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On the Performance of RIS-Assisted Dual-Hop UAV Communication Systems
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.
... This phenomenon can be explained by the "multiplicative fading" effect, that is, the path loss of the cascade BS-RIS-Receiver link is the product rather than the sum of path loss of BS-RIS link and RIS-User link. To counter this new type of "double decline", active RIS was put forward in [17]- [19]. The main feature of active RIS different from passive one lies in the structure of each reflecting element. ...
... In active RIS, each reflecting element has a power amplifier, and can simultaneously adjust phase and amplification of signal. In [17]- [19], active RIS assisted uplink and downlink systems were studied respectively, and some algorithms were also presented to design the phase shift and amplification factor of active RIS. Numerical experiments showed that this kind of active RIS can achieve higher transmission rate in comparison with passive RIS, thus overcoming the basic limitation of "double fading" effect. ...
... Denoting the optimal solution of the problem (19) as {V * k , µ * }, V * k will always satisfy rank(V * k ) = 1. Proof. ...
In this paper, we investigate simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS), which enables to communicate with users both sides by transmitting signals to users forward and reflecting signals to users backward simultaneously. We consider a communication system with a STAR-RIS, a base station (BS) and many users to maximize the minimum user energy efficiency (EE) by jointly optimizing active beamforming, transmission and reflection coefficients with power constraint. Considering the fractional programming form of the formulated problem, we adopte Dinkelbach algorithm to transform the problem into the form of convex function subtraction, and then divide it into two sub-problems optimizing the beamforming vectors at BS and coefficient matrices at STAR-RIS separately. With the two subproblems fixed, an Alternating Optimization (AO) algorithm is proposed to solve the maximize minimum user EE fair optimization problem. Numerical results demonstrate that STAR-RIS can enhance energy and spectrum efficiency of the STAR-RIS communication system, and the proposed algorithm maximizes the minimum EE effectively to ensure user fairness.
... However, it has been observed that in dual-hop communication systems with co-channel interference, RIS can help mitigate the impact of interference from nearby channels [24]. Ref. [25] proposed a study on the effect of different system parameters, including the number and placement of RIS elements, on the performance of a RIS-assisted communication system. Here, the authors concluded that the most effective RIS configuration for optimal performance depends on the specific communication scenario and network requirements. ...
... Substituting Eqs. (25) and (32) ...
... where T 1 = (a + a e + n + 2). Plugging (25) and (32) ...
The appearance of sixth-generation networks has resulted in the proposal of several solutions to tackle signal loss. One of these solutions is the utilization of reconfigurable intelligent surfaces (RIS), which can reflect or refract signals as required. This integration offers significant potential to improve the coverage area from the sender to the receiver. In this paper, we present a comprehensive framework for analyzing the secrecy performance of a RIS-aided mixed radio frequency (RF)-free space optics (FSO) system, for the first time. Our study assumes that a secure message is transmitted from a RF transmitter to a FSO receiver through an intermediate relay. The RF link experiences Rician fading while the FSO link experiences M\'alaga distributed turbulence with pointing errors. We examine three scenarios: 1) RF-link eavesdropping, 2) FSO-link eavesdropping, and 3) a simultaneous eavesdropping attack on both RF and FSO links. We evaluate the secrecy performance using analytical expressions to compute secrecy metrics such as the average secrecy capacity, secrecy outage probability, strictly positive secrecy capacity, effective secrecy throughput, and intercept probability. Our results are confirmed via Monte-Carlo simulations and demonstrate that fading parameters, atmospheric turbulence conditions, pointing errors, and detection techniques play a crucial role in enhancing secrecy performance.
... In recent reports, the RIS has been used to aid UAV communications [3,4,7,8,[14][15][16][17][18][19][20][21][22][23]. Specifically, different scenarios were investigated such as RISs aided ground-to-air (G2A) [3,8,14,15], air-to-ground (A2G) [4,7,16,20,22,23], and both G2A and A2G communications [17,21]. ...
... In recent reports, the RIS has been used to aid UAV communications [3,4,7,8,[14][15][16][17][18][19][20][21][22][23]. Specifically, different scenarios were investigated such as RISs aided ground-to-air (G2A) [3,8,14,15], air-to-ground (A2G) [4,7,16,20,22,23], and both G2A and A2G communications [17,21]. Additionally, the case that the UAV carry RISs to support data transmission was also studied [18,19,21]. ...
... As a result, the number of users in NOMA systems is greatly increased. On the other hand, the advantages of utilizing RISs and UAV in wireless communications have been demonstrated in recent reports [3,4]. Specifically, the RIS can reflect signals without signal processing leading to a reduction in computation complexity [5,6]. ...
This paper examines the performance of a wireless system where two reconfigurable intelligent surfaces (RISs) and an unmanned aerial vehicle (UAV) are combined to aid multiple ground users adopting nonorthogonal multiple access (NOMA) scheme. Specifically, the UAV acts as an aerial relay station, the first and the second RISs are respectively used to aid ground-to-air (G2A) and air-to-ground (A2G) communications. Besides the reflected link from the RISs, the direct link between a ground base station (S) and the UAV, the UAV and ground users are exploited to increase the received power at UAV and ground users. We successfully derive the theoretical formulas of outage probability (OP) and ergodic capacity (EC) of RIS-aided G2A/A2G-NOMA system over Nakagami-m fading channel. The derived formulas are validated by Monte-Carlo simulations, and the impacts of system parameters on the OP and EC are fully evaluated to achieve useful insights into the system behaviors. Numerical results clarify that by using two RISs where every RIS is equipped with 100 reflecting elements (REs), the OP of RIS-aided G2A/A2G-NOMA system is greatly lower than the OP of the G2A/A2G-NOMA system without RISs, in which there are only S-UAV and UAV-ground users direct links. In the case of EC, the benefit of using RISs focuses on a low transmit power regime. It is because the EC of the RIS-aided G2A/A2G-NOMA system is only higher than the EC of the system without RISs in the low transmit power region. For the high transmit power regions, the ECs with and without RISs are similar.
... Low Power Wide Area Networks (LPWANs) [3,4] is a promising solution to these challenges and has been deployed in IoT applications such as environment monitoring [5], smart irrigation [6], and unmanned aerial vehicle (UAV) communication systems [7]. As another key technology of future wireless networks, the UAVs may be used as either a relaying station decoding and forwarding the signals of a ground source reflected by a reconfigurable intelligent surface (RIS) [8] or an aerial base station to increase the performance of a dual-hop underwater wireless optical communication (UWOC) system where an autonomous underwater vehicle is receiving transmissions through a over-sea surface relay (R) [9]. Since the Rician fading model is valid for both the UAV-RIS link in the RIS assisted UAV system [8] and the UAV-R link in the dual-hop UWOC system [9], the proposed union bounds can also be applied in these scenarios if LoRa modulation is adopted in the communication links to the UAVs. ...
... As another key technology of future wireless networks, the UAVs may be used as either a relaying station decoding and forwarding the signals of a ground source reflected by a reconfigurable intelligent surface (RIS) [8] or an aerial base station to increase the performance of a dual-hop underwater wireless optical communication (UWOC) system where an autonomous underwater vehicle is receiving transmissions through a over-sea surface relay (R) [9]. Since the Rician fading model is valid for both the UAV-RIS link in the RIS assisted UAV system [8] and the UAV-R link in the dual-hop UWOC system [9], the proposed union bounds can also be applied in these scenarios if LoRa modulation is adopted in the communication links to the UAVs. ...
... Conditional on symbol m being transmitted, the DFT of the de-chirped signal (8) can be rewritten aš ...
In this paper the symbol error performance of LoRa modulation is addressed for flat Rician block fading channels. First the exact symbol error probability of the LoRa modulation on Rician fading is derived. Then the upper and lower union bounds are employed on the derived symbol error probability. The proposed bounds are compared against the exact symbol error probability, the numerical evaluation of the symbol error probability and the state-of-art approximation of the LoRa symbol error probability. Numerical results show that while the proposed upper bound is very tight to the exact symbol error probability, there is approximately a 2.5 dB gap for the lower bound.
... where h 1,n and h 2,n denote the channel coefficients between the UE and the n th element of the LoS IRS, and the n th element of the IRS and the LoS BS, respectively. The channel coefficients are modeled independently across different IRS elements, as in [31]- [33]. ...
... The probabilities P(A D,P L ) and P(A I,P L ) are obtained by applying the total probability theorem to (33) and (34), and averaging over x and z, respectively. Proposition 3. The probability that the PL value of the direct link is lower than or equal to the PL value of the indirect link is ...
p>Intelligent reflecting surfaces (IRSs) have become a popular topic in recent years for their great potential for controlling the radio link environment of wireless networks. With this controlled environment, the coverage can be increased. This paper examines the coverage analysis of IRS-aided networks, considering both two-dimensional buildings and the product-distance path loss model for the first time. Leveraging the tools from stochastic geometry, the locations of base stations (BSs), buildings, and IRSs are modeled with a homogeneous Poisson point process (PPP). A Gamma approximation for the distribution of the nearest line-of-sight (LoS)-neighbor distance is proposed, leading to a closed-form expression for the distribution of the product-distance. Feasible BSs are defined as BSs which are reachable via an IRS deployed on a specific facade of a building, and the ratio of feasible BSs is derived. Simulations are performed, which confirm the proposed analytical methods. In the numerical results, it is observed that the IRSs can introduce up to a 45\% coverage boost, and the effect of the IRS length on the coverage probability is limited beyond 1.2 meters at 60 GHz.</p
... However, the conventional reflection-only PRIS has only half of the service area, which means that the transmitter and receiver must be located on the same side of the PRIS. With the development of metasurface technology, a new type of simultaneously transmitting and reflecting surface (STARS) was proposed, which expands the service scope to full space [9]. ...
... Before calculating the outage probability of RU m , we derive the PDF expression for Z = |h m | + X φ 2 . By combining (8), (9) and (13), the PDF expression for Z is calculated as ...
The active simultaneously transmitting/reflecting surface (ASTARS) is considered a promising technique to achieve full spatial coverage and overcome multiplicative fading caused by cascaded paths. This paper investigates the performance of ASTARS-assisted rate-splitting multiple-access networks (ASTARS-RSMA) with multiple transmission users (TUs) and reflection users (RUs). The energy-splitting configurations of ASTARS and the effects of imperfect/perfect successive interference cancellation (SIC) on ASTARS-RSMA networks are considered in the analysis. We derive new exact and asymptotic expressions of the outage probability with imperfect/perfect SIC for TUs and RUs. On this basis, we further calculate the diversity orders of TUs and RUs. Moreover, the system throughput and energy efficiency (EE) of ASTARS-RSMA are evaluated in the delay-limited mode. The simulation results confirm the accuracy of the theoretical expressions and show that (i) the outage probability and system throughput with imperfect/perfect SIC of ASTARS-RSMA exceed that of passive simultaneously transmitting/reflecting surface (PSTARS)-assisted RSMA when the number of elements is not too large; (ii) although ASTARS increases power consumption compared to PSTARS, it can bring further EE improvements to RSMA networks.
... where h 1,n and h 2,n denote the channel coefficients between the UE and the n th element of the LoS IRS, and the n th element of the IRS and the LoS BS, respectively. The channel coefficients are modeled independently across different IRS elements, as in [31]- [33]. ...
... The probabilities P(A D,P L ) and P(A I,P L ) are obtained by applying the total probability theorem to (33) and (34), and averaging over x and z, respectively. Proposition 3. The probability that the PL value of the direct link is lower than or equal to the PL value of the indirect link is ...
p>Intelligent reflecting surfaces (IRSs) have become a popular topic in recent years for their great potential for controlling the radio link environment of wireless networks. With this controlled environment, the coverage can be increased. This paper examines the coverage analysis of IRS-aided networks, considering both two-dimensional buildings and the product-distance path loss model for the first time. Leveraging the tools from stochastic geometry, the locations of base stations (BSs), buildings, and IRSs are modeled with a homogeneous Poisson point process (PPP). A Gamma approximation for the distribution of the nearest line-of-sight (LoS)-neighbor distance is proposed, leading to a closed-form expression for the distribution of the product-distance. Feasible BSs are defined as BSs which are reachable via an IRS deployed on a specific facade of a building, and the ratio of feasible BSs is derived. Simulations are performed, which confirm the proposed analytical methods. In the numerical results, it is observed that the IRSs can introduce up to a 45\% coverage boost, and the effect of the IRS length on the coverage probability is limited beyond 1.2 meters at 60 GHz.</p
... RISs offers wireless communication link several advantages over technologies such as optical relay systems. These advantages of RISs have recently studied and triggered intensive investigations of the technology [16]- [28]. ...
... The gain of system is given by ℎ , in which the quantity is deterministic in contrast to ℎ and . The PDF of SNR's system, ( ), can be evaluated as [28]. ...
... They are random variables. The PDF of SNR's system, ( ), is evaluated as [20]. ...
... We assume that SISO sub-channel turbulence processes are independent, uncorrelated, identically distributed, PDF ( ) can be decreased to a product of the first-order PDF of each element. The (17) and formula contact between probability density function, the PDF of end-to-end SNR, ( ), in the case of weak turbulence channels can be given as (20). ...
span lang="EN-US">Free-space optical (FSO) communication can provide the cost-efficient, secure, high data-rate communication links required for applications. For example, it provides broadband internet access and backhauling for the fifth-generation (5G) and the sixth-generation (6G) communication networks. However, previous solutions to deal with signal loss caused by obstructions and atmospheric turbulence. In these solutions, reconfigurable intelligent surfaces (RISs) are considered hardware technology to improve the performance of optical wireless communication systems. This study investigates the pointing error effects for RIS-aided FSO links under atmospheric turbulence channels. We analyze the performance of RIS-aided FSO links influenced by pointing errors, atmospheric attenuation, and turbulence for the subcarrier quadrature amplitude modulation (SC-QAM) technique. Atmospheric turbulence is modeled using log-normal distribution for weak atmospheric turbulence. Several numerical outcomes obtained for different transmitter beam waist radius and pointing error displacement standard deviation are shown to quantitatively illustrate the average symbol error rate (ASER).</span
... where h 1,n and h 2,n denote the channel coefficients between the UE and the n th element of the LoS IRS, and the n th element of the IRS and the LoS BS, respectively. The channel coefficients are modeled independently across different IRS elements, as in [31]- [33]. ...
... The probabilities P(A D,P L ) and P(A I,P L ) are obtained by applying the total probability theorem to (33) and (34), and averaging over x and z, respectively. ...
p>Intelligent reflecting surfaces (IRSs) have become a popular topic in recent years for their great potential for controlling the radio link environment of wireless networks. With this controlled environment, the coverage can be increased. This paper examines the coverage analysis of IRS-aided networks, considering both two-dimensional buildings and the product-distance path loss model for the first time. Leveraging the tools from stochastic geometry, the locations of base stations (BSs), buildings, and IRSs are modeled with a homogeneous Poisson point process (PPP). A Gamma approximation for the distribution of the nearest line-of-sight (LoS)-neighbor distance is proposed, leading to a closed-form expression for the distribution of the product-distance. Feasible BSs are defined as BSs which are reachable via an IRS deployed on a specific facade of a building, and the ratio of feasible BSs is derived. Simulations are performed, which confirm the proposed analytical methods. In the numerical results, it is observed that the IRSs can introduce up to a 45\% coverage boost, and the effect of the IRS length on the coverage probability is limited beyond 1.2 meters at 60 GHz.</p
... In [5], The transmission performance of the system is improved by the joint optimization of active and passive beamforming. The authors of [6] obtained the SNR distribution of RIS-assisted communication system by using the central limit theorem. The authors extend similar work to the more general Rician channel [7]. ...
... We set k ε the range from 6 10 − to 5 10 − and set the block length b=200 and the convergence accuracy to . Four users are randomly generated around the AP with a maximum distance of 50 m and a minimum distance of 5 m. ...
Phasor measurement unit (PMU) has been widely applied in power grids. For these applications, there is usually a high requirement for data transmission delay and reliability; therefore, short data packets are often used for transmission. Therefore, a downlink communication model for reconfigurable intelligent surface (RIS) assisted multi-PMU is proposed in the paper. We propose a joint optimization problem to maximize the total achievable system rate. Finally, an alternate iterative algorithm based on SCA is proposed. The results show that the proposed algorithm can effectively improve the sum rate.
... One solution is to adopt Reconfigurable Intelligent Surface (RIS), which is a programming surface structure that enables spectral-and energy-efficient wireless communications [8]. The combination of RIS and UAV has led to significant gains in coverage [9] and throughput [10]. However, their objective function is not linked with the secrecy rate. ...
... 3) Reward : Our goal is to optimize SEE as defined in (9). However, in practice, we find that directly employing (9) as reward function may lead to poor convergence and performance. ...
This paper investigates the physical layer security (PLS) issue in reconfigurable intelligent surface (RIS) aided millimeter-wave rotary-wing unmanned aerial vehicle (UAV) communications under the presence of multiple eavesdroppers and imperfect channel state information (CSI). The goal is to maximize the worst-case secrecy energy efficiency (SEE) of UAV via a joint optimization of flight trajectory, UAV active beamforming and RIS passive beamforming. By interacting with the dynamically changing UAV environment, real-time decision making per time slot is possible via deep reinforcement learning (DRL). To decouple the continuous optimization variables, we introduce a twin twin-delayed deep deterministic policy gradient (TTD3) to maximize the expected cumulative reward, which is linked to SEE enhancement. Simulation results confirm that the proposed method achieves greater secrecy energy savings than the traditional twin-deep deterministic policy gradient DRL (TDDRL)-based method.
Code is provided in https://github.com/yjwong1999/Twin-TD3
... The focus of our survey is not only on energy optimization in general but also the opti-276 mization methods employed in energy efficient UAV-assisted wireless communication 277 networking. 278 • We categorized the methods according to their type (i.e., conventional and ML) and 279 investigated each energy optimization method accordingly. 280 • For the sake of completeness. ...
... RISs can be combined with UAVs to improve system throughput [277], network 1899 coverage and reliability [278,279] or EE [280][281][282][283]. By acting as relays for signals without 1900 the need for additional external power supply, RISs achieve high EE when combined with 1901 UAVs in cellular networks. ...
Wireless communication networks have been witnessing unprecedented demand due to the increasing number of connected devices and emerging bandwidth-hungry applications. Although there are 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 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 where it is needed. However, UAVs mostly have limited energy storage, hence, 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 trends in the literature.
... The authors proposed an optimization algorithm to maximize the average achievable rate. In a more recent study, [5] considered a scenario where a source sends a signal to a UAV through a stationary RIS and then to a destination. The authors modeled the signal-tonoise ratio (SNR) statistics of the ground-to-air (G2A) link by considering the distribution of channel gains between the source and a RIS element to be Rayleigh, while the channel between the RIS element and UAV is Rician. ...
... For P a , inserting (5) in (19) gives ...
In this paper, we study the performance of multiple reconfigurable intelligent surfaces (RISs)-aided unmanned aerial vehicle (UAV) communication networks over Nakagami-$m$ fading channels. For that purpose, we used accurate closed-form approximations for the channel distributions to derive closed-form approximations for the outage probability (OP), average symbol error probability (ASEP), and the average channel capacity assuming independent non-identically distributed (i.ni.d.) channels. Furthermore, we derive the asymptotic OP at the high signal-to-noise ratio (SNR) regime to get more insights into the system performance. We also study some practical scenarios related to RISs, UAV, and destination locations and illustrate their impact on the system performance through simulations. Finally, we provide an optimization problem on the transmit power of each channel.
... In [8], an RIS-assisted fingerprint base method was proposed, where an RIS was placed on a wall for indoor multi-user localization. In [9][10][11], an RIS was attached to a building to pass on communications among a mobile UAV, a fixed-position UAV, and ground users. In addition, an RIS can also be applied underground or underwater. ...
... Unlike the previous algorithms based on Q-learning, the PPO algorithm is based on a ratio between the current policy π θ (s t |a t ) that will be learned and the baseline policy π θ k (s t |a t ) that was obtained from previous experiences. The ratio between these policies is notated as R t (θ) and is shown in (10). ...
A reconfigurable intelligent surface (RIS) is a development of conventional relay technology that can send a signal by reflecting the signal received from a transmitter to a receiver without additional power. RISs are a promising technology for future wireless communication due to their improvement of the quality of the received signal, energy efficiency, and power allocation. In addition, machine learning (ML) is widely used in many technologies because it can create machines that mimic human mindsets with mathematical algorithms without requiring direct human assistance. Meanwhile, it is necessary to implement a subfield of ML, reinforcement learning (RL), to automatically allow a machine to make decisions based on real-time conditions. However, few studies have provided comprehensive information related to RL algorithms—especially deep RL (DRL)—for RIS technology. Therefore, in this study, we provide an overview of RISs and an explanation of the operations and implementations of RL algorithms for optimizing the parameters of RIS technology. Optimizing the parameters of RISs can offer several benefits for communication systems, such as the maximization of the sum rate, user power allocation, and energy efficiency or the minimization of the information age. Finally, we highlight several issues to consider in implementing RL algorithms for RIS technology in wireless communications in the future and provide possible solutions.
... where h 1,n and h 2,n denote the channel coefficients between UE and n th element of LoS IRS, and n th element of IRS and LoS BS, respectively. The channel coefficients are modeled independently across different IRS elements, similar to [27], [28], [29]. ...
... The probabilities P(A D,P L ) and P(A I,P L ) are obtained by applying the total probability theorem to (29) and (30), and averaging over x and z, respectively. ...
p>Intelligent reflecting surfaces (IRSs) have become a popular topic in recent years for their great potential for controlling the radio link environment of wireless networks. With this controlled environment, the coverage can be increased. In this paper, unlike the other works related to the IRS-aided coverage analysis, both two-dimensional buildings and the product-distance path loss model are considered for the indirect link for the first time. Leveraging the tools from stochastic geometry, the locations of base stations (BSs), buildings, and IRSs are modeled with a homogeneous Poisson point process (PPP). Then, a Gamma approximation for the distribution of the nearest-LoS-neighbor distance is proposed for analytical tractability. With the proposed approximation, a closed-form expression for the distribution of the product-distance is derived. A method to obtain the ratio of feasible BSs which indicates whether a BS is reachable with an IRS depending on the facade of the building it is deployed is proposed. Simulations are performed which confirm the proposed analytical methods. In the numerical results, it is observed that the IRSs can introduce up to a 45\% coverage boost, and the effect of the IRS length on the coverage probability is limited beyond 1.2 meters at 60 GHz.</p
... The UAV-RIS system described in [20] effectively overcame the blockage between the IoTDs and the BS, however, the battery-powered UAV presented the challenge of limited service time. In terms of the decode-and-forward-based RIS-assisted UAV communication system described in [25], the fixed RIS was able to significantly improve the coverage and average capacity of the UAV communication system, whereas the frame-based RIS-assisted transmission protocol outlined in [26] enhanced the coverage and communication quality of the UAV-user link. Furthermore, the resource management problem of the UAV-RIS system was studied in [27] to minimize the energy consumption of the system by joint optimization of UAV deployment, phase shift, and the UAV-RIS-user association. ...
... As the table shows, all the above-related works mainly focused on maximizing the system's total rate and minimizing energy consumption. Although the work in [10], [11], [25], [26] and [12] guaranteed the communication QoS requirement of UTs, the active energy efficiency solution for RIS-assisted communication systems has not yet been considered. The successful paradigms of the RIS-aided SWIPT framework outlined in [7], [13], [32] and [33] can harvest energy on the time-domain. ...
Integrating unmanned aerial vehicles with RIS (UAV–RIS) can offer ubiquitous deployment services in communication-disabled areas, but is limited by the on-board energy of the UAVs. In this paper, a novel energy harvesting (EH) scheme on top of the UAV–RIS system, called EH-RIS scheme, is developed for the next generation high performance wireless system. The proposed EH-RIS scheme extends the simultaneous wireless information and power transfer (SWIPT) system by splitting the passive reflected arrays on the geometric space for transporting information and harvesting energy simultaneously. However, pedestrian mobility, and rapid channel changes post challenges to efficient resource allocation in wireless systems. Thus, a robust deep reinforcement learning (DRL)-based algorithm is developed to improve the proposed EH-RIS scheme for guaranteeing the quality of service (QoS) under dynamic wireless environments. The simulation results demonstrate the effectiveness and efficiency of the proposed robust DRL-based EH-RIS system, which not only outperform the existing state-of-the-art solutions but also approach to the performance of the exhaustive search method.
... where h 1,n and h 2,n denote the channel coefficients between UE and n th element of LoS IRS, and n th element of IRS and LoS BS, respectively. The channel coefficients are modeled independently across different IRS elements, similar to [27], [28], [29]. ...
... The probabilities P(A D,P L ) and P(A I,P L ) are obtained by applying the total probability theorem to (29) and (30), and averaging over x and z, respectively. ...
p>Intelligent reflecting surfaces (IRSs) have become a popular topic in recent years for their great potential for controlling the radio link environment of wireless networks. With this controlled environment, the coverage can be increased. In this paper, unlike the other works related to the IRS-aided coverage analysis, both two-dimensional buildings and the product-distance path loss model are considered for the indirect link for the first time. Leveraging the tools from stochastic geometry, the locations of base stations (BSs), buildings, and IRSs are modeled with a homogeneous Poisson point process (PPP). Then, a Gamma approximation for the distribution of the nearest-LoS-neighbor distance is proposed for analytical tractability. With the proposed approximation, a closed-form expression for the distribution of the product-distance is derived. A method to obtain the ratio of feasible BSs which indicates whether a BS is reachable with an IRS depending on the facade of the building it is deployed is proposed. Simulations are performed which confirm the proposed analytical methods. In the numerical results, it is observed that the IRSs can introduce up to a 45\% coverage boost, and the effect of the IRS length on the coverage probability is limited beyond 1.2 meters at 60 GHz.</p
... In [14], Zheng et al. pursue a theoretical performance comparison between non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) in the IRS-assisted downlink communication, in which the minimizing transmit power problem is formulated, taking into account the discrete unit-modulus reflection constraint applied to each IRS element. In [15], Yangs, et al. provide an IRS-assisted UAV scheme to improve the coverage and effectiveness of UAV communication systems. In this scheme, the IRS mounted on a building is utilized to reflect signals originating from a ground source towards a UAV. ...
In the next-generation network, intelligent reflecting surface (IRS), non-orthogonal multiple access (NOMA), and simultaneous wireless information and power transfer (SWIPT) are promising wireless communication techniques to effectively improve system sum rates. In traditional unmanned aerial vehicles (UAV) communication systems, the sum rate and coverage are greatly affected when there is an occlusion on the direct transmission link. To solve this problem, the IRS technology is introduced to improve the poor channel conditions. However, most of the previous research on IRS-assisted UAV to optimize system sum rate only considers frameworks that utilize the partially joint-combining techniques of IRS, NOMA, and SWIPT. In this paper, in order to further improve the sum rate of the system, we simultaneously integrate IRS, NOMA, and SWIPT technologies and establish a sum rate maximization optimization problem when the direct link is blocked. Then, an alternative optimization (AO) algorithm based on the maximizing system sum rate is proposed to solve the non-convex optimization problem, in which the IRS location and phase, the reflecting amplitude coefficient, UAV forwarding altitude, and power splitting factor are considered. To let the non-convex and non-linear function be transformed into a convex one, we first use an iterative approach to optimize the position of the IRS. After that, an optimization problem is constructed to maximize the system sum rate with the constraints of the IRS phase shifts, successful successive interference cancellation (SIC), maximum transmit power of base station (BS), and UAV. Numerical results show that the proposed algorithm outperforms the traditional orthogonal multiple access (OMA) and algorithms without IRS-assisted links in terms of the system sum rate.
... 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. ...
This paper analyzes the performance of multiple reconfigurable intelligent surfaces (RISs)-aided networks with opportunistic RIS scheduling. 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-
m
fading environment. Then, the approximate expressions for outage probability (OP) and average symbol error probability (ASEP) 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 (SNR) 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.
... where Z = N i=1 i b i , E S is the average power of the signal andγ 1 = L S L R E S /N 1 = 10 (g S +g R )/10 E S /N 1 r µ S S r µ R R is the average SNR of the S-R link. From [9], we apply the K G distribution to model the statistical distribution of γ 1 . Thus, the PDF and CDF of γ 1 are obtained as ...
In this work, we propose a non-terrestrial vehicle communication network where the communication between the satellite and the terrestrial source is assisted by a unmanned aerial vehicle (UAV) used as a relay. In particular, a reconfigurable intelligent surface (RIS) is used in the RF channel to reflect the signals of the terrestrial user to the relay with a fixed amplification gain, while free-space optical (FSO) is applied to the relay-satellite link to obtain a high speed transmission. For such a dual-hop system, assuming that the FSO channel experiences
$\mathcal {M}$
-distributed fading with pointing errors, the expressions for the outage probability (OP) and average bit error rate (ABER) are evaluated in closed-forms. In addition, the high signal-to-noise ratio (SNR) analyses for the OP and ABER are developed and the lower and upper bounds on the average channel capacity (ACC) are calculated to obtain further insights. Results show that the diversity order of the proposed system is
$\min \lbrace k_{w},m_{w}\rbrace$
, where these two parameters are related to the Nakagami-
m
distribution parameters and the number of RIS elements. Finally, we take the shadowing effects into consideration, and it can be seen that the shadowing effects significantly degrade the system performance.
... It will need nearly no power to reflect the legitimate signal, which is now the famous topic in recent five years [25,26]. [27] researched the OP of the UAV-assisted communication networks. ...
Integrated satellite unmanned aerial vehicle relay networks (ISUAVRN-s) are the famous topic these years. This paper investigates the average secrecy capacity (ASC) for reconfigurable intelligent surface (RIS)-enabled ISUAVRNs. Especially, an eve is considered to steal the legitimate information for the considered secrecy systems. Besides, we get the detailed analysis for the ASC of the regarded secrecy systems with reconfigurable intelligent surface's help. Furthermore , to gain deep results of the major parameters on the ASC in high signal-to-noise ratio regime, the approximate investigations are further gotten, which gives the efficient method to value the secrecy analysis. At last, some representative computer results are obtained to prove the theoretical findings.
... RIS is a passive digitally programmable planar, which has the advantages of low cost, high EE, and flexible deployment [39]. RIS can change the wireless propagation environment in real-time by adjusting the phase shift matrix [40], [41]. It is utilized to reflect signals from the source and enhance or weaken the incident signal according to demands, thus it can be applied to cooperative communication and enhance security [42], [43]. ...
In this paper, we investigate the outage behavior of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-empowered cognitive non-terrestrial vehicle network with non-orthogonal multiple access (NOMA), in which Rician fading channels and imperfect/perfect successive interference cancellation (ipSIC/pSIC) are considered. In the network, STAR-RIS is utilized to assist the transmission and reflection of the superposed signals from a secondary unmanned aerial vehicle. Under the interference constraints of primary vehicle users, we derive the exact expressions of outage probability for all secondary vehicle users to evaluate the system parameters. Moreover, to gain deeper insights, the asymptotic expressions of outage probability along with the diversity order are obtained. Then, from numerical results, we confirm the correctness of our theoretical analysis and discuss the influences of key parameters on system performance. Compared with STAR-RIS-orthogonal multiple access (OMA)/non-STAR-RIS scheme, the superiority of our scheme is verified.
... • We provide an analytical expression for the network's ergodic secrecy capacity (ESC) by deriving the closedform EC expressions for Bob and Eves in terms of the Meijer-G functions. Most existing works on A-RIS consider the non-central chi-square (NCCS) distribution based on the central limit theorem to describe the statistical model of the RIS channel [23], [24]. However, RIS is proven to benefit from LoS communication, which is better described by the Nakagami-m distribution. ...
We propose a novel approach for enhancing physical layer security (PLS) in wireless networks by utilizing a combination of reconfigurable intelligent surfaces (RIS) and artificial noise (AN). The proposed aerial RIS (A-RIS) concept utilizes a RIS-attached unmanned aerial vehicle (UAV) that hovers over the network area to improve the signal quality for legitimate users and jam that of illegitimate ones. We propose a method of virtually partitioning the RIS, such that the partition phase shifts are configured to improve the intended signal at a legitimate user while simultaneously increasing the impact of AN on illegitimate users. Closed-form expressions for both legitimate and illegitimate users' ergodic secrecy capacity (ESC) are derived and validated. Then, optimization problems are formulated to maximize network ESC by optimizing the 3D deployment of the A-RIS and RIS portions for users subject to predefined quality-of-service constraints. Simulation results validate closed-form solutions and demonstrate that the proposed joint A-RIS deployment and partitioning framework can significantly improve network security compared to benchmarks where RIS and AN are separately used without deployment optimization. Additionally, thanks to the availability of closed-form optimal RIS portions, the proposed deployment approaches converge in less than a second, making it suitable for dynamic A-RIS deployment.
... Denote ( ), the SNR's PDF. It is evaluated as [26]. where (⋅) is PDF of the S-RIS, (⋅) PDF of the RIS-D. ...
span>Optical wireless communication (OWC) has attracted significant interest recently in academia and industry. Free-space optical (FSO) communication systems are where free space acts as a communication channel between transceivers that are line of sight (LOS) for the successful transmission of optical signals. The FSO transmissions through the atmosphere, nevertheless, bring significant challenges, besides the uncertainty of atmospheric channels, especially the signal fading due to the atmospheric turbulence, attenuation and pointing errors caused by the random beam misalignments between transceivers, signal obstruction due to buildings or trees can pre-vent the transmitted message to reach the destination. This study theoretically investigates the average symbol error rate (ASER) of reconfigurable intelligent surfaces (RIS) assisted FSO link over log-normal turbulence channels. The RIS effect is examined by considering the influence of link distance, transmitted optical power, and quadrature amplitude modulation (QAM) scheme on the ASER.</span
... Besides, UAV-assisted wireless relay networks can also be enhanced by the usage of RISs. In [147], 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. [148] 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. ...
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.
... Besides, UAV-assisted wireless relay networks can also be enhanced by the usage of RISs. In [147], 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. [148] 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. ...
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
The authors propose a secure intelligent reflecting surface (IRS) assisted transmission system in the presence of a ground eavesdropper, where the source in the ground transmits confidential information to unmanned aerial vehicle (UAV), IRS is deployed to promote the transmission rate of the source to UAV. Closed‐form expression for secure transmission non‐outage probability is derived, and secure transmission performance is analyzed. Simulation validates the correctness of the derivation. Compared with benchmarks, results show that the IRS can improve the secure transmission of ground to UAV.
Radio over free space optics (RoFSO) is one of the potential technologies that can satisfy the requirements of 5G services in a smart city. However, as RoFSO is line-of-sight (LOS) communication, one of its limitations is the occurrence of a skip zone in the targeted areas. In this work, a reconfigurable intelligent surface (RIS) is proposed as the solution to overcome this connection difficulty, which prevents signal blocking by generating LOS connections. These RIS modules extend the communication channel coverage, making it more intelligent and controllable. The performance analysis based on outage probability, ergodic channel capacity, and bit error rate has been performed using heterodyne detection. Malaga distribution has been used to model atmospheric turbulence. The exact closed-form expressions of the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio are derived. Exploiting these derived statistics, system performance is investigated through the ergodic channel capacity, outage probability, and average bit error rate for M-ary quadrature amplitude modulation and two binary modulation schemes: non-coherent binary frequency-shift keying and coherent binary phase-shift keying. Numerical results are compared among different turbulence conditions, link lengths, and scattering errors. The results show that the proposed RIS-assisted RoFSO technology has the potential to be effective for 5G smart city applications.
In the unmanned aerial vehicle (UAV) enabled wireless communication system, the rapid channel change caused by high-speed UAV may cause the beam misalignment problem. It is a challenge to fast establish and maintain the communications link between UAV and ground base stations. To address this challenge, an IRS-aided high-speed UAV adaptive beam alignment (IRS-UAV-ABA) scheme utilizing double-layer long short-term memory (DL-LSTM) spatial angle prediction model is proposed to improve the effectiveness and reliability of the UAV communication. Firstly, DL-LSTM enabled spatial angle prediction is modeled to facilitate the calculation of the beam coverage adapted to the UAV real-time flight trajectory. Secondly, the non-convex optimization problem for maximizing the received signal-to-noise ratio of moving UAV is formulated to obtain the suboptimal solutions of the BS’ beamforming vector and the IRS’ phase shift matrix. Finally, with the aforementioned beamforming vectors, the phase shift matrix and the adaptive beam coverage, the adaptive dual-beams alignment from BS to UAV and from IRS to UAV can be achieved by the proposed DL-LSTM-enabled IRS-UAV-ABA algorithm. Simulation results verify that the proposed scheme not only can effectively avoid the beam misalignment caused by the mobility and perturbation of UAV, but also has superior spectrum efficiency and stable beamforming gain.
This paper focuses on an intelligent reflecting surface (IRS)-assisted sidelink transmission system. Specifically, two relay strategies, i.e., decode-and-forward (DF) and lossy-forward (LF), are compared, wherein the impacts of introducing IRS are taken into account. The system performances in terms of outage probabilities are mathematically derived, assuming transmission channels suffer from block Rayleigh fading. It is found that when the number of IRS elements is small, the advantages of LF over DF can be observed, as long as the relay is not too close to the source. Moreover, it has been shown that the more IRS elements there are, the less sensitive the system performances will be to the relay schemes and relay locations.
In the complicated or hard-to-reach communication areas, sometimes much higher average achievable rate (AAR) or wider coverage is in urgent and temporary need. To address this challenge, this article proposes a novel amplify-and-forward (AF) relay-based cooperation communication scheme with reconfigurable intelligent surface (RIS)-enabled unmanned aerial vehicle (UAV) enhancing. The UAV acts as a moving RIS in the sky, which provides additional links to enhance the performance of the AF relay-based cooperation on the ground. Aiming to maximize the AAR of the proposed scheme, we jointly optimize the trajectory of UAV and the phase shift matrices of RIS over total time frames. At first, we decompose the original intractable problem into two subproblems of the phase shift matrices optimization and trajectory optimization. Subsequently, the optimal phase shift matrices are achieved by aligning the phases of received signals. Since the trajectory optimization subproblem is still complicated and nonconvex, we apply the successive convex approximation (SCA) method to reformulate it into a convex problem and further solve it by iterative method. Simulation results demonstrate the superiorities of the proposed scheme as compared with the benchmark schemes and also reveal the effects of various factors on its performance.
The rapid development of unmanned aerial vehicle (UAV) technology provides flexible communication services to terrestrial nodes. Energy efficiency is crucial to the deployment of UAVs, especially rotary-wing UAVs whose propulsion power is sensitive to the wind effect. In this paper, we first derive a three-dimensional (3D) generalised propulsion energy consumption model (GPECM) for rotary-wing UAVs under the consideration of stochastic wind modeling and 3D force analysis. Based on the GPECM, we study a UAV-enabled downlink communication system, where a rotary-wing UAV flies subject to stochastic wind disturbance and provides communication services for ground users (GUs). We aim to maximize the energy efficiency (EE) of the UAV by jointly optimizing the 3D trajectory and user scheduling among the GUs based on the GPECM. We formulate the problem as stochastic optimization, which is difficult to solve due to the lack of real-time wind information. To address this issue, we propose an offline-based online adaptive (OBOA) design with two phases, namely, an offline phase and an online phase. In the offline phase, we average the wind effect on the UAV by leveraging stochastic programming (SP) based on wind statistics; then, in the online phase, we further optimize the instantaneous velocity to adapt the real-time wind. Simulation results show that the optimized trajectories of the UAV in both two phases can better adapt to the wind in changing speed and direction, and achieves a higher EE compared with the windless scheme. In particular, our proposed OBOA design can be applied in the scenario with dramatic wind changes, and makes the UAV adjust its velocity dynamically to achieve a better performance in terms of EE.
In this work, we propose an unmanned aerial vehicle (UAV)-based reconfigurable intelligent surface (RIS)-assisted dual-hop power line communication and radio frequency (PLC-RF) system, in which a UAV-based RIS (URIS) located in the sky forwards signals from the relay to the terrestrial user. More specifically, we assume that the PLC channel suffers from a log-normal distribution, and the URIS-assisted RF link follows the Rician distribution. With the amplify-and-forward (AF) and decode-and-forward (DF) relaying models, the closed-form expressions for the outage probability and average bit-error-rate are derived. In addition, by taking into account the hardware impairments, the outage probability of the non-ideal hardware system is derived. Finally, the Monte Carlo simulation verifies the analytical results, which demonstrates the effect of impulsive noise and the number of the reflecting surface units on the system performance.
Enhanced quality of service (QoS) and resource allocation are important measurements in a mobile communication system. Compared to traditional mobile networks, 6G uses spectrum from 0.3 to 3 terahertz (THz) and millimeter wave (mmWave) from 30 to 300 GHz (GHz), making QoS and proper resource allocation difficult. Unmanned aerial vehicles (UAVs) have garnered attention in academia and industry because of their high mobility, elastic placement, cost-efficiency, and easy combination with other communication systems. Similarly, reconfigurable intelligent surfaces (RISs) also gain the attention of the wireless research community owing to their precise signal reflection, spectrum, and energy efficiency, as well as their capability of regulating waveforms such as amplitude, frequency, polarization, and phase using passive reflections. The UAV has several weaknesses and is plagued by eavesdroppers, blockage, etc. Likewise, deploying RIS also is ineffective in standalone scenarios due to the user's mobility. Thus, we comprehensively review recent studies on UAV and RIS technologies. Furthermore, we have discussed the challenges of future communication technologies evolving RIS-assisted UAVs and future trends. We aim to provide sufficient information for the research on UAV and RIS's revolutionized phases, problems, and research trends.
Reconfigurable intelligent surfaces (RISs) are artificial planar structures able to offer a unique way of manipulating propagated wireless signals. Commonly composed of a number of reconfigurable passive cell components and basic electronic circuits, RISs can almost freely perform a set of wave modification functionalities, in order to realize programmable wireless environments (PWEs). However, a more energy-efficient way to realize a PWE is through dynamically relocating static metasurfaces that perform a unique functionality. In this paper, we employ a UAV swarm to dynamically deploy a set of low-cost passive metasurfaces that are able to perform only one electromagnetic functionality, but with the benefit of requiring no power. Specifically, the UAV-mounted static metasurfaces are carefully positioned across the sky to create cascaded channels for improved user service and security hardening. The performance evaluation results, based on ray tracing, demonstrate the potential of the proposed method.
To address the scarcity of spectrum resource incurred by explosively increasing traffic load in existing fifth generation (5G) networks, we consider exploiting Reconfigurable intelligent surface (RIS) to harmonize the coexistence of cellular networks and WiFi system on the unlicensed bands. In particular, RIS is capable of not only improving the performance of new radio on unlicensed bands (NR-U) by increasing the signal to interference and noise ratio (SINR) of cellular users, but also decreasing the interference to the existing WiFi system caused by NR-U. Thus, we investigate the performance of RIS aided coexisting networks both in single cell and multiple cell scenarios. For the single cell scenario, we aim to improve the worst transmission rate of UEs in NR-U network. The problem is solved by a proposed alternating optimization-based solution. For the multiple cell scenario, cell cooperation is exploited jointly with multiple RISs to maximize the sum-rate of all UEs. Although the problem is more challenging than that of single cell case, we propose a multi-agent reinforcement learning (MARL) algorithm to solve it. Numeral results verify the effectiveness of our proposed schemes and show that deploying RISs is a promising solution for the enhancement of NR-U networks.
In this paper, we investigate the effects of jamming caused by a malicious unmanned aerial vehicle (UAV) on the performance of a free-space optical (FSO) communication system using a legitimate UAV as a relay and utilizing an intelligent reflecting surface (IRS) to improve the coverage. We statistically characterize the combined effects of the three impairments on the performance of the considered system: atmospheric turbulence (AT), pointing error (PE), and angle of arrival (AoA) fluctuation of both the UAVs. Incorporating the aforementioned effects, closed-form expressions of the overall average bit error rate (ABER) and outage probability (OP) for the considered system are derived. We also present a comprehensive study of the proposed system by considering different locations of the malicious UAV jammer and IRS and draw some interesting conclusions. Useful insights into the system performance are obtained through the asymptotic analysis. Furthermore, the analytically derived expressions are verified through Monte-Carlo simulations.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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°.
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.
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.
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.
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.
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