1,721,145 research outputs found
NOMA-Aided Joint Radar and Multicast-Unicast Communication Systems
No full textThe novel concept of non-orthogonal multiple access (NOMA) aided joint radar
and multicast-unicast communication (Rad-MU-Com) is investigated. Employing the
same spectrum resource, a multi-input-multi-output (MIMO) dual-functional
radar-communication (DFRC) base station detects the radar-centric users
(R-user), while transmitting mixed multicast-unicast messages both to the
R-user and to the communication-centric user (C-user). In particular, the
multicast information is intended for both the R- and C-users, whereas the
unicast information is only intended for the C-user. More explicitly, NOMA is
employed to facilitate this double spectrum sharing, where the multicast and
unicast signals are superimposed in the power domain and the superimposed
communication signals are also exploited as radar probing waveforms. First, a
beamformer-based NOMA-aided joint Rad-MU-Com framework is proposed for the
system having a single R-user and a single C-user. Based on this framework, the
unicast rate maximization problem is formulated by optimizing the beamformers
employed, while satisfying the rate requirement of multicast and the predefined
accuracy of the radar beam pattern. The resultant non-convex optimization
problem is solved by a penalty-based iterative algorithm to find a high-quality
near-optimal solution. Next, the system is extended to the scenario of multiple
pairs of R- and C-users, where a cluster-based NOMA-aided joint Rad-MU-Com
framework is proposed. A joint beamformer design and power allocation
optimization problem is formulated for the maximization of the sum of the
unicast rate at each C-user, subject to the constraints on both the minimum
multicast rate for each R&C pair and on accuracy of the radar beam pattern for
detecting multiple R-users. The resultant joint optimization problem is
efficiently solved by another penalty-based iterative algorithm developed.Comment: 14 pages, 12 figures, this work is accepeted for the publication in
IEEE Journal on Selected Areas in Communication
RIS-aided near-field communications for 6G: opportunities and challenges
No full textReconfigurable intelligent surface (RIS)-aided nearfield communications is investigated. First, the necessity of investigating RIS-aided near-field communications and the advantages brought about by the unique spherical-wave-based near-field propagation are discussed. Then, the family of patch-array-based RISs and metasurface-based RISs are introduced along with their respective near-field channel models. A pair of fundamental performance limits of RIS-aided near-field communications, namely their power scaling law and effective degrees-of-freedom, are analyzed for both patch-array-based and metasurface-based RISs, which reveals the potential performance gains that can be achieved. Furthermore, the associated near-field beam training and beamforming design issues are studied, where a twostage hierarchical beam training approach and a low-complexity element-wise beamforming design are proposed for RIS-aided near-field communications. Finally, a suite of open research problems is highlighted for motivating future research
Hybrid reinforcement learning for STAR-RISs: a coupled phase-shift model based beamformer
No full textA simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted multi-user downlink multiple-input single-output (MISO) communication system is investigated. In contrast to the existing ideal STAR-RIS model assuming an independent transmission and reflection phase-shift control, a practical coupled phase-shift model is considered. Then, a joint active and passive beamforming optimization problem is formulated for minimizing the long-term transmission power consumption, subject to the coupled phase-shift constraint and the minimum data rate constraint. Despite the coupled nature of the phase-shift model, the formulated problem is solved by invoking a hybrid continuous and discrete phase-shift control policy. Inspired by this observation, a pair of hybrid reinforcement learning (RL) algorithms, namely the hybrid deep deterministic policy gradient (hybrid DDPG) algorithm and the joint DDPG & deep-Q network (DDPG-DQN) based algorithm are proposed. The hybrid DDPG algorithm controls the associated high-dimensional continuous and discrete actions by relying on the hybrid action mapping. By contrast, the joint DDPG-DQN algorithm constructs two Markov decision processes (MDPs) relying on an inner and an outer environment, thereby amalgamating the two agents to accomplish a joint hybrid control. Simulation results demonstrate that the STARRIShas superiority over other conventional RISs in terms of its energy consumption. Furthermore, both the proposed algorithms outperform the baseline DDPG algorithm, and the joint DDPGDQN algorithm achieves a superior performance, albeit at anincreased computational complexity
STAR: Simultaneous Transmission And Reflection for 360° Coverage by Intelligent Surfaces
No full textA novel simultaneously transmitting and reflecting (STAR) system design relying on reconfigurable intelligent surfaces (RISs) is conceived. First, an existing prototype is reviewed and the potential benefits of STAR-RISs are discussed. Then, the key differences between conventional reflecting-only RISs and STAR-RISs are identified from the perspectives of hardware design, physics principles, and communication system design. Furthermore, the basic signal model of STAR-RISs is introduced, and three practical protocols are proposed for their operation, namely energy splitting, mode switching, and time switching. Based on the proposed protocols, a range of promising application scenarios are put forward for integrating STAR-RISs into next-generation wireless networks. By considering the downlink of a typical RIS-aided multiple-input single-output (MISO) system, numerical case studies are provided for revealing the superiority of STAR-RISs over other baselines, when employing the proposed protocols. Finally, several open research problems are discussed
Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted UAV communications
No full textA novel air-to-ground communication paradigm is conceived, where an unmanned aerial vehicle (UAV)-mounted base station (BS) equipped with multiple antennas sends information to multiple ground users (GUs) with the aid of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS). In contrast to the conventional RIS whose main function is to reflect incident signals, the STAR-RIS is capable of both transmitting and reflecting the impinging signals from either side of the surface, thereby leading to full-space 360 degree coverage. However, the transmissive and reflective capabilities of the STAR-RIS require more complex transmission/reflection coefficient design. Therefore, in this work, a sum-rate maximization problem is formulated for the joint optimization of the UAV’s trajectory, the active beamforming at the UAV, and the passive transmission/reflection beamforming at the STAR-RIS. This cutting-edge optimization problem is also subject to the UAV’s flight safety, to the maximum flight duration constraint, as well as to the GUs’ minimum data rate requirements. Given the unknown locations of obstacles prior to the UAV’s flight, we provide an online decision making framework employing reinforcement learning (RL) to simultaneously adjust both the UAV’s trajectory as well as the active and passive beamformer. To enhance the system’s robustness against the associated uncertainties caused by limited sampling of the environment, a novel “distributionally-robust” RL (DRRL) algorithm is proposed for offering an adequate worst-case performance guarantee. Our numerical results unveil that: 1) the STAR-RIS assisted UAV communications benefit from significant sum-rate gain over the conventional reflecting-only RIS; and 2) the proposed DRRL algorithm achieves both more stable and more robust performance than the state-of-the-art RL algorithms
Joint Location and Beamforming Design for STAR-RIS Assisted NOMA Systems
No full textSimultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) communication systems are investigated in its vicinity, where a STAR-RIS is deployed within a predefined region for establishing communication links for users. Both beamformer-based NOMA and cluster-based NOMA schemes are employed at the multi-antenna base station (BS). For each scheme, the STAR-RIS deployment location, the passive transmitting and reflecting beamforming (BF) of the STAR-RIS, and the active BF at the BS are jointly optimized for maximizing the weighted sum-rate (WSR) of users. To solve the resultant non-convex problems, an alternating optimization (AO) algorithm is proposed, where successive convex approximation (SCA) and semi-definite programming (SDP) methods are invoked for iteratively addressing the non-convexity of each sub-problem. Numerical results reveal that 1) the WSR performance canbe significantly enhanced by optimizing the specific deployment location of the STAR-RIS; 2) both beamformer-based and clusterbased NOMA prefer asymmetric STAR-RIS deployment
Simultaneously Transmitting and Reflecting Intelligent Omni-Surfaces: Modeling and Implementation
Full text linkGiven the rapid development of advanced electromagnetic (EM) manipulation technologies, researchers have turned their attention to the investigation of smart surfaces for enhancing radio coverage. Simultaneously transmitting and reflecting (STAR) intelligent omni-surfaces (IOSs) constitute one of the most promising categories. Although previous research contributions have demonstrated the benefits of simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOSs) in terms of wireless communication performance gains, several important issues remain unresolved, including practical hardware implementations and accurate physical models. In this article, we address these by discussing four practical hardware implementations of STAR-IOSs as well as three hardware modeling techniques and five channel modeling methods. We thus clarify the taxonomy of smart surface technologies in support of further investigating the family of STAR-IOSs.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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