Article

Reconfigurable Intelligent Surfaces With Reflection Pattern Modulation: Beamforming Design and Performance Analysis

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Abstract

Recent research on reconfigurable intelligent surfaces (RISs) suggests that RISs can perform passive beamforming and information transfer (PBIT) simultaneously via smart reflections. In this paper, we propose an RIS-enhanced multiple-input single-output system with reflection pattern modulation (RPM) to achieve PBIT, where the joint active and passive beamforming is carefully designed by taking into account the communication outage probability. We formulate an optimization problem to maximize the average received signal power by jointly optimizing the active beamforming at the access point (AP) and passive beamforming at the RIS under the assumption that the RISb’s state information is statistically known by the AP, and propose a high-quality suboptimal solution based on the alternating optimization technique. Moreover, a closed-form expression for the asymptotic outage probability of the proposed scheme in Rician fading is derived. The achievable rate of the proposed scheme is also investigated under the assumption that the transmitted symbols are drawn from a finite constellation. Simulation results validate the effectiveness of the proposed scheme and reveal the effect of various system parameters on the achievable rate. It is shown that the proposed scheme outperforms, in terms of achievable rate, the conventional RIS-assisted system without information transfer.

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... Reconfigurable intelligent surface (RIS), as a new type of transmission relay, has ability to correct the wireless channel through a highly controllable software, which paves the way for an intelligent and programmable wireless environment [26][27][28]. Because of its novelty and greater gain, the RISassisted NOMA networks have attracted a large part of research effort [29][30][31]. ...
... When ρ ⟶ ∞, θ ⟶ 0. Then, by substituting (28) into (18), and further taking the first term of each series representation, i.e., n = 0 and l = 0, we can obtain (27). The proof is completed. ...
... Remark 8. Upon substituting (27) into (25), the diversity order of the nearby user n with pSIC is equal to nK ½Eðφ n Þ 2 /2Varðφ n Þ, which is related to the order of the nearby user, and Nakagami-m parameter m ′ and Ω Corollary 9. The asymptotic outage probability expression of the distant user m for RIS-assisted non-terrestrial NOMA network when ρ ⟶ ∞ can be given by ...
Article
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This paper considers the application of reconfigurable intelligent surface (RIS) to non-terrestrial non-orthogonal multiple access (NOMA) networks. More specifically, the performance of a pair of non-orthogonal users for RIS assisted non-terrestrial NOMA networks is investigated over large-scale fading and Nakagami- m fading cascaded channel. The exact and asymptotic expressions of outage probability are derived for the nearby user and distant user with the imperfect successive interference cancellation (SIC) and perfect SIC schemes. Based on the approximated results, the diversity orders of these two users are obtained in the high signal-to-noise ratios. The simulation results are used to verify the theoretical derivations and find that: 1) The outage behaviors of RIS assisted non-terrestrial NOMA networks outperforms than that of orthogonal multiple access; 2) By increasing the number of reflecting elements of RIS and Nagakami- m fading factors m ′ and Ω , RIS-assisted non-terrestrial NOMA networks are able to achieve the enhanced outage performance.
... Further to avoid the needs of the highly directive beam to convey the RIS information, a new scenario is proposed in [123][124][125], where RIS implicitly encodes information bits in the ON/OFF states of reflective elements. In [123], a passive beamforming and information transmission (PBIT) scheme is presented, where RIS transmits the information bits via ON/OFF states of the reflecting elements. ...
... In [123], a passive beamforming and information transmission (PBIT) scheme is presented, where RIS transmits the information bits via ON/OFF states of the reflecting elements. Further, an RIS-aided reflection pattern modulation (RPM) scheme is presented in [124], where index of the set of ON state reflective elements is used to transmits the information bits. However, in PBIT and RPM schemes, RIS does not transmit its own information, instead, shares its reflection configuration with AP. ...
... i) PBIT and RPM schemes proposed in [123,124] [118] only AP conveys the information, while an RIS reflects the impinging wave. However, in the proposed scheme, RIS and AP both transmit their information simultaneously to the Rx. ...
Thesis
... Yet, having the off state of all IRS elements concurrently deflects the purpose of deploying an IRS as it leads to a low spectral efficiency in end-to-end information transmission in the primary system. To further improve the spectral efficiency of IRS-assisted systems, [3] and [8] adopted a higher order IRS modulation by exploiting spatial modulation over IRS elements. However, the problem formulations in [3] and [8] did not take into account the quality-of-service (QoS) requirement of decoding IRS symbols at the SU. ...
... To further improve the spectral efficiency of IRS-assisted systems, [3] and [8] adopted a higher order IRS modulation by exploiting spatial modulation over IRS elements. However, the problem formulations in [3] and [8] did not take into account the quality-of-service (QoS) requirement of decoding IRS symbols at the SU. As such, the performance of the SU cannot be guaranteed. ...
... As such, the performance of the SU cannot be guaranteed. Moreover, all the aforementioned papers, i.e., [3], [6]- [8], ideally assumed that the SU is capable to perform sophisticated multiuser detection or successive interference cancellation for decoding the information symbols of the AP and the IRS jointly, which is generally impossible for a lowcost SU. Besides, without knowing the symbols transmitted by the AP, the effective channel state information (CSI) is generally unknown at the desired SU as it is a product of instantaneous CSI and AP symbols. ...
Preprint
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This paper investigates multiuser multi-input single-output downlink symbiotic radio communication systems assisted by an intelligent reflecting surface (IRS). Different from existing methods ideally assuming the secondary user (SU) can jointly decode information symbols from both the access point (AP) and the IRS via multiuser detection, we consider a more practical SU that only non-coherent detection is available. To characterize the non-coherent decoding performance, a practical upper bound of the average symbol error rate (SER) is derived. Subsequently, we jointly optimize the beamformer at the AP and the phase shifts at the IRS to maximize the average sum-rate of the primary system taking into account the maximum tolerable SER constraint for the SU. To circumvent the couplings of variables, we exploit the Schur complement that facilitates the design of a suboptimal beamforming algorithm based on successive convex approximation. Our simulation results show that compared with various benchmark algorithms, the proposed scheme significantly improves the average sum-rate of the primary system, while guaranteeing the decoding performance of the secondary system.
... In this way, the signal-to-noise ratio (SNR), data rate, security, and/or the coverage probability can be potentially adjusted by the RIS. By leveraging advantages of the RIS, capacity and rate improvement analysis [13]- [15], power efficiency optimization [16], [17], communication reliability [18], [19], physical layer security (PLS) [20] in most of applications benefiting by RIS-assisted networks. In the normal case of many PLS techniques, the legitimate and illegitimate nodes work in the same direction and they have similar directional processing such as artificial noise (AN), conventional beamforming and directional modulation. ...
... Numerical results show that applying the RISs with higher meta-surface can improve the secrecy performance significantly. APPENDIX A Substituting (2) and (5) into (15), P D1 is expressed as ...
Preprint
p>We focus on the secure performance metrics at the legitimate users, i.e. secure outage probability (SOP) and secrecy capacity, to quantify the secrecy performance of NOMA-RIS-aided IoT systems. We assume the RIS is placed between the access point and the legitimate devices, and is expected to enhance the link security through the smart phase shift mechanism of metasurface elements in RIS. We first present analytical results for the SOP and secrecy capacity. Next, an iterative search algorithm is adopted to exhibit optimal SOP for further insights and analysis. In order to help the base station allocate power coefficients to NOMA users properly, an efficient deep-neural network (DNN)-based secure metric prediction scheme is adopted to achieve the secure performance. Our derivations and simulations indicate that the number of meta-surface elements of the RIS and the average signal-to-noise ratio at the base station contribute the most to the system performance enhancement. </p
... 2) IRS-Aided Spatial Modulation: Apart from passive beamforming, IRS can also be used to transfer additional low-rate information by embedding/encoding implicit information onto its reflection pattern, which shares a similar concept with "spatial modulation" and thus referred to as "reflection modulation" [225]. At the receiver side, the information from both the transmitter and IRS needs to be detected for achieving enhanced rate performance. ...
... At the receiver side, the information from both the transmitter and IRS needs to be detected for achieving enhanced rate performance. Specifically, for coherent detection, it is indispensable to acquire the explicit CSI for differentiating the information mapped to different reflection states [225], [226]. Attentive to this, the authors in [226] proposed an MMSE-based cascaded channel estimation scheme that jointly designs the encoder of the transmitted signal and IRS reflection pattern. ...
Preprint
Full-text available
Intelligent reflecting surface (IRS) has emerged as a key enabling technology to realize smart and reconfigurable radio environment for wireless communications, by digitally controlling the signal reflection via a large number of passive reflecting elements in real time. Different from conventional wireless communication techniques that only adapt to but have no or limited control over dynamic wireless channels, IRS provides a new and cost-effective means to combat the wireless channel impair-ments in a proactive manner. However, despite its great potential, IRS faces new and unique challenges in its efficient integration into wireless communication systems, especially its channel estimation and passive beamforming design under various practical hardware constraints. In this paper, we provide a comprehensive survey on the up-to-date research in IRS-aided wireless communications, with an emphasis on the promising solutions to tackle practical design issues. Furthermore, we discuss new and emerging IRS architectures and applications as well as their practical design problems to motivate future research.
... Further, average bit error rate (ABER) of RIS-assisted wireless networks is studied in [3]. The concept of index modulation (IM) is exploited in [4], [5] to implicitly convey information via activating subsets of reflecting elements. In [6], an RIS transmits the local data to receiver (Rx). ...
... The following are the key novelties of the proposed scheme: 1) IM schemes proposed in [4], [5], implicitly convey the RIS data via ON/OFF reflection states of the reflecting elements. Further, the number of ON-state reflecting elements varies over time, which results in high outage probability due to significant fluctuation in the received signal power. ...
Article
In this paper, we present a reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) modulation and reflection phase modulation (RPM) scheme, where RIS embeds its own information in the reflection phase shift of the reflected radio frequency (RF) signal. More specifically, the RIS simultaneously performs two tasks: i) reflecting the impinging RF signal with a discrete phase shift and ii) embedding the information bits in the phase shift. We perform joint detection for the RPM and SSK symbols using a maximum likelihood detector, and a unified analytical framework is presented for theoretical analysis of the average bit error rate (ABER) and ergodic capacity of the proposed scheme. All numerical results are thoroughly verified through Monte Carlo simulation. Numerical results reveal that the proposed scheme outperforms the conventional SSK scheme.
... Previous contributions on RIS are mainly focused on maximizing the spectral efficiency/achievable rate or minimizing the transmission power [12]- [21]. In [12], Wu and Zhang minimized the transmission power in the downlink of RISaided multi-user MIMO systems, where the popular alternating optimization and semi-define relaxation (SDR) methods were employed for jointly optimizing the active transmission beamforming (TBF) of the BS and the passive beamforming, represented by the RIS phase shift matrix. ...
... In [20], Xu et al. designed their discrete phase shift matrix based on low resolution digital-to-analog converters, and derived the lower bound of the asymptotic rate. Furthermore, the problem of maximizing the achievable rate of RIS users was studied by Lin et al. [21], where the novel concept of reflection pattern modulation was employed. ...
Article
Full-text available
Reconfigurable intelligent surfaces (RIS) have been actively researched as a potential technique for future wireless communications, which intelligently ameliorate the signal propagation environment. In the conventional design, each RIS element configures and reflects its received signal independently of all other RIS elements, which results in a diagonal phase shift matrix. By contrast, we propose a novel RIS architecture, where the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment , which increases the flexibility in the design of RIS phase shifts, hence, potentially improving the system performance. The resultant RIS phase shift matrix also has off-diagonal elements, as opposed to the pure diagonal structure of the conventional design. Compared to the state-of-art fully-connected/group-connected RIS structures, our proposed RIS architecture has lower complexity, while attaining a higher channel gain than the group-connected RIS structure, and approaching that of the fully-connected RIS structure. We formulate and solve the problem of maximizing the achievable rate of our proposed RIS architecture by jointly optimizing the transmit beamforming and the non-diagonal phase shift matrix based on alternating optimization and semi-define relaxation (SDR) methods. Moreover, the closed-form expressions of the channel gain, the outage probability and bit error ratio (BER) are derived. Simulation results demonstrate that our proposed RIS architecture results in an improved performance in terms of the achievable rate compared to the conventional architecture, both in single-user as well as in multiuser scenarios. Index Terms-Reconfigurable intelligent surfaces (RIS), channel gain, outage probability, average bit error ratio (BER), joint beamforming.
... Previous contributions on RIS are mainly focused on maximizing the spectral efficiency/achievable rate or minimizing the transmission power [12]- [21]. In [12], Wu and Zhang minimized the transmission power in the downlink of RISaided multi-user MIMO systems, where the popular alternating optimization and semi-define relaxation (SDR) methods were employed for jointly optimizing the active transmission beamforming (TBF) of the BS and the passive beamforming, represented by the RIS phase shift matrix. ...
... In [20], Xu et al. designed their discrete phase shift matrix based on low resolution digital-to-analog converters, and derived the lower bound of the asymptotic rate. Furthermore, the problem of maximizing the achievable rate of RIS users was studied by Lin et al. [21], where the novel concept of reflection pattern modulation was employed. ...
Preprint
Full-text available
Reconfigurable intelligent surfaces (RIS) have been actively researched as a potential technique for future wireless communications, which intelligently ameliorate the signal propagation environment. In the conventional design, each RIS element configures and reflects its received signal independently of all other RIS elements, which results in a diagonal phase shift matrix. By contrast, we propose a novel RIS architecture, where the incident signal impinging on one element can be reflected from another element after an appropriate phase shift adjustment, which increases the flexibility in the design of RIS phase shifts, hence, potentially improving the system performance. The resultant RIS phase shift matrix also has off-diagonal elements, as opposed to the pure diagonal structure of the conventional design. Compared to the state-of-art fully-connected/group-connected RIS structures, our proposed RIS architecture has lower complexity, while attaining a higher channel gain than the group-connected RIS structure, and approaching that of the fully-connected RIS structure. We formulate and solve the problem of maximizing the achievable rate of our proposed RIS architecture by jointly optimizing the transmit beamforming and the non-diagonal phase shift matrix based on alternating optimization and semi-define relaxation (SDR) methods. Moreover, the closed-form expressions of the channel gain, the outage probability and bit error ratio (BER) are derived. Simulation results demonstrate that our proposed RIS architecture results in an improved performance in terms of the achievable rate compared to the conventional architecture, both in single-user as well as in multi-user scenarios.
... 2) IRS-Aided Spatial Modulation: Apart from passive beamforming, IRS can also be used to transfer additional lowrate information by embedding/encoding implicit information onto its reflection pattern, which shares a similar concept with "spatial modulation" and thus referred to as "reflection modulation" [231]. At the receiver side, the information from both the transmitter and IRS needs to be detected for achieving enhanced rate performance. ...
... At the receiver side, the information from both the transmitter and IRS needs to be detected for achieving enhanced rate performance. Specifically, for coherent detection, it is indispensable to acquire the explicit CSI for differentiating the information mapped to different reflection states [231], [232]. Attentive to this, the authors in [232] proposed an MMSE-based cascaded channel estimation scheme that jointly designs the encoder of the transmitted signal and IRS reflection pattern. ...
Article
Full-text available
Intelligent reflecting surface (IRS) has emerged as a key enabling technology to realize smart and reconfigurable radio environment for wireless communications, by digitally controlling the signal reflection via a large number of passive reflecting elements in real time. Different from conventional wireless communication techniques that only adapt to but have no or limited control over dynamic wireless channels, IRS provides a new and cost-effective means to combat the wireless channel impairments in a proactive manner. However, despite its great potential, IRS faces new and unique challenges in its efficient integration into wireless communication systems, especially its channel estimation and passive beamforming design under various practical hardware constraints. In this paper, we provide a comprehensive survey on the up-to-date research in IRS-aided wireless communications, with an emphasis on the promising solutions to tackle practical design issues. Furthermore, we discuss new and emerging IRS architectures and applications as well as their practical design problems to motivate future research.
... Numerous studies have been conducted by combining RISs with many research fields. Assisting a relay system with an RIS [10], RIS-based index modulation [11], RIS-enabled reflection modulation [12], implementation of non-orthogonal multiple access (NOMA) by using RISs [13], [14], and channel modeling in the presence of an RIS [15]- [17] can be given as examples. These studies have achieved a great success with the help of RISs by avoiding or abiding some of the negative effects of them, such as the double pathloss problem [18], [19], and lossy reflection through reflecting elements. ...
... where ∠· denotes the phase of a complex term. Next, we should specify the value for an optimal by fixing the optimal value of as in (12). It is mentioned earlier that there is a maximum power level for the amplified signal. ...
Preprint
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Reconfigurable intelligent surfaces (RISs) have recently attracted the attention of the community as a potential candidate for next generation of wireless communication networks. Various studies have been carried out on this technology, which allows the control of the signal propagation environment. However, when an RIS is used in its inherently passive structure, it appears to be only a supportive technology for communications, while suffering from a multiplicative path loss. Therefore, researchers have lately begun to focus on RIS hardware designs with minimal active elements to further boost the benefits of this technology. In this paper, we present a simple hardware architecture for RISs including a single variable gain amplifier for reflection amplification to confront the multiplicative path loss. The end-to-end signal model for communication systems assisted with the proposed amplifying RIS design is presented, together with an analysis for the capacity maximization and the theoretical bit error probability performance, which is verified by computer simulations. In addition, the advantages of the proposed amplifying RIS design compared to its passive counterpart are discussed. It is shown that the proposed RIS-based system significantly eliminates the double fading problem appearing in conventional passive RIS-assisted systems as well as improves the energy efficiency.
... Furthermore, the authors in [10] propose a reflection modulation (RM) scheme for RIS systems and investigate various receiver structures and their performance in terms of BER. Moreover, an RM based on beam-patterns is presented in [11] and expressions for the achievable rate and the outage probability are derived. Union bounds on the BER performance of RIS-based spaceshift keying (SSK) are obtained by the authors in [12] by using passive beam-forming and space-time block coding techniques. ...
... Using (8c), (11), and (12), the PEP in (9) can be written as ...
... The outage probability of IRS-aided multiple-input-singleoutput (MISO) systems was also studied. In [14], considering reflection pattern modulation (RPM), a closed-form approximation for the asymptotic outage probability over Rician channels was obtained using Gamma approximation. In [15], with maximum-ratio transmission (MRT), the expression of the outage probability and its asymptotically-optimal form were given, while the phase shifts were optimized to minimize the outage probability. ...
Preprint
Intelligent reflecting surfaces (IRSs) are promising enablers for high-capacity wireless communication systems by constructing favorable channels between the transmitter and receiver. However, general, accurate, and tractable outage analysis for IRS-aided multiple-input-multiple-output (MIMO) systems is not available in the literature. In this paper, we first characterize the mutual information (MI) of IRS-aided MIMO systems by capitalizing on large random matrix theory (RMT). Based on this result, a closed-form approximation for the outage probability is derived and a gradient-based algorithm is proposed to minimize the outage probability with statistical channel state information (CSI). We also investigate the diversity-multiplexing tradeoff (DMT) with the finite signal-to-noise ratio (SNR). Based on these theoretical results, we further study the impact of the IRS size on system performance. In the high SNR regime, we provide closed-form expressions for the ergodic mutual information (EMI) and outage probability as a function of the IRS size, which analytically reveal that the benefit of increasing the IRS size saturates quickly. Simulation results validate the accuracy of the theoretical analysis and confirm the increasing cost for deploying larger IRSs to improve system performance. For example, for an IRS-aided MIMO system with 20 antennas at both the transmitter and receiver, we need to double the size of the IRS to increase the throughout from 90% to 95% of its maximum value.
... This effectively allows controlling the wireless environment. Two implementations of RIS are proposed: RIS-aided wireless communications (RAWC) [7] and RIS-based information transmission (RBIT) [8]. The former uses passive reflectors to do interference suppression and signal steering, while the latter acts as a backscatter where the signals are modulated by the RIS array. ...
Preprint
Cell-Free networking is one of the prime candidates for 6G networks. Despite being capable of providing the 6G needs, practical limitations and considerations are often neglected in current research. In this work, we introduce the concept of federations to dynamically scale and select the best set of resources, e.g., antennas, computing and data resources, to serve a given application. Next to communication, 6G systems are expected to provide also wireless powering, positioning and sensing, further increasing the complexity of such systems. Therefore, each federation is self-managing and is distributed over the area in a cell-free manner. Next to the dynamic federations, new accompanying terminology is proposed to design cell-free systems taking into account practical limitations such as time synchronization and distributed processing. We conclude with an illustration with four federations, serving distinct applications, and introduce two new testbeds to study these architectures and concepts.
... In order to improve this, both the LS and the minimum mean squared error (MMSE) methods of [25]- [27] activate all of the RIS elements based on the classic DFT matrix, so that the direct link and all reflected links become orthogonal, which facilitates their dedicated channel estimation. Fourthly, as the number of RIS elements increases, the pilot overhead of estimating both the direct link and the RIS-reflected links can be further reduced by grouping together a set of adjacent RIS elements [23], [28], [29] and upon configuring the RIS based on the statistical knowledge of the CSI [30], by compressive sensing [31], [32], deep learning [33] and matrix factorization [34]- [37]. For multi-user RIS systems, a novel three-phase channel estimation method was proposed in [24], which exploited the commonality of the BS-RIS link shared by all users. ...
Article
Full-text available
Reconfigurable intelligent surfaces (RIS) constitute a revolutionary technique of beneficially reconfiguring the smart radio environment. However, despite the fact that wireless propagation is of time-varying nature, most of the existing RIS contributions focus on time-invariant scenarios for the following reasons. Firstly, it becomes impractical to instantaneously feed back the control signal based on the doubly selective non-line-ofsight (NLoS) fading scenario. Secondly, channel estimation conceived for the high-mobility and high-dimensional RIS-assisted links has to take into account the spatial-domain (SD), timedomain (TD), and frequency-domain (FD) correlations imposed by the angle-of-arrival/departure (AoA/AoD), the Doppler and the orthogonal frequency-division multiplexing (OFDM) operations, respectively, where none of the existing solutions can be directly applied. Thirdly, it is far from trivial to maximize the NLoS channel powers on all subcarriers by a common set of RIS reflecting coefficients. Fourthly, in the face of double selectivity, it becomes inevitable to encounter either inter-symbol interference (ISI) or inter-channel interference (ICI) during the signal detection in the TD or in the FD, respectively. Against this background, firstly, we focus our attention on line-of-sight (LoS) dominated unmanned aerial vehicle (UAV) scenarios. Secondly, we conceive new minimum mean squared error (MMSE) channel estimation methods for doubly selective fading, which perodically transmit pilot symbols embedded into the TD and FD over the SD in order to beneficially exploit the correlations in the three domains. Thirdly, the RIS coefficients are optimized by a low-complexity algorithm based on the LoS representation of the end-to-end system model. Fourthly, tailor-made interference cancallation techniques are devised for improving the signal detection both in the FD and in the TD. Our simulation results are examined in six frequency bands licensed in 5G, which confirms that the employment of RIS is capable of achieving substantial performance improvements.
... These two interact with each other according to the dynamic wireless environments. The MAC protocol is designed and optimized on a frame-by-frame basis, 1 Channel estimation in RIS-assisted wireless systems is an ongoing field of research with approaches ranging from cascade channel estimation via passive RISs to compressed sensing channel estimation via RISs with minimal and basic sensing capabilities [45], [46]. 2 Compared with the desired reflected signal, the interference power caused by reflections via the remaining RISs in non-overlapping frequency bands is relatively low [47], and can be ignored. 3 The new mobile users will be regarded as an existing user in the next frame once it has joined in the current frame, and the BS will update the value of K based on the dynamic network. ...
Article
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The envisioned wireless networks of the future entail the provisioning of massive numbers of connections, heterogeneous data traffic, ultra-high spectral efficiency, and low latency services. This vision is spurring research activities focused on defining a next generation multiple access (NGMA) protocol that can accommodate massive numbers of users in different resource blocks, thereby, achieving higher spectral efficiency and increased connectivity compared to conventional multiple access schemes. In this article, we present a multiple access scheme for NGMA in wireless communication systems assisted by multiple reconfigurable intelligent surfaces (RISs). In this regard, considering the practical scenario of static users operating together with mobile ones, we first study the interplay of the design of NGMA schemes and RIS phase configuration in terms of efficiency and complexity. Based on this, we then propose a multiple access framework for RIS-assisted communication systems, and we also design a medium access control (MAC) protocol incorporating RISs. In addition, we give a detailed performance analysis of the designed RIS-assisted MAC protocol. Our extensive simulation results demonstrate that the proposed MAC design outperforms the benchmarks in terms of system throughput and access fairness, and also reveal a trade-off relationship between the system throughput and fairness.
... The observation of the evolving states of the wireless environment from the orchestration controller, via the collection of the CSI vectors in (33), is one of the key practical challenges with the proposed DRL formulation, as presented in Section V-B. The role of the controller is to associate the CSI observations to desirable RIS phase configurations. ...
Preprint
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The emerging technology of Reconfigurable Intelligent Surfaces (RISs) is provisioned as an enabler of smart wireless environments, offering a highly scalable, low-cost, hardware-efficient, and almost energy-neutral solution for dynamic control of the propagation of electromagnetic signals over the wireless medium, ultimately providing increased environmental intelligence for diverse operation objectives. One of the major challenges with the envisioned dense deployment of RISs in such reconfigurable radio environments is the efficient configuration of multiple metasurfaces with limited, or even the absence of, computing hardware. In this paper, we consider multi-user and multi-RIS-empowered wireless systems, and present a thorough survey of the online machine learning approaches for the orchestration of their various tunable components. Focusing on the sum-rate maximization as a representative design objective, we present a comprehensive problem formulation based on Deep Reinforcement Learning (DRL). We detail the correspondences among the parameters of the wireless system and the DRL terminology, and devise generic algorithmic steps for the artificial neural network training and deployment, while discussing their implementation details. Further practical considerations for multi-RIS-empowered wireless communications in the sixth Generation (6G) era are presented along with some key open research challenges. Differently from the DRL-based status quo, we leverage the independence between the configuration of the system design parameters and the future states of the wireless environment, and present efficient multi-armed bandits approaches, whose resulting sum-rate performances are numerically shown to outperform random configurations, while being sufficiently close to the conventional Deep Q-Network (DQN) algorithm, but with lower implementation complexity.
... The UAV, RIS and UE, as previously mentioned, have a UPA antenna with M U , M R and M G elements, respectively. Due to our XL-MIMO RIS assumption, the channel model we propose corresponds to each subsection/group of elements in the XL-MIMO RIS that we are using to serve different UEs, similar to the approach proposed in [23] to reflect sharp beams towards specific destinations. We assume that these groups have sufficient spatial separation thereby neglecting interference among them. ...
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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.
... For the RIS-based configuration to deploy its full potential, Channel State Information (CSI) for the end-to-end wireless link is required. Its estimation is, in general, a challenging task due to the absence of baseband processing capability of the (baseline) RISs [3]; most available RIS designs only contain basic circuitry for low-resolution phase control, that is, they lack the ability to also transmit training signals [2]. Moreover, the size of the Channel Estimation (CE) problem increases with the number of the RIS elements, and of course the number of antennas and users in a Multi-User (MU) Multiple-Input Multiple-Output (MIMO) system, rendering it a very challenging task in practically large RIS-empowered networks. ...
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The accurate estimation of Channel State Information (CSI) is of crucial importance for the successful operation of Multiple-Input Multiple-Output (MIMO) communication systems, especially in a Multi-User (MU) time-varying environment and when employing the emerging technology of Reconfigurable Intelligent Surfaces (RISs). Their predominantly passive nature renders the estimation of the channels involved in the user-RIS-base station link a quite challenging problem. Moreover, the time-varying nature of most of the realistic wireless channels drives up the cost of real-time channel tracking significantly, especially when RISs of massive size are deployed. In this paper, we develop a channel tracking scheme for the uplink of RIS-enabled MU MIMO systems in the presence of channel fading. The starting point is a tensor representation of the received signal and we rely on its PARAllel FACtor (PARAFAC) analysis to both get the initial estimate and track the channel time variation. Simulation results for various system settings are reported, which validate the feasibility and effectiveness of the proposed channel tracking approach.
... For each scheme, the symbol error probability (SEP) of the system was analyzed and an enormous benefit was demonstrated compared to the conventional wireless system without the RIS. An RIS was deployed in a multiple-input single-output (MISO) system in [10], where the RIS, in addition to beamforming, also conveys its own information data via reflection pattern modulation (RPM). An alternating optimization (AO) algorithm was used to optimize active and passive beamforming, respectively, at the transmitter and RIS, in order to maximize the signal power at the receiver. ...
Preprint
... To realize the potential gain of mmWave massive MIMO systems, the acquisition of accurate channel state information (CSI) is a top-priority task [5]- [7]. However, the channel estimation for massive MIMO systems equipped with a large number of antennas at mmWave frequencies faces unprecedented challenges. ...
... Channel estimation is required to design the passive beamformer weights. Although the BS-user, BS-RIS and RIS-user channels can be estimated separately[47], the estimation of the direct and the cascaded channels is sufficient for beamformer design[48,49]. ...
Article
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Machine learning (ML) has attracted a great research interest for physical layer design problems, such as channel estimation, thanks to its low complexity and robustness. Channel estimation via ML requires model training on a dataset, which usually includes the received pilot signals as input and channel data as output. In previous works, model training is mostly done via centralized learning (CL), where the whole training dataset is collected from the users at the base station (BS). This approach introduces huge communication overhead for data collection. In this paper, to address this challenge, we propose a federated learning (FL) framework for channel estimation. We design a convolutional neural network (CNN) trained on the local datasets of the users without sending them to the BS. We develop FL-based channel estimation schemes for both conventional and RIS (intelligent reflecting surface) assisted massive MIMO (multiple-input multiple-output) systems, where a single CNN is trained for two different datasets for both scenarios. We evaluate the performance for noisy and quantized model transmission and show that the proposed approach provides approximately 16 times lower overhead than CL, while maintaining satisfactory performance close to CL. Furthermore, the proposed architecture exhibits lower estimation error than the state-of-the-art ML-based schemes.
... However, the number of active elements is not fixed, resulting in fluctuating received power. To solve this problem, the authors in [25] propose to activate a fixed number of reflecting elements per transmission, and use their indices for information transfer. In order to avoid power loss resulting from inactive reflecting elements, all reflecting elements can be activated to reflect signals. ...
Preprint
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Reconfigurable intelligent surface (RIS)-aided symbiotic active/passive transmission is a promising communication paradigm, which is able to improve the propagation environment while transmitting additional information. In this paper, a novel scheme, termed RIS-aided number modulation (RIS-NM), is proposed for symbiotic active/passive communications. In RIS-NM, the RIS elements are divided into in-phase (I-) and quadrature (Q-) subsets depending on their phase shift configurations, and the number of elements in the I-subset (or Q-subset, equivalently) is used to convey the RIS's private information. A low-complexity yet near-optimal detector is designed for RIS-NM by shrinking the search space of constellation points. We then investigate a special case of RIS-NM, termed RIS-aided number shift keying (RIS-NSK), in which the radio-frequency source transmits unmodulated carrier signals. Statistic channel state information (CSI)-based maximum-likelihood (ML) detection is developed for RIS-NSK. We analyze the bit error rate (BER) performance of RIS-NM/NSK over Rician fading channels. BER upper bounds are derived in closed-form for RIS-NM by assuming instantaneous CSI-based ML detection, while an approximate BER expression is obtained for RIS-NSK by assuming statistic CSI-based ML detection. Furthermore, we extend RIS-NM to multiple-input multiple-output scenarios. Our simulation results in terms of BER corroborate the performance analysis and the superiority of RIS-NM over the state-of-the-art RIS-aided symbiotic active/passive transmission scheme.
... However, in recent studies, a novel IM technique, reflection modulation (RM), has been developed to utilize the reflecting elements as information transmitting units [29]. In recent RM systems, using ON/OFF keying mechanism of the passive reflection elements, an RIS has been deployed to carry information [30]- [33]. ...
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Reconfigurable intelligent surface (RIS)-empowered communication has emerged as a novel concept for customizing future wireless environments in a cost- and energy-efficient way. However, due to double path loss, existing fully passive RIS systems that purely reflect the incident signals into preferred directions attain an unsatisfactory performance improvement over the traditional wireless networks in certain conditions. To overcome this bottleneck, we propose a novel transmission scheme, named hybrid reflection modulation (HRM), exploiting both active and passive reflecting elements at the RIS and their combinations, which enables to convey information without using any radio frequency (RF) chains. In the HRM scheme, the active reflecting elements using additional power amplifiers are able to amplify and reflect the incoming signal, while the remaining passive elements can simply reflect the signals with appropriate phase shifts. Based on this novel transmission model, we obtain an upper bound for the average bit error probability (ABEP), and derive achievable rate of the system using an information theoretic approach. Moreover, comprehensive computer simulations are performed to prove the superiority of the proposed HRM scheme over existing fully passive, fully active and reflection modulation (RM) systems.
... Considering more conventional IM designs, (Li et al., 2021;Basar, 2020) put forward RIS-aided receive IM schemes, which maximize the signal powers of the target receive antennas. However, in Lin et al., 2020), novel reflection modulation (RM) concepts, which innovatively utilize the RISs for delivering additional information, are proposed. Above all, main limitation of the aforementioned studies is the lack of comprehensive practical insights on considered system configurations. ...
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Reconfigurable intelligent surface (RIS)-empowered communication is a revolutionary technology that enables to manipulate wireless propagation environment via smartly controllable low-cost reflecting surfaces. However, in order to outperform conventional communication systems, an RIS-aided system with solely passive reflection requires an extremely large surface. To meet this challenge, the concept of active RIS, which performs simultaneous amplification and reflection on the incident signal at the expense of additional power consumption, has been recently introduced. In this paper, deploying an active RIS, we propose a novel beamforming concept, over-the-air beamforming, for RIS-aided multi-user multiple-input single-output (MISO) transmission schemes without requiring any pre/post signal processing hardware designs at the transmitter and receiver sides. In the proposed over-the-air beamforming-based transmission scheme, the reflection coefficients of the active RIS elements are customized to maximize the sum-rate gain. To tackle this issue, first, a non-convex quadratically constrained quadratic programming (QCQP) problem is formulated. Then, using semidefinite relaxation (SDR) approach, this optimization problem is converted to a convex feasibility problem, which is efficiently solved using the CVX optimization toolbox. Moreover, taking inspiration from this beamforming technique, a novel high-rate receive index modulation (IM) scheme with a low-complexity sub-optimal detector is developed. Through comprehensive simulation results, the sum-rate and bit error rate (BER) performance of the proposed designs are investigated.
... Intelligent Reflecting Surface (IRS) is one of the key technologies for 6G, which has the advantages of low energy consumption and simple deployment [2][3][4]. Specifically, IRS is a planar surface comprising a large number of reconfigurable passive electrical components that can adjust the phase shift of incident signal according to different channel conditions, leading to more reliable communication links, greater transmission capacity and higher spectral and energy efficiency [5,6]. ...
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In order to investigate the effect of cooperative Intelligent Reflecting Surface (IRS) in improving spectral efficiency, this paper explores the joint design of active and passive beamforming based on a double IRS-assisted model. First, considering the maximum power constraint of the active vector and the unit modulus constraint of the cooperative passive vector, we establish the non-linear and non-convex optimization problem of multi-user maximization weighted sum rate (WSR). Then, we propose an alternating optimization (AO) algorithm to design the active vector and the cooperative passive vector based on fractional programming (FP) and successive convex approximations (SCA). In addition, we conduct a study on the optimization of the passive reflection vector under discrete phase shift. The simulation results show that the proposed beamforming scheme of double IRS-assisted model performs better than the conventional single IRS-assisted model.
... Since the reflected signal power fluctuates over time with a various number of reflecting elements switched on, the outage probability of PBIT can be unacceptably high. As a remedy, an RIS-based reflection pattern modulation (RIS-RPM) scheme has been proposed in [64], in which a subset of reflecting elements are switched on to implement efficient beamforming at the cost of conveying less extra information via the RIS. In these ON/OFF RIS-phase modulation schemes, the reflected power is less than maximum since only a part of reflecting elements are switched on. ...
Preprint
Reconfigurable intelligent surface (RIS) has been recognized as an essential enabling technique for the sixth-generation (6G) mobile communication network. Specifically, an RIS is comprised of a large number of small and low-cost reflecting elements whose parameters are dynamically adjustable with a programmable controller. Each of these elements can effectively reflect a phase-shifted version of the incident electromagnetic wave. By adjusting the wave phases in real time, the propagation environment of the reflected signals can be dynamically reconfigured to enhance communication reliability, boost transmission rate, expand cellular coverage, and strengthen communication security. In this paper, we provide an overview on RIS-assisted wireless communications. Specifically, we elaborate on the state-of-the-art enabling techniques of RISs as well as their corresponding substantial benefits from the perspectives of RIS reflection and RIS modulation. With these benefits, we envision the integration of RIS into emerging applications for 6G. In addition, communication security is of unprecedented importance in the 6G network with ubiquitous wireless services in multifarious verticals and areas. We highlight potential contributions of RIS to physical-layer security in terms of secrecy rate and secrecy outage probability, exemplified by a typical case study from both theoretical and numerical aspects. Finally, we discuss challenges and opportunities on the deployment of RISs in practice to motivate future research.
... In RBIT, RIS acts as an information transmitter which modulates information message in the reflection coefficients of the REs [97,98]. The merits of RBIT systems lie in that: (1) RBIT is achieved via passive reflection, which is energy and cost effective; (2) The large number of REs on the RIS can provide considerable spatial multiplexing or substantial diversity. ...
Thesis
Recently, the emergence of reconfigurable intelligent surface (RIS) has attracted heated attention from both industry and academia. A RIS is a planar surface that consists of a large number of low-cost passive reflecting elements. By carefully adjusting the phase shifts of the reflecting elements, an RIS can reshape the wireless environment for better communication. In this thesis, we focus on two subjects: (i) To study the modeling and optimization of RIS-aided communication systems. (ii) To study RIS-aided spatial modulation, especially the detection using deep learning techniques. Chapter 1 introduces the concept of smart radio environments and RIS. In 5G and future communications, RIS is a key technique to achieve seamless connectivity and less energy consumption at the same time. Chapter 2 introduces RIS-aided communication systems. The reflection principle, channel estimation problem and system design problem are introduced in detail. State-of-the-art research on the problems of channel estimation and system design are overviewed. Chapter 3 investigates the distribution of the signal-to-noise ratio (SNR) as a random variable in an RIS-aided multiple-input multiple-output (MIMO) system. Rayleigh fading and line-of-sight propagation are considered separately. The theoretical derivation and numerical simulation prove that the SNR is equivalent in distribution to the product of three (Rayleigh fading) or two (line-of-sight propagation) independent random variables. Chapter 4 studies the behavior of interference in an RIS-aided MIMO system, where each base station serves a user equipment (UE) through an RIS. The interference at a UE is caused by its non-serving RIS. It is proven that the interference-to-noise ratio is equivalent in distribution to the product of a Chi-squared random variable and a random variable which can be approximated with a Gamma distribution. Chapter 5 focuses on RIS-aided spatial modulation. First, we introduce deep learning aided detection for MIMO systems. Then, by generalizing RIS-aided spatial modulation systems as a special case of traditional spatial modulation systems, we investigate deep learning based detection for RIS-aided spatial modulation systems. Numerical results validate the proposed data-based and model-based deep learning detection schemes for RIS-aided spatial modulation systems. Finally, Chapter 6 concludes the thesis and discusses possible future research directions.
Article
This paper investigates a novel intelligent reflecting surface (IRS)-based symbiotic radio (SR) system architecture consisting of a transmitter, an IRS, and an information receiver (IR). The primary transmitter communicates with the IR and at the same time assists the IRS in forwarding information to the IR. Based on the IRSs symbol period, we distinguish two scenarios, namely, commensal SR (CSR) and parasitic SR (PSR), where two different techniques for decoding the IRS signals at the IR are employed. We formulate bit error rate (BER) minimization problems for both scenarios by jointly optimizing the active beamformer at the base station and the phase shifts at the IRS, subject to a minimum primary rate requirement. Specifically, for the CSR scenario, a penalty-based algorithm is proposed to obtain a high-quality solution, where semi-closed-form solutions for the active beamformer and the IRS phase shifts are derived based on Lagrange duality and Majorization-Minimization methods, respectively. For the PSR scenario, we apply a bisection search-based method, successive convex approximation, and difference of convex programming to develop a computationally efficient algorithm, which converges to a locally optimal solution. Simulation results demonstrate the effectiveness of the proposed algorithms and show that the proposed SR techniques are able to achieve a lower BER than benchmark schemes.
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.
Article
Novel communication technology based on large intelligent surface (LIS), which manipulates the phase of the incident waves to enhance the communication quality, has been proposed as a promising candidate for future wireless communication systems and standards. In order to improve the spectral efficiency, the concept of LIS assisted communications has been introduced into the realm of index modulation (IM) recently. In this paper, a practical LIS based generalized spatial modulation (LIS-GSM) scheme is proposed by modulating the transmit signals through both transmitter and LIS jointly while designing the pattern of receive antennas in a more flexible way. Furthermore, a unified framework for the derivation of the theoretical average bit error rate (ABER) based on the maximum likelihood detector is also presented. Simulation results demonstrate that LIS-GSM is more robust in terms of ABER as well as receive antenna pattern compared with conventional LIS-SM.
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The demanding objectives for the future sixth generation (6G) of wireless communication networks have spurred recent research efforts on novel materials and radio-frequency front-end architectures for wireless connectivity, as well as revolutionary communication and computing paradigms. Among the pioneering candidate technologies for 6G belong the reconfigurable intelligent surfaces (RISs), which are artificial planar structures with integrated electronic circuits that can be programmed to manipulate the incoming electromagnetic field in a wide variety of functionalities. Incorporating RISs in wireless networks has been recently advocated as a revolutionary means to transform any wireless signal propagation environment to a dynamically programmable one, intended for various networking objectives, such as coverage extension and capacity boosting, spatiotemporal focusing with benefits in energy efficiency and secrecy, and low electromagnetic field exposure. Motivated by the recent increasing interests in the field of RISs and the consequent pioneering concept of the RIS-enabled smart wireless environments, in this paper, we overview and taxonomize the latest advances in RIS hardware architectures as well as the most recent developments in the modeling of RIS unit elements and RIS-empowered wireless signal propagation. We also present a thorough overview of the channel estimation approaches for RIS-empowered communications systems, which constitute a prerequisite step for the optimized incorporation of RISs in future wireless networks. Finally, we discuss the relevance of the RIS technology in the latest wireless communication standards, and highlight the current and future standardization activities for the RIS technology and the consequent RIS-empowered wireless networking approaches.
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The emerging technology of reconfigurable intelligent surfaces (RISs) is provisioned as an enabler of smart wireless environments, offering a highly scalable, low-cost, hardware-efficient, and almost energy-neutral solution for dynamic control of the propagation of electromagnetic signals over the wireless medium, ultimately providing increased environmental intelligence for diverse operation objectives. One of the major challenges with the envisioned dense deployment of RISs in such reconfigurable radio environments is the efficient configuration of multiple metasurfaces with limited, or even the absence of, computing hardware. In this article, we consider multiuser and multi-RIS-empowered wireless systems and present a thorough survey of the online machine learning approaches for the orchestration of their various tunable components. Focusing on the sum-rate maximization as a representative design objective, we present a comprehensive problem formulation based on deep reinforcement learning (DRL). We detail the correspondences among the parameters of the wireless system and the DRL terminology, and devise generic algorithmic steps for the artificial neural network training and deployment while discussing their implementation details. Further practical considerations for multi-RIS-empowered wireless communications in the sixth-generation (6G) era are presented along with some key open research challenges. Different from the DRL-based status quo, we leverage the independence between the configuration of the system design parameters and the future states of the wireless environment, and present efficient multiarmed bandits approaches, whose resulting sum-rate performances are numerically shown to outperform random configurations, while being sufficiently close to the conventional deep $Q$ network (DQN) algorithm, but with lower implementation complexity.
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The demanding objectives for the future sixth generation (6G) of wireless communication networks have spurred recent research efforts on novel materials and radio-frequency front-end architectures for wireless connectivity, as well as revolutionary communication and computing paradigms. Among the pioneering candidate technologies for 6G belong the reconfigurable intelligent surfaces (RISs), which are artificial planar structures with integrated electronic circuits that can be programmed to manipulate the incoming electromagnetic field in a wide variety of functionalities. Incorporating RISs in wireless networks have been recently advocated as a revolutionary means to transform any wireless signal propagation environment to a dynamically programmable one, intended for various networking objectives, such as coverage extension and capacity boosting, spatiotemporal focusing with benefits in energy efficiency and secrecy, and low electromagnetic field exposure. Motivated by the recent increasing interests in the field of RISs and the consequent pioneering concept of the RIS-enabled smart wireless environments, in this paper, we overview and taxonomize the latest advances in RIS hardware architectures as well as the most recent developments in the modeling of RIS unit elements and RIS-empowered wireless signal propagation. We also present a thorough overview of the channel estimation approaches for RIS-empowered communications systems, which constitute a prerequisite step for the optimized incorporation of RISs in future wireless networks. Finally, we discuss the relevance of the RIS technology in the latest wireless communication standards, and highlight the current and future standardization activities for the RIS technology and the consequent RIS-empowered wireless networking approaches.
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Reconfigurable intelligent surface (RIS)-assisted communication have recently attracted the attention of the wireless communication community as a potential candidate for the next 6-th generation (6G) of wireless networks. Various studies have been carried out on the RIS technology, which is capable of enabling the control of the signal propagation environment by network operators. However, when an RIS is used in its inherently passive structure, it appears to be only a supportive technology for communications, while suffering from a multiplicative path loss. Therefore, researchers have lately begun to focus on RIS hardware designs with minimal active elements to further boost the benefits of this technology. In this paper, we present a simple RIS hardware architecture including a single and variable gain amplifier for reflection amplification to confront the multiplicative path loss. The end-to-end signal model for communication systems assisted with the proposed amplifying RIS design is presented, together with an analysis focusing on the capacity maximization and theoretical bit error probability performance, which is corroborated by computer simulations. In addition, the major advantages of the proposed amplifying RIS design compared to its passive counterpart are discussed. It is shown that the proposed RIS-based wireless system significantly eliminates the double fading problem appearing in conventional passive RIS-assisted systems and improves the communication energy efficiency.
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In this paper, we combine of two new technologies: full-duplex (FD) transmission and intelligent reflecting surface (IRS) in a wireless communication system for investigating. Specifically, we evaluate the performance of an IRS aided bidirectional FD communication system in a practical scenario where imperfect self-interference (SI) cancellation and hardware impairments (HIs) are taken into consideration. We successfully derive the closed-form expressions of ergodic capacity (EC) and symbol error rate (SER) of the IRS aided FD-HI system over Rayleigh fading channels. We confirm the correctness of the derived expressions via Monte-Carlo simulations. To clarify the effects of residual SI and HIs, we compare the performance of the IRS aided FD-HI system with that of the IRS aided FD-ideal hardware (ID), half-duplex (HD)-HI, and HD-ID systems. Numerical results clarify a strong impact of residual SI and HIs on the EC and SER of the IRS aided FD-HI system. Thus, the EC and SER of the IRS aided FD-HI system go to the saturated values in a high signal-to-noise (SNR) regime even large number of reflecting elements in the IRS. Therefore, depending on the residual SI and HI levels as well as the requirements about the EC and SER in practice, we can use appropriately transmit power of terminals and number of reflecting elements in the IRS for enhancing the performance and saving the energy consumption of the IRS aided FD-HI system. Index Terms-Intelligent reflecting surface, full-duplex communications , self-interference cancellation, ergodic capacity, symbol error rate.
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Future wireless networks are expected to evolve toward an intelligent and software reconfigurable paradigm enabling ubiquitous communications between humans and mobile devices. They will also be capable of sensing, controlling, and optimizing the wireless environment to fulfill the visions of low-power, high-throughput, massively-connected, and low-latency communications. A key conceptual enabler that is recently gaining increasing popularity is the HMIMOS that refers to a low-cost transformative wireless planar structure comprised of sub-wavelength metallic or dielectric scattering particles, which is capable of shaping electromagnetic waves according to desired objectives. In this article, we provide an overview of HMIMOS communications including the available hardware architectures for reconfiguring such surfaces, and highlight the opportunities and key challenges in designing HMIMOS-enabled wireless communications.
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Reconfigurable intelligent surfaces (RISs) have the potential of realizing the emerging concept of smart radio environments by leveraging the unique properties of metamaterials and large arrays of inexpensive antennas. In this article, we discuss the potential applications of RISs in wireless networks that operate at high-frequency bands, e.g., millimeter wave (30-100 GHz) and sub-millimeter wave (greater than 100 GHz) frequencies. When used in wireless networks, RISs may operate in a manner similar to relays. The present paper, therefore, elaborates on the key differences and similarities between RISs that are configured to operate as anomalous reflectors and relays. In particular, we illustrate numerical results that highlight the spectral efficiency gains of RISs when their size is sufficiently large as compared with the wavelength of the radio waves. In addition, we discuss key open issues that need to be addressed for unlocking the potential benefits of RISs for application to wireless communications and networks.
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Transmission through reconfigurable intelligent surfaces (RISs), which control the reflection/scattering characteristics of incident waves in a deliberate manner to enhance the signal quality at the receiver, appears as a promising candidate for future wireless communication systems. In this paper, we bring the concept of RIS-assisted communications to the realm of index modulation (IM) by proposing RIS-space shift keying (RIS-SSK) and RIS-spatial modulation (RIS-SM) schemes. These two schemes are realized through not only intelligent reflection of the incoming signals to improve the reception but also utilization of the IM principle for the indices of multiple receive antennas in a clever way to improve the spectral efficiency. Maximum energy-based suboptimal (greedy) and exhaustive search-based optimal (maximum likelihood) detectors of the proposed RIS-SSK/SM schemes are formulated and a unified framework is presented for the derivation of their theoretical average bit error probability. Extensive computer simulation results are provided to assess the potential of RIS-assisted IM schemes as well as to verify our theoretical derivations. Our findings also reveal that RIS-based IM, which enables high data rates with remarkably low error rates, can become a potential candidate for future wireless communication systems in the context of beyond multiple-input multiple-output (MIMO) solutions.
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The rate and energy efficiency of wireless channels can be improved by deploying software-controlled metasurfaces to reflect signals from the source to destination, especially when the direct path is weak. While previous works mainly optimized the reflections, this letter compares the new technology with classic decode-and-forward (DF) relaying. The main observation is that very high rates and/or large metasurfaces are needed to outperform DF relaying, both in terms of minimizing the total transmit power and maximizing the energy efficiency, which also includes the dissipation in the transceiver hardware.
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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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Abstract An emerging and promising vision of wireless networks consists of coating the environmental objects with reconfigurable metasurfaces that are capable of modifying the radio waves impinging upon them according to the generalized law of reflection. By relying on tools from point processes, stochastic geometry, and random spatial processes, we model the environmental objects with a modified random line process of fixed length and with random orientations and locations. Based on the proposed modeling approach, we develop the first analytical framework that provides one with the probability that a randomly distributed object that is coated with a reconfigurable metasurface acts as a reflector for a given pair of transmitter and receiver. In contrast to the conventional network setup where the environmental objects are not coated with reconfigurable metasurfaces, we prove that the probability that the typical random object acts as a reflector is independent of the length of the object itself. The proposed analytical approach is validated against Monte Carlo simulations, and numerical illustrations are given and discussed.
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In this article, we have provided general, comprehensive coverage of the SDR technique, from its practical deployments and scope of applicability to key theoretical results. We have also showcased several representative applications, namely MIMO detection, BÂ¿ shimming in MRI, and sensor network localization. Another important application, namely downlink transmit beamforming, is described in [1]. Due to space limitations, we are unable to cover many other beautiful applications of the SDR technique, although we have done our best to illustrate the key intuitive ideas that resulted in those applications. We hope that this introductory article will serve as a good starting point for readers who would like to apply the SDR technique to their applications, and to locate specific references either in applications or theory.
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Intelligent reflecting surface (IRS) has emerged as an enabling technology to achieve smart and reconfigurable wireless communication environment cost-effectively. Prior works on IRS mainly consider its passive beamforming design and performance optimization without the inter-IRS signal reflection, which thus do not unveil the full potential of multi-IRS assisted wireless networks. In this paper, we study a double-IRS assisted multi-user communication system with the cooperative passive beamforming design that captures the multiplicative beamforming gain from the inter-IRS channel. Under the general channel setup with the co-existence of both double- and single-reflection links, we jointly optimize the (active) receive beamforming at the base station (BS) and the cooperative (passive) reflect beamforming at the two distributed IRSs (deployed near the BS and users, respectively) to maximize the minimum signal-to-interference-plus-noise ratio (SINR) of all users. Moreover, for the single-user and multi-user setups, we analytically show the superior performance of the double-IRS cooperative system over the conventional single-IRS system in terms of the maximum signal-to-noise ratio (SNR) and multi-user effective channel rank, respectively. Simulation results validate our analytical results and show the practical advantages of the proposed double-IRS system with cooperative passive beamforming designs.
Article
Reconfigurable intelligent surfaces (RISs) are regarded as a promising emerging hardware technology to improve the spectrum and energy efficiency of wireless networks by artificially reconfiguring the propagation environment of electromagnetic waves. Due to the unique advantages in enhancing wireless channel capacity, RISs have recently become a hot research topic. In this article, we focus on three fundamental physical-layer challenges for the incorporation of RISs into wireless networks, namely, channel state information acquisition, passive information transfer, and low-complexity robust system design. We summarize the state-of-the-art solutions and explore potential research directions. Furthermore, we discuss other promising research directions of RISs, including edge intelligence and physical-layer security.
Article
In this letter, we study efficient channel estimation for an intelligent reflecting surface (IRS)-assisted orthogonal frequency division multiplexing (OFDM) system to achieve minimum training time. First, a fast channel estimation scheme with reduced OFDM symbol duration is proposed for arbitrary frequency-selective fading channels. Next, under the typical condition that the IRS-user channel is line-of-sight (LoS) dominant, another fast channel estimation scheme based on the novel concept of sampling-wise IRS reflection variation is proposed. Moreover, the pilot signal and IRS training reflection pattern are jointly optimized for both proposed schemes. Finally, the proposed schemes are compared in terms of training time and channel estimation performance via simulations, as well as against benchmark schemes.
Article
To achieve the full passive beamforming gains of intelligent reflecting surface (IRS), accurate channel state information (CSI) is indispensable but practically challenging to acquire, due to the excessive amount of channel parameters to be estimated which increases with the number of IRS reflecting elements as well as that of IRS-served users. To tackle this challenge, we propose in this paper two efficient channel estimation schemes for different channel setups in an IRS-assisted multiuser broadband communication system employing the orthogonal frequency division multiple access (OFDMA). The first channel estimation scheme, which estimates the CSI of all users in parallel simultaneously at the access point (AP), is applicable for arbitrary frequency-selective fading channels. In contrast, the second channel estimation scheme, which exploits a key property that all users share the same (common) IRS-AP channel to enhance the training efficiency and support more users, is proposed for the typical scenario with line-of-sight (LoS) dominant user-IRS channels. For the two proposed channel estimation schemes, we further optimize their corresponding training designs (including pilot tone allocations for all users and IRS time-varying reflection pattern) to minimize the channel estimation error. Moreover, we derive and compare the fundamental limits on the minimum training overhead and the maximum number of supportable users of these two schemes. Simulation results verify the effectiveness of the proposed channel estimation schemes and training designs, and show their significant performance improvement over various benchmark schemes.
Article
Reconfigurable intelligent surfaces (RISs) are an emerging transmission technology for application to wireless communications. RISs can be realized in different ways, which include (i) large arrays of inexpensive antennas that are usually spaced half of the wavelength apart; and (ii) metamaterial-based planar or conformal large surfaces whose scattering elements have sizes and inter-distances much smaller than the wavelength. Compared with other transmission technologies, e.g., phased arrays, multi-antenna transmitters, and relays, RISs require the largest number of scattering elements, but each of them needs to be backed by the fewest and least costly components. Also, no power amplifiers are usually needed. For these reasons, RISs constitute a promising software-defined architecture that can be realized at reduced cost, size, weight, and power (C-SWaP design), and are regarded as an enabling technology for realizing the emerging concept of smart radio environments (SREs). In this paper, we (i) introduce the emerging research field of RIS-empowered SREs; (ii) overview the most suitable applications of RISs in wireless networks; (iii) present an electromagnetic-based communication-theoretic framework for analyzing and optimizing metamaterial-based RISs; (iv) provide a comprehensive overview of the current state of research; and (v) discuss the most important research issues to tackle. Owing to the interdisciplinary essence of RIS-empowered SREs, finally, we put forth the need of reconciling and reuniting C. E. Shannon’s mathematical theory of communication with G. Green’s and J. C. Maxwell’s mathematical theories of electromagnetism for appropriately modeling, analyzing, optimizing, and deploying future wireless networks empowered by RISs.
Article
Reconfigurable intelligent surfaces (RISs) have recently emerged as an innovative technology for improving the coverage, throughput, and energy/spectrum efficiency of future wireless communications. In this paper, we propose a new transmission protocol for wideband RIS-assisted single-input multiple-output (SIMO) orthogonal frequency division multiplexing (OFDM) communication systems, where each transmission frame is divided into multiple sub-frames to execute channel estimation simultaneously with passive beamforming. As the training symbols are discretely distributed over multiple sub-frames, the channel state information (CSI) associated with RIS cannot be estimated at once. As such, we propose a new channel estimation method to progressively estimate the associated CSI over consecutive sub-frames, based on which the passive beamforming at the RIS is fine-tuned to improve the achievable rate for data transmission. In particular, during the channel training, the RIS plays two roles of embedding training reflection states for progressive channel estimation and performing passive beamforming for data transmission on the data tones. Based on the estimated CSI in each sub-frame, we formulate an optimization problem to maximize the average achievable rate by designing the passive beamforming at the RIS. which needs to balance the received signal power over different sub-carriers and different receive antennas. As the formulated problem is non-convex and thus difficult to solve optimally, we propose two efficient algorithms to find high-quality solutions. Simulation results validate the effectiveness of the proposed channel estimation and beamforming optimization methods. It is shown that the proposed joint channel estimation and passive beamforming scheme is able to drastically improve the average achievable rate and reduce the delay for data transmission as compared to existing schemes.
Article
This paper investigates the passive beamforming and information transfer (PBIT) technique for multiuser multipleinput multiple-output (Mu-MIMO) systems with the aid of reconfigurable intelligent surfaces (RISs), where the RISs enhance the primary communication via passive beamforming (P-BF) and at the same time deliver additional information by the on-off reflecting modulation (in which the RIS information is carried by the on/off state of each reflecting element). For the P-BF design, we propose to maximize the achievable user sum rate of the RIS-aided Mu-MIMO channel and formulate the problem as a two-step stochastic program. A sample average approximation (SAA) based iterative algorithm is developed for the efficient PBF design of the considered scheme. To strike a balance between complexity and performance, we further propose a simplified P-BF algorithm by approximating the stochastic program as a deterministic alternating optimization problem. For the receiver design, the signal detection at the receiver is a bilinear estimation problem since the RIS information is multiplicatively modulated onto the reflected signals of the reflecting elements. To solve this bilinear estimation problem, we develop a turbo message passing (TMP) algorithm in which the factor graph associated with the problem is divided into two modules: one for the estimation of the user signals and the other for the estimation of the on-off state of each RIS element. The two modules are executed iteratively to yield a near-optimal low-complexity solution. Furthermore, we extend the design of the Mu-MIMO PBIT scheme from single- RIS to multi-RIS, by leveraging the similarity between the single- RIS andmulti-RIS system models. Extensive simulation results are provided to demonstrate the advantages of our P-BF and receiver designs.
Article
Recently, the reconfigurable intelligent surface (RIS), benefited from the breakthrough on the fabrication of programmable meta-material, has been speculated as one of the key enabling technologies for the future six generation (6G) wireless communication systems scaled up beyond massive multiple input multiple output (Massive-MIMO) technology to achieve smart radio environments. Employed as reflecting arrays, RIS is able to assist MIMO transmissions without the need of radio frequency chains resulting in considerable reduction in power consumption. In this paper, we investigate the joint design of transmit beamforming matrix at the base station and the phase shift matrix at the RIS, by leveraging recent advances in deep reinforcement learning (DRL). We first develop a DRL based algorithm, in which the joint design is obtained through trial-and-error interactions with the environment by observing predefined rewards, in the context of continuous state and action. Unlike the most reported works utilizing the alternating optimization techniques to alternatively obtain the transmit beamforming and phase shifts, the proposed DRL based algorithm obtains the joint design simultaneously as the output of the DRL neural network. Simulation results show that the proposed algorithm is not only able to learn from the environment and gradually improve its behavior, but also obtains the comparable performance compared with two state-of-the-art benchmarks. It is also observed that, appropriate neural network parameter settings will improve significantly the performance and convergence rate of the proposed algorithm.
Article
Intelligent reflecting surface (IRS) is a promising new technology for achieving both spectrum and energy efficient wireless communication systems in the future. However, existing works on IRS mainly consider frequency-flat channels and assume perfect knowledge of channel state information (CSI) at the transmitter. Motivated by the above, in this paper we study an IRS-enhanced orthogonal frequency division multiplexing (OFDM) system under frequency-selective channels and propose a practical transmission protocol with channel estimation. First, to reduce the overhead in channel training as well as exploit the channel spatial correlation, we propose a novel IRS elements grouping method, where each group consists of a set of adjacent IRS elements that share a common reflection coefficient. Based on this method, we propose a practical transmission protocol where only the combined channel of each group needs to be estimated, thus substantially reducing the training overhead. Next, with any given grouping and estimated CSI, we formulate the problem to maximize the achievable rate by jointly optimizing the transmit power allocation and the IRS passive array reflection coefficients. Although the formulated problem is non-convex and thus difficult to solve, we propose an efficient algorithm to obtain a high-quality suboptimal solution for it, by alternately optimizing the power allocation and the passive array coefficients in an iterative manner, along with a customized method for the initialization. Simulation results show that the proposed design significantly improves the OFDM link rate performance as compared to the case without using IRS. Moreover, it is shown that there exists an optimal size for IRS elements grouping which achieves the maximum achievable rate due to the practical trade-off between the training overhead and IRS passive beamforming flexibility.
Article
The integration of intelligent reflecting surface (IRS) to multiple access networks is a cost-effective solution for boosting spectrum/energy efficiency and enlarging network coverage/connections. However, due to the new capability of IRS in reconfiguring the wireless propagation channels, it is fundamentally unknown which multiple access scheme is superior in the IRS-assisted wireless network. In this letter, we pursue a theoretical performance comparison between non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) in the IRS-assisted downlink communication, for which the transmit power minimization problems are formulated under the discrete unit-modulus reflection constraint on each IRS element. We analyze the minimum transmit powers required by different multiple access schemes and compare them numerically, which turn out to not fully comply with the stereotyped superiority of NOMA over OMA in conventional systems without IRS. Moreover, to avoid the exponential complexity of the brute-force search for the optimal discrete IRS phase shifts, we propose a low-complexity solution to achieve near-optimal performance.
Article
Large intelligent surface (LIS) has emerged as a promising new solution to improve the energy and spectrum efficiency of wireless networks. A LIS, composed of a large number of low-cost nearly-passive reconfigurable reflecting elements, enhances wireless communications by reflecting impinging electromagnetic waves. In this paper, we propose a novel passive beamforming and information transfer (PBIT) technique by adopting spatial modulation on the index of the LIS elements, in which the LIS simultaneously enhances the primary communication (by passive beamforming) and sends its private data to the receiver (by spatial modulation). We develop a passive beamforming method to improve the average receive signal-to-noise ratio (SNR). We also establish a two-step approach at the receiver to retrieve the information from both the transmitter and the LIS. Numerical results show that the proposed PBIT system, especially with the optimized passive beamforming, significantly outperforms the system without LIS enhancement. Furthermore, a tradeoff between the passive-beamforming gain and the information rate of the LIS has been demonstrated.
Article
In the intelligent reflecting surface (IRS)-enhanced wireless communication system, channel state information (CSI) is of paramount importance for achieving the passive beamforming gain of IRS, which, however, is a practically challenging task due to its massive number of passive elements without transmitting/receiving capabilities. In this letter, we propose a practical transmission protocol to execute channel estimation and reflection optimization successively for an IRS-enhanced orthogonal frequency division multiplexing (OFDM) system. Under the unit-modulus constraint, a novel reflection pattern at the IRS is designed to aid the channel estimation at the access point (AP) based on the received pilot signals from the user, for which the channel estimation error is derived in closed-form. With the estimated CSI, the reflection coefficients are then optimized by a low-complexity algorithm based on the resolved strongest signal path in the time domain. Simulation results corroborate the effectiveness of the proposed channel estimation and reflection optimization methods.