressions for the PS ratios at each SU and secondary relay's power control factor. Furthermore, in the second step, interference alignment (IA) is used to design proper precoding and decoding matrices for managing the interference between secondary and primary networks. We choose IA matrices based on the minimum mean square error (MMSE) iterative algorithm. The simulation results demonstrate a significant decrease in the outage probability for the proposed scheme compared to the benchmark schemes, with an average reduction of more than two orders of magnitude achieved

    Energy-Efficient Cooperative Cognitive Relaying Schemes for Cognitive Radio Networks

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    We investigate a cognitive radio network in which a primary user (PU) may cooperate with a cognitive radio user (i.e., a secondary user (SU)) for transmissions of its data packets. The PU is assumed to be a buffered node operating in a time-slotted fashion where the time is partitioned into equal-length slots. We develop two schemes which involve cooperation between primary and secondary users. To satisfy certain quality of service (QoS) requirements, users share time slot duration and channel frequency bandwidth. Moreover, the SU may leverage the primary feedback message to further increase both its data rate and satisfy the PU QoS requirements. The proposed cooperative schemes are designed such that the SU data rate is maximized under the constraint that the PU average queueing delay is maintained less than the average queueing delay in case of non-cooperative PU. In addition, the proposed schemes guarantee the stability of the PU queue and maintain the average energy emitted by the SU below a certain value. The proposed schemes also provide more robust and potentially continuous service for SUs compared to the conventional practice in cognitive networks where SUs transmit in the spectrum holes and silence sessions of the PUs. We include primary source burstiness, sensing errors, and feedback decoding errors to the analysis of our proposed cooperative schemes. The optimization problems are solved offline and require a simple 2-dimensional grid-based search over the optimization variables. Numerical results show the beneficial gains of the cooperative schemes in terms of SU data rate and PU throughput, average PU queueing delay, and average PU energy savings

    Maximum Throughput of a Secondary User Cooperating with an Energy-Aware Primary User

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    This paper proposes a cooperation protocol between a secondary user (SU) and a primary user (PU) which dedicates a free frequency subband for the SU if cooperation results in energy saving. Time is slotted and users are equipped with buffers. Under the proposed protocol, the PU releases portion of its bandwidth for secondary transmission. Moreover, it assigns a portion of the time slot duration for the SU to relay primary packets and achieve a higher successful packet reception probability at the primary receiver. We assume that the PU has three states: idle, forward, and retransmission states. At each of these states, the SU accesses the channel with adaptive transmission parameters. The PU cooperates with the SU if and only if the achievable average number of transmitted primary packets per joule is higher than the number of transmitted packets per joule when it operates alone. The numerical results show the beneficial gains of the proposed cooperative cognitive protocol.Comment: Accepted WiOpt 201
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