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STAR-RISs: Simultaneous Transmitting and Reflecting Reconfigurable Intelligent Surfaces
TL;DR: In this paper, the authors proposed a general hardware model for simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) and compared the diversity gain of the RISs with that of the conventional RISs.
Abstract: In this letter, simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) are studied. Compared with the conventional reflecting-only RISs, the coverage of STAR-RISs is extended to 360 degrees via simultaneous transmission and reflection. A general hardware model for STAR-RISs is presented. Then, channel models are proposed for the near-field and the far-field scenarios, base on which the diversity gain of the STAR-RISs is analyzed and compared with that of the conventional RISs. Numerical simulations are provided to verify analytical results and to demonstrate that full diversity order can be achieved on both sides of the STAR-RIS.
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TL;DR: In this article, the authors investigated a STAR-IOS-aided downlink NOMA network with randomly deployed users, and derived the diversity gains of paired users by the M-fold convolution model.
Abstract: Simultaneous transmitting and reflecting intelligent omini-surfaces
(STAR-IOSs) are able to achieve full coverage "smart radio environments". By
splitting the energy or altering the active number of STAR-IOS elements,
STAR-IOSs provide high flexibility of successive interference cancellation
(SIC) orders for non-orthogonal multiple access (NOMA) systems. Based on the
aforementioned advantages, this paper investigates a STAR-IOS-aided downlink
NOMA network with randomly deployed users. We first propose three tractable
channel models for different application scenarios, namely the central limit
model, the curve fitting model, and the M-fold convolution model. More
specifically, the central limit model fits the scenarios with large-size
STAR-IOSs while the curve fitting model is extended to evaluate multi-cell
networks. However, these two models cannot obtain accurate diversity orders.
Hence, we figure out the M-fold convolution model to derive accurate diversity
orders. We consider three protocols for STAR-IOSs, namely, the energy splitting
(ES) protocol, the time switching (TS) protocol, and the mode switching (MS)
protocol. Based on the ES protocol, we derive analytical outage probability
expressions for the paired NOMA users by the central limit model and the curve
fitting model. Based on three STAR-IOS protocols, we derive the diversity gains
of NOMA users by the M-fold convolution model. The analytical results reveal
that the diversity gain of NOMA users is equal to the active number of STAR-IOS
elements. Numerical results indicate that 1) in high signal-to-noise ratio
regions, the central limit model performs as an upper bound, while a lower
bound is obtained by the curve fitting model; 2) the TS protocol has the best
performance but requesting more time blocks than other protocols; 3) the ES
protocol outperforms the MS protocol as the ES protocol has higher diversity
gains.
22 citations
Posted Content•
TL;DR: In this paper, a novel simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) design is proposed in a non-orthogonal multiple access (NOMA) enhanced coordinated multi-point transmission (CoMP) network.
Abstract: In this letter, a novel simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) design is proposed in a non-orthogonal multiple access (NOMA) enhanced coordinated multi-point transmission (CoMP) network. Based on the insights of signal-enhancement-based (SEB) and signal-cancellation-based (SCB) designs, we propose a novel simultaneously-signal-enhancement-and-cancellation-based (SSECB) design, where the inter-cell interferences and desired signals can be simultaneously eliminated and boosted. Our simulation results demonstrate that: i) the inter-cell interference can be perfectly eliminated, and the desired signals can be enhanced simultaneously with the aid of a large number of RIS elements; ii) the proposed SSECB design is capable of outperforming the conventional SEB and SCB designs.
3 citations
References
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TL;DR: In this article, the authors developed energy-efficient designs for both the transmit power allocation and the phase shifts of the surface reflecting elements subject to individual link budget guarantees for the mobile users.
Abstract: The adoption of a reconfigurable intelligent surface (RIS) for downlink multi-user communication from a multi-antenna base station is investigated in this paper. We develop energy-efficient designs for both the transmit power allocation and the phase shifts of the surface reflecting elements subject to individual link budget guarantees for the mobile users. This leads to non-convex design optimization problems for which to tackle we propose two computationally affordable approaches, capitalizing on alternating maximization, gradient descent search, and sequential fractional programming. Specifically, one algorithm employs gradient descent for obtaining the RIS phase coefficients, and fractional programming for optimal transmit power allocation. Instead, the second algorithm employs sequential fractional programming for the optimization of the RIS phase shifts. In addition, a realistic power consumption model for RIS-based systems is presented, and the performance of the proposed methods is analyzed in a realistic outdoor environment. In particular, our results show that the proposed RIS-based resource allocation methods are able to provide up to 300% higher energy efficiency in comparison with the use of regular multi-antenna amplify-and-forward relaying.
1,967 citations
TL;DR: This paper overviews the current research efforts on smart radio environments, the enabling technologies to realize them in practice, the need of new communication-theoretic models for their analysis and design, and the long-term and open research issues to be solved towards their massive deployment.
Abstract: Future wireless networks are expected to constitute a distributed intelligent wireless communications, sensing, and computing platform, which will have the challenging requirement of interconnecting the physical and digital worlds in a seamless and sustainable manner. Currently, two main factors prevent wireless network operators from building such networks: (1) the lack of control of the wireless environment, whose impact on the radio waves cannot be customized, and (2) the current operation of wireless radios, which consume a lot of power because new signals are generated whenever data has to be transmitted. In this paper, we challenge the usual “more data needs more power and emission of radio waves” status quo, and motivate that future wireless networks necessitate a smart radio environment: a transformative wireless concept, where the environmental objects are coated with artificial thin films of electromagnetic and reconfigurable material (that are referred to as reconfigurable intelligent meta-surfaces), which are capable of sensing the environment and of applying customized transformations to the radio waves. Smart radio environments have the potential to provide future wireless networks with uninterrupted wireless connectivity, and with the capability of transmitting data without generating new signals but recycling existing radio waves. We will discuss, in particular, two major types of reconfigurable intelligent meta-surfaces applied to wireless networks. The first type of meta-surfaces will be embedded into, e.g., walls, and will be directly controlled by the wireless network operators via a software controller in order to shape the radio waves for, e.g., improving the network coverage. The second type of meta-surfaces will be embedded into objects, e.g., smart t-shirts with sensors for health monitoring, and will backscatter the radio waves generated by cellular base stations in order to report their sensed data to mobile phones. These functionalities will enable wireless network operators to offer new services without the emission of additional radio waves, but by recycling those already existing for other purposes. This paper overviews the current research efforts on smart radio environments, the enabling technologies to realize them in practice, the need of new communication-theoretic models for their analysis and design, and the long-term and open research issues to be solved towards their massive deployment. In a nutshell, this paper is focused on discussing how the availability of reconfigurable intelligent meta-surfaces will allow wireless network operators to redesign common and well-known network communication paradigms.
1,504 citations
TL;DR: An overview of HMIMOS communications including the available hardware architectures for reconfiguring such surfaces are provided, and the opportunities and key challenges in designingHMIMOS-enabled wireless communications are highlighted.
Abstract: Future wireless networks are expected to evolve toward an intelligent and software reconfigurable paradigm enabling ubiquitous communications between humans and mobile devices. They will also be capable of sensing, controlling, and optimizing the wireless environment to fulfill the visions of low-power, high-throughput, massively- connected, and low-latency communications. A key conceptual enabler that is recently gaining increasing popularity is the HMIMOS that refers to a low-cost transformative wireless planar structure comprised of sub-wavelength metallic or dielectric scattering particles, which is capable of shaping electromagnetic waves according to desired objectives. In this article, we provide an overview of HMIMOS communications including the available hardware architectures for reconfiguring such surfaces, and highlight the opportunities and key challenges in designing HMIMOS-enabled wireless communications.
925 citations
TL;DR: In this paper, the authors make a careful study of the use of metasurfaces to transform the impinging optical wave front, and propose a method to enable advanced control of electromagnetic waves over deeply subwavelength thicknesses.
Abstract: Metasurfaces are engineered systems that enable advanced control of electromagnetic waves over deeply subwavelength thicknesses. Researchers make a careful study of the use of metasurfaces to transform the impinging optical wave front.
347 citations
Posted Content•
TL;DR: A comprehensive overview of the state-of-the-art on RISs, with focus on their operating principles, performance evaluation, beamforming design and resource management, applications of machine learning to RIS-enhanced wireless networks, as well as the integration of RISs with other emerging technologies is provided in this article.
Abstract: Reconfigurable intelligent surfaces (RISs), also known as intelligent reflecting surfaces (IRSs), have received significant attention for their potential to enhance the capacity and coverage of wireless networks by smartly reconfiguring the wireless propagation environment. Therefore, RISs are considered a promising technology for the sixth-generation (6G) communication networks. In this context, we provide a comprehensive overview of the state-of-the-art on RISs, with focus on their operating principles, performance evaluation, beamforming design and resource management, applications of machine learning to RIS-enhanced wireless networks, as well as the integration of RISs with other emerging technologies. We describe the basic principles of RISs both from physics and communications perspectives, based on which we present performance evaluation of multi-antenna assisted RIS systems. In addition, we systematically survey existing designs for RIS-enhanced wireless networks encompassing performance analysis, information theory, and performance optimization perspectives. Furthermore, we survey existing research contributions that apply machine learning for tackling challenges in dynamic scenarios, such as random fluctuations of wireless channels and user mobility in RIS-enhanced wireless networks. Last but not least, we identify major issues and research opportunities associated with the integration of RISs and other emerging technologies for application to next-generation networks.
323 citations