373 research outputs found
IRS-assisted UAV Communications: A Comprehensive Review
Intelligent reflecting surface (IRS) can smartly adjust the wavefronts in
terms of phase, frequency, amplitude and polarization via passive reflections
and without any need of radio frequency (RF) chains. It is envisaged as an
emerging technology which can change wireless communication to improve both
energy and spectrum efficiencies with low energy consumption and low cost. It
can intelligently configure the wireless channels through a massive number of
cost effective passive reflecting elements to improve the system performance.
Similarly, unmanned aerial vehicle (UAV) communication has gained a viable
attention due to flexible deployment, high mobility and ease of integration
with several technologies. However, UAV communication is prone to security
issues and obstructions in real-time applications. Recently, it is foreseen
that UAV and IRS both can integrate together to attain unparalleled
capabilities in difficult scenarios. Both technologies can ensure improved
performance through proactively altering the wireless propagation using smart
signal reflections and maneuver control in three dimensional (3D) space. IRS
can be integrated in both aerial and terrene environments to reap the benefits
of smart reflections. This study briefly discusses UAV communication, IRS and
focuses on IRS-assisted UAC communications. It surveys the existing literature
on this emerging research topic and highlights several promising technologies
which can be implemented in IRS-assisted UAV communication. This study also
presents several application scenarios and open research challenges. This study
goes one step further to elaborate research opportunities to design and
optimize wireless systems with low energy footprint and at low cost. Finally,
we shed some light on future research aspects for IRS-assisted UAV
communication
Reconfigurable Intelligent Surfaces Aided mmWave NOMA: Joint Power Allocation,Phase Shifts, and Hybrid Beamforming Optimization
In this paper, an reconfigurable intelligent surface (RIS)-aided millimeter
wave (mmWave) non-orthogonal multiple access (NOMA) system is considered. In
particular, we consider an RIS-aided mmWave-NOMA downlink system with a hybrid
beamforming structure. To maximize the achievable sum-rate under a minimum rate
constraint for the users and a minimum transmit power constraint, a joint RIS
phase shifts, hybrid beamforming, and power allocation problem is formulated.
To solve this non-convex optimization problem, we develop an alternating
optimization algorithm. Specifically, first, the non-convex problem is
transformed into three subproblems, i.e., power allocation, joint phase shifts
and analog beamforming optimization, and digital beamforming design. Then, we
solve the power allocation problem under fixed phase shifts of the RIS and
hybrid beamforming. Finally, given the power allocation matrix, an alternating
manifold optimization (AMO)-based method and a successive convex approximation
(SCA)-based method are utilized to design the phase shifts, analog beamforming,
and transmit beamforming, respectively. Numerical results reveal that the
proposed alternating optimization algorithm outperforms state-of-the-art
schemes in terms of sum-rate. Moreover, compared to a conventional mmWave-NOMA
system without RIS, the proposed RIS-aided mmWave-NOMA system is capable of
improving the achievable sum-rate of the system
Intelligent Reflective Surface Deployment in 6G: A Comprehensive Survey
Intelligent reflecting surfaces (IRSs) are considered a promising technology
that can smartly reconfigure the wireless environment to enhance the
performance of future wireless networks. However, the deployment of IRSs still
faces challenges due to highly dynamic and mobile unmanned aerial vehicle (UAV)
enabled wireless environments to achieve higher capacity. This paper sheds
light on the different deployment strategies for IRSs in future terrestrial and
non-terrestrial networks. Specifically, in this paper, we introduce key
theoretical concepts underlying the IRS paradigm and discuss the design aspects
related to the deployment of IRSs in 6G networks. We also explore
optimization-based IRS deployment techniques to improve system performance in
terrestrial and aerial IRSs. Furthermore, we survey model-free reinforcement
learning (RL) techniques from the deployment aspect to address the challenges
of achieving higher capacity in complex and mobile IRS-assisted UAV wireless
systems. Finally, we highlight challenges and future research directions from
the deployment aspect of IRSs for improving system performance for the future
6G network.Comment: 16 pages, 3 Figures, 7 table
Joint Design for Simultaneously Transmitting And Reflecting (STAR) RIS Assisted NOMA Systems
Different from traditional reflection-only reconfigurable intelligent
surfaces (RISs), simultaneously transmitting and reflecting RISs (STAR-RISs)
represent a novel technology, which extends the half-space coverage to
full-space coverage by simultaneously transmitting and reflecting incident
signals. STAR-RISs provide new degrees-of-freedom (DoF) for manipulating signal
propagation. Motivated by the above, a novel STAR-RIS assisted non-orthogonal
multiple access (NOMA) (STAR-RIS-NOMA) system is proposed in this paper. Our
objective is to maximize the achievable sum rate by jointly optimizing the
decoding order, power allocation coefficients, active beamforming, and
transmission and reflection beamforming. However, the formulated problem is
non-convex with intricately coupled variables. To tackle this challenge, a
suboptimal two-layer iterative algorithm is proposed. Specifically, in the
inner-layer iteration, for a given decoding order, the power allocation
coefficients, active beamforming, transmission and reflection beamforming are
optimized alternatingly. For the outer-layer iteration, the decoding order of
NOMA users in each cluster is updated with the solutions obtained from the
inner-layer iteration. Moreover, an efficient decoding order determination
scheme is proposed based on the equivalent-combined channel gains. Simulation
results are provided to demonstrate that the proposed STAR-RIS-NOMA system,
aided by our proposed algorithm, outperforms conventional RIS-NOMA and RIS
assisted orthogonal multiple access (RIS-OMA) systems
Joint Beamforming Design for RIS-enabled Integrated Positioning and Communication in Millimeter Wave Systems
Integrated positioning and communication (IPAC) system and reconfigurable
intelligent surface (RIS) are both considered to be key technologies for future
wireless networks. Therefore, in this paper, we propose a RIS-enabled IPAC
scheme with the millimeter wave system. First, we derive the explicit
expressions of the time-of-arrival (ToA)-based Cram\'er-Rao bound (CRB) and
positioning error bound (PEB) for the RIS-aided system as the positioning
metrics. Then, we formulate the IPAC system by jointly optimizing active
beamforming in the base station (BS) and passive beamforming in the RIS to
minimize the transmit power, while satisfying the communication data rate and
PEB constraints. Finally, we propose an efficient two-stage algorithm to solve
the optimization problem based on a series of methods such as the exhaustive
search and semidefinite relaxation (SDR). Simulation results show that by
changing various critical system parameters, the proposed RIS-enabled IPAC
system can cater to both reliable data rates and high-precision positioning in
different transmission environments
RIS-Aided Cell-Free Massive MIMO Systems for 6G: Fundamentals, System Design, and Applications
An introduction of intelligent interconnectivity for people and things has
posed higher demands and more challenges for sixth-generation (6G) networks,
such as high spectral efficiency and energy efficiency, ultra-low latency, and
ultra-high reliability. Cell-free (CF) massive multiple-input multiple-output
(mMIMO) and reconfigurable intelligent surface (RIS), also called intelligent
reflecting surface (IRS), are two promising technologies for coping with these
unprecedented demands. Given their distinct capabilities, integrating the two
technologies to further enhance wireless network performances has received
great research and development attention. In this paper, we provide a
comprehensive survey of research on RIS-aided CF mMIMO wireless communication
systems. We first introduce system models focusing on system architecture and
application scenarios, channel models, and communication protocols.
Subsequently, we summarize the relevant studies on system operation and
resource allocation, providing in-depth analyses and discussions. Following
this, we present practical challenges faced by RIS-aided CF mMIMO systems,
particularly those introduced by RIS, such as hardware impairments and
electromagnetic interference. We summarize corresponding analyses and solutions
to further facilitate the implementation of RIS-aided CF mMIMO systems.
Furthermore, we explore an interplay between RIS-aided CF mMIMO and other
emerging 6G technologies, such as next-generation multiple-access (NGMA),
simultaneous wireless information and power transfer (SWIPT), and millimeter
wave (mmWave). Finally, we outline several research directions for future
RIS-aided CF mMIMO systems.Comment: 30 pages, 15 figure
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