Article

# On the Performance of RIS-Assisted Dual-Hop UAV Communication Systems

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## Abstract

In this paper, to further improve the coverage and performance of unmanned aerial vehicle (UAV) communication systems, we propose a reconfigurable intelligent surface (RIS)-assisted UAV scheme where an RIS installed on a building is used to reflect the signals transmitted from the ground source to an UAV, and the UAV is deployed as a relay to forward the decoded signals to the destination. To model the statistical distribution of the RIS-assisted ground-to-air (G2A) links, we develop a tight approximation for the probability density function (PDF) of the instantaneous signal-to-noise ratio (SNR). Thanks to this distribution, analytical expressions of outage probability, average bit error rate (BER), and average capacity are derived. Results show that the use of RISs can effectively improve the coverage and reliability of UAV communication systems.

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... In [33], the OP, average bit error rate, and ergodic capacity of RIS-assisted dual-hop UAV communication systems were quantified, where the S-RIS and RIS-UAV links were modeled as Rayleigh and mixture-Gamma (MG) RVs, respectively. Finally, in [34], the authors formulated coverage and link performance problems of an aerial-terrestrial communication system and presented machine learning enabled RIS-assisted transmission strategies. ...
... Finally, by applying (18) in (36), we get (33). This concludes the proof. ...
... This concludes the proof. From (33), it becomes evident that the key parameters that influence the outage performance of an RIS-assisted UAV wireless systems are the fading conditions and the number of MAs, which are captured by the parameters k A , m A and Ξ, the geometric losses and the SNR, i.e. γ, as well as the spectral efficiency of the transmission scheme, r th , that depends on γ th since r th = log 2 (γ th + 1). Due to the fact that ...
Article
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In this paper, we analyze the performance of a reconfigurable intelligent surface (RIS)-assisted unmanned aerial vehicle (UAV) wireless system that is affected by mixture-gamma small-scale fading, stochastic disorientation and misalignment, as well as transceivers hardware imperfections. First, we statistically characterize the end-to-end channel for both cases, i.e., in the absence as well as in the presence of disorientation and mis-alignment, by extracting closed-form formulas for the probability density function (PDF) and the cumulative distribution function (CDF). Building on the aforementioned expressions, we extract novel closed-form expressions for the outage probability (OP) in the absence and the presence of disorientation and misalignment as well as hardware imperfections. In addition, high signal-to-noise ratio OP approximations are derived, leading to the extraction of the diversity order. Finally, an OP floor due to disorientation and misalignment is presented.
... In [17], a theoretical framework was presented to analyze the performance of an integrated UAV-IRS relaying system. Furthermore, in [18], the secure transmission problem was considered in an UAV and IRS assisted millimeter wave (mmWave) networks in the presence of an eavesdropper. In addition, [19] showed that the use of IRSs can effectively improve the coverage and reliability of mmWave UAV communication systems. ...
... To deal with non-convex constraints, we first use S-procedure to convert the worst-case secrecy rate constraints into LMIs. Since the logarithmic function is a monotonically increasing function, by considering the channel uncertainty in (18) and (20) and ...
... However, even with 90 REs, the transmission power with the passive IRS is more than that with the active IRS equipped with M * (optimum M ) antennas, as the active IRS can directly amplify the incident signal. On the contrary, the active IRS only takes few REs (15)(16)(17)(18)(19)(20) or so) to reach its optimal secrecy performance under different amplitude gains. With the less REs, the active IRS is of smaller surface size and more suitable to the loadlimited UAV secure communication. ...
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Unmanned Aerial Vehicles (UAV)-enabled communication is a promising solution for secure air-to-ground (A2G) networks due to the additional secure degrees of freedom afforded by mobility. However, the jittering characteristics caused by the random airflow and the body vibration of the UAV itself have a non-negligible impact on the performance of UAV communication. Considering the impact of UAV jittering, this paper propose a robust and secure transmission design assisted by an novel active intelligent reflecting surface (IRS),where the reflecting elements in IRS not only adjust the phase shift but also amplify the amplitude of signals. Specifically, under the worst-case secrecy rate constraints, we aim to minimize the transmission power by the robust joint design of active IRS's reflecting coefficient and beamforming at the UAV-borne base station (UBS). However,it is challenging to solve the joint optimization problem due to its non-convexity. To tackle this problem, the non-convex problem is reformulated with linear approximation for the channel variations and linear matrix inequality transformed by S-procedure and Schur's complement. Then, we decouple this problem into two sub-problems, namely, passive beamforming and active IRS's reflecting coefficient optimization, and solve them through alternate optimization (AO). Finally, the numerical results demonstrate the potential of active IRS on power saving under secure transmission constraints and the impact of UAV jittering.
... At each location of the RIS, the overall system's gain is given by ℎ , where the quantity is deterministic in contrast to ℎ and , which are random variables. Thus, the SNR's PDF, ( ), which can be calculated from the SNRs, ℎ and , can be evaluated as [27,Eq. (5)] ...
... This implies that under such conditions, the system does not reach the specific rate . The OP, , which defines this failure, can be readily calculated from (27) by finding ( ℎ ). ...
... The values of and for selected modulation schemes, namely, CBFSK, NBFSK, CBPSK, and DBPSK, are respectively given by the pairs (0.5, 0.5), (1, 0.5), (0.5, 1), and (1, 1). Substituting (27) into (34) leads to = eq 2Γ( ) ...
Article
One of the main problems faced by communication systems is the presence of skip-zones in the targeted areas. With the deployment of the fifth-generation mobile network, solutions are proposed to solve the signal loss due to obstruction by buildings, mountains, and atmospheric or weather conditions. Among these solutions, reconfigurable intelligent surfaces (RIS), which are newly proposed modules, may be exploited to reflect the incident signal in the direction of dead zones, increase communication coverage, and make the channel smarter and controllable. This paper tackles the skip-zone problem in terrestrial free-space optical (T-FSO) systems using RIS. Considering link distances and jitter ratios at the RIS position, we carry out a performance analysis of RIS-aided T-FSO links affected by turbulence and pointing errors, for both heterodyne detection and intensity modulation-direct detection techniques. Turbulence is modeled using the Gamma-Gamma distribution. We analyze the model and provide exact closed-form expressions of the probability density function, cumulative distribution function, and moment generating function of the end-to-end signal-to-noise ratio. Capitalizing on these statistics, we evaluate the system performance through the outage probability, ergodic channel capacity, and average bit error rate for selected binary modulation schemes. Numerical results, validated through simulations, obtained for different RIS positions and link distances ratio values, reveal that RIS-based T-FSO performs better when the RIS module is located near the transmitter.
... Recently, intelligent reflecting surface (IRS), also known as (a.k.a.) reconfigurable intelligent surface (RIS), has been extensively studied in a variety of new technical challenges such as cascaded channel estimation [1], [2], passive beamforming [3]- [7], passive information transfer [8], [9], and physical layer security [10]- [12], as well as new IRS-aided communication scenarios such as cell-free multiple-input multiple-output (MIMO) [13]- [16], unmanned aerial vehicles (UAV) communications [17]- [19], Terahertz communications [20]- [22], and non-orthogonal multiple access (NOMA) [23]- [25]. An IRS is made of a large number of low-cost reconfigurable elements, a.k.a. ...
... From (14), all the Tx-IRS-Rx links share a common total path loss η 0 . From (15), (18), (19), and (22), the total path phase shift of the received signal experienced in the t p -m k,l -r q link (in exponential) is h (k,l),p h q,(k,l) θ k,l , with h (k,l),p defined in (18), h q,(k,l) defined in (19), and θ k,l defined in (22), k ∈ I Qx , l ∈ I Qy , p ∈ I Nt , q ∈ I Nr . Therefore, H is given by (24). ...
... From (14), all the Tx-IRS-Rx links share a common total path loss η 0 . From (15), (18), (19), and (22), the total path phase shift of the received signal experienced in the t p -m k,l -r q link (in exponential) is h (k,l),p h q,(k,l) θ k,l , with h (k,l),p defined in (18), h q,(k,l) defined in (19), and θ k,l defined in (22), k ∈ I Qx , l ∈ I Qy , p ∈ I Nt , q ∈ I Nr . Therefore, H is given by (24). ...
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In this paper, we build up a new intelligent reflecting surface (IRS) aided multiple-input multiple-output (MIMO) channel model, named the cascaded LoS MIMO channel. The proposed channel model consists of a transmitter (Tx) and a receiver (Rx) both equipped with uniform linear arrays (ULAs), and an IRS used to enable communications between the transmitter and the receiver through the line-of-sight (LoS) links seen by the IRS. To model the reflection of electromagnetic waves at the IRS, we take into account the curvature of the wavefront on different reflecting elements (REs), which is distinct from most existing works that take the plane-wave assumption. Based on the established model, we study the spatial multiplexing capability and input-output mutual information (MI) of the cascaded LoS MIMO system. We generalize the notion of Rayleigh distance originally coined for the single-hop MIMO channel to the full multiplexing region (FMR) for the cascaded LoS MIMO channel, where the FMR is, roughly speaking, the union of Tx-IRS and IRS-Rx distance pairs that enable full multiplexing communication between the Tx and the Rx. We propose a new passive beamforming (PB) strategy named reflective focusing, which aims to coherently superimpose the waves originating from a transmit antenna, reflected by the IRS, and focused on a receive antenna. With reflective focusing, we derive an inner bound of the FMR, and provide the corresponding orientation settings of the antenna arrays that enable full multiplexing. We further employ the MI to measure the quality of the cascaded LoS MIMO channel, and formulate an optimization problem to maximize the MI over PB and antenna array orientations. We give analytical solutions to the problem under asymptotic conditions such as high or low signal-to-noise ratio (SNR) regimes. For general cases, we propose an alternating optimization method to solve the problem.
... Assuming a high phase-shift resolution, i.e., 2 bn ≫ 1, and perfect channel state information (CSI), the n-th RIS is able to generate ideal phase-shifts such that the phase errors can be zero, i.e., δ nl = 0, ∀l, ∀n. It is noted that the ideal phase-shift assumption has been extensively used for single-RIS-aided systems [6], [9], [10], [31], and [11], multi-RISaided systems [4] and [16], and multi-hop multi-RIS-aided networks [18]. As a result, θ * nl = φ 0 − (φ nl + ψ nl ), which yields a synthesized received signal at D having the largest amplitude. ...
... ∼ LogNormal(ν Z , ζ Z ) as stated in Theorem 2, we can derive the PDF of Z 2 , however, the resultant PDF of Z 2 using this method leads to a non-closed-form expression of the integral in (31). To circumvent this problem, we perform Step 2 of the proposed statistic characterization framework again, and after some mathematical manipulation steps, we obtain Z 2 approx. ...
... , ζ Z 2 = ln µZ (4) (µZ (2)) 2 . From (31), an approximate closed-form expression for the EC of the ERA scheme can be obtained as ...
Article
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[ Matlab code with detailed simulation of phase-shifts | https://github.com/trinhudo/Multi-RIS ] In this paper, we study the statistical characterization and modeling of distributed multi-reconfigurable intelligent surface (RIS)-aided wireless systems. Specifically, we consider a practical system model where the RISs with different geometric sizes are distributively deployed, and wireless channels associated to different RISs are assumed to be independent but not identically distributed (i.n.i.d.). We propose two purpose-oriented multi-RIS-aided schemes, namely, the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) schemes. A mathematical framework, which relies on the method of moments, is proposed to statistically characterize the end-to-end (e2e) channels of these schemes. It is shown that either a Gamma distribution or a Log-Normal distribution can be used to approximate the distribution of the magnitude of the e2e channel coefficients in both schemes. With these findings, we evaluate the performance of the two schemes in terms of outage probability (OP) and ergodic capacity (EC), where tight approximate closed-form expressions for the OP and EC are derived. Representative results show that the ERA scheme outperforms the ORA scheme in terms of OP and EC. In addition, under i.n.i.d. fading channels, the reflecting element settings and location settings of RISs have a significant impact on the system performance of both the ERA or ORA schemes.
... From [6], we obtain the maximum output signal for the RIS by setting ...
... With the similar reason, there is no transmission link between the interference nodes and RIS. Since the RISs just passively reflect signals without additional power to optimize the phases of the reflection and maximize the signal-to-noise ratio (SNR), which eventually improves the endto-end quality of the desired link of IS-UAV-TNs [6], [7]. Hence the impact of interference nodes on RIS are marginal to the entire network. ...
... The channel and system parameters given in Tables I [2] and Table II [ 1], respectively. From [6] and [15], we set A = 1, L ViR = L SVi = L = 20,λ 1 =λ RD =γ. Besides, we set k S = k R = k υ = k ν = k,λ ID =λ IR = γ I and δ 2 R = δ 2 D = 1. ...
Article
This letter studies the performance of reconfigurable intelligent surface-assisted integrated satellite-unmanned aerial vehicle (UAV)-terrestrial networks with hardware impairments (HIs) and interference in the presence of unavailable direct link. A UAV is considered as a relay to forward the intended signal to the satellite. Due to the practical constraints, HIs and interference are considered in this letter. Specifically, the closed-form expression for the outage probability (OP) is obtained to evaluate the impacts of RIS and key parameters on the system performance. Besides, in order to evaluate the system performance at high signal-to-noise ratios (SNRs), the asymptotic analysis for the OP is also derived. Finally, simulations are provided to verify the correctness of the theoretical results.
... Since the signals are forwarded via reflection, no transmit power is required and no noise is induced at the IRS. The integration of IRS for UAV relay can provide more potentials to improve the quality of the target links and weaken the quality of the interference links [80], [88], [235]- [237]. The IRS can be deployed on the ground buildings to assist the communication between a UAV relay and ground users. ...
... For both scenarios, the placement/trajectory of the UAV relay and the passive beamforming of the IRS can be jointly exploited and optimized for the improvement of the system performance [237]. For example, an IRS-assisted UAV relaying system was investigated in [235], where an IRS is installed on a building to assist the link between a source node and a UAV relay, and the DF relaying protocol is employed at the UAV to serve the destination node. The outage probability, average BER, and average capacity were derived for the considered system [235], and it was shown that the performance highly depends on the horizontal position and altitude of the UAV relay. ...
... For example, an IRS-assisted UAV relaying system was investigated in [235], where an IRS is installed on a building to assist the link between a source node and a UAV relay, and the DF relaying protocol is employed at the UAV to serve the destination node. The outage probability, average BER, and average capacity were derived for the considered system [235], and it was shown that the performance highly depends on the horizontal position and altitude of the UAV relay. The authors in [236] considered an ultra reliable low latency communications (URLLC) scenario facilitated by UAV relay and IRS, where the UAV positioning, passive beamforming of the IRS, and the blocklength were jointly optimized for minimization of the total decoding error rate over the link between the ground transmitter and the ground user. ...
Article
Unmanned aerial vehicles (UAVs) have found widespread commercial, civilian, and military applications. Wireless communication has always been one of the core technologies for UAV. However, the communication capacity is becoming a bottleneck for UAV to support more challenging application scenarios. The heavily-occupied sub-6 GHz frequency band is not sufficient to meet the ultra high-data-traffic requirements. The utilization of the millimeter-wave (mmWave) frequency bands is a promising direction for UAV communications, where large antenna arrays can be packed in a small area on the UAV to perform three-dimensional (3D) beamforming. On the other hand, UAVs serving as aerial access points or relays can significantly enhance the coverage and quality of service of the terrestrial mmWave cellular networks. In this paper, we provide a comprehensive survey on mmWave beamforming enabled UAV communications and networking. The technical potential of and challenges for mmWave-UAV communications are presented first. Then, we provide an overview on relevant mmWave antenna structures and channel modeling. Subsequently, the technologies and solutions for UAV-connected mmWave cellular networks and mmWave-UAV ad hoc networks are reviewed, respectively. Finally, we present open issues and promising directions for future research in mmWave beamforming enabled UAV communications and networking.
... The performance of RIS in the mixed RF/FSO dual-hop system was presented in [26] where the authors considered opportunistic users scheduling. In addition, performance of RIS-empowered UAV relaying network was investigated in [27]. ...
... iv. Relative to [27] where the RIS assisted an unmanned aerial vehicle (UAV) relaying node, the UAV employed DF relay protocol for signal transmission and the system considered a single user at the destination. In this paper, however, an AF relaying protocol is proposed for the HAPS and the system consider multiple users at the destination. ...
... Assuming thatγ 1 → ∞ and following the approach in [27], the first term of (23) can be expressed as: ...
Article
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This research paper studies the performance analysis of a reconfigurable intelligent surface (RIS)-assisted high altitude platform station (HAPS) relaying communication network for multiple ground users. Owing to transmission blockage between the source and the destination, the source transmits its information to the HAPS via the RIS where the HAPS acts as an amplify-and-forward (AF) relay node for sending the source information to ground users. To characterize the transmission links, the channels between the source-RIS-HAPS are assumed to follow approximate generalized-K fading distribution while the links between the HAPS and multiple ground users experience Shadowing-Rician fading distribution. Since HAPS employs AF relaying scheme, the closed-form expressions of the cumulative distribution function (CDF) and probability density function (PDF) of the system end-to-end signal-to-noise ratio (SNR) are derived by exploiting the Meijer’s G and Fox’s H functions. Based on these expressions, the analytical expressions of the system outage probability, average bit error rate (BER) and average capacity are obtained to examine the system performance. Moreover, the asymptotic expressions are obtained to provide more physical insight about the system. In addition, the results unveil the significant effect of the system and channel parameters on proposed system. The results show that the system with RIS offers better performance compared the system without RIS. The analytical results are verified by Monte-Carlo simulation to justify the accuracy of the derived expressions.
... As a result of these efforts two system models have been developed. In the first, the RIS is attached to the UAV [19]- [24], while in the second, the RIS is attached to a fixed position, such as a wall, and has a clear line of sight (LOS) to the RIS [25]- [33]. For both system models, great effort have been made to optimize and analyze their performance. ...
... In [31], the active beamforming of the UAV, the coefficients of the RIS MAs, and the trajectory of the UAV were jointly optimized to arXiv:2201.12056v1 [eess.SP] 28 Jan 2022 [32], the authors investigated the joint optimization of UAV trajectory, RIS PS, sub-band allocation, and power management to maximize the minimum average achievable rate of all users in a RIS-assisted UAV wireless system operating in the terahertz (THz) band. Finally, in [33], the OP, average bit error rate, and ergodic capacity of RIS-assisted dual-hop UAV communication systems were quantified, where the S-RIS and RIS-UAV links were modeled as Rayleigh and mixture-Gamma (MG) RVs, respectively. ...
... Finally, by applying (18) in (36), we get (33). This concludes the proof. ...
Preprint
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In this paper, we analyze the performance of a reconfigurable intelligent surface (RIS)-assisted unmanned aerial vehicle (UAV) wireless system that is affected by mixture-gamma small-scale fading, stochastic disorientation, and misalignment, as well as transceivers hardware imperfections. First, we statistically characterize the end-to-end channel for both cases, i.e., in the absence as well as in the presence of disorientation and misalignment, by extracting closed-form formulas for the probability density function (PDF) and the cumulative distribution function (CDF). Building on the aforementioned expressions, we extract novel closed-form expressions for the outage probability (OP) in the absence and the presence of disorientation and misalignment as well as hardware imperfections. In addition, high signal-to-noise ratio OP approximations are derived, leading to the extraction of the diversity order. Finally, an OP floor due to disorientation and misalignment is presented.
... When such metamaterials are deployed in metasurfaces, their effective parameters can be tailored to realize a desired transformation on the transmitted, received, or impinging waves [14]- [17]. With the availability of new degrees of freedom useful to improve the network performance, the environment will be no more perceived as a passive entity, but as a meaningful support for wireless communications based applications [18]- [23], e.g., energy transfer [24], vehicular networks [25], unmanned aerial vehicle (UAV) communications [26], physical layer security [24], cognitive radio [27], electromagnetic fields (EMF)-aware beamforming [28]- [30], and many others [31]. In this context, wireless localization with RISs [32], [33] has not yet received a large attention, albeit they represent a promising candidate for enhancing positioning and orientation estimation capabilities in next-generation cellular networks for various 6G applications, e.g., augmented reality and self-driving cars [25], [34]- [36]. ...
... for the Synchronous and Asynchronous signalling, respectively, where τ BM and τ RM , ρ BM and ρ BRM , and θ BM , φ BM , θ RM φ RM are the TOAs, RSSI, and AOAs, respectively. All the main parameters that are required to infer the location and orientation of the UE are included in (26). 2 The first approach with the measurement parameter vector expressed in (25) will be also referred to as "Direct" because it directly estimates the state. On the other side, the second approach in (26) will be also named "Two-stage" because it uses intermediate parameters to infer the UE position and orientation. ...
... Starting from (25)- (26), the CRLB on the UE state vector can be written from [58, (178)] as ...
Article
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Next-generation cellular networks will witness the creation of smart radio environments (SREs), where walls and objects can be coated with reconfigurable intelligent surfaces (RISs) to strengthen the communication and localization coverage by controlling the reflected multipath. In fact, RISs have been recently introduced not only to overcome communication blockages due to obstacles but also for high-precision localization of mobile users in GPS denied environments, e.g., indoors. Towards this vision, this paper presents the localization performance limits for communication scenarios where a single next generation NodeB base station (gNB), equipped with multiple-antennas, infers the position and the orientation of a user equipment (UE) in a RIS-assisted SRE. We consider a signal model that is valid also for near-field propagation conditions, as the usually adopted far-field assumption does not always hold, especially for large RISs. For the considered scenario, we derive the Cramer-Rao lower bound (CRLB) for assessing the ultimate localization and orientation performance of synchronous and asynchronous signaling schemes. In addition, we propose a closed-form RIS phase profile that well suits joint communication and localization, and we perform extensive numerical results to assess the performance of our scheme for various localization scenarios and for various RIS phase design. Numerical results show that the proposed scheme can achieve remarkable performance, even in asynchronous signaling and that the proposed phase design approaches the numerical optimal phase design that minimizes the CRLB.
... Hence, the error variance of the channels is given by var ĥ d = var (ĝ m ) = σ 2 T P I N , m = 1, · · · , M + 1. (16) By comparing (16) with (14), the DFT training scheme reduces the channel error by a factor equal to 1/T for the direct channelĥ d and by a factor equal to 1/(2T ) for the RIS-reflected channelsĝ m , ∀m. Recently, the DFT training scheme has been extended to the multiple-input multiple-output (MIMO) case in [47] and to the orthogonal frequency division multiplexing (OFDM) system in [48]. ...
... Then, the training phase shift matrix Φ can be set equal to the first M + 1 columns of the matrix D 2 B . Then, the error covariance matrix is calculated as in (16), which means that setting the training phase shift matrix as the Hadamard matrix results in the minimum MSE of the estimator. Compared to the DFT matrix, the main advantage is that only two discrete phase shifts, namely {0, π}, are required for channel training, which can reduce the hardware complexity, and is thus an appealing solution from the implementation standpoint. ...
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In the past as well as present wireless communication systems, the wireless propagation environment is regarded as an uncontrollable black box that impairs the received signal quality, and its negative impacts are compensated for by relying on the design of various sophisticated transmission/reception schemes. However, the improvements through applying such schemes operating at two endpoints (i.e., transmitter and receiver) only are limited even after five generations of wireless systems. Reconfigurable intelligent surface (RIS) or intelligent reflecting surface (IRS) have emerged as a new and revolutionary technology that can configure the wireless environment in a favorable manner by properly tuning the phase shifts of a large number of passive and low-cost reflecting elements, thus standing out as a promising candidate technology for the next-/sixth-generation (6G) wireless system. However, to reap the performance benefits promised by RIS/IRS, efficient signal processing techniques are crucial, for a variety of purposes such as channel estimation, transmission design, radio localization, and so on. In this paper, we provide a comprehensive overview of recent advances on RIS/IRS-aided wireless systems from the signal processing perspective. We also highlight promising research directions that are worthy of investigation in the future.
... In [32], the authors have considered a downlink transmission system and proposed an algorithm to jointly optimize the UAV trajectory and RIS's passive beamforming in order to maximize the achievable rate. While the performance of RIS-aided UAV relaying system has been studied in [33], the authors in [34] have considered an UAV-RIS system and proposed a algorithm to design UAV trajectory and passive beamforming at the RIS. Moreover, NOMA technique has been adopted in [34] to further improve the spectral efficiency as well as a decaying learning rate deep Qnetwork (D-DQN)-based algorithm has been proposed to design trajectory and passive beamforming. ...
... Furthermore, the total CE error is obtained by adding the three CE error values given in (33). We have considered different values for total CE error in Fig. 6 and have compared the AOP performance under imperfect CSI scenario with the perfect CSI case (i.e., No CE error). ...
Article
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Rate-splitting multiple access (RSMA) has emerged as a novel generalized multiple access technology which can control the interference in multiuser communication systems. Reconfigurable intelligent surface (RIS) has also been proven to improve the spectral efficiency in the next-generation (beyond 5G (B5G) and 6G) wireless communication networks. Thus, in this paper, we explore the RSMA scheme for RIS-enabled unmanned aerial vehicle (UAV) based multiuser vehicular communication network in the presence of co-channel interference. The communication between the UAV and the desired vehicle takes place through an RIS along with the direct link and is interfered by multiple interfering vehicles operating in the same spectrum. After deriving the approximate cumulative distribution function (CDF) of common and private stream signal-to-interference-plus-noise ratios (SINRs) at the desired vehicle in the interference limited scenario, we obtain an expression for the average outage probability (AOP) over the complete flying duration of the UAV. Furthermore, the optimum RSMA power allocation coefficients in each time slot of UAV flight duration are obtained by minimizing the Sum-AOP over all the desired vehicles. The performance of the proposed RSMA based RIS-aided vehicular communication network is compared with (a) RSMA based network without RIS and (b) non-orthogonal multiple access (NOMA) based RIS-aided counterpart. The numerical results reveal the superiority of RSMA over NOMA and the significance of deploying RIS in considered vehicular communication network. The analytical results are corroborated with Monte Carlo simulations. Index Terms-Interference-limited communication, outage probability, rate-splitting multiple access (RSMA), reconfigurable intelligent surface (RIS), unmanned aerial vehicle (UAV), vehicular communication.
... There has been an increased research interest to assess the applicability of RIS based wireless systems for intelligent vehicular communications [6]- [15], radio-frequency (RF) [16]- [24], free-space-optics (FSO) [25]- [30], and terahertz (THz) wireless systems [31]- [33]. The authors in [9] applied series expansion and central limit theorem (CLT) to approximate the outage probability performance of the RIS-assisted vehicular communication assuming Rayleigh and Rician fading channels. ...
... (21) where f γ F SO (γ) and f γ R2V (γ) are the PDF of SNR for FSO and RF links, respectively. We use (15) and (18) with a simple transformation of random variable to get the PDF of SNR ...
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Reconfigurable intelligent surface (RIS) is a promising technology to avoid signal blockage by creating line-of-sight (LOS) connectivity for free-space optical (FSO) and radio frequency (RF) wireless systems. There is limited research on the use of multiple RIS between a source and destination for wireless communications. This paper analyzes the performance of a RIS-assisted multi-hop transmission for vehicular communications by employing multiple RIS to enable LOS communication and reliable connectivity for a hybrid FSO and RF system. We develop an analytical framework to derive statistical results of the signal-to-noise ratio (SNR) of a multi-RIS communication system over general fading models. We use decode-and-forward (DF) and fixed-gain (FG) relaying protocols to mix multi-RIS transmissions over RF and FSO technologies, and derive probability density and distribution functions for both the relaying schemes by considering independent and non-identical double generalized gamma (dGG) distribution models for vehicular RF transmissions and atmospheric turbulence for FSO system combined with zero-boresight pointing errors. We analyze the performance of a moving vehicle connected to one of the RIS modules by deriving exact analytical expressions of the outage probability, average bit-error rate (BER), and ergodic capacity in terms of Fox's H-function. We present asymptotic analysis and diversity order of the outage probability in the high SNR regime to provide a better insight into the system performance. We use computer simulations to demonstrate the effect of multiple RIS modules, fading parameters, and pointing errors on the RIS-aided multi-hop transmissions for the considered vehicular communication system.
... The transmitted signals from the ground device are forwarded at the aerial platform, then reflected at TRIS, prior to being received by the ground receiver. Typically, the TRIS can be employed to reflect the signals transmitted from a ground source to a UAV, and the UAV acts, in turn, as a relaying station employed for transmitting the signal to the destination by using a decode-and-forward (DF) protocol [65], [66]. In this way, such a network combines the reconfigurable channel abilities of RISs and the flexible deployment of UAVs in a novel way. ...
... The first theoretical study about TRIS-assisted NTNs is conducted by Yang et al in [66] where a TRIS-assisted UAV is used to relay the communication between a source and a destination node, as illustrated in Fig. 5. Under the assumption of a Rician distribution for TRIS-UAV and UAV-gU channels, and Rayleigh distribution for the TRIS-gU channel, the authors provided an approximation of the cascaded channel distribution and derived the SNR of the RIS-assisted groundair system. ...
Preprint
Non-terrestrial communications have emerged as a key enabler for seamless connectivity in the upcoming generation networks. This kind of network can support high data rate communications among aerial platforms (i.e., unmanned aerial vehicles (UAVs), high-altitude platforms (HAPs), and satellites) and cellular networks, achieving anywhere and anytime connections. However, there are many practical implementation limitations, especially overload power consumption, high probability of blockage, and dynamic propagation environment. Fortunately, the recent technology reconfigurable intelligent surface (RIS) is expected to be one of the most cost-efficient solutions to address such issues. RIS with low-cost elements can bypass blockages and create multiple line-of-sight (LoS) links, and provide controllable communication channels. In this paper, we present a comprehensive literature review on the RIS-assisted non-terrestrial networks (RANTNs). Firstly, the framework of the RANTNs is introduced with detailed discussion about distinct properties of RIS in NTNs and the two types of RIS, that is, terrestrial RISs (TRISs), and aerial RISs (ARISs), and the classification of RANTNs including RIS-assisted air-to-ground (A2G)/ground-to-air (G2A), ARIS-assisted ground-to-ground (G2G), and RIS-assisted air-to-air (A2A) communications. In combination with next-generation communication technologies, the advanced technologies in RANTNs are discussed. Then we overview the literature related to RANTNs from the perspectives of performance analysis and optimization, followed by the widely used methodologies. Finally, open challenges and future research direction in the context of the RANTNs are highlighted.
... Enlightened by these great potentials of integrating the RIS into UAV communications, a proliferation of innovative studies have been devoted to performance optimization of their integration [8]- [11]. In more detail, the RIS was exploited to assist the UAV relaying system and shown to improve the coverage and reliability of UAV communication systems significantly in [8]. ...
... Enlightened by these great potentials of integrating the RIS into UAV communications, a proliferation of innovative studies have been devoted to performance optimization of their integration [8]- [11]. In more detail, the RIS was exploited to assist the UAV relaying system and shown to improve the coverage and reliability of UAV communication systems significantly in [8]. Moreover, by jointly optimizing the location of UAV and the reflection coefficients of RIS, the UAV equipped with an RIS was leveraged to achieve transmission capacity maximization in [9]. ...
... Under the maximum transmit power constraints, the work [14] aims to maximize the minimum SINR, and the work [15] aims to maximize the weighted-sum-rate (WSR) of all users. Moreover, RISs have also been studied under other communication setups, such as secure communication [16], [17], unmanned aerial vehicle (UAV) communication [18], [19], and simultaneous wireless information and power transfer (SWIPT) systems [20], [21]. Note that in all these works on joint active-passive beamforming design, the active beamforming at the BS is fully digital as in most of the multiple-input-multiple-output (MIMO) beamforming literature, which requires each antenna to be connected to one RF chain, and hence has a high hardware cost. ...
... Find the next point b i+1 using retraction according to (21); 5: Calculate the Riemannian gradient grad f (b i+1 ) according to (18); 6: Calculate the transport T bi→bi+1 (η i ) according to (20); 7: Choose the Polak-Ribiere parameter λ 1 ; 8: Calculate the conjugate direction η i+1 according to (19); 9: i ← i + 1; ...
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This paper considers a reconfigurable intelligent surface (RIS)-aided millimeter wave (mmWave) downlink communication system where hybrid analog-digital beamforming is employed at the base station (BS). We formulate a power minimization problem by jointly optimizing hybrid beamforming at the BS and the response matrix at the RIS, under the signal-to-interference-plus-noise ratio (SINR) constraints at all users. The problem is highly challenging to solve due to the non-convex SINR constraints as well as the unit-modulus phase shift constraints for both the RIS reflection coefficients and the analog beamformer. A two-layer penalty-based algorithm is proposed to decouple variables in SINR constraints, and manifold optimization is adopted to handle the non-convex unit-modulus constraints. {We also propose a low-complexity sequential optimization method, which optimizes the RIS reflection coefficients, the analog beamformer, and the digital beamformer sequentially without iteration.} Furthermore, the relationship between the power minimization problem and the max-min fairness (MMF) problem is discussed. Simulation results show that the proposed penalty-based algorithm outperforms the state-of-the-art semidefinite relaxation (SDR)-based algorithm. Results also demonstrate that the RIS plays an important role in the power reduction.
... In [40], the authors designed a system model where the UAV acts as a relay between the RIS transmitter and ground receiver. The authors aimed to maximize the coverage area and improve the reliability of the system. ...
... The simulations have shown that, for constant SNR, the RIS can significantly reduce the bit error rate (BER) and improve the coverage, and reliability of the system. In [41], the authors considered a system model similar to [40] assisted by VLC. Compared to the limitations of radio waves, the VLC provides high bandwidth and immunity to the interference from the electromagnetic signals [42]. ...
Article
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Reconfigurable intelligent surface (RIS)-aided communication is considered as an exciting research topic in academic and industrial communities since it provides an emerging affordable solution to achieve high quality and secure next-generation wireless systems. Especially, the deployment of RIS in multi-user wireless networks promises to reduce system hardware costs, signal processing complexity, as well as energy consumption due to small size, lightweight and ability to actively shape the wireless propagation environment. Further, by realizing a cost-effective radio environment, RIS-aided communication can be implemented to be an appealing technology for future integration with other emerging wireless applications and communication systems. Despite the positive appeal, RISs face new challenges that hinder integrating efficiently into wireless networks, such as network secrecy performance and system sum-rates, as well as achieving efficient deployment design in highly dynamic and time-varying wireless environments. To this end, we overview recent state-of-the-art techniques to address the above issues faced in the integration of RISs with various emerging multi-user communication techniques, such as Unmanned Aerial Vehicles (UAVs), Non-Orthogonal Multiple Access (NOMA), Millimeter Wave (mmWave) and Terahertz (THz) communications, Physical Layer Security (PLS), massive antennas, and Simultaneous Wireless Information and Power Transfer (SWIPT). Finally, we highlight promising future research directions of RIS-aided communication in Cell-Free Massive Multiple-Input-Multiple-Output (MIMO) systems, Rate-Splitting Multiple Access (RSMA), Light Fidelity (LiFi), and Cognitive Radio (CR) systems.
... Therefore, several works have studied various RIS-assisted UAV communication system. In general, these studies fall into two categories, one for terrestrial RIS [7]- [10] and one for aerial RIS [11]- [15]. For the first category, the UAV trajectory and the phase shift of the RIS mounted on the building surface are jointly designed to intensify different utilities such as communication coverage [7], energy efficiency [8], confidentiality [9] , and communication rates [10]. ...
... In general, these studies fall into two categories, one for terrestrial RIS [7]- [10] and one for aerial RIS [11]- [15]. For the first category, the UAV trajectory and the phase shift of the RIS mounted on the building surface are jointly designed to intensify different utilities such as communication coverage [7], energy efficiency [8], confidentiality [9] , and communication rates [10]. ...
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This paper investigates the unmanned aerial vehicle (UAV)-aided two-way reflecting (TWR) communication system under the probabilistic line-of-sight (LoS) channel model, where a UAV equipped with an RIS is deployed to assist two ground nodes in their information exchange. An optimization problem with the objective of maximizing the expected achievable rate is formulated to design the communication scheduling, the RIS's phase, and the UAV trajectory. To solve such a non-convex problem, we propose an efficient iterative algorithm to obtain suboptimal its solution. Simulation results show that our proposed design provides new insights into the elevation angle-distance trade-off for the TWR communication system, and improves the rate by $28\%$ compared to the scheme under the conventional LoS channel model.
... RISs are passive with dedicated energy sources as they can shape the wave incident upon it. RIS is also important from the energy efficiency perspective as the intelligent reflecting surfaces (IRSs) do not involve any encoding/decoding or amplification of impinged signal while significantly improving the signal power at the intended receiver [5]. Similarly, RISs can easily be deployed on various surfaces, such as houses, cars, or indoor areas. ...
... Similarly, in [10], a RIS assisted dual-hop visible light communication(VLC)/RF system in an indoor scenario was studied where the closedform expression for bit error rate (BER) and outage probability (OP) is derived. The coverage and reliability of unmanned area vehicle communication systems improvement using RIS are explored in [5] and closed-form expression for BER and average capacity is derived. More recently, the authors in [11] have examined the secrecy performance of a RISaided wireless communication system. ...
Article
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In this letter, we analyze the performance of reconfigurable intelligent surface (RIS) assisted mixed power line communication (PLC)/ radio frequency (RF) system in smart grid application. In a smart grid, the data concentrator (DC) plays an important role in communication between the home appliances through the access point (AP). The DC can communicate using the existing PLC up to the AP, and the AP interacts with the home appliances with advanced communication technology like RIS-based RF communication. The modulation scheme is considered here as binary phase-shift keying (BPSK) modulation. Based on the system model, a closed-form expression for average bit error probability (ABEP) and outage probability (OP) are derived and analyzed by varying the various parameters like the number of reflectors in the RIS, impulsive noise scenario in the PLC channel.
... In [14], the UAVs are used as intermediate aerial nodes between the macro and small cell tiers to improve the coverage and increase the capacity of heterogeneous networks. To improve coverage and performance of an UAV system, the authors of [15] proposed a reconfigurable intelligent surface (RIS)-assisted UAV scheme where the UAV operates as a relay to forward the signals from the RIS to the destination. ...
... Similar to (14), the asymptotic OP with ipSIC can be given asP Upon substituting the conditions into (13), we obtain (18). The user k with ipSIC reaches the zero diversity order by replacing (19) into (15). This is due to the impact of residual ipSIC interference. ...
Article
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We evaluate a downlink non-orthogonal multiple access (NOMA)-enable UAV-aided communication system to address the demand of spectrum usage of unmanned aerial vehicles (UAVs). In this paper, multiple NOMA users are served by an UAV to improve the effective spectrum usage. Over Nakagami-m fading model, the performance of the system was investigated based on the analysis of outage probabilities (OPs) of the NOMA users, the ergodic rate, and symbol error rate of the system under two scenarios, i.e., perfect successive interference cancellation (pSIC) and imperfect successive interference cancellation (ipSIC). Additionally, the effects of the system parameters such as the transmit power and altitude of the UAV, the coefficients of channel model on the system performance were studied. The results demonstrate that the performance of the NOMA-based system is better compared with that of the conventional orthogonal multiple access (OMA)-based system in terms of OP, throughput and ergodic rate. Considering outage probabilities at the users, the system in the ipSIC case achieves the same performance as the pSIC case at a low transmit power of the UAV. In addition, an increase in the height of the UAV decreases the ergodic capacity of each user.
... where r p th = 2 Rp − 1 is a threshold. Hereby, we consider a coherent phase shift design [37] to enhance the performance of primary user UE. In other words, it is assumed that the phase shifts from the IRS elements are adjusted in such a way that the reflected signals are co-phased, i.e., φ n = −∠h b,n h n,e where ∠x indicates the phase of a complex number x. ...
Article
Ambient backscatter communication (AmBC) is an emerging technology that has the potential to offer spectral- and energy-efficient solutions for the next generation wireless communications networks, especially for the Internet of Things (IoT). Intelligent reflecting surfaces (IRSs) are also perceived to be an integral part of the beyond 5G systems to complement the traditional relaying scheme. To this end, this paper proposes a novel system design that enables the co-existence of a backscattering secondary system with the legacy primary system. This co-existence is primarily driven by leveraging the AmBC technique in IRS-assisted unmanned aerial vehicle (UAV) networks. More specifically, an aerial-IRS mounted on a UAV is considered to be employed for cooperatively relaying the transmitted signal from a terrestrial primary source node to a user equipment on the ground. Meanwhile, capitalizing on the AmBC technology, a backscatter capable terrestrial secondary node transmits its information to a terrestrial secondary receiver by modulating and backscattering the ambient relayed radio frequency signals from the UAV-IRS. We comprehensively analyze the performance of the proposed design framework with co-existing systems by deriving the outage probability and ergodic spectral efficiency expressions. Moreover, we also investigate the asymptotic behavior of outage performance in high transmit power regimes for both primary and secondary systems. Importantly, we analyze the performance of the primary system by considering two different scenarios, i.e., optimal phase shifts design and random phase shifting at IRS. Finally, based on the analytical performance assessment, we present numerical results to provide various useful insights and also provide simulation results to corroborate the derived theoretical results.
... As an emerging technology, it is artificial planar surface made of electromagnetic (EM) materials which can effectively control the characteristics of incident signal such as phase, amplitude, and frequency [3,4]. Compared to the conventional relaying and multiantenna technologies, it requires no RF chain and additional power for signal amplification, transmission, and regeneration [5]. Therefore, they are more economical and environmental friendly compared to active antenna systems counterpart [6]. ...
Article
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Reconfigurable intelligent surface (RIS) has been suggested as a promising solution to prevent wireless communication systems from transmission blockage. In this paper, the performance of reconfigurable intelligent surface in cooperative decode-and-forward relaying for hybrid radio frequency (RF)/free space optical (FSO) system is evaluated where parallel transmission of information occurs on the system downlink. In this network, the RF links in the system are assumed to follow Nakagami-m distributions while the FSO link is subjected to Gamma-Gamma distribution. Thus, the exact closed-form expressions of the system outage probability and average bit error rate are obtained to quantify the system performance. The accuracy of these expressions is justified by Monte-Carlo simulations. Also, to get more physical insight from the derived outage probability expression, the asymptotic outage probability under the condition of higher signal-to-noise ratio (SNR) is provided. In addition, the results illustrate that the system and channel parameters significantly affect the performance of the concerned system. Furthermore, the results show that RIS-hybrid downlink system offers better performance than hybrid downlink system without RIS. Under the RIS system, the results demonstrate that RIS-hybrid downlink system outperforms RIS-FSO downlink system.
... The idea of incorporating UAVs and RIS has gained traction in the last couple of years. Recently, there have been certain works that have provided definitions and optimization scenarios to tackle the direct links between UAVs and User Equipments (UEs) as well as links between UAV and UE with the aid of RIS [5]- [8]. However, the issues of the existence and capacity limitation of the link from Base This work has been submitted to IEEE for possible publication. ...
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The usage of Reconfigurable Intelligent Surfaces (RIS) in conjunction with Unmanned Ariel Vehicles (UAVs) is being investigated as a way to provide energy-efficient communication to ground users in dense urban areas. In this paper, we devise an optimization scenario to reduce overall energy consumption in the network while guaranteeing certain Quality of Service (QoS) to the ground users in the area. Due to the complex nature of the optimization problem, we provide a joint UAV trajectory and RIS phase decision to minimize transmission power of the UAV and Base Station (BS) that yields good performance with lower complexity. So, the proposed method uses a Successive Convex Approximation (SCA) to iteratively determine a joint optimal solution for UAV Trajectory, RIS phase and BS and UAV Transmission Power. The approach has, therefore, been analytically evaluated under different sets of criterion.
... This communication mode is mainly designed with the purpose of energy saving. Generally, developing future cellular IoT faces many challenges, such as adapting to more users with low latency requirements, dynamically identifying active users, and reliably receiving their information [11]. For the traditional authorization-based random access scheme, uncoordinated users can compete for physical layer resource blocks for data transmission. ...
Article
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This paper performs a comprehensive survey on transmission schemes for the large-scale Internet of things (IoT) networks with nonorthogonal multiple access (NOMA). By solving the interference among users, NOMA can significantly improve the frequency reuse efficiency and support multiple users to use the same frequency resources. It is considered to be one of the most effective technologies for the next-generation wireless communication. However, there are still many challenges on the transmission schemes for the large-scale NOMA system, including the short-data packet transmission, active user detection, channel estimation, and data detection. In order to meet these challenges, this paper first reviews the short-packet transmission in the large-scale NOMA systems and then reviews the active user detection and channel estimation technologies of the considered systems.
... Due to the fact that cooperative relaying technique offers wide operation range and established reliable communication under heavy channel environments and huge obstacles, RIS has been recently extended to cooperative transmission. The RIS research topics have been widely studied in open literature, specifically, in Yang et al. (2020b) where the performance of RIS-empowered UAV relaying network was evaluated. Also, the performance of i. ...
Article
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In this paper, the performance analysis of a reconfigurable intelligent surface (RIS) in a dual-hop decode-and-forward (DF) relay empowered asymmetric radio frequency (RF)/free space optical (FSO) systems is presented. The RIS-assisted RF network is subjected to Nakagami-m distribution while the RIS-assisted FSO networks experience Gamma–Gamma distribution in which both atmospheric turbulence and pointing errors are considered. Thus, the closed-form expressions for the system outage probability and average bit error rate (ABER) are derived with limited number of reflecting elements at RIS-assisted RF network and multiple number of reflecting elements at RIS-assisted FSO network. Further, to obtain more insight about the system characteristic, the asymptotic closed-form expressions are derived at high signal-to-noise ratio (SNR) for the system performance metrics. The results illustrate the impact of the system and channel parameters on the proposed system in terms of atmospheric turbulence, pointing errors under beam width condition, m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$m$$\end{document}-fading parameter, and number of reflecting elements. The correctness of the derived analytical expressions is validated via the Monte-Carlo simulations.
... By realizing the IoT devices with RISs and by utilizing the concept of backscattering, the sensed data can be piggybacked into the shape of the reflected signals without any overhead and energy cost [17]. Employing RISs also has a strong potential in boosting the converge and performance of UAVs-assisted future wireless networks [18]. Furthermore, RISs can significantly assist in enhancing the security of wireless links at the physical layer of protocol stack. ...
Article
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The upcoming beyond 5G (B5G)/6G wireless networks target various innovative technologies, services, and interfaces such as edge computing, ultra-reliable and low-latency communication (URLLC), backscatter communications, and TeraHertz (THz) technology-enabled inter-chip high-capacity communications, to name a few. Although there are ongoing advances in the system/network level, it is crucial to advance the device-level design to efficiently support these novel technologies by addressing various practical constraints in terms of power, computational capacity, and storage capacity limitations. This need for device-level innovation ultimately demands significant enhancements in today's consumer electronics (CE) framework, i.e., CE advancement towards Consumer Electronics 2.0". Considering the contemporary latency requirements of future CE applications (e.g., entertainment, gaming, etc), to enhance the commercial potential of edge processing as a service", it is envisioned that URLLC will further evolve as enhanced-URLLC (e-URLLC) in the B5G era. In this regard, this paper proposes a novel edge computing-enabled e-URLLC framework, named edge computing for CE (ECCE), to support advancements and to initiate discussions on the need for the next-generation CE. Starting with the discussion on recent trends and advances in CE, the proposed framework and its importance in the 6G wireless era are described. Subsequently, several potential technologies and tools to enable the implementation of the proposed ECCE framework are identified along with some interesting open research topics and future recommendations.
... Meanwhile, the interior and glasses of cars/high-speed trains/airplanes also can be coated with IRS that can provide high-speed Internet to passengers while reducing their EM field exposure and decreasing energy consumption. Moreover, IRS-assisted unmanned aerial vehicles (UAVs) communications [83]− [89] can be employed in 6G networks to effectively enhance the coverage in remote areas, such as remote rural and desert areas. It is worth noting that IRS deployed in UAV networks can harvest energy from the incident signals, thus not requiring dedicated energy supply [87] . ...
Preprint
The intelligent information society, which is highly digitized, intelligence inspired and globally data driven, will be deployed in the next decade. The next 6G wireless communication networks are the key to achieve this grand blueprint, which is expected to connect everything, provide full dimensional wireless coverage and integrate all functions to support full-vertical applications. Recent research reveals that intelligent reflecting surface (IRS) with wireless environment control capability is a promising technology for 6G networks. Specifically, IRS can intelligently control the wavefront, e.g., the phase, amplitude, frequency, and even polarization by massive tunable elements, thus achieving fine-grained 3-D passive beamforming. In this paper, we first give a blueprint of the next 6G networks including the vision, typical scenarios and key performance indicators (KPIs). Then, we provide an overview of IRS including the new signal model, hardware architecture and competitive advantages in 6G networks. Besides, we discuss the potential application of IRS in the connectivity of 6G networks in detail, including intelligent and controllable wireless environment, ubiquitous connectivity, deep connectivity and holographic connectivity. At last, we summarize the challenges of IRS application and deployment in 6G networks. As a timely review of IRS, our summary will be of interest to both researchers and practitioners engaging in IRS for 6G networks.
... Besides, UAV-assisted wireless relay networks can also be enhanced by the usage of RISs. In [145], researchers ensured wireless LoS links among user and eNB by modeling RIS reflection factors and UAV routes together in user mobility based RIS-assisted mmWave downlink transmission resulting in enhancement of per user data rate and of realizable downlink LoS likelihood. [146] approximated the instant SINR PDF and obtained the logical expressions of per-user capacity, symbol error rate (SER) and outage possibility within RIS-aided UAV relaying applications. ...
Preprint
Sixth generation (6G) internet of things (IoT) networks will modernize the applications and satisfy user demands through implementing smart and automated systems. Intelligence-based infrastructure, also called reconfigurable intelligent surfaces (RISs), have been introduced as a potential technology striving to improve system performance in terms of data rate, latency, reliability, availability, and connectivity. A huge amount of cost-effective passive components are included in RISs to interact with the impinging electromagnetic waves in a smart way. However, there are still some challenges in RIS system, such as finding the optimal configurations for a large number of RIS components. In this paper, we first provide a complete outline of the advancement of RISs along with machine learning (ML) algorithms and overview the working regulations as well as spectrum allocation in intelligent IoT systems. Also, we discuss the integration of different ML techniques in the context of RIS, including deep reinforcement learning (DRL), federated learning (FL), and FL-deep deterministic policy gradient (FL-DDPG) techniques which are utilized to design the radio propagation atmosphere without using pilot signals or channel state information (CSI). Additionally, in dynamic intelligent IoT networks, the application of existing integrated ML solutions to technical issues like user movement and random variations of wireless channels are surveyed. Finally, we present the main challenges and future directions in integrating RISs and other prominent methods to be applied in upcoming IoT networks. <br
... Several studies have considered the application of IRSs with non-orthogonal multiple access (NOMA) technology to address the challenges of random fluctuation in propagation environments and blocking, see e.g., [109]- [112] and references therein. The works in [113]- [115] investigate the application of IRSs in UAVs networks to enhance the communication coverage and improve the quality of service. ...
Article
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The commercial fifth-generation (5G) wireless communications networks have already been deployed with the aim of providing high data rates. However, the rapid growth in the number of smart devices and the emergence of the Internet of Everything (IoE) applications, which require an ultra-reliable and low-latency communication, will result in a substantial burden on the 5G wireless networks. As such, the data rate that could be supplied by 5G networks will unlikely sustain the enormous ongoing data traffic explosion. This has motivated research into continuing to advance the existing wireless networks toward the future generation of cellular systems, known as sixth generation (6G). Therefore, it is essential to provide a prospective vision of the 6G and the key enabling technologies for realizing future networks. To this end, this paper presents a comprehensive review/survey of the future evolution of 6G networks. Specifically, the objective of the paper is to provide a comprehensive review/survey about the key enabling technologies for 6G networks, which include a discussion about the main operation principles of each technology, envisioned potential applications, current state-of-the-art research, and the related technical challenges. Overall, this paper provides useful information for industries and academic researchers and discusses the potentials for opening up new research directions.
... For UAV-aided edge computing, UAV-aided architectures have been proposed [5][6][7], where a UAV serves as a node that is involved in various tasks-for instance, an edge computing server that executes the computational tasks of nodes, and a relay station to offload computational tasks [8]. Although UAVs provide some advantages to edge computing systems, such systems still need guaranteed reliability to enhance the computational task completion and efficient energy management of the system [9,10]. As the energy of a UAV is typically limited, there may be instances when the battery capacity level of the UAV is not sufficient for computation, and the UAV may therefore provide an incomplete computation service to the user. ...
Article
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An edge computing system is a distributed computing framework that provides execution resources such as computation and storage for applications involving networking close to the end nodes. An unmanned aerial vehicle (UAV)-aided edge computing system can provide a flexible configuration for mobile ground nodes (MGN). However, edge computing systems still require higher guaranteed reliability for computational task completion and more efficient energy management before their widespread usage. To solve these problems, we propose an energy efficient UAV-based edge computing system with energy harvesting capability. In this system, the MGN makes requests for computing service from multiple UAVs, and geographically proximate UAVs determine whether or not to conduct the data processing in a distributed manner. To minimize the energy consumption of UAVs while maintaining a guaranteed level of reliability for task completion, we propose a stochastic game model with constraints for our proposed system. We apply a best response algorithm to obtain a multi-policy constrained Nash equilibrium. The results show that our system can achieve an improved life cycle compared to the individual computing scheme while maintaining a sufficient successful complete computation probability.
... Besides, UAV-assisted wireless relay networks can also be enhanced by the usage of RISs. In [145], researchers ensured wireless LoS links among user and eNB by modeling RIS reflection factors and UAV routes together in user mobility based RIS-assisted mmWave downlink transmission resulting in enhancement of per user data rate and of realizable downlink LoS likelihood. [146] approximated the instant SINR PDF and obtained the logical expressions of per-user capacity, symbol error rate (SER) and outage possibility within RIS-aided UAV relaying applications. ...
Preprint
Sixth generation (6G) internet of things (IoT) networks will modernize the applications and satisfy user demands through implementing smart and automated systems. Intelligence-based infrastructure, also called reconfigurable intelligent surfaces (RISs), have been introduced as a potential technology striving to improve system performance in terms of data rate, latency, reliability, availability, and connectivity. A huge amount of cost-effective passive components are included in RISs to interact with the impinging electromagnetic waves in a smart way. However, there are still some challenges in RIS system, such as finding the optimal configurations for a large number of RIS components. In this paper, we first provide a complete outline of the advancement of RISs along with machine learning (ML) algorithms and overview the working regulations as well as spectrum allocation in intelligent IoT systems. Also, we discuss the integration of different ML techniques in the context of RIS, including deep reinforcement learning (DRL), federated learning (FL), and FL-deep deterministic policy gradient (FL-DDPG) techniques which are utilized to design the radio propagation atmosphere without using pilot signals or channel state information (CSI). Additionally, in dynamic intelligent IoT networks, the application of existing integrated ML solutions to technical issues like user movement and random variations of wireless channels are surveyed. Finally, we present the main challenges and future directions in integrating RISs and other prominent methods to be applied in upcoming IoT networks.
... RISs can be combined with UAVs to improve system throughput [249], network coverage and reliability [250], [251] or EE [252]- [255]. By acting as relays for signals without the need for additional external power supply, RISs achieve high EE when combined with UAVs in cellular networks. ...
Preprint
Wireless communication networks have been witnessing an unprecedented demand due to the increasing number of connected devices and emerging bandwidth-hungry applications. Albeit many competent technologies for capacity enhancement purposes, such as millimeter wave communications and network densification, there is still room and need for further capacity enhancement in wireless communication networks, especially for the cases of unusual people gatherings, such as sport competitions, musical concerts, etc. Unmanned aerial vehicles (UAVs) have been identified as one of the promising options to enhance the capacity due to their easy implementation, pop up fashion operation, and cost-effective nature. The main idea is to deploy base stations on UAVs and operate them as flying base stations, thereby bringing additional capacity to where it is needed. However, because the UAVs mostly have limited energy storage, their energy consumption must be optimized to increase flight time. In this survey, we investigate different energy optimization techniques with a top-level classification in terms of the optimization algorithm employed; conventional and machine learning (ML). Such classification helps understand the state of the art and the current trend in terms of methodology. In this regard, various optimization techniques are identified from the related literature, and they are presented under the above mentioned classes of employed optimization methods. In addition, for the purpose of completeness, we include a brief tutorial on the optimization methods and power supply and charging mechanisms of UAVs. Moreover, novel concepts, such as reflective intelligent surfaces and landing spot optimization, are also covered to capture the latest trend in the literature.
... In [9], the channel distributions of a dual-hop RIS-aided scheme and a transmitting scheme are derived. Additionally, for the RIS-assisted unmanned aerial vehicle (UAV) communication systems, the RIS is used to reflect the signals transmitted from the ground source to a UAV, for which the corresponding outage probability, average bit error rate, and average capacity are given in [10]. In [11], a non-iterative twostage channel estimation framework is proposed for the DOA estimation in the point-to-point RIS-aided millimeter-wave (mmWave) multi-input and multi-output (MIMO) system. ...
Article
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The conventional direction of arrival (DOA) estimation methods are performed with multiple receiving channels. In this paper, a changeling DOA estimation problem is addressed in a different scenario with only one full-functional receiving channel. A new unmanned aerial vehicle (UAV) swarm system using multiple lifted reconfigurable intelligent surface (RIS) is proposed for the DOA estimation. The UAV movement degrades the DOA estimation performance significantly, and the existing atomic norm minimization (ANM) methods cannot be used in the scenario with array perturbation. Specifically, considering the position perturbation of UAVs, a new atomic norm-based DOA estimation method is proposed, where an atomic norm is defined with the parameter of the position perturbation. Then, a customized semi-definite programming (SDP) method is derived to solve the atomic norm-based method, where different from the traditional SDP method, an additional transforming matrix is formulated. Moreover, a gradient descent method is applied to refine the estimated DOA and the position perturbation further. Simulation results show that the proposed method achieves much better DOA estimation performance in the RIS-aided UAV swarm system with only one receiving channel than various benchmark schemes.
... RIS operates with the help of varactor and PIN diodes, where varactor is able to adjust the resonant frequency of RIS and PIN diodes are able to make the structures behave as reflectors or absorbers of the radio frequency (RF) waves by connecting or disconnecting to the ground based on the external voltage supply. RIS, being a passive unit, has been recently used for various applications [4] ranging from device-to-device [5], non-orthogonal multiple access [6], wireless power transfer [7] to free-space optical systems [8] and unmanned aerial vehicular systems [9] with RIS being deployed as a reflector or as an access point to implement a RF chain free transmitter. ...
Article
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Reconfigurable intelligent surfaces (RIS) hold the potential to revolutionize the wireless communications industry via the dynamic control of the wireless channels to achieve unprecedented high data rates, high spectral and energy efficiencies, and low error rates, towards meeting the advanced specifications of beyond 5G. In this paper, we propose a practical channel model for RIS-assisted MIMO communication systems which considers the effects of RIS configurations, operating environments, path loss, scattering, operating frequency, etc. We derive the probability density function and cumulative distribution function of such comprehensive channel model by utilizing a double generalized K distribution, and obtain closed-form expressions for the system's error rate, outage probability, and channel capacity. The analysis highlights that RIS-assisted systems perform better in indoor environments as compared to outdoor use cases due to the presence of less scatterers in the former.
... Owing to broadcast nature of RF link, the eavesdropper attempts to intercept the secret information of the ground users from the HAP relay node. As a result of this, the eavesdropper link is assumed to follow Rican distribution with the PDF of instantaneous SNR γ e defined as [40,41]: ...
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In this paper, the secrecy performance of a mixed free space optical (FSO)/radio frequency (RF) integrated satellite-high altitude platform (HAP) relaying networks for terrestrial multiusers with the existence of an eavesdropper is investigated. In this network, FSO is adopted to establish the link between the satellite and HAP for which it experiences Gamma-Gamma distributions under different detection schemes (i.e., heterodyne and intensity modulation direct detection). The transmission between the amplify-and-forward (AF) relaying HAP and terrestrial multiusers is through the RF and is modeled as shadowed-Rician fading distribution. Owning to broadcasting nature of RF link, it is assumed that an eavesdropper attempts to intercept the users’ confidential message, and the eavesdropper link is subjected to Rician distributions. Specifically, the closed-form expression for the system equivalent end-to-end cumulative distribution function is derived by exploiting the Meijer’s G and Fox’s H functions. Based on this expression, the exact closed-form expressions of the system connection outage probability, secrecy outage probability, and strictly positive secrecy capacity are obtained under the different detection schemes at HAP. Moreover, the asymptotic analyze of the system secrecy outage probability is provided to obtain more physical insights. Furthermore, the accuracy of all the derived analytical closed-form expressions is verified through the Monte-Carlo simulations. In addition, the impact of atmospheric turbulence, pointing errors, shadowing severity parameters, and Rician factor are thoroughly evaluated. Under the same system conditions, the results depict that heterodyne detection outperforms the intensity modulation direct detection.
... For example, when employing a RIS of 100 pieces, UAV energy efficiency is boosted five times. Authors in [11] suggested a RIS-assisted UAV method, in which a RIS mounted on a building is used to reflect signals broadcast from a ground source to a UAV. The UAV is deployed as a relay to transfer the decoded signals to the destination. ...
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With possible new use cases and demanding requirements of future 5th generation (5G) and beyond cellular networks, the future of mobile communications sounds promising. However, the propagation medium has been considered a randomly acting agent between the transmitter and the receiver. With the advent of the digital age of wireless communications, the received signal quality is degrading due to the uncontrollable interactions of the transmitted radio waves with the surrounding artifacts. This paper presents a comprehensive literature review on reconfigurable intelligent surfaces (RISs) and assisted application areas. With the RIS, the network operators can control radio waves’ scattering, reflection, and refraction characteristics by resolving the harmful properties of environmental wireless propagation. Further, the RIS can effectively control the wavefront, such as amplitude, phase, frequency, and even polarization, without requiring complex encoding, decoding, or radio wave processing techniques. Motivated by technological advances, the metasurfaces, reflectarrays, phase shift, and liquid crystals are potential candidates for implementing RIS. Thus, they can be considered the front runner for realizing the 5G and beyond network. Furthermore, the current research activities in the evolving field of wireless networks operated by RIS are reviewed and discussed thoroughly. Finally, to fully explore the potential of RISs in wireless networks, the fundamental research issues to be addressed have been discussed.
... They derived closed-form expressions for the outage probability (OP) and bit error rate (BER) of AF and decode-and-forward (DF) relaying schemes. In [9], the authors utilized RIS to improve the quality of the source signal, which is sent to the destination through an unmanned aerial vehicle. The secrecy OP of RIS-assisted network has been derived in [10] in the presence of a direct link and eavesdropper. ...
Preprint
This paper analyzes the performance of multiple reconfigurable intelligent surfaces (RISs)-aided networks. The paper also provides some optimization results on the number of reflecting elements on RISs and the optimal placement of RISs. We first derive accurate closed-form approximations for RIS channels' distributions assuming independent non-identically distributed (i.ni.d.) Nakagami-\emph{m} fading environment. Then, the approximate expressions for outage probability (OP) and average symbol error probability are derived in closed-form. Furthermore, to get more insights into the system performance, we derive the asymptotic OP at the high signal-to-noise ratio regime and provide closed-form expressions for the system diversity order and coding gain. Finally, the accuracy of our theoretical analysis is validated through Monte-Carlo simulations. The obtained results show that the considered RIS scenario can provide a diversity order of $\frac{a}{2}K$, where $a$ is a function of the Nakagami fading parameter $m$ and the number of meta-surface elements $N$, and $K$ is the number of RISs.
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In this paper, the outage performance of an unmanned aerial vehicle (UAV)-based relay in downlink satellite communication network under the hovering fluctuations is investigated. The satellite establishes communication link with the UAV via the free space optical (FSO) access and the radio frequency (RF) access links the UAV to the ground user. The UAV position and its stability significantly affect the RF and FSO link in the network. Thus, the RF link line of sight and the quality of the FSO link are respectively affected by the relative position of the UAV and hovering. As a result of this, the outage probability and outage capacity of the concerned system are determined subject to different UAV detection techniques under the decode-and-forward relaying protocol. To obtain explicit insight about the system performance, the system outage probability asymptotic expression is derived at high signal-to-noise ratio (SNR). Moreover, the results illustrate the impact of atmospheric turbulence, pointing error, angle-of-arrival (AOA) fluctuation and UAV position on the system performance. The accuracy of the derived outage expressions is validated by the Monte-Carlo simulations.
Article
In this work, we investigate the performance of an all-optical dual hop free-space optical (FSO) communication system using an unmanned aerial vehicle (UAV) as a relay and utilizing a reconfigurable optical intelligent reflecting surface (ROIRS) to improve the coverage. We statistically characterize the combined effects of the three impairments: atmospheric turbulence (AT), angle of arrival (AoA) fluctuation of UAV, and the geometric and misalignment loss (GML) caused by the fluctuations of the transmitter, ROIRS, and the position and orientation of the UAV. Incorporating the aforementioned effects and the non-orthogonality of the received beam at the UAV and the receiver, closed-form expressions of the overall outage probability and the average bit error rate for the considered system are derived. Furthermore, the analytically derived expressions are verified by Monte-Carlo simulations.
Preprint
In this paper, we investigate the performance of an RIS-aided wireless communication system subject to outdated channel state information that may operate in both the near- and far-field regions. In particular, we take two RIS deployment strategies into consideration: (i) the centralized deployment, where all the reflecting elements are installed on a single RIS and (ii) the distributed deployment, where the same number of reflecting elements are placed on multiple RISs. For both deployment strategies, we derive accurate closed-form approximations for the ergodic capacity, and we introduce tight upper and lower bounds for the ergodic capacity to obtain useful design insights. From this analysis, we unveil that an increase of the transmit power, the Rician-K factor, the accuracy of the channel state information and the number of reflecting elements help improve the system performance. Moreover, we prove that the centralized RIS-aided deployment may achieve a higher ergodic capacity as compared with the distributed RIS-aided deployment when the RIS is located near the base station or near the user. In different setups, on the other hand, we prove that the distributed deployment outperforms the centralized deployment. Finally, the analytical results are verified by using Monte Carlo simulations.
Chapter
Intelligent reflecting surface (IRS), a promising technology, can intelligently reflect the signals and improve the propagation environment, which makes it further achieve spectrum efficiency for wireless systems in the future. In this paper, we investigate an IRS-assisted full-duplex (FD) unmanned aerial vehicle (UAV) relay system with a source node and a destination node. Due to the obstacles, the source node can not communicate with the UAV-based mobile relay. The IRS on the wall of a building can reflect the signals from the source node to the relay. We aim to maximize the average rate of the system by jointly optimizing the phase shifter of the IRS, the trajectory of the UAV, and the transmit power of the UAV-based relay. On account of the non-convexity of the formulated problem, we propose a novel algorithm based on iteration that divides the problem into three sub-problems by the block coordinate descent and applies the successive convex approximation (SCA) method for obtaining an approximate optimal solution. Simulation results demonstrate the efficiency of our algorithm.
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In this paper, we propose a mixed radio frequency (RF)/free space optical (FSO) unmanned aerial vehicle (UAV) communication system, based on modulating retro-reflector (MRR) and reconfigurable intelligent surface (RIS), which adopts the hybrid L-ary pulse position modulation-binary phase shift keying-subcarrier intensity modulation (L-PPM-BPSK-SIM). More specifically, the RF channel follows Rayleigh distribution, while the FSO channel obeys Gamma–Gamma distribution that considers atmospheric turbulence and pointing error. For decode-and-forward (DF) relay, the MRR is installed on the UAV to reduce its weight, size, and power consumption. In particular, the RIS is used as user terminal along with the RF signal generator to achieve signal enhancement. Based on this, closed expressions for the outage probability, average bit error rate (BER) and average channel capacity of the end-to-end uplink and downlink are derived. Numerical results confirm that while the relay limitation is solved by MRR, RIS significantly reduces the outage probability and average BER as well as obviously increases the average channel capacity. Furthermore, the hybrid L-PPM-BPSK-SIM with average symbol length greater than eight can effectively improve the average BER performance of the system.
Preprint
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The conventional direction of arrival (DOA) estimation methods are performed with multiple receiving channels. In this paper, a changeling DOA estimation problem is addressed in a different scenario with only one full-functional receiving channel. A new unmanned aerial vehicle (UAV) swarm system using multiple lifted reconfigurable intelligent surface (RIS) is proposed for the DOA estimation. The UAV movement degrades the DOA estimation performance significantly, and the existing atomic norm minimization (ANM) methods cannot be used in the scenario with array perturbation. Specifically, considering the position perturbation of UAVs, a new atomic norm-based DOA estimation method is proposed, where an atomic norm is defined with the parameter of the position perturbation. Then, a customized semi-definite programming (SDP) method is derived to solve the atomic norm-based method, where different from the traditional SDP method, an additional transforming matrix is formulated. Moreover, a gradient descent method is applied to refine the estimated DOA and the position perturbation further. Simulation results show that the proposed method achieves much better DOA estimation performance in the RIS-aided UAV swarm system with only one receiving channel than various benchmark schemes.
Preprint
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This work performs the statistical QoS analysis of a Rician block-fading reconfigurable intelligent surface (RIS)-assisted D2D link in which the transmit node operates under delay QoS constraints. First, we perform mode selection for the D2D link, in which the D2D pair can either communicate directly by relaying data from RISs or through a base station (BS). Next, we provide closed-form expressions for the effective capacity (EC) of the RIS-assisted D2D link. When channel state information at the transmitter (CSIT) is available, the transmit D2D node communicates with the variable rate $r_t(n)$ (adjustable according to the channel conditions); otherwise, it uses a fixed rate $r_t$. It allows us to model the RIS-assisted D2D link as a Markov system in both cases. We also extend our analysis to overlay and underlay D2D settings. To improve the throughput of the RIS-assisted D2D link when CSIT is unknown, we use the HARQ retransmission scheme and provide the EC analysis of the HARQ-enabled RIS-assisted D2D link. Finally, simulation results demonstrate that: i) the EC increases with an increase in RIS elements, ii) the EC decreases when strict QoS constraints are imposed at the transmit node, iii) the EC decreases with an increase in the variance of the path loss estimation error, iv) the EC increases with an increase in the probability of ON states, v) EC increases by using HARQ when CSIT is unknown, and it can reach up to $5\times$ the usual EC (with no HARQ and without CSIT) by using the optimal number of retransmissions.
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Reconfigurable intelligent surfaces (RISs), also known as intelligent reflecting surfaces (IRSs), or large intelligent surfaces (LISs), <sup>1</sup> 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) of 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 multiantenna 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 applications to next-generation networks. <sup>1</sup> Without loss of generality, we use the name of RIS in the remainder of this paper. </fn
Conference Paper
This paper deals with the use of Reconfigurable Intelligent Surfaces (RISs) for radar surveillance in Non-Line Of Sight (N-LOS) scenarios. First of all, the geometry of the scene and the new system concept is described with emphasis on the required operative modes and the role played by the RIS. Then, the specific radar equation (including the RIS effect) is developed to manage the coverage requirements in the challenging region where the LOS is not present. Hence, a digression on the use of the radar timeline for the new operative mode is presented together with a discussion on the resolution issues for the range, azimuth, and Doppler domains. Finally, the interplay among the system parameters and, in particular, those involving the RIS is discussed and analyzed via numerical simulations.
Chapter
The diverse and stringent requirements of 6G networks, detailed in the present book, give rise to fully flexible, end-to-end, software-defined network paradigm, where every part of the network can be configured and controlled via software. However, the wireless environment per se – i.e. the wireless medium or channel – is generally assumed uncontrollable, thus, imposing inherent limitations to 6G networks, that arise by the very nature of wireless operation. To this end, the emergence of the newly introduced concept of reconfigurable intelligent surfaces (RISs) challenges the fundamental status quo that the wireless environment is fixed by nature, paving the way for the realization of end-to-end fully reconfigurable 6G networks. RISs are network devices that can control and manipulate the radio waves traversing through the wireless channel. Since RISs reside within the wireless channel and not at the communication endpoints, they can control the wireless channels from within. In this context, the present chapter introduces the concept of RISs within the 6G framework, elaborating on their advantages and limitations. RISs are then compared with other transmission technologies, e.g. phased arrays, multi-antenna transmitters, and relays, while demonstrating the trade-offs governing their operation and applicability. Subsequently, the different types of RIS implementations are presented from a theoretical standpoint, along the available prototypes in the literature; thus, demonstrating how the RIS technology is already a reality, ushering wireless networks into the 6G era.
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Challenging environments comprise a range of scenarios, which share the fact that it is extremely difficult to establish a communication link using conventional technology due to many impairments typically associated with the propagation medium and increased signal scattering. Specifically, underwater and underground media are known to absorb electromagnetic radiation, which heavily affects the overall path loss. Industrial and disaster environments can be viewed as rich scattering environments with corresponding substantial multipath propagation leading to intersymbol interference and deterioration of signal quality. Although the challenges for the design of communication networks, and specifically the Internet of Things (IoT), in such environments are known, there is no common enabler or solution for all these applications. Reconfigurable intelligent surfaces (RISs) have been introduced to improve the signal propagation characteristics by focusing the signal power in the preferred direction, thus making the communication environment ’smart’. While the usual application of RIS is related to blockage avoidance, the very same technique can be used to reduce the effect of multipath and even partially compensate the signal absorption via passive beamforming. Due to the beneficial properties of RIS, its use in challenging environments can become the aforementioned enabler and a game changing technology. However, various aspects of RIS deployment and system design need to be addressed in order to fully benefit from this technology. In this paper, we discuss potential use cases, deployment strategies and design aspects for RIS devices in underwater IoT, underground IoT as well as Industry 4.0 and emergency networks. Furthermore, we provide a potential hardware architecture and derive the expected signal quality improvements with increasing number of RIS elements. The numerical results reveal substantial performance gains of up to 20 dB per decade. In addition, novel research challenges to be addressed in this context are described.
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Intelligent reflecting surface (IRS) that enables the control of wireless propagation environment has recently emerged as a promising cost-effective technology for boosting the spectral and energy efficiency of future wireless communication systems. Prior works on IRS are mainly based on the ideal phase shift model assuming full signal reflection by each of its elements regardless of the phase shift, which, however, is practically difficult to realize. In contrast, we propose in this paper a practical phase shift model that captures the phase-dependent amplitude variation in the element-wise reflection design. Based on the proposed model and considering an IRS-aided multiuser system with one IRS deployed to assist in the downlink communications from a multi-antenna access point (AP) to multiple single-antenna users, we formulate an optimization problem to minimize the total transmit power at the AP by jointly designing the AP transmit beamforming and the IRS reflect beamforming, subject to the users’ individual signal-to-interference-plus-noise ratio (SINR) constraints. Iterative algorithms are proposed to find suboptimal solutions to this problem efficiently by utilizing the alternating optimization (AO) as well as penalty-based optimization techniques. Moreover, to draw essential insight, we analyze the asymptotic performance loss of the IRS-aided system that employs practical phase shifters but assumes the ideal phase shift model for beamforming optimization, as the number of IRS elements goes to infinity. Simulation results unveil substantial performance gains achieved by the proposed beamforming optimization based on the practical phase shift model as compared to the conventional ideal model.
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The future of mobile communications looks exciting with the potential new use cases and challenging requirements of future 6th generation (6G) and beyond wireless networks. Since the beginning of the modern era of wireless communications, the propagation medium has been perceived as a randomly behaving entity between the transmitter and the receiver, which degrades the quality of the received signal due to the uncontrollable interactions of the transmitted radio waves with the surrounding objects. The recent advent of reconfigurable intelligent surfaces in wireless communications enables, on the other hand, network operators to control the scattering, reflection, and refraction characteristics of the radio waves, by overcoming the negative effects of natural wireless propagation. Recent results have revealed that reconfigurable intelligent surfaces can effectively control the wavefront, e.g., the phase, amplitude, frequency, and even polarization, of the impinging signals without the need of complex decoding, encoding, and radio frequency processing operations. Motivated by the potential of this emerging technology, the present article is aimed to provide the readers with a detailed overview and historical perspective on state-of-the-art solutions, and to elaborate on the fundamental differences with other technologies, the most important open research issues to tackle, and the reasons why the use of reconfigurable intelligent surfaces necessitates to rethink the communication-theoretic models currently employed in wireless networks. This article also explores theoretical performance limits of reconfigurable intelligent surface-assisted communication systems using mathematical techniques and elaborates on the potential use cases of intelligent surfaces in 6G and beyond wireless networks.
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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.
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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.
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This paper analyzes the performance of dual-hop mixed RF/unified FSO DF relaying, where heterodyne detection (HD) and intensity modulation-direct detection (IM-DD) are considered for FSO detection. Specifically, we derive closed-form expressions for the outage probability, average BER, and ergodic capacity of this system. In doing that, we utilize, for the first time to best of our knowledge, a precise channel capacity result for IM-DD channel. Moreover, this is the first time that not only the (IM-DD input-independent) but also the (IM-DD cost-dependent) AWGN channel is considered in such analysis. This work uses general fading models, namely Nakagami-m and Malaga (M) fading with pointing error for RF and FSO links, respectively. Also, in BER derivation, we assume that the modulation schemes in the two hops can be different, since not all modulation schemes are suitable for IM-DD FSO links. Additionally, the system performance is investigated asymptotically at high SNRs where a new non-reported diversity order and coding gain analysis is shown. Interestingly, we find that in the FSO hop, based on transmitted power, all the FSO detectors result in the same diversity order. Furthermore, we provide simulation results which confirm the accuracy of the derived analytical and asymptotic expressions.
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Relaying is an effective technique to achieve reliable wireless connectivity in harsh communication environment. However, most of the existing relaying schemes are based on relays with fixed locations, or \emph{static relaying}. In this paper, we consider a novel \emph{mobile relaying} technique, where the relay nodes are assumed to be capable of moving at high speed. Compared to static relaying, mobile relaying offers a new degree of freedom for performance enhancement via careful relay trajectory design. We study the throughput maximization problem in mobile relaying systems by optimizing the source/relay transmit power along with the relay trajectory, subject to practical mobility constraints (on the relay speed and initial/final relay locations), as well as the \emph{information-causality constraint} at the relay owing to its decode-store-and-forward (DSF) strategy. It is shown that for fixed relay trajectory, the throughput-optimal source/relay power allocations over time follow a "staircase" water filling (WF) structure, with \emph{non-increasing} and \emph{non-decreasing} water levels at the source and relay, respectively. On the other hand, with given power allocations, the throughput can be further improved by optimizing the relay trajectory via successive convex optimization. An iterative algorithm is thus proposed to optimize the power allocations and relay trajectory alternately. Furthermore, for the special case with free initial and final relay locations, the jointly optimal power allocation and relay trajectory are derived. Numerical results show that by optimizing the trajectory of the relay and power allocations adaptive to its induced channel variation, mobile relaying is able to achieve significant throughput gains over the conventional static relaying.
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Novel analytic solutions are derived for integrals that involve the generalized Marcum Q-function, exponential functions and arbitrary powers. Simple closed-form expressions are also derived for the specific cases of the generic integrals. The offered expressions are both convenient and versatile, which is particularly useful in applications relating to natural sciences and engineering, including wireless cpmmunications and signal processing. To this end, they are employed in the derivation of the channel capacity for fixed rate and channel inversion in the case of correlated multipath fading and switched diversity.
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Aerial platforms have recently gained a significant popularity as key enablers for rapid deployable relief networks where coverage is provided by onboard radio heads. These platforms are capable of delivering essential wireless communication for public safety agencies in remote areas or during the aftermath of natural disasters. In this paper we provide an analytical approach to optimize the altitude of such platforms in order to allow the maximum exploitation of their coverage capabilities over an urban environment. The solution is presented in a simple mathematical form, which is a function of the maximum allowed pathloss and of the statistical parameters of the urban environment as defined by the International Telecommunication Union. Furthermore, we present a closed form formula for predicting the probability of geometrical line of sight between a low altitude platform and a corresponding ground receiver based on the same environment statistical parameters.
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Recent studies have shown that reconfigurable intelligent surfaces (RISs) can help in extending the wireless coverage. However, no work has quantitatively analyzed the coverage. In this letter, we present the coverage analysis for an RIS-aided communication system. For comparison, the coverage analysis for relaying systems and direct-link transmission systems are also derived. Furthermore, we provide the analysis for the probability of signal-to-noise ratio (SNR) gain of the RIS over the direct link transmission. Finally, we analyze the delay outage rate of the RIS in achieving ultra-reliable low-latency transmissions. The results show that using the RIS can expand the coverage, increase the SNR gain, and reduce latency of the links.
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In this letter, we propose a dual-hop reconfigurable intelligent surface (RIS)-based free space optical and radio frequency (FSO-RF) communication system, where an RIS is utilized to improve the coverage and system performance. Taking both the atmospheric turbulence and pointing errors into consideration, we derive exact closed-form expressions for the outage probability and bit error rate (BER) in terms of the Meijer’s G-function, generalized hypergeometric function, and Marcum Q-function. Furthermore, to obtain the diversity order, an asymptotic outage analysis is also obtained. Finally, the correctness of the analytical results is verified by Monte-Carlo simulation results.
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Thanks to the line-of-sight (LoS) transmission and flexibility, unmanned aerial vehicles (UAVs) effectively improve the throughput of wireless networks. Nevertheless, the LoS links are prone to severe deterioration by complex propagation environments, especially in urban areas. Reconfigurable intelligent surfaces (RISs), as a promising technique, can significantly improve the propagation environment and enhance communication quality by intelligently reflecting the received signals. Motivated by this, the joint UAV trajectory and RIS’s passive beamforming design for a novel RIS-assisted UAV communication system is investigated to maximize the average achievable rate in this letter. To tackle the formulated non-convex problem, we divide it into two subproblems, namely, passive beamforming and trajectory optimization. We first derive a closed-form phase-shift solution for any given UAV trajectory to achieve the phase alignment of the received signals from different transmission paths. Then, with the optimal phase-shift solution, we obtain a suboptimal trajectory solution by using the successive convex approximation (SCA) method. Numerical results demonstrate that the proposed algorithm can considerably improve the average achievable rate of the system.
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IRS is a new and revolutionizing technology that is able to significantly improve the performance of wireless communication networks, by smartly reconfiguring the wireless propagation environment with the use of massive low-cost passive reflecting elements integrated on a planar surface. Specifically, different elements of an IRS can independently reflect the incident signal by controlling its amplitude and/or phase and thereby collaboratively achieve fine-grained 3D passive beamforming for directional signal enhancement or nulling. In this article, we first provide an overview of the IRS technology, including its main applications in wireless communication, competitive advantages over existing technologies, hardware architecture as well as the corresponding new signal model. We then address the key challenges in designing and implementing the new IRS-aided hybrid (with both active and passive components) wireless network, as compared to the traditional network comprising active components only. Finally, numerical results are provided to show the great performance enhancement with the use of IRS in typical wireless networks.
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In this letter, we investigate the secrecy performance of an unmanned aerial vehicle (UAV)-to-UAV system, where a UAV acts as the source ($S$) transmitting information to a legitimate UAV receiver while a group of UAVs trying to eavesdrop the information delivery between $S$ and legitimate UAV receiver. The locations of the legitimate UAV receiver and the eavesdropping UAVs are randomly distributed in the coverage space of $S$. We first characterize the statistical characteristics of the signal-to-noise ratio over $S$ to the legitimate UAV receiver links; and then the closed-form analytical expressions for secrecy outage probability and the average secrecy capacity have also been derived accordingly. Finally, Monte-Carlo simulations are carried out to verify our proposed analytical models.
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In this paper, we propose an UAV-based relaying system with energy harvesting functionality. In particular, we assume that this system is operated in urban communication environments, where the channel between the UAV and the land destination is modeled as shadowed-Rician fading or shadowed- Rayleigh fading. Based on this setting, outage probability analysis for different urban environment parameters is derived. Finally, Monte Carlo simulations are conducted to verify the accuracy of our analytical results.
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The use of unmanned aerial vehicles (UAVs) that serve as aerial base stations is expected to become predominant in the next decade. However, in order for this technology to unfold its full potential it is necessary to develop a fundamental understanding of the distinctive features of air-to-ground (A2G) links. As a contribution in this direction, this paper proposes a generic framework for the analysis and optimization of the A2G systems. In contrast to the existing literature, this framework incorporates both height-dependent path loss exponent and small-scale fading, and unifies a widely used ground-to-ground channel model with that of A2G for analysis of large-scale wireless networks. We derive analytical expressions for the optimal UAV height that minimizes the outage probability of a given A2G link. Moreover, our framework allows us to derive a height-dependent closed-form expression and a tight lower bound for the outage probability of an \textit{A2G cooperative communication} network. Our results suggest that the optimal location of the UAVs with respect to the ground nodes does not change by the inclusion of ground relays. This enables interesting insights in the deployment of future A2G networks, as the system reliability could be adjusted dynamically by adding relaying nodes without requiring changes in the position of the corresponding UAVs.
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In this paper, we consider a two-way relaying system where two users communicate with each other with the help of multiple relays and in which every node is equipped with a single antenna. We assume that the relay employs the amplify-and-forward protocol and propose the Nth worst relay selection scheme. We intend to analyze the system performance of our considered system under two different fading models: lognormal shadowing channels and generalized-K fading channels. More specifically, the outage probability and bit-error-rate expressions of the proposed scheme are derived. Finally, numerical results are provided to verify and illustrate our mathematical analysis.
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In this study, highly accurate closed-form approximations to the probability density function of the sum of independent identically distributed (i.i.d.) generalised-K fading envelopes are derived. These approximations are valid for a wide range of values of the distribution parameters and number of summands. Based on the previously derived formulas, simple precise approximations for the outage probability and the average bit error probability of equal-gain combining receivers operating on i.i.d. generalised-K fading channels are proposed. Extensive numerically evaluating and computer simulation results are presented to demonstrate the proposed analysis.
Conference Paper
The stratospheric platform or high altitude platform (HAP) is currently new proposal of wireless infrastructure in the near future. To provide high quality services, investigation of stratospheric platform channel is essential. This paper proposes the definition and the analysis of the narrowband wireless channel for the link between stratospheric platform and terrestrial users based on experiment in "semi-urban" environment. Narrowband channel characteristics are presented in terms of Ricean factor (K factor) and local mean received power over a wide range of elevation angles ranging from 10° to 90°. Finally in this paper, we performed average bit error probability evaluations based on the proposed channels model. For a given environment where the measurements were conducted, we found a better performance for elevation angles greater than 40°.
Article
Error performance is one of the main performance measures and the derivation of its closed-form expression has proved to be quite involved for certain communication systems operating over composite fading channels. In this letter, a unified closed-form expression, applicable to different binary modulation schemes, for the bit error rate of dual-branch selection diversity based systems undergoing independent but not necessarily identically distributed generalized-K fading is derived in terms of the extended generalized bivariate Meijer G-function.
Article
Stratospheric platforms have been recently proposed as a new wireless infrastructure for realizing the next generation of communication systems. To provide high quality services, an investigation of the wireless stratospheric platform channel is essential. This paper proposes a definition and describes an analysis of the wireless channel for the link between stratospheric platforms and terrestrial mobile users based on an experiment in a semi-urban environment. Narrowband channel characteristics are presented in terms of Ricean factor ( K factor) and local mean received power over a wide range of elevation angles ranging from 10° to 90°. Finally, we evaluated average bit error probability based on the proposed channel model to examine the channel performance. For the environment in which the measurements were conducted, we find that elevation angles greater than 40° yield better performance.
Article
A scheme to approximate the distribution of the sum of M independent, non-identically distributed Ricean random variables by the κ-μ distribution is proposed. To this end, appropriate κ-μ distribution parameters are derived. The summands are assumed to have arbitrary mean powers and arbitrary fading parameters. The differences between exact and approximate distribution curves are almost imperceptible.
Article
We quantify the performance of wireless transmissions over random fading channels at high signal-to-noise ratio (SNR). The performance criteria we consider are average probability of:error and outage probability. We show that as functions of the average SNR, they can both be characterized by two parameters: the diversity and coding gains. They both exhibit identical diversity orders, but their coding gains in decibels differ by a constant. The diversity and coding gains are found to depend on the behavior of-the random SNR's probability density function only at the origin, or equivalently, on the decaying order of the corresponding moment generating function (i.e., how fast the moment generating function goes to zero as its argument goes to infinity). Diversity and coding gains for diversity combining systems are expressed in terms of the diversity branches' individual diversity and coding gains, where the branches can come from any diversity technique such as space, time, frequency, or, multipath. The proposed analysis offers a simple and unifying approach to evaluating the performance of uncoded and (possibly space-time) coded transmissions over fading channels, and the method applies to almost all digital modulation schemes, including M-ary phaseshift keying, quadrature amplitude modulation, and frequency-shift keying with coherent or noncoherent detection.
Article
The capacity of multiple-input multiple-output (MIMO) wireless channels is limited by both the spatial fading correlation and rank deficiency of the channel. While spatial fading correlation reduces the diversity gains, rank deficiency due to double scattering or keyhole effects decreases the spatial multiplexing gains of multiple-antenna channels. In this paper, taking into account realistic propagation environments in the presence of spatial fading correlation, double scattering, and keyhole effects, we analyze the ergodic (or mean) MIMO capacity for an arbitrary finite number of transmit and receive antennas. We assume that the channel is unknown at the transmitter and perfectly known at the receiver so that equal power is allocated to each of the transmit antennas. Using some statistical properties of complex random matrices such as Gaussian matrices, Wishart (1928) matrices, and quadratic forms in the Gaussian matrix, we present a closed-form expression for the ergodic capacity of independent Rayleigh-fading MIMO channels and a tight upper bound for spatially correlated/double scattering MIMO channels. We also derive a closed-form capacity formula for keyhole MIMO channels. This analytic formula explicitly shows that the use of multiple antennas in keyhole channels only offers the diversity advantage, but provides no spatial multiplexing gains. Numerical results demonstrate the accuracy of our analytical expressions and the tightness of upper bounds.
The k −µ distribution and the η −µ distribution
• M D Yacoub
M. D. Yacoub, "The k −µ distribution and the η −µ distribution," IEEE Antenn Propag M, vol. 49, no. 1, pp. 68-81, Feb. 2007.
IEEE Wireless Commun. Lett.
• L Yang
• Y Yang
• M O Hasna
• M Alouini
Proc. IEEE 20th Int. Workshop Signal Process. Adv. Wireless Commun.
• C Huang
• G C Alexandropoulos
• C Yuen
• M Debbah