No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
STAR-RISs: A Correlated T&R Phase-Shift Model and Practical Phase-Shift Configuration Strategies
Abstract
A
correlated
transmission and reflection (T&R) phase-shift model is proposed for passive lossless simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs). A STAR-RIS-aided two-user downlink communication system is investigated for both orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA). To evaluate the impact of the correlated T&R phase-shift model on the communication performance, three phase-shift configuration strategies are developed, namely the primary-secondary phase-shift configuration (PS-PSC), the diversity preserving phase-shift configuration (DP-PSC), and the T/R-group phase-shift configuration (TR-PSC) strategies. Furthermore, we derive the outage probabilities for the three proposed phase-shift configuration strategies as well as for those for the random phase-shift configuration and the independent phase-shift model, which constitute performance lower and upper bounds, respectively. Then, the diversity order of each strategy is investigated based on the obtained analytical results. It is shown that the proposed DP-PSC strategy achieves full diversity order simultaneously for users located on both sides of the STAR-RIS. Moreover, power scaling laws are derived for the three proposed strategies and for the random phase-shift configuration. Numerical simulations reveal a performance gain if the users on both sides of the STAR-RIS are served by NOMA instead of OMA. Moreover, it is shown that the proposed DP-PSC strategy yields the same diversity order as achieved by STAR-RISs under the independent phase-shift model and a comparable power scaling law with only 4 dB reduction in received power.
... where τ ∈ {ipSIC, pSIC}. P ipSIC out,r , P pSIC out,r and P out,t are given by (14), (15) and (17), respectively. ...
... where P ipSIC out,r , P pSIC out,r and P out,t are obtained from (14), (15) and (17), respectively. ...
... In Fig. 3, we plot the outage probability of U r and U t versus the system power budget. The outage probability curves of U t and U r with pSIC/ipSIC curves are plotted by (14), (15) and (17), respectively. This figure demonstrates that the analytical expressions derived match the simulation results exactly, which validates the accuracy of the analytical methods applied. ...
The novel active simultaneously transmitting and reflecting surface (ASTARS) has recently received a lot of attention due to its capability to conquer the multiplicative fading loss and achieve full-space smart radio environments. This paper introduces the ASTARS to assist non-orthogonal multiple access (NOMA) communications, where the paring users are uniformly distributed within the service area. We design the independent reflection/transmission phase-shift controllers of ASTARS to align the phases of cascaded channels at pairing users. We derive new approximate and asymptotic expressions of the outage probability and ergodic data rate for ASTARS-NOMA networks in the presence of perfect/imperfect successive interference cancellation (pSIC/ipSIC). The diversity orders and multiplexing gains for ASTARS-NOMA are derived to provide more insights. Furthermore, the system throughputs of ASTARS-NOMA are investigated in both delay-tolerant and delay-limited transmission modes. The numerical results are presented and show that: 1) ASTARS-NOMA with pSIC outperforms ASTARS assisted-orthogonal multiple access (ASTARS-OMA) in terms of outage probability and ergodic data rate; 2) The outage probability of ASTARS-NOMA with pSIC/ipSIC can be further reduced within a certain range by increasing the power amplification factors; and 3) The system throughputs of ASTARS-NOMA are superior to that of ASTARS-OMA in both delay-limited and delay-tolerant transmission modes.
... where τ ∈ {ipSIC, pSIC}. P ipSIC out,r , P pSIC out,r and P out,t are given by (14), (15) and (17), respectively. ...
... where P ipSIC out,r , P pSIC out,r and P out,t are obtained from (14), (15) and (17), respectively. ...
... In Fig. 3, we plot the outage probability of U r and U t versus the system power budget. The outage probability curves of U t and U r with pSIC/ipSIC curves are plotted by (14), (15) and (17), respectively. This figure demonstrates that the analytical expressions derived match the simulation results exactly, which validates the accuracy of the analytical methods applied. ...
The novel active simultaneously transmitting and reflecting surface (ASTARS) has recently received a lot of attention due to its capability to conquer the multiplicative fading loss and achieve full-space smart radio environments. This paper introduces the ASTARS to assist non-orthogonal multiple access (NOMA) communications, where the stochastic geometry theory is used to model the spatial positions of pairing users. We design the independent reflection/transmission phase-shift controllers of ASTARS to align the phases of cascaded channels at pairing users. We derive new closed-form and asymptotic expressions of the outage probability and ergodic data rate for ASTARS-NOMA networks in the presence of perfect/imperfect successive interference cancellation (pSIC). The diversity orders and multiplexing gains for ASTARS-NOMA are derived to provide more insights. Furthermore, the system throughputs of ASTARS-NOMA are investigated in both delay-tolerant and delay-limited transmission modes. The numerical results are presented and show that: 1) ASTARS-NOMA with pSIC outperforms ASTARS assisted-orthogonal multiple access (ASTARS-OMA) in terms of outage probability and ergodic data rate; 2) The outage probability of ASTARS-NOMA can be further reduced within a certain range by 2 increasing the power amplification factors; 3) The system throughputs of ASTARS-NOMA are superior to that of ASTARS-OMA in both delay-limited and delay-tolerant transmission modes. Index terms-Active simultaneously transmitting and reflecting surface, non-orthogonal multiple access, stochastic geometry.
... The coupled phase-shift STAR-RIS model was proposed in [26]. Then,in [27], the extension of the previous work, with a more in-depth explanation of the Torebek Toregozhin is pursuing a B.Sc.degree in Electrical and Computer Engineering at Nazarbayev University. In 2020, he graduated from Almaty's Nazarbayev Intellectual School of Chemical and Biological direction. ...
... Meanwhile, the network layer configuration according to the programmable wireless environments paradigm featuring programmable metasurfaces is considered in [38][39][40]. 2 We limit our system model to two paired users due to the propagation error and complexity restrictions as in [13,20,[22][23][24][25][26][27][28][29][30]. ...
... where 1 = ( ), 2 = ( ), 3 = ( + 1 2 ), 4 = ( + 1 2 ), 5 = ( + 1), and 6 = ( + 1). Hence, the expression in (27) can be obtained by substituting (30) into (28). ...
... Indeed, in the case of a passive and lossless IOS, the difference between the phase value in transmission and reflection must fulfill one condition and therefore, they cannot be independently adjusted. In [6], some practical phase-shift configurations for IOS communications were developed by taking this electromagnetic constraint into account. Unfortunately, this constraint often hinders the design of IOS elements to achieve a balanced amplitude with independent phase shift for the transmission and refl ection (T&R) modes. ...
... The main states are: linear polarizations (vertical, horizontal and slant) and circular polarizations. To the best of our knowledge, polarization states are overlooked in the IOS communications literature [6]. This limits the potential of IOSs at the electromagnetic level since the T&R coefficients are intimately linked and, therefore, T&R waves are closely coupled. ...
... Typically, IOSs used for modeling communication scenarios are passive and lossless. This leads to the fact that the diff erence between the refl ected and transmitted phase values for each IOS element must be ± 90° [6]. Now, when considering polarized elements for the IOS, this constraint is removed and there is more flexibility in adjusting these phase values to obtain the desired passive beamforming pattern in the full space. ...
p>Wireless communications based on reconfigurable intelligent surfaces (RIS) have upsurged as a key research topic in the communication-theoretic arena. Recently, the RIS concept has evolved into the so-called simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOS, or simply IOS), which extend the RIS functionality by incorporating transmission (in addition to reflection) capabilities. To realize STAR-IOS, full and independent reconfiguration capabilities for both reflected and transmitted waves are crucially required. However, such full independent reconfiguration has thus far been hurdled by the intimate coupling between the transmission and reflection behaviour of the IOS elements. To overcome this challenge and realize the full potential of reconfigurable IOS-aided systems, we here advocate for the use of the polarization domain in the design and operation of STAR-IOS. Thanks to the polarization- dependent features of IOS elements, fully independent (reconfigurable) transmission and reflection modes can be delivered, thus bringing key performance improvements and opportunities for new communication scenarios.</p
... Indeed, in the case of a passive and lossless IOS, the difference between the phase value in transmission and reflection must fulfill one condition and therefore, they cannot be independently adjusted. In [6], some practical phase-shift configurations for IOS communications were developed by taking this electromagnetic constraint into account. Unfortunately, this constraint often hinders the design of IOS elements to achieve a balanced amplitude with independent phase shift for the transmission and refl ection (T&R) modes. ...
... The main states are: linear polarizations (vertical, horizontal and slant) and circular polarizations. To the best of our knowledge, polarization states are overlooked in the IOS communications literature [6]. This limits the potential of IOSs at the electromagnetic level since the T&R coefficients are intimately linked and, therefore, T&R waves are closely coupled. ...
... Typically, IOSs used for modeling communication scenarios are passive and lossless. This leads to the fact that the diff erence between the refl ected and transmitted phase values for each IOS element must be ± 90° [6]. Now, when considering polarized elements for the IOS, this constraint is removed and there is more flexibility in adjusting these phase values to obtain the desired passive beamforming pattern in the full space. ...
Research in wireless communications based on reconfigurable intelligent surfaces (RIS) has surged in the communication-theoretic arena. Recently, the RIS concept has moved into the area of so-called simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOS, or simply IOS), which extend the RIS functionality by incorporating transmission (in addition to reflection) capabilities. Development of STAR-IOS’ full and independent reconfiguration capabilities for both reflected and transmitted waves is crucial. However, such full independent reconfiguration has thus far been hampered by the intimate coupling between the transmission and reflection behavior of IOS elements. To overcome this challenge and realize the full potential of reconfigurable IOS-aided systems, in this article we advocate for the use of the polarization domain in the design and operation of STAR-IOS. Thanks to the polarization-dependent features of IOS elements, fully independent (reconfigurable) transmission and reflection modes can be delivered, thus bringing key performance improvements in, and opportunities for, new communication scenarios.
... This is non-trivial to realize in practice. The authors of [10] have studied the coupled phase shift model of STAR-RIS, where only the phase shifts of refraction and reflection are coupled while the amplitudes are changed randomly. However, with the amplification function, the amplitude and phase shift for MF-RIS are coupled [11], where the amplitude and the cosine of phase shift satisfy a specific constraint. ...
... When the RIS is passive and lossless, the conservation of energy theory needs to be satisfied, which means that the energy of the incident signal is equal to the sum of the energies of the refracted and reflected signals, i.e., (β t m ) 2 +(β r m ) 2 = 1. In addition, the electric and magnetic impedances should be purely imaginary, i.e., ℜ(Z e,m ) = 0 and ℜ(Z m,m ) = 0 [10]. In this case, by introducing T m = β t m e θ t m and R m = β r m e θ r m , the amplitude and phase shift coefficients satisfy β r m β t m cos(θ t m −θ r m ) = 0. ...
... We set the total power budget P total = 20 dBm, where P BS = P RIS = P total /2 for the MF-RIS-aided networks, and P BS = P total for the passive RIS-aided networks. For the performance comparison, we consider the following four benchmarks: 1) ideal MF-RIS; 2) ideal STAR-RIS [1]; 3) coupled STAR-RIS [10]; 4) SF-RIS, where one M/2 refracting-only and one M/2 reflecting-only RIS are deployed adjacent to each other. Fig. 2 shows the sum-rate versus P total . ...
In this paper, a multi-functional reconfigurable intelligent surface (MF-RIS) assisted non-orthogonal multiple access multiuser network is investigated. In contrast to existing studies assuming that the amplitude and phase shift coefficients of the refraction and reflection can be adjusted independently, we propose a practical model for the MF-RIS, where the refraction and reflection coefficients are highly coupled. Then, we investigate a sum-rate maximization problem by jointly optimizing the transmit beamforming and MF-RIS coefficients, subject to the coupled amplitude and phase shift constraints. To address the formulated non-convex problem, we propose an efficient iterative algorithm based on the fractional programming theory. Finally, numerical results show that: 1) The coupled MF-RIS scheme is superior to the simultaneous transmitting and reflecting RIS (STAR-RIS) and single-functional RIS (SF-RIS) schemes under the same power budget; 2) The performance gap between the ideal MF-RIS and coupled MF-RIS decreases with the increase of the total power budget.
... In order to reduce the hardware complexity and the information exchange overhead between the BSR and STAR-RIS, in this work, we assume that all elements have the same energy-splitting coefficients, 2 In this paper, to characterize the maximum performance gain of the STAR-RIS, we consider an independent phase-shift model for the reflection and transmission coefficients. However, in practical implementations, particularly in the case of fully passive STAR-RIS, the phase-shift coefficients are coupled, leading to performance loss and complicating the reflection and transmission beamforming design [31]- [33]. Therefore, to fully evaluate the potentials of STAR-RIS under the practical limitations, the performance analysis of STAR-RIS-assisted BAC-NOMA system with coupled phase-shift model is worth investigating in the future. ...
... where R is independent of , and represents the ergodic capacity of the BSN . Using (33), the upper bound on the effective capacity of BSN R is given by ...
... Proof. Using (33), the upper bound on the effective capacity of BSN T is given by ...
... In order to reduce the hardware complexity and the information exchange overhead between the BSR and STAR-RIS, in this work, we assume that all elements have the same energy-splitting coefficients, 2 In this paper, to characterize the maximum performance gain of the STAR-RIS, we consider an independent phase-shift model for the reflection and transmission coefficients. However, in practical implementations, particularly in the case of fully passive STAR-RIS, the phase-shift coefficients are coupled, leading to performance loss and complicating the reflection and transmission beamforming design [31]- [33]. Therefore, to fully evaluate the potentials of STAR-RIS under the practical limitations, the performance analysis of STAR-RIS-assisted BAC-NOMA system with coupled phase-shift model is worth investigating in the future. ...
... where R is independent of , and represents the ergodic capacity of the BSN . Using (33), the upper bound on the effective capacity of BSN R is given by ...
... Proof. Using (33), the upper bound on the effective capacity of BSN T is given by ...
While targeting the energy-efficient connectivity of the Internet-of-things (IoT) devices in the sixth-generation (6G) networks, in this paper, we explore the integration of non-orthogonal multiple access-based backscatter communication (BAC-NOMA) and simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs). To this end, first, for the performance evaluation of the STAR-RIS-assisted BAC-NOMA system, we derive the statistical distribution of the channels under Nakagami-fading. Second, by leveraging the derived statistical channel distributions, we present the effective capacity analysis under the delay quality-of-service (QoS) constraint. In particular, we derive the closed-form expressions for the effective capacity of the reflecting and transmitting backscatter nodes (BSNs) under the energy-splitting protocol of STAR-RIS. To obtain more insight into the performance of the considered system, we provide the asymptotic analysis, and derive the upper bound on the effective capacity, which represents the ergodic capacity. Our simulation results validate the analytical analysis, and reveal the effectiveness of the STAR-RIS-assisted BAC-NOMA system over the conventional RIS (C-RIS)-and orthogonal multiple access (OMA)-based counterparts. Finally, to highlight the trade-off between the effective capacity and energy consumption, we analyze the link-layer energy efficiency. Overall, this paper provides useful guidelines for the performance analysis and design of the STAR-RIS-assisted BAC-NOMA systems. Index Terms-Backscatter communication (BackCom), non-orthogonal multiple access (NOMA), simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS), effective capacity.
... Moreover, for STARS with passive-lossless elements, the phase shifts of the T&R coefficients are coupled. In [209], a correlated T&R phase-shift model was proposed for STARS. 2) STARS enhanced NOMA: Compared to conventional transmitting/reflecting-only RISs, STARS provides more DoFs for facilitating the T&R NOMA. ...
... Such a T&R NOMA operation addresses the issue when employing transmitting/reflecting-only RISs in NOMA, where the users surrounding the transmitting/reflecting-only RIS might have a similar channel condition. To further illustrate the benefits of T&R NOMA, Fig. 9b depicts the outage probabilities for STAR-NOMA and STAR-OMA users [209]. It can be observed that with the aid of STARS, NOMA significantly outperforms OMA for both users' outage probabilities. ...
... The STARS-NOMA system was first proposed in [210] where the achievable sum rate was maximized by jointly optimizing the decoding order, power allocation coefficients, active beamforming, T&R beamforming. In terms of performance analysis of the STAR-NOMA network, various studies have been conducted to evaluate the bit error rate (BER) performance [211], the impact of the correlated T&R phase-shift [209], the ergodic rate for users over Rician fading channels [212], the outage probability of users over spatially correlated channels [213], and the performance of an uplink STARS-NOMA network [214]. Apart from these studies, recent works focused on STARS-NOMA networks with the more complex distribution of users and scatters in the environment. ...
Wireless communication systems to date primarily rely on the orthogonality of resources to facilitate the design and implementation, from user access to data transmission. Emerging applications and scenarios in the sixth generation (6G) wireless systems will require massive connectivity and transmission of a deluge of data, which calls for more flexibility in the design concept that goes beyond orthogonality. Furthermore, recent advances in signal processing and learning have attracted considerable attention, as they provide promising approaches to various complex and previously intractable problems of signal processing in many fields. This article provides an overview of research efforts to date in the field of signal processing and learning for next-generation multiple access, with an emphasis on massive random access and non-orthogonal multiple access. The promising interplay with new technologies and the challenges in learning-based NGMA are discussed.
... We make realistic assumptions regarding RIS (either regular or STAR) by modeling the small-scale and large-scale fading and considering three different feasibility sets based on the models in [7], [41]- [44]. We propose three main approaches to optimize reflecting/transmitting coefficients in STAR-RIS. ...
... In a multicell BC, intercell interference may highly degrade the system performance especially for cell-edge users, which should be handled by a joint optimization of transmit parameters at BSs. We assume perfect, global and instantaneous channel state information (CSI) at all transceivers similar to many other works on RIS (either STAR or regular) such as [27], [29]- [31], [33], [34], [36], [44], [62]- [66]. Additionally, it should be noted that in this work, we focus on resource allocation for URLLC systems in which it is common to assume perfect, global and instantaneous CSI [23], [24], [58], [67]- [70]. ...
... Here, we briefly describe the model here and refer the readers to [33], [37], [7, Sec. II] or [44] for a detailed description/review on the features of RIS and/or STAR-RIS. ...
This paper proposes a general optimization framework to improve the spectral and energy efficiency (EE) of ultra-reliable low-latency communication (URLLC) simultaneous-transfer-and-receive (STAR) reconfigurable intelligent surface (RIS)-assisted interference-limited systems with finite block length (FBL). This framework can solve a large variety of optimization problems in which the objective and/or constraints are linear functions of the rates and/or EE of users. Additionally, the framework can be applied to any interference-limited system with treating interference as noise as the decoding strategy at receivers. We consider a multi-cell broadcast channel as an example and show how this framework can be specialized to solve the minimum-weighted rate, weighted sum rate, global EE and weighted EE of the system. We make realistic assumptions regarding the (STAR-)RIS by considering three different feasibility sets for the components of either regular RIS or STAR-RIS. Our results show that RIS can substantially increase the spectral and EE of URLLC systems if the reflecting coefficients are properly optimized. Moreover, we consider different three different transmission strategies for STAR-RIS as energy splitting (ES), mode switching (MS), and time switching (TS). We show that STAR-RIS can outperform a regular RIS when the regular RIS cannot cover all the users. Furthermore, it is shown that the ES scheme outperforms the MS and TS schemes.
... Indeed, in the case of a passive and lossless IOS, the difference between the phase value in transmission and reflection must fulfill one condition and therefore, they cannot be independently adjusted. In [6], some practical phase-shift configurations for IOS communications were developed by taking this electromagnetic constraint into account. Unfortunately, this constraint often hinders the design of IOS elements to achieve a balanced amplitude with independent phase shift for the transmission and reflection (T&R) modes. ...
... The main states are: linear polarizations (vertical, horizontal and slant) and circular polarizations. To the best of our knowledge, polarization states are overlooked in the IOS communications literature [6]. This limits the potential of IOSs at the electromagnetic level since the T&R coefficients are intimately linked and, therefore, T&R waves are closely coupled. ...
... Typically, IOSs used for modeling communication scenarios are passive and lossless. This leads to the fact that the difference between the reflected and transmitted phase values for each IOS element must be ±90 o [6]. Now, when considering polarized elements for the IOS, this constraint is removed and there is more flexibility in adjusting these phase values to obtain the desired passive beamforming pattern in the full space. ...
p>Wireless communications based on reconfigurable
intelligent surfaces (RIS) have upsurged as a key research topic in the communication-theoretic arena. Recently, the RIS concept has evolved into the so-called simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOS, or simply IOS), which extend the RIS functionality by incorporating transmission (in addition to reflection) capabilities. To realize STAR-IOS, full and independent reconfiguration capabilities for both reflected and transmitted waves are crucially required. However, such full independent reconfiguration has thus far been hurdled by the intimate coupling between the transmission and reflection behaviour of the IOS elements. To overcome this challenge and realize the full potential of reconfigurable IOS-aided systems, we here advocate for the use of the polarization domain in the design and operation of STAR-IOS. Thanks to the polarization- dependent features of IOS elements, fully independent (reconfigurable) transmission and reflection modes can be delivered, thus bringing key performance improvements and opportunities for new communication scenarios.
[Comment: This work has been accepted in IEEE Communications Magazine]</p
... Indeed, both transceivers should be in the reflection space of a regular RIS so that it can modulate the channel. To address this issue, simultaneously reflect and transmit RIS (STAR-RIS) is proposed that can reflect and transmit simultaneously, which provides a full 360 • coverage [12]- [16]. Thus, STAR-RIS may be able to support a wider range of applications compared to a regular RIS. ...
... In the sets T U and T I , the phases of θ r mi and θ t mi can be independently optimized. In another model, not only the amplitudes of θ r mi and θ t mi are dependent, but also their phases are related as in the following set [12], [16,Lemma 1] ...
... where {Θ} = {Θ r ,Θ t } is given by (11). The convergence of the proposed scheme is ensured by (12). ...
This paper proposes an energy-efficient scheme for multicell multiple-input, multiple-output (MIMO) simultaneous transmit and reflect (STAR) reconfigurable intelligent surfaces (RIS)-assisted broadcast channels by employing rate splitting (RS) and improper Gaussian signaling (IGS). Regular RISs can only reflect signals. Thus, a regular RIS can assist only when the transmitter and receiver are in the reflection space of the RIS. However, a STAR-RIS can simultaneously transmit and reflect, thus providing a 360-degrees coverage. In this paper, we assume that transceivers may suffer from I/Q imbalance (IQI). To compensate for IQI, we employ IGS. Moreover, we employ RS to manage intracell interference. We show that RIS can significantly improve the energy efficiency (EE) of the system when RIS components are carefully optimized. Additionally, we show that STAR-RIS can significantly outperform a regular RIS when the regular RIS cannot cover all the users. We also show that RS can highly increase the EE comparing to treating interference as noise.
... While, the independent information is transmitted to each PU for the unicasting signal model. For each model, we aim to minimize the transmit power at the BS by optimizing the active beamforming at the BS and the reflection and transmission coefficients at the STAR-RIS under the practical phase correlation constraints [35]. Unlike [27]- [34], the implementation of STAR-RIS results in the joint design of reflection and transmission coefficients, and thus the methods proposed in [27]- [34] are inappropriate for the considered problems in this work. ...
... θ r m ∈ [0, 2π) and θ t m ∈ [0, 2π) are the corresponding phase shifts of the m-th element. According to [35] and [38], it is known that the reflection coefficients and transmission coefficients of the STAR-RIS need to satisfy the following constraints ...
... H q,i ), andχ q = Tr(H q W ). Then, the left-hand sides of the constraints C13 − C15 are respectively recast as (35) and (36) are all non-convex, which results in the challenge of solving P6.2. To transform P6.2 into a convex one, we also exploit the lower-bounds of (34), (35) and (36), which are respectively given by ...
In this paper, we propose a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) empowered transmission scheme for symbiotic radio (SR) systems to make more flexibility for network deployment and enhance system performance. The STAR-RIS is utilized to not only beam the primary signals from the base station (BS) towards multiple primary users on the same side of the STAR-RIS, but also achieve the secondary transmission to the secondary users on another side. We consider both the broadcasting signal model and unicasting signal model at the BS. For each model, we aim for minimizing the transmit power of the BS by designing the active beamforming and simultaneous reflection and transmission coefficients under the practical phase correlation constraint. To address the challenge of solving the formulated problem, we propose a block coordinate descent based algorithm with the semidefinite relaxation, penalty dual decomposition and successive convex approximation methods, which decomposes the original problem into one sub-problem about active beamforming and the other sub-problem about simultaneous reflection and transmission coefficients, and iteratively solve them until the convergence is achieved. Numerical results indicate that the proposed scheme can reduce up to 150.6% transmit power compared to the backscattering device enabled scheme.
... Indeed, in the case of a passive and lossless IOS, the phases in transmission and reflection operation cannot be independently adjusted. In [6], some practical phase-shift configurations for IOS communications were developed by taking this electromagnetic constraint into account. Unfortunately, this constraint often hinders the design of IOS elements to achieve a balanced amplitude with independent phase shift for the transmission and reflection (T&R) modes. ...
... The main states are: linear polarizations (vertical, horizontal and slant) and circular polarizations. To the best of our knowledge, polarization states are overlooked in the IOS communications literature [6]. This limits the potential of IOSs at the electromagnetic level since the T&R coefficients are intimately linked and, therefore, T&R waves are closely coupled. ...
... Typically, IOSs used for modeling communication scenarios are passive and lossless. This leads to the fact that the difference between the reflected and transmitted phase values for each IOS element must be ±90 o [6]. Now, when considering polarized elements for the IOS, this constraint is removed and there is more flexibility in adjusting these phase values to obtain the desired passive beamforming pattern in the full space. ...
p>Wireless communications based on reconfigurable intelligent surfaces (RIS) have upsurged as a key research topic in the communication-theoretic arena. Recently, the RIS concept has evolved into the so-called simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOS, or simply IOS), which extend the RIS functionality by incorporating transmission (in addition to reflection) capabilities. To realize STAR-IOS, full and independent reconfiguration capabilities for both reflected and transmitted waves are crucially required.
However, such full independent reconfiguration has thus far been hurdled by the intimate coupling between the transmission and reflection behaviour of the IOS elements. To overcome this challenge and realize the full potential of reconfigurable IOS-aided systems, we here advocate for the use of the polarization domain in the design and operation of STAR-IOS. Thanks to the polarization-dependent features of IOS elements, fully independent (reconfigurable) transmission and reflection modes can be delivered, thus bringing key performance improvements and opportunities for new communication scenarios. We here discuss the principles, benefits, and potential implementations of the proposed polarization-aided IOS, along with some key opportunities and associated challenges.
[Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible]</p
... To employ a regular RIS, both the transmitter and receiver should be in the reflection space of the regular RIS, which may restrict its applicability. To address this issue, simultaneously-reflectand-transmit (STAR-) RISs can be employed, which provide a 360 • coverage [20]- [25]. In a STAR-RIS, each component can not only transmit, but also can reflect at the same time. ...
... Then the other steps remain the same. Specifically, to obtain new {Θ}, we should solve (18) for T U , (20) for T I , and (21) for T N . Then we should normalize the solutions for the feasibility sets T I and T N according to (22) and update {Θ} based on (23). ...
This paper proposes schemes to improve the spectral efficiency of a multiple-input multiple-output (MIMO) broadcast channel (BC) with I/Q imbalance (IQI) at transceivers by employing a combination of improper Gaussian signaling (IGS), non-orthogonal multiple access (NOMA) and simultaneously transmit and reflect (STAR) reconfigurable intelligent surface (RIS). When there exists IQI, the output RF signal is a widely linear transformation of the input signal, which may make the output signal improper. To compensate for IQI, we employ IGS, thus generating a transmit improper signal. We show that IGS alongside with NOMA can highly increase the minimum rate of the users. Moreover, we propose schemes for different operational modes of STAR-RIS and show that STAR-RIS can significantly improve the system performance. Additionally, we show that IQI can highly degrade the performance especially if it is overlooked in the design.
... Besides, the channels are modeled as G ≜ √ l brḠ ∈ C N ×Nt and h H u k ≜ l ru kh H u k ∈ C 1×N , where l xy is the large-scale path loss of the link x-to-y and xy ∈ {br, ru k } denotes the direction of the link from x to y. Furthermore, the small-scale fading coefficientsḠ ∈ C N ×Nt andh H u k ∈ C 1×Nt are modeled as Rician fading channels [29], [48], given bȳ ...
... Now, we can conclude that the solution of problem (70) satisfies the KKT conditions of problem (64). Besides, we employ the majorization-minimization algorithm [69] to approximate problem (70) as a sequence of convex optimization problems, i.e., problem (29). Since our proposed Algorithm 1 satisfies the conditions stated in [69,Eqs (2) and (3)], we can conclude that the converged solution of Algorithm 1 also satisfies the KKT conditions of problem (64). ...
Considering a simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS)-aided dual-functional radar-communications (DFRC) system, this paper proposes a symbol-level precoding-based scheme for concurrent securing confidential information transmission and performing target sensing, where the public signals intended for multiple unclassified users are exploited to deceive the multiple potential malicious radar targets. Specifically, the STAR-RIS-aided DFRC system design is formulated as a joint optimization problem that determines the transmission waveform signal, the transmission and reflection coefficients of STAR-RIS. The objective is to maximize the average received radar sensing power subject to the quality-of-service constraints for multiple communication users, the security constraint for multiple potential eavesdroppers, as well as various practical waveform design restrictions. However, the formulated problem is challenging to handle due to its nonconvexity. Furthermore, the high dimensionality of the optimization variables also renders existing optimization algorithms inefficient. To address these issues, we propose a distance-majorization induced low-complexity algorithm to obtain an efficient solution, which converts the nonconvex joint design problem into a sequence of subproblems that can be solved in closed-form, relieving the required high computational burden of the conventional approaches, e.g., the interior point method. Simulation results confirm the effectiveness of the STAR-RIS in improving the DFRC performance. Besides, by comparing with the state-of-the-art alternating direction method of multipliers (ADMM) algorithm, simulation results validate the efficiency of our proposed optimization algorithm and show that it enjoys excellent scalability for different number of T-R elements equipped at the STAR-RIS.
... Here, 'lossless' means that there is no additional energy loss during transmission and reflection, i.e., β T m + β R m = 1. Under this constraint, it can be shown that the transmission phase shift, θ T m , and reflection phase shift, θ R m , are coupled subject to specific values of phase-shift differences as follows [23]: ...
The ultimate goal of next generation multiple access (NGMA) is to support massive terminals and facilitate multiple functionalities over the limited radio resources of wireless networks in the most efficient manner possible. However, the random and uncontrollable wireless radio environment is a major obstacle to realizing this NGMA vision. Given the prominent feature of achieving 360{\deg} smart radio environment, simultaneously transmitting and reflecting surfaces (STARS) are emerging as one key enabling technology among the family of reconfigurable intelligent surfaces for NGMA. This paper provides a comprehensive overview of the recent research progress of STARS, focusing on fundamentals, performance analysis, and full-space beamforming design, as well as promising employments of STARS in NGMA. In particular, we first introduce the basics of STARS by elaborating on the foundational principles and operating protocols as well as discussing different STARS categories and prototypes. Moreover, we systematically survey the existing performance analysis and beamforming design for STARS-aided wireless communications in terms of diverse objectives and different mathematical approaches. Given the superiority of STARS, we further discuss advanced STARS applications as well as the attractive interplay between STARS and other emerging techniques to motivate future works for realizing efficient NGMA.
... Thus, with a minor adaptation, the proposed scheme of RIS assignment and element allocation can still be worked. of energy conservation, leading to correlations between T&R coefficients [42]. ...
The integration of reconfigurable intelligent surfaces (RISs) and grant-free non-orthogonal multiple access (GF-NOMA) has emerged as a promising solution for enhancing spectral efficiency and massive connectivity in future wireless networks. This paper proposes a GF-NOMA communication network enabled by simultaneously transmitting and reflecting RISs (STAR-RIS). In the proposed GF-NOMA, all user equipments (UEs) have instantaneous access to resource blocks (RBs) without the need for grant acquisition and power control as in the traditional grant-based NOMA schemes. Specifically, we have considered two regimes of interest: 1) the max-min fair (MMF) regime and 2) the max-sum throughput (MST) regime. To achieve the required power disparity, a two-level power control mechanism is proposed; initially, the UEs are clustered according to their channel gains. Additionally, we introduce a multi-level GF-NOMA (MGF-NOMA) scheme that adjusts the transmit power levels for each UE in the cluster. The second level of power disparity is achieved through the assignment of STAR-RISs to the clusters and optimal partitioning of STAR-RIS to support each of the cluster members. Specifically, we have also derived the closed-form equations for the optimal partitioning of STAR-RIS within the clusters for both regimes of interest. Simulation results demonstrate that the proposed STAR-RIS-aided MGF-NOMA yields a gain of 60% and 20% in the MST regime with active and passive RIS realization, respectively. Furthermore, the active and passive RIS-based MGF-NOMA achieve nearly the equivalent fairness that can be obtained through optimal power control in the MMF regime. The finding emphasizes the potential of integrating STAR-RIS with GF-NOMA as a robust and promising solution for future wireless communication systems.
... Moreover, we have also examine the performance of the phase-shift coupled STAR-RIS design, i.e., ES-PC-STAR-RIS-SPC with all other schemes. The results in Fig. 5 demonstrate that the consideration of phase-shift coupled design perform slightly inferior to the ideally ES mode and achieves a significant performance gain over the conventional RIS as validated in existing research works [29], [56], [57]. Fig. 7 shows the performance behavior of the STAR-RISaided NOMA system with counterpart conventional schemes. ...
Next-generation wireless applications are expected to enable extended ultra-reliable low-latency communication (URLLC) to support high data rates along with ultra-reliability and low-latency features beyond the capabilities of existing core services. There is a need to transition from conventional architectures to more efficient and robust multiple-access schemes to meet these consolidated requirements in resource-constrained systems. This study explores the utilization of simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) in non-orthogonal multiple access (NOMA) systems to enable spectrally efficient URLLC, even under the imperfect channel state information. In particular, we focus on maximizing spectral efficiency by jointly designing robust beamforming at the base station and STAR-RIS subject to given URLLC requirements. Due to the non-convexity of the formulated problem, we propose an alternating optimization framework that obtains sub-optimal solutions to the problems of beamforming design at the BS and STAR-RIS, respectively by exploiting
S
-procedure and successive convex approximation. Simulation results confirm that the STAR-RIS-NOMA system can significantly boost the spectral efficiency by 10-15% compared to conventional reflecting-only RIS while guaranteeing the strict URLLC requirements. Specifically, among all the possible modes of STAR-RIS, the time-splitting mode provides better spectral efficiency than other modes owing to its better interference management.
... As pointed out by Zhu et al. (2014), for STAR-RISs using passive lossless materials, the corresponding electric impedance and magnetic impedance should be purely imaginary numbers. Under this constraint, transmission phase shift (φ t m ) and reflection phase shift (φ r m ) are coupled subject to specific values of phase-shift differences (Zhu et al., 2014;Xu JQ et al., 2022b): ...
Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been attracting significant attention in both academia and industry for their advantages of achieving 360° coverage and enhanced degrees-of-freedom. This article first identifies the fundamentals of STAR-RIS, by discussing the hardware models, channel models, and signal models. Then, three representative categorizing approaches for STAR-RISs are introduced from the phase-shift, directional, and energy consumption perspectives. Furthermore, the beamforming design of STAR-RISs is investigated for both independent and coupled phase-shift cases. As a recent advance, a general optimization framework, which has high compatibility and provable optimality regardless of the application scenarios, is proposed. As a further advance, several promising applications are discussed to demonstrate the potential benefits of applying STAR-RISs in sixth-generation wireless communication. Lastly, a few future directions and research opportunities are highlighted.
... where β is the attenuation factor of 1 m [22], ϱ ij ∈ [0, 2π] is the angle for reflecting signals. ...
This paper investigates a deep learning-based algorithm to optimize the unmanned aerial vehicle (UAV) trajectory and reconfigurable intelligent surface (RIS) reflection coefficients in UAV-RIS-aided cell-free (CF) hybrid non-orthogonal multiple-access (NOMA)/orthogonal multiple-access (OMA) networks. The practical RIS reflection model and user grouping optimization are considered in the proposed network. A double cascade correlation network (DCCN) is proposed to optimize the RIS reflection coefficients , and based on the results from DCCN, an inverse-variance deep reinforcement learning (IV-DRL) algorithm is introduced to address the UAV trajectory optimization problem. Simulation results show that the proposed algorithms significantly improve the performance in UAV-RIS-assisted CF networks.
... To achieve this goal, one solution is to rely on new physical layer technologies. Recently, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been proposed [3][4][5]. ...
Recently, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have received significant research interest. The employment of large STAR-RIS and high-frequency signaling inevitably make the near-field propagation dominant in wireless communications. In this work, a STAR-RIS aided near-field multiple-input multiple-multiple (MIMO) communication framework is proposed. A weighted sum rate maximization problem for the joint optimization of the active beamforming at the base station (BS) and the transmission/reflection-coefficients (TRCs) at the STAR-RIS is formulated. The non-convex problem is solved by a block coordinate descent (BCD)-based algorithm. In particular, under given STAR-RIS TRCs, the optimal active beamforming matrices are obtained by solving a convex quadratically constrained quadratic program. For given active beamforming matrices, two algorithms are suggested for optimizing the STAR-RIS TRCs: a penalty-based iterative (PEN) algorithm and an element-wise iterative (ELE) algorithm. The latter algorithm is conceived for STAR-RISs with a large number of elements. Numerical results illustrate that: i) near-field beamforming for STAR-RIS aided MIMO communications significantly improves the achieved weighted sum rate compared with far-field beamforming; ii) the near-field channels facilitated by the STAR-RIS provide enhanced degrees-of-freedom and accessibility for the multi-user MIMO system; and iii) the BCD-PEN algorithm achieves better performance than the BCD-ELE algorithm, while the latter has a significantly lower computational complexity.
... • Coupled Phase-Shift Model: As pointed out by [42], for STAR-RISs using passive lossless materials, the corresponding electric impedance and magnetic impedances should be purely imaginary numbers. Under this constraint, transmission phase shift (φ t m ) and reflection phase shift (φ r m ) are coupled subject to specific values of phaseshift differences as follows [42], [43]. ...
Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been attracting significant attention in both academia and industry for their advantages of achieving 360{\deg} coverage and enhanced degrees of freedom. This article first identifies the fundamentals of STAR-RIS, by discussing the hardware models, channel models, and signal models. Then, three representative categorizing approaches for STAR-RIS are introduced from phase-shift, directional, and energy consumption perspectives. Furthermore, the beamforming design of STAR-RIS is investigated for both independent and coupled phase-shift cases. A general optimization framework is proposed as the recent advances, which has high compatibility and provable optimality regardless of the application scenarios. As a further advance, several promising applications are discussed to demonstrate the potential benefits of applying STAR-RIS in the sixth-generation wireless network. Lastly, a few future directions and research opportunities are highlighted for motivating future work.
... However, the conventional reflecting-only RIS requires that the transceivers be on the same side of the RIS, which is limited in practical communication systems. Recently, the simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) can serve users on both sides of the RIS and achieve 360 • full-space coverage of wireless signals [12,13]. The incident signal is reflected to the reflection area via the STAR-RIS and simultaneously refracted to the transmission area, realizing widespread coverage and more flexible network deployments. ...
This paper explores the potential application of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR‐RIS) in improving the physical layer security (PLS) of millimetre wave (mmWave) non‐orthogonal multiple access (NOMA) uplink communications. In particular, the legitimate users on both sides of STAR‐RIS send confidential information simultaneously to the base station (BS) while keeping it secret from the eavesdroppers near the BS by exploiting the STAR‐RIS to proactively regulate the electromagnetic propagation environment. By jointly designing the transmit power, the active beamforming at the BS, and the reflecting/transmitting coefficients at the STAR‐RIS, the authors' goal is to maximize the minimum secrecy capacity subject to the successive interference cancellation decoding order constraints. Due to the non‐convexity of the formulated problem, an efficient algorithm is proposed by capitalizing on alternating optimization, a penalty‐based approach, successive convex approximation, and semi‐definite relaxation. Firstly, a two‐layer iterative algorithm based on the penalty for the joint beamforming optimization sub‐problem is proposed. Then, the non‐convex problem is transformed into a convex positive semi‐definite programming problem for the transmit power sub‐problem. Numerical simulation results reveal that the STAR‐RIS‐aided system has more tremendous advantages and effectiveness in improving the PLS of mmWave NOMA uplink communication compared with the benchmark schemes.
Device-to-device (D2D) communication offers significant potential for future wireless networks but faces challenges such as limited range, signal blockage, and interference. Reconfigurable intelligent surfaces (RISs) can mitigate these issues by dynamically controlling signal reflections. However, existing RIS-assisted D2D systems often rely on impractical infinite-resolution phase shifters and achieve limited coverage. This paper addresses these limitations by proposing a novel D2D communication system using simultaneous transmitting and reflecting RISs (STAR-RISs) with coupled and low-resolution phase shifters for cost-effective and full-space coverage D2D communication. We further introduce irregular STAR-RIS configurations where a subset of elements is strategically activated for enhanced spatial diversity. To optimize this system, a unified cross-entropy optimization (CEO) framework is developed for joint optimization of element selection (for irregular configurations) and the continuous amplitudes for transmission and reflection, along with the discrete phase shifts. Simulation results reveal that the proposed CEO-based algorithm achieves significantly higher sum rates compared to the benchmark algorithms. Furthermore, irregular STAR-RIS configurations provide additional gains in both sum rate and energy efficiency.
Wireless networks are increasingly relying on machine learning (ML) paradigms to provide various services at the user level. Yet, it remains impractical for users to offload their collected data set to a cloud server for centrally training their local ML model. Federated learning (FL), which aims to collaboratively train a global ML model by leveraging the distributed wireless computation resources across users without exchanging their local information, is therefore deemed as a promising solution for enabling intelligent wireless networks in the data-driven society of the future. Recently, reconfigurable intelligent metasurfaces (RIMs) have emerged as a revolutionary technology, offering a controllable means for increasing signal diversity and reshaping transmission channels, without implementation constraints traditionally associated with multi-antenna systems. In this paper, we present a comprehensive survey of recent works on the applications of FL to RIM-aided communications. We first review the fundamental basis of FL with an emphasis on distributed learning mechanisms, as well as the operating principles of RIMs, including tuning mechanisms, operation modes, and deployment options. We then proceed with an in-depth survey of literature on FL-based approaches recently proposed for the solution of three key interrelated problems in RIM-aided wireless networks, namely: channel estimation (CE), passive beamforming (PBF) and resource allocation (RA). In each case, we illustrate the discussion by introducing an expanded FL (EFL) framework in which only a subset of active users partake in the distributed training process, thereby allowing to reduce transmission overhead. Lastly, we discuss some current challenges and promising research avenues for leveraging the full potential of FL in future RIM-aided extremely large-scale multiple-input-multiple-output (XL-MIMO) networks.
The achievable rate performance of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided massive multiple-input multiple-output (MIMO) systems is investigated. Specifically, the achievable user rates are derived for three operating protocols of the STAR-RIS, namely the energy-splitting (ES), mode-switching (MS), and time-switching (TS) with both unicast and multicast transmissions. This analysis is useful in evaluating the system performance under imperfectly estimated channel state information (CSI), spatially correlated fading, pilot contamination, and statistical CSI based phase-shift optimization, transmit power control, and user signal decoding. For the high signal-to-noise ratio regime, the asymptotic achievable rates are also derived, and they serve as benchmarks or upper bounds for the rate performance comparisons for systems operating under the above transmission impediments. The composite uplink channels are estimated through linear minimum mean square error estimation techniques, and the phase-shift matrices at the STAR-RIS are optimized to maximize the effective average channel gains to minimize the channel estimation overhead. The base-station optimizes the transmit power based on the max-min criterion to attain a system-wide common user rate by negating the near-far effects of the downlink composite channels. Our numerical and simulation results validate our theoretical analysis and convergence of phase-shift and transmit power optimization algorithms. Our analytical and simulation results are useful in investigating the performance gains/comparisons among the ES, MS, and TS protocols for unicast and multicast transmissions to enable 360° smart coverage extensions with passive STAR-RIS aided massive MIMO.
Recent research in the wireless communications field has focused on the reconfigurable intelligent surface (RIS), which can enhance energy and spectrum efficiency by reconfiguring radio waves to propagate in a specific direction. In this study, we applied the simultaneous transmit and reflect (STAR)-RIS to an underwater wireless optical communication (UWOC) system. In contrast to a conventional RIS, a STAR-RIS allows multiple users to transfer data simultaneously in all directions by obtaining a UWOC channel despite underwater turbulence, beam attenuation, blockage, and pointing errors. Underwater turbulence-induced fading was obtained by using exponential and generalized Gamma distributions, and numerical analysis were performed to evaluate the outage probability, bit error rate (BER), and channel capacity of a direct UWOC channel, conventional RIS assisted UWOC channel, and STAR-RIS assisted UWOC channel. The BER performance was also evaluated according to the number of reflective elements, type of modulation scheme, impact of pointing errors, blockage levels, and varying the transmit-reflection coefficient. Monte-Carlo simulations were performed to validate the analytical results for the BER and outage probability with respect to the average signal-to-noise ratio. Experiments were performed to demonstrate the received luminous intensity performance with respect to the incident beam luminous intensity of the proposed STAR-RIS assisted UWOC channel.
In this work, we investigate the effect of phase noise in downlink simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided massive multiple-input multiple-output (mMIMO) systems, where users are located on both sides of the STAR-RIS panel. We assume that users only know the knowledge of statistical channel state information (CSI), and account for correlated Rayleigh fading to meet realistic conditions. The use-and-then-forget (UatF) bound of the sum-rate has been derived by assuming maximum ratio transmission (MRT) precoding. Notably, a projected gradient ascent method (PGAM) algorithm has been used to optimize the amplitudes and phase shifts of the STAR-RIS panel simultaneously. Our simulations show that the sum-rate can be improved as the noise concentration parameter of the Von Mises (VM) distribution increases.
This paper investigates the performance of downlink simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted cell-free (CF) massive multiple-input multiple-output (mMIMO) systems, where user equipments (UEs) are located on both sides of the RIS. We account for correlated Rayleigh fading and multiple antennas per access point (AP), while the maximum ratio (MR) beamforming is applied for the design of the active beamforming in terms of instantaneous channel state information (CSI). Firstly, we rely on an aggregated channel estimation approach that reduces the overhead required for channel estimation while providing sufficient information for data processing. We obtain the normalized mean square error (NMSE) of the channel estimate per AP, and design the passive beamforming (PB) of the surface based on the long-time statistical CSI. Next, we derive the received signal in the asymptotic regime of numbers of APs and surface elements. Then, we obtain a closed-form expression of the downlink achievable rate for arbitrary numbers of APs and STAR-RIS elements under statistical CSI. Finally, based on the derived expressions, the numerical results show the feasibility and the advantages of deploying a STAR-RIS into conventional CF mMIMO systems. In particular, we theoretically analyze the properties of STAR-RIS-assisted CF mMIMO systems and reveal explicit insights in terms of the impact of channel correlation, the number of surface elements, and the pilot contamination on the achievable rate.
This letter considers a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided non-orthogonal multiple access (NOMA) system in the presence of unbalanced transmission and reflection (T&R) users. It takes into account the correlated T&R phase shifts and proposes various phase shift configuration strategies, namely blind phase shifting (BPS), STAR-RIS partitioning (SPS), secondary user STAR-RIS partitioning (SSPS), and near-field broadcasting (NBS). The ergodic sum rate is then analyzed under the mentioned strategies. Numerical and simulation results demonstrate the NBS strategy’s superiority over other strategies. However, its effectiveness is constrained by the distance and size of the STAR-RIS, making it more suitable for short-range applications. The results also reveal that the SPS strategy outperforms the BPS and SSPS strategies in low-power scenarios and with limited STAR-RIS elements allocated for the secondary user. The SSPS strategy shows substantial improvement in high-power scenarios, making it well-suited for systems such as millimeter-wave and terahertz wireless communications.
In this article, we propose a novel simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted nonorthogonal multiple access (NOMA) system. Unlike most of the STAR-RIS-assisted NOMA works, we target scalable phase shift design for our proposed system that requires a reduced channel estimation overhead. Within this perspective, we propose novel algorithms to partition the STAR-RIS surface among the available users. These algorithms aim to determine the proper number of transmitting/reflecting elements needs to be assigned to each user in order to maximize the system sum rate while guaranteeing the quality-of-service requirements for individual users. For the proposed system, we derive closed-form analytical expressions for the outage probability (OP) and its corresponding asymptotic behavior under different user deployments. Finally, Monte Carlo simulations are performed in order to verify the correctness of the theoretical analysis. It is shown that the proposed system outperforms the classical NOMA and orthogonal multiple access systems in terms of OP and sum rate, under spatial correlation and phase errors.
In this paper, we propose a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) empowered transmission scheme for symbiotic radio (SR) systems to make more flexibility for network deployment and enhance system performance. The STAR-RIS is utilized to not only beam the primary signals from the base station (BS) towards multiple primary users on the same side of the STAR-RIS, but also achieve the secondary transmission to the secondary users on another side. We consider both the broadcasting signal model and unicasting signal model at the BS. For each model, we aim for minimizing the transmit power of the BS by designing the active beamforming and simultaneous reflection and transmission coefficients under the practical phase correlation constraint. To address the challenge of solving the formulated problem, we propose a block coordinate descent based algorithm with the semidefinite relaxation, penalty dual decomposition and successive convex approximation methods, which decomposes the original problem into one sub-problem about active beamforming and the other sub-problem about simultaneous reflection and transmission coefficients, and iteratively solve them until the convergence is achieved. Numerical results indicate that the proposed scheme can reduce up to 150.6% transmit power compared to the backscattering device enabled scheme.
The reconfigurable intelligent surface (RIS) is a promising technology to provide smart radio environment. In contrast to the well-studied patch-array-based RISs, this work focuses on the metasurface-based RISs and simultaneously transmitting and reflecting (STAR)-RISs where the elements have millimeter or even molecular sizes. For these meticulous metasurface structures, near-field effects are dominant and a continuous electric current distribution should be adopted for capturing their electromagnetic response instead of discrete phase-shift matrices. Exploiting the electric current distribution, a Green’s function method based channel model is proposed. Based on the proposed model, performance analysis is carried out for both transmitting/reflecting-only RISs and STAR-RISs. 1) For the transmitting/reflecting-only RIS-aided single-user scenario, closed-formed expressions for the near-field/far-field boundary and the end-to-end channel gain are derived. Then, degrees-of-freedom (DoFs) and the power scaling laws are obtained. It is proved that the near-field channel exhibits higher DoFs than the far-field channel. It is also confirmed that when communication distance increases beyond the field boundary, the near-field power scaling law degrades to the well-known far-field result. 2) For the STAR-RIS-aided multi-user scenario, three practical STAR-RIS configuration strategies are proposed, namely power splitting (PS), selective element grouping (SEG), and random element grouping (REG) strategies. The channel gains for users are derived within both the pure near-field regime and the hybrid near-field and far-field regime. Finally, numerical results confirm that: 1) metasurface-based RISs are able to to outperform patch-array-based RISs, 2) the received power scales quadratically with the number of elements within the far-field regime and scales linearly within the near-field regime, and 3) for STAR-RISs, SEG has the highest near-field channel gain among the three proposed strategies and PS yields the highest DoFs for the near-field channel.
A hardware model for active simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) is proposed consisting of reflection-type amplifiers. The amplitude gains of the STAR element are derived for both coupled and independent phase-shift scenarios. Based on the proposed hardware model, an active STAR-RIS-aided two-user downlink communication system is investigated. Closed-form expressions are obtained for the outage probabilities of both the coupled and independent phase-shift scenarios. To obtain further insights, scaling laws and diversity orders are derived for both users. Analytical results confirm that active STAR-RIS achieves the same diversity orders as passive ones while their scaling laws are different. It is proved that average received SNRs scale with
M
and
M
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>
for active and passive STAR-RISs, respectively. Numerical results show that active STAR-RISs outperform passive STAR-RISs in terms of outage probability especially when the number of elements is small.
A simultaneously transmitting and reflecting surface (STARS) aided terahertz (THz) communication system is proposed. A novel power consumption model is proposed that depends on the type and resolution of the STARS elements. The spectral efficiency (SE) and energy efficiency (EE) are maximized in both narrowband and wideband THz systems by jointly optimizing the hybrid beamforming at the base station (BS) and the passive beamforming at the STARS. 1) For narrowband systems, independent phase-shift STARSs are investigated first. The resulting complex joint optimization problem is decoupled into a series of subproblems using penalty dual decomposition. Low-complexity element- wise algorithms are proposed to optimize the analog beamforming at the BS and the passive beamforming at the STARS. The proposed algorithm is then extended to the case of coupled phase-shift STARS. 2) For wideband systems, the spatial wideband effect at the BS and STARS leads to significant performance degradation due to the beam split issue. To address this, true time delayers (TTDs) are introduced into the conventional hybrid beamforming structure for facilitating wideband beamforming. An iterative algorithm based on the quasi-Newton method is proposed to design the coefficients of the TTDs. Finally, our numerical results confirm the superiority of the STARS over the conventional reconfigurable intelligent surface (RIS). It is also revealed that i) there is only a slight performance loss in terms of SE and EE caused by coupled phase shifts of the STARS in both narrowband and wideband systems, and ii) the conventional hybrid beamforming achieves comparable SE performance and much higher EE performance compared with the full-digital beamforming in narrowband systems but not in wideband systems, where the TTD-based hybrid beamforming is required for mitigating wideband beam split.
The recent development of metasurfaces, which may enable several use cases by modifying the propagation environment, is anticipated to substantially affect the performance of 6G wireless communications. Metasurface elements can produce passive sub-wavelength scattering to enable a smart radio environment. STAR-RIS, which refers to reconfigurable intelligent surfaces (RIS) that can transmit and reflect concurrently (STAR), is gaining popularity. In contrast to the widely studied RIS, which can only reflect the wireless signal and serve users on the same side as the transmitter, the STAR-RIS can reflect and refract (transmit), enabling 360-degree wireless coverage, thus serving users on both sides of the transmitter. This paper presents a comprehensive review of the STAR-RIS, focusing on the most recent schemes for diverse use cases in 6G networks, resource allocation, and performance evaluation. We begin by laying the foundation for RIS (passive, active, STAR-RIS), and then discuss the STAR-RIS protocols, advantages, and applications. In addition, we categorize the approaches within the domain of use scenarios, which include increasing coverage, enhancing physical layer security (PLS), maximizing sum rate, improving energy efficiency (EE), and reducing interference. Next, we will discuss the various strategies for resource allocation and measures for performance evaluation. We aimed to elaborate, compare, and evaluate the literature regarding setup, channel characteristics, methodology, and objectives. In conclusion, we examine this field’s open research problems and potential future prospects.
Given the rapid development of advanced electromagnetic manipulation technologies, researchers have turned their attentions to the investigation of smart surfaces for enhancing the radio coverage. Simultaneously transmitting and reflecting intelligent omni-surfaces (STAR-IOSs) constitute one of the most promising categories. Although previous research contributions have demonstrated the benefits of STAR-IOSs in terms of its wireless communication performance gains, several important issues remain unresolved, including both their practical hardware implementations and their accurate physical models. In this paper, we address these issues by discussing four practical hardware implementations of STAR-IOSs, as well as three hardware modeling techniques and five channel modeling methods. We clarify the taxonomy of the smart surface technologies in support of further investigating the family of STAR-IOSs.
A novel simultaneously transmitting and reflecting (STAR) system design relying on reconfigurable intelligent surfaces (RISs) is conceived. First, an existing prototype is reviewed and the potential benefits of STAR-RISs are discussed. Then, the key differences between conventional reflecting-only RISs and STAR-RISs are identified from the perspectives of hardware design, physics principles, and communication system design. Furthermore, the basic signal model of STAR-RISs is introduced, and three practical protocols are proposed for their operation, namely energy splitting, mode switching, and time switching. Based on the proposed protocols, a range of promising application scenarios are put forward for integrating STAR-RISs into next-generation wireless networks. By considering the downlink of a typical RIS-aided multiple-input single-output (MISO) system, numerical case studies are provided for revealing the superiority of STAR-RISs over other baselines, when employing the proposed protocols. Finally, several open research problems are discussed.
In this letter, we study efficient uplink channel estimation design for a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted two-user communication systems. We first consider the time switching (TS) protocol for STAR-RIS and propose an efficient scheme to separately estimate the channels of the two users with optimized training (transmission/reflection) pattern. Next, we consider the energy splitting (ES) protocol for STAR-RIS under the practical coupled phase-shift model and devise a customized scheme to simultaneously estimate the channels of both users. Although the problem of minimizing the resultant channel estimation error for the ES protocol is difficult to solve, we propose an efficient algorithm to obtain a high-quality solution by jointly designing the pilot sequences, power-splitting ratio, and training patterns. Numerical results show the effectiveness of the proposed designs and reveal that the STAR-RIS under the TS protocol achieves a smaller channel estimation error than the ES case.
The novel concept of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is investigated, where incident signals can be transmitted and reflected to users located at different sides of the surface. In particular, the fundamental coverage range of STAR-RIS aided two-user communication networks is studied. A sum coverage range maximization problem is formulated for both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA), where the resource allocation at the access point and the transmission and reflection coefficients at the STAR-RIS are jointly optimized to satisfy the communication requirements of users. For NOMA, we transform the non-convex decoding order constraint into a linear constraint and the resulting problem is convex, which can be optimally solved. For OMA, we first show that the optimization problem for given time/frequency resource allocation is convex. Then, we employ the one dimensional search-based algorithm to obtain the optimal solution. Numerical results reveal that the coverage can be significantly extended by the STAR-RIS compared with conventional RISs.
In this letter, simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) are studied. Compared with the conventional reflecting-only RISs, the coverage of STAR-RISs is extended to 360 degrees via simultaneous transmission and reflection. A general hardware model for STAR-RISs is presented. Then, channel models are proposed for the near-field and the far-field scenarios, based on which the diversity gain of the STAR-RISs is analyzed and compared with that of the conventional RISs. Numerical simulations are provided to verify analytical results and to demonstrate that full diversity order can be achieved on both sides of the STAR-RIS.
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.
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.
This paper investigates the physical layer security of non-orthogonal multiple access (NOMA) in large-scale networks with invoking stochastic geometry. Both single-antenna and multiple-antenna aided transmission scenarios are considered, where the base station (BS) communicates with randomly distributed NOMA users. In the single-antenna scenario, we adopt a protected zone around the BS to establish an eavesdropper-exclusion area with the aid of careful channel-ordering of the NOMA users. In the multiple-antenna scenario, artificial noise is generated at the BS for further improving the security of a beamforming-aided system. In order to characterize the secrecy performance, we derive new exact expressions of the security outage probability for both single-antenna and multiple-antenna aided scenarios. To obtain further insights, 1) for the single antenna scenario, we perform secrecy diversity order analysis of the selected user pair. The analytical results derived demonstrate that the secrecy diversity order is determined by the specific user having the worse channel condition among the selected user pair; and 2) for the multiple-antenna scenario, we derive the asymptotic secrecy outage probability, when the number of transmit antennas tends to infinity. Monte Carlo simulations are provided for verifying the analytical results derived and to show that: i)~The security performance of the NOMA networks can be improved by invoking the protected zone and by generating artificial noise at the BS; and ii)~The asymptotic secrecy outage probability is close to the exact secrecy outage probability.
Transmission and reflection are two fundamental properties of the electromagnetic wave propagation through obstacles. Full control of both the magnitude and phase of the transmission and reflection independently are important issue for free manipulation of electromagnetic wave propagation. Here we employed the equivalent principle, one fundamental theorem of electromagnetics, to analyze the required surface electric and magnetic impedances of a passive metasurface to produce either arbitrary transmission magnitude and phase or arbitrary reflection magnitude and phase. Based on the analysis, a tunable metasurface is proposed. It is shown that the transmission phase can be tuned by 360° with the unity transmissivity or the transmissivity can be tuned from 0 to 1 while the transmission phase is kept around 0°. The reflection magnitude and phase can also been tuned similarly with the proposed metasurface. The proposed design may have many potential applications, such as the dynamic EM beam forming and scanning.
The recent development of metasurfaces has motivated their potential use for improving the performance of wireless communication networks by manipulating the propagation environment through nearly passive sub-wavelength scattering elements arranged on a surface. However, most studies of this technology focus on reflective metasurfaces, that is, the surface reflects the incident signals toward receivers located on the same side of the transmitter, which restricts the coverage to one side of the surface. In this article, we introduce the concept of an intelligent omni-surface (IOS), which is able to serve mobile users on both sides of the surface to achieve full-dimensional communications by jointly engineering its reflective and refractive properties. The working principle of the IOS is introduced, and a novel hybrid beamforming scheme is proposed for IOS-based wireless communications. Moreover, we present a prototype of IOS-based wireless communications and report experimental results. Furthermore, potential applications of IOSs to wireless communications together with relevant research challenges are discussed.
The novel concept of simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) is investigated, where the incident wireless signal is divided into transmitted and reflected signals passing into both sides of the space surrounding the surface, thus facilitating a
full-space
manipulation of signal propagation. Based on the introduced basic signal model of ‘STAR’, three practical operating protocols for STAR-RISs are proposed, namely energy splitting (ES), mode switching (MS), and time switching (TS). Moreover, a STAR-RIS aided downlink communication system is considered for both unicast and multicast transmission, where a multi-antenna base station (BS) sends information to two users, i.e., one on each side of the STAR-RIS. A power consumption minimization problem for the joint optimization of the active beamforming at the BS and the passive transmission and reflection beamforming at the STAR-RIS is formulated for each of the proposed operating protocols, subject to communication rate constraints of the users. For ES, the resulting highly-coupled non-convex optimization problem is solved by an iterative algorithm, which exploits the penalty method and successive convex approximation. Then, the proposed penalty-based iterative algorithm is extended to solve the mixed-integer non-convex optimization problem for MS. For TS, the optimization problem is decomposed into two subproblems, which can be consecutively solved using state-of-the-art algorithms and convex optimization techniques. Finally, our numerical results reveal that: 1) the TS and ES operating protocols are generally preferable for unicast and multicast transmission, respectively; and 2) the required power consumption for both scenarios is significantly reduced by employing the proposed STAR-RIS instead of conventional reflecting/transmitting-only RISs.
Reconfigurable intelligent surfaces (RISs), also known as intelligent reflecting surfaces (IRSs), or large intelligent surfaces (LISs),
<sup>1</sup>
have received significant attention for their potential to enhance the capacity and coverage of wireless networks by smartly reconfiguring the wireless propagation environment. Therefore, RISs are considered a promising technology for the sixth-generation (6G) of communication networks. In this context, we provide a comprehensive overview of the state-of-the-art on RISs, with focus on their operating principles, performance evaluation, beamforming design and resource management, applications of machine learning to RIS-enhanced wireless networks, as well as the integration of RISs with other emerging technologies. We describe the basic principles of RISs both from physics and communications perspectives, based on which we present performance evaluation of multiantenna assisted RIS systems. In addition, we systematically survey existing designs for RIS-enhanced wireless networks encompassing performance analysis, information theory, and performance optimization perspectives. Furthermore, we survey existing research contributions that apply machine learning for tackling challenges in dynamic scenarios, such as random fluctuations of wireless channels and user mobility in RIS-enhanced wireless networks. Last but not least, we identify major issues and research opportunities associated with the integration of RISs and other emerging technologies for applications to next-generation networks.
<sup>1</sup>
Without loss of generality, we use the name of RIS in the remainder of this paper.
</fn
The reconfigurable intelligent surface (RIS) is one of the promising technologies contributing to the next generation smart radio environment. A novel physics-based RIS channel model is proposed. In the model, the signal reflected through the scatters and the RIS elements are jointly studied as multipath components of the overall received envelope. This novel strategy simplifies the mathematical structure of the channel gain and is able to compactly derive the distribution of the overall channel. For the case of continuous phase shifts, the distribution depends on the number of elements of the RIS and the observing direction of the receiver. For the case of discrete phase shifts, the distribution further depends on the number of phase quantization levels. The scaling law of the average received power is obtained from the scale factor of the distribution. For the application scenarios where RIS functions as an anomalous reflector, we investigate the performance of single RIS-assisted multiple access networks for time-division multiple access (TDMA), frequency-division multiple access (FDMA), and non-orthogonal multiple access (NOMA). Closed-form expressions for the outage probability of the proposed channel model are derived. It is proved that a constant diversity order exists, which is independent of the number of RIS elements. Simulation results are presented to confirm that the proposed model applies effectively to the element-based RISs.
The emergence of visible light communication (VLC) technology as a solution to solve radio frequency impediments, such as spectrum shortage, is continuously appealing. In addition to its large and unlicensed bandwidth, VLC provides a high level of security in a closed room with zero radio frequency interference. However, loss of the VLC signal is experienced when the receiver rotates or moves. This challenge requires a special solution for integration into portable devices. On the other hand, re-configurable intelligent surface (RIS) is a technology exploited in radio frequency to solve dead zones and loss of signal. RIS elements are characterized by tunable physico-chemical characteristics including physical depth and refractive index. In this article, we exploit these RIS attributes to steer the incident light beam, offer the VLC receiver a large range of rotation angle, and improve its field-of-view. We show that instead of using convex, parabolic, or spherical lenses, adopting a meta-lens with artificial muscles or a thin-film Liquid-Crystal with embedded Titanium dioxide nano-disk, a VLC receiver can detect light rays at a high incidence angle with high precision and considerable improvement in the detected light intensity, even with a miniaturized single photodetector.
In this paper, we introduce a physics-consistent analytical characterization of the free-space path-loss of a wireless link in the presence of a reconfigurable intelligent surface. The proposed approach is based on the vector generalization of Green’s theorem. The obtained path-loss model can be applied to two-dimensional homogenized metasurfaces, which are made of sub-wavelength scattering elements and that operate either in reflection or transmission mode. The path-loss is formulated in terms of a computable integral that depends on the transmission distances, the polarization of the radio waves, the size of the surface, and the desired surface transformation. Closed-form expressions are obtained in two asymptotic regimes that are representative of far-field and near-field deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.
Reconfigurable intelligent surfaces (RISs) are an emerging technology for application to wireless networks. We introduce a physics and electromagnetic (EM) compliant communication model for analyzing and optimizing RIS-assisted wireless systems. The proposed model has four main notable attributes: (i) it is
end-to-end
, i.e., it is formulated in terms of an equivalent channel that yields a one-to-one mapping between the voltages fed into the ports of a transmitter and the voltages measured at the ports of a receiver; (ii) it is
EM-compliant
, i.e., it accounts for the generation and propagation of the EM fields; (iii) it is
mutual coupling aware
, i.e., it accounts for the mutual coupling among the sub-wavelength unit cells of the RIS; and (iv) it is
unit cell aware
, i.e., it accounts for the intertwinement between the amplitude and phase response of the unit cells of the RIS.
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.
This paper investigates a downlink multiple-input single-output intelligent reflecting surface (IRS) aided non-orthogonal multiple access (NOMA) system, where a base station (BS) serves multiple users with the aid of IRSs. Our goal is to maximize the sum rate of all users by jointly optimizing the active beamforming at the BS and the passive beamforming at the IRS, subject to successive interference cancellation decoding rate conditions and IRS reflecting elements constraints. In term of the characteristics of reflection amplitudes and phase shifts, we consider ideal and non-ideal IRS assumptions. To tackle the formulated non-convex problems, we propose efficient algorithms by invoking alternating optimization, which design the active beamforming and passive beamforming alternately. For the ideal IRS scenario, the two subproblems are solved by invoking the successive convex approximation technique. For the non-ideal IRS scenario, constant modulus IRS elements are further divided into continuous phase shifts and discrete phase shifts. To tackle the passive beamforming problem with continuous phase shifts, a novel algorithm is developed by utilizing the sequential rank-one constraint relaxation approach, which is guaranteed to find a locally optimal rank-one solution. Then, a quantization-based scheme is proposed for discrete phase shifts. Finally, numerical results illustrate that: i) the system sum rate can be significantly improved by deploying the IRS with the proposed algorithms; ii) 3-bit phase shifters are capable of achieving almost the same performance as the ideal IRS; iii) the proposed IRS-aided NOMA systems achieve higher system sum rate than the IRS-aided orthogonal multiple access system.
The field quantities H, B, E, D, satisfy some interface and boundary conditions on the boundary surface of two media. On the boundary surfaces there can be present electric or magnetic single or double charge or current layers. This article describes interface and boundary conditions for quantities H, B, E, D and for scalar and vector potentials. To the best knowledge of the author some of these conditions have not been published.
This handbook, now available in paperback, brings together a comprehensive collection of mathematical material in one location. It also offers a variety of new results interpreted in a form that is particularly useful to engineers, scientists, and applied mathematicians. The handbook is not specific to fixed research areas, but rather it has a generic flavor that can be applied by anyone working with probabilistic and stochastic analysis and modeling. •Classic results are presented in their final form without derivation or discussion, allowing for much material to be condensed into one volume. •Concise compilation of disparate formulae saves time in searching different sources. •Focused application has broad interest for many disciplines: engineers, computer scientists, statisticians, physicists; as well as for any researcher working in probabilistic and stochastic analysis and modeling in the natural or social sciences. •The material is timeless, with intrinsic value to practicing engineers and scientists. Excerpts from reviews of the hardbound edition: This is a unique book and an invaluable reference for engineers and scientists in the fields of electrical engineering, mathematics and statistics. There is no other single reference book that covers Gaussian and Gaussian-related distributions of random variables that are encountered in research work and practice. This is a reference book that I recommend to all my graduate students working in the area of telecommunication system analysis and design. -John Proakis, Professor Emeritus, Northeastern University and Adjunct Professor, University of California, San Diego The reference book Probability Distributions Involving Gaussian Random Variables, authored by Dr. Marvin Simon, has become, in a very short time frame, one of the most useful aids to research in the field of digital communications that has come out in many years. It has numerous results that can save researchers in the field endless hours of work. It has replaced various other well known resources because of its concentration of relevant, timely, and easily accessible results. - Larry Milstein, UCSD There are a small number of reference works that have proven so invaluable that I have purchased a home copy in addition to my office copy. This handbook is one of them. - Dr. Norman C. Beaulieu, University of Alberta The Gaussian distribution and those derived from it are at the very core of a huge number of problems in multi-disciplinary fields of engineering, mathematics and science. The book, with its comprehensive information in analytical, tabular, and graphical form, is an invaluable tool for scientists and engineers. - Sergio Benedetto, Politecnico di Torino More testimonials can be found in the front of this edition. Copyright © 2002 by Springer Science+Business Media, LLC. All rights reserved.
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.
DOCOMO conducts world’s first successful trial of transparent dynamic metasurface
- N Docomo
Fundamentals of Massive MIMO
On the interface and boundary conditions of electromagnetic fields
- Vg