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Abstract

Reconfigurable intelligent surface (RIS) is a revolutionary technology to achieve spectrum-, energy-, and cost-efficient wireless networks. This paper considers an RIS-assisted downlink non-orthogonal-multiple-access (NOMA) system. To optimize the rate performance and ensure user fairness, we maximize the minimum decoding signal-to-interference-plus-noise-ratio (equivalently the rate) of all users, by jointly optimizing the (active) transmit beamforming at the base station (BS) and the phase shifts (i.e., passive beamforming) at the RIS. A combined-channel-strength based user-ordering scheme for NOMA decoding is first proposed to decouple the user-ordering design and the joint beamforming design. Efficient algorithms are further proposed to solve the non-convex problem, by leveraging the block coordinated descent and semidefinite relaxation (SDR) techniques. For the single-antenna BS setup, the optimal power allocation at the BS and the asymptotically optimal phase shifts at the RIS are obtained in closed forms. For the multiple-antenna BS setup, it is shown that the rank of the SDR solution of the transmit beamforming design is upper bounded by two. Also, the proposed algorithms are analyzed in terms of convergence and complexity. Simulation results show that the RIS-assisted NOMA system can enhance the rate performance significantly, compared to traditional NOMA without RIS and traditional orthogonal multiple access with/without RIS.

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... In [19], it was examined the RIS phase shifter design, between the coherent discrete and the random discrete comprehensively. In [20], a capacity fairnes performance of RIS SISO NOMA and RIS MIMO NOMA over Ricean fading channel was investigated. In [21], the symmetrical splitting of RIS elements for downlink CCUs and CEUs was proposed, and it was shown that RIS improved the BER performances of both CCUs and CEUs. ...
... SYSTEM MODEL Based on [23] where N RIS elements split among NOMA user and the incoming NOMA signal is reflected to corresponding user. In this paper, we denotes reflecting element allocation L k and prior to [20], a PD-NOMA downlink scheme is considered with single antenna equipped at base station. The initialization step from Fig.1 is that all the user symbols are modulated and mapped with an M-ary constellation which is denoted by S k . ...
... Additionally, ζ 2 represents the user 2 channel gain and noise power ρ2 N0 since the received signal y 2 . Similarly, with the in-phase component of SC, the BEP for SC of quadrature component had the same formulation with (20). Therefore P e2 = (P e2,ℜ + P e2,ℑ )/2. ...
Preprint
Reconfigurable intelligent surface (RIS) as a supportive technology for aiding downlink non-orthogonal multiple access (NOMA) can enhance the bit error rate (BER) performance. In this paper, a novel BER-aware reflecting elements allocation (REA) on an RIS is proposed to maintain the BER order among paired RIS-NOMA users. The RIS REA is useful for minimizing the average user BER, ompared with a system that allocates the same number of elements to all users. Additionally, the Ricean fading is considered instead of Rayleigh fading as it is more practical and general. Furthermore,an REA optimization objective function for equalizing the user BER is proposed. In order to solve the problem, a modified exhaustive search is proposed to reduce complexity. The distribution of the objective function is observed first; subsequently, the exhaustive search range is determined. Both the analytical and simulation results show that the proposed algorithm can minimize the average user BER.
... It is worth mentioning that different from the works in [13]- [15], where downlink NOMA is considered, in the RIS-SSN considered in this letter, the relative strength of the received signals from different transmitters is generally different at different receivers since they experience different channel attenuations. This means that the optimal decoding orders at different receivers are generally different, which makes problem (P1) to be a complicated mixed-combinatorial-andcontinuous optimization problem. ...
... One way to tackle this is to exhaustively search all the possible decoding orders and select the one which maximizes the objective function [13]. Another way is to heuristically select the decoding orders by simple computation [14], [15]. We choose the latter approach due to its low computational complexity, which is especially effective when the number of transceiver pairs is large. ...
... Generally, the optimal Θ * obtained by solving (P2-a) is not rank-one. We use the Gaussian randomization method to obtain a rank-one solution [15]. To be more specific, we first compute the eigenvalue decomposition of Θ * as Θ * = UΣU H , and then generate random vectorsθ r = UΣ 1 2 r, where r ∼ CN (0, I N +1 ). ...
Article
In this letter, a reconfigurable intelligent surface (RIS)-enabled successive interference cancellation (SIC)-based spectrum sharing (SS) scheme is proposed for SS networks where multiple transceiver pairs communicate over the same spectrum at the same time. The key idea is to apply the SIC technique at each receiver and utilize RIS simultaneously, so as to reduce the interference among different transceiver pairs and to focus energy. Specifically, to minimize the total power consumption at all the transmitters under given data rate constraints, the decoding order at each receiver, the reflection coefficients at RIS, and the transmit power at each transmitter are jointly optimized. The formulated problem is a mixed-combinatorial-and-continuous optimization problem and thus is difficult to solve. To address this, a low-complexity but effective decoding order determining method is first proposed. Given the decoding orders, the reflection coefficients and the transmit power are alternately optimized with the help of the semidefinite relaxation method. Substantial performance gains are shown compared with traditional RIS-enabled SS scheme without SIC.
... A. Related Works 1) Related Works for Conventional RIS-aided NOMA: Recent research contributions have evaluated the conventional RIS-aided NOMA systems in several aspects. Several optimization methods are firstly proposed to cope with the integrated signals [20]- [22]. More specifically, the works propose joint passive beamforming designs for multi-cluster multiple-input-singleoutput (MISO) NOMA systems in [20], for RIS enhanced massive NOMA systems in [21], and for NOMA systems with user-ordering designs in [22]. ...
... Several optimization methods are firstly proposed to cope with the integrated signals [20]- [22]. More specifically, the works propose joint passive beamforming designs for multi-cluster multiple-input-singleoutput (MISO) NOMA systems in [20], for RIS enhanced massive NOMA systems in [21], and for NOMA systems with user-ordering designs in [22]. With the aid of beamforming designs, the optimized physical channel models are evaluated by [23]- [25]. ...
... is approximately expressed as 2 F 1 2t + 2, t − n + 1 2 ; t + n + 5 2 ; 1 . Hence, the Laplace transform is finally derived as L f |h rf RU,m h BR,m| (x) (s) = ∞ t=0 ∞ n=0 4 t−n+1 √ πk t 1 k t 2 [(1 + k 1 ) (1 + k 2 )] t+1 (t!) 2 (n!) 2 exp (k 1 + k 2 ) Γ 2n + 2, 2t + 2 t + n + 5 2 × 2 F 1 2t + 2, t − n + 1 2 ; t + n + Firstly, we substitute the CDF of the Gamma distribution, (20), into the definition of the outage probability of the reflecting user, (22). Hence, we obtain the integration as ...
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Simultaneous transmitting and reflecting intelligent omini-surfaces (STAR-IOSs) are able to achieve full coverage "smart radio environments". By splitting the energy or altering the active number of STAR-IOS elements, STAR-IOSs provide high flexibility of successive interference cancellation (SIC) orders for non-orthogonal multiple access (NOMA) systems. Based on the aforementioned advantages, this paper investigates a STAR-IOS-aided downlink NOMA network with randomly deployed users. We first propose three tractable channel models for different application scenarios, namely the central limit model, the curve fitting model, and the M-fold convolution model. More specifically, the central limit model fits the scenarios with large-size STAR-IOSs while the curve fitting model is extended to evaluate multi-cell networks. However, these two models cannot obtain accurate diversity orders. Hence, we figure out the M-fold convolution model to derive accurate diversity orders. We consider three protocols for STAR-IOSs, namely, the energy splitting (ES) protocol, the time switching (TS) protocol, and the mode switching (MS) protocol. Based on the ES protocol, we derive analytical outage probability expressions for the paired NOMA users by the central limit model and the curve fitting model. Based on three STAR-IOS protocols, we derive the diversity gains of NOMA users by the M-fold convolution model. The analytical results reveal that the diversity gain of NOMA users is equal to the active number of STAR-IOS elements. Numerical results indicate that 1) in high signal-to-noise ratio regions, the central limit model performs as an upper bound, while a lower bound is obtained by the curve fitting model; 2) the TS protocol has the best performance but requesting more time blocks than other protocols; 3) the ES protocol outperforms the MS protocol as the ES protocol has higher diversity gains.
... In [19], it was examined the RIS phase shifter design, between the coherent discrete and the random discrete comprehensively. In [20], a capacity fairnes performance of RIS SISO NOMA and RIS MIMO NOMA over Ricean fading channel was investigated. In [21], the symmetrical splitting of RIS elements for downlink CCUs and CEUs was proposed, and it was shown that RIS improved the BER performances of both CCUs and CEUs. ...
... SYSTEM MODEL Based on [23] where N RIS elements split among NOMA user and the incoming NOMA signal is reflected to corresponding user. In this paper, we denotes reflecting element allocation L k and prior to [20], a PD-NOMA downlink scheme is considered with single antenna equipped at base station. The initialization step from Fig.1 is that all the user symbols are modulated and mapped with an M-ary constellation which is denoted by S k . ...
... Additionally, ζ 2 represents the user 2 channel gain and noise power ρ2 N0 since the received signal y 2 . Similarly, with the in-phase component of SC, the BEP for SC of quadrature component had the same formulation with (20). Therefore P e2 = (P e2,ℜ + P e2,ℑ )/2. ...
Preprint
Reconfigurable intelligent surface (RIS) as a supportive technology for aiding downlink non-orthogonal multiple access (NOMA) can enhance the bit error rate (BER) performance. In this paper, a novel BER-aware reflecting elements allocation (REA) on an RIS is proposed to maintain the BER order among paired RIS-NOMA users. The RIS REA is useful for minimizing the average user BER, ompared with a system that allocates the same number of elements to all users. Additionally, the Ricean fading is considered instead of Rayleigh fading as it is more practical and general. Furthermore,an REA optimization objective function for equalizing the user BER is proposed. In order to solve the problem, a modified exhaustive search is proposed to reduce complexity. The distribution of the objective function is observed first; subsequently, the exhaustive search range is determined. Both the analytical and simulation results show that the proposed algorithm can minimize the average user BER.
... However, most existing works assume that RISs can only reflect the incident signal [6][7][8][9][10][11][12][13][14][15][16]. As a result, the communication system can not exploit the benefits of the RIS when the transmitter and receiver are not located on the same side of the RIS. ...
... Nevertheless, these research contributions primarily considered the OMA scheme. Driven by the benefits of RIS and NOMA, the combination of the two has also been investigated in many prior works [11][12][13][14][15][16]. Specifically, the authors of [11] considered an RIS-aided NOMA system with single-inputsingle-output (SISO) and MISO setup, respectively, where the formulated minimum rate maximization problem was solved optimally using an iterative algorithm. ...
... Driven by the benefits of RIS and NOMA, the combination of the two has also been investigated in many prior works [11][12][13][14][15][16]. Specifically, the authors of [11] considered an RIS-aided NOMA system with single-inputsingle-output (SISO) and MISO setup, respectively, where the formulated minimum rate maximization problem was solved optimally using an iterative algorithm. The total transmit power was minimized in an RIS-empowered MISO-NOMA network by designing the joint beamforming using an efficient difference-of-convex method [12]. ...
... For instance, in a downlink multiple-input-single-output (MISO) system, when the users have mutually orthogonal channels, traditional spatial division multiple access (SDMA) can achieve better performance. e other drawback of NOMA is that under the consideration of user fairness, the performances of strong users are affected heavily by the power allocation to the users with weak channel conditions [4]. rough using various new technologies such as ultradense network (UDN), massive multiple-input multipleoutput (MIMO), and millimeter-wave (mmWave) communication, the fifth-generation (5G) wireless network can achieve an almost 1000-fold network capacity increase and ubiquitous wireless connectivity for at least 100 billion devices [5]. ...
... In order to reconstruct a modulated symbol from x A (n), its real and imaginary parts are first put through a sign function and then added together as (11) where j denotes the imaginary unit. Finally, the data symbol of user B can be estimated by 4 Mobile Information Systems ...
... e channel is assumed to be a quasistatic fading channel, i.e., the channel remains constant within the block duration time, and changes to new independent realizations for the next block duration time. In simulation, we set ρ 0 � −30dB, , d 0 � 50m, , α RU � α AR � 2.2, and K 1 � K 2 � 10,, which are almost the typical values used in much of the literature that studies STAR-RIS [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Moreover, ideal channel estimation is assumed and no channel coding is considered. ...
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Reconfigurable intelligent surface (RIS) is a promising solution to build a programmable wireless propagation environment through reflecting the incident signals with a large number of low-cost passive elements. Recently, a novel concept of simultaneous transmitting and reflecting RIS (STAR-RIS) has been proposed, where incident signals can be transmitted and reflected to users located on different sides of the surface. Therefore, there are two coefficients that needed to be configured for each element of STAR-RIS. One is used to adjust the phase shifts and amplitudes of the transmitted signal; the other is configured for the reflected signal. In some harsh situations where the direct links between access points (AP) and users are blocked, STAR-RIS can be deployed to provide additional signal propagation paths. With the aid of STAR-RIS, multiple users can share the same time-frequency resources and transmit signals in a power-domain nonorthogonal multiple access (NOMA) case. However, when the users move into cell edge where they have almost the same transmit power and distance to AP, the system performance will degrade largely. To solve this problem, a novel element selection method is proposed, which can reshape the channel to a favorable propagation environment, for NOMA by only activating the appropriate elements in STAR-RIS.
... Besides sum-rate maximization, the user fairness is also a critical performance metric for MU networks. The authors in [125] aim at maximizing the minimum signal-tointerference plus noise ratio (SINR) of all users, which can be treated as a fairness measure, in an IRS-assisted nonorthogonal multiple access (NOMA) system by jointly optimizing the BS's power allocation and the IRS's passive beamforming. The authors in [126] study the problem of joint active and passive beamforming for IRS-assisted MU massive MIMO downlink communications, aiming to maximize the users' minimum SINR. ...
... (b) The IRS-assisted MU MISO downlink transmissions, e.g., [117], [119], [120], [123], [124], [128], [129], [134], [139], [141]. The special case with singleantenna BS is studied in [114], [125]. (c) The IRS-assisted downlink transmissions to multi-antenna receivers (MIMO), e.g., [121], [130]. ...
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This paper presents a comprehensive literature review on applications and design aspects of the intelligent reflecting surface (IRS) in the future wireless networks. Conventionally, the network optimization has been limited to transmission control at two endpoints, i.e., end users and network controller. The fading wireless channel is uncontrollable and becomes one of the main limiting factors for performance improvement. The IRS is composed of a large array of scattering elements, which can be individually configured to generate additional phase shifts to the signal reflections. Hence, it can actively control the signal propagation properties in favor of signal reception, and thus realize the notion of a smart radio environment. As such, the IRS's phase control combined with the conventional transmission control can potentially bring performance gain compared to the conventional wireless networks without using the IRS. In this survey, we first introduce basic concepts of the IRS and the realizations of its reconfigurability. Then, we focus on applications of the IRS in wireless communications. We overview different performance metrics and analytical approaches to characterize the performance improvement of IRS-assisted wireless networks. To exploit the performance gain, we discuss the joint optimization of the IRS's phase control and the transceivers' transmission control in different network design problems, e.g., rate maximization and power minimization problems. Furthermore, we extend the discussion of IRS-assisted wireless networks to some emerging wireless applications. Finally, we highlight important practical challenges and future research directions of realizing IRS-assisted wireless communications in beyond 5G networks.
... After obtaining the optimal decoding order, the joint power allocation and phase shift optimization problems were tackled by using the alternative optimization (AO) algorithm and semidefinite relaxation (SDR) method. Moreover, the authors expanded the work in [30] into multiantenna scenarios, and the block coordinate descent (BCD) and SDR were utilized to solve the maximization of target signalinterference-noise-ratio [31]. The authors in [32] proposed a joint optimization problem of power allocation, reflection matrix, and decoding order in multi-cell IRS-assisted systems with NOMA, where the decoding order and IRS reflection matrix were jointly designed for improving SE. ...
... Note that both (31) and ( ...
Article
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Non-orthogonal multiple access (NOMA) is a promising candidate for the sixth generation wireless communication networks due to its high spectrum efficiency (SE), energy efficiency (EE), and better connectivity. It can be applied in cognitive radio networks (CRNs) to further improve SE and user connectivity. However, the interference caused by spectrum sharing and the utilization of non-orthogonal resources can downgrade the achievable performance. In order to tackle this issue, intelligent reflecting surface (IRS) is exploited in a downlink multiple-input-single-output (MISO) CRN with NOMA. To realize a desirable tradeoff between SE and EE, a multi-objective optimization (MOO) framework is formulated under both the perfect and imperfect channel state information (CSI). An iterative block coordinate descent (BCD)-based algorithm is exploited to optimize the beamforming design and IRS reflection coefficients iteratively under the perfect CSI case. A safe approximation and the S-procedure are used to address the non-convex infinite inequality constraints of the problem under the imperfect CSI case. Simulation results demonstrate that the proposed scheme can achieve a better balance between SE and EE than baseline schemes. Moreover, it is shown that both SE and EE of the proposed algorithm under the imperfect CSI can be significantly improved by exploiting IRS.
... 최근 채널 환경을 제어할 수 있는 LIS와 중첩 코딩 을 이용하여 높은 주파수 효율을 가진 NOMA를 결합 한 LIS-NOMA가 연구되고 있다 [13][14][15] . 두 기술의 결합 시 LIS는 반사되는 전파 신호를 임의로 조절할 수 있 으므로, 반사 소자의 위상 변화를 조정하여 채널 강도 를 향상시켜 원신호 검출 성능을 돕는다 [13] . ...
... 최근 채널 환경을 제어할 수 있는 LIS와 중첩 코딩 을 이용하여 높은 주파수 효율을 가진 NOMA를 결합 한 LIS-NOMA가 연구되고 있다 [13][14][15] . 두 기술의 결합 시 LIS는 반사되는 전파 신호를 임의로 조절할 수 있 으므로, 반사 소자의 위상 변화를 조정하여 채널 강도 를 향상시켜 원신호 검출 성능을 돕는다 [13] . [ ...
... However, modeling links as Rayleigh/Nakagamim distributions is imprecise in RIS based systems, due to substantial losses in non-line of sight (LOS) components and the presence of a strong LOS component. Therefore, authors in [29]- [32] considered the Rician distribution to correctly model the BS-RIS-user link. To improve the rate performance, the combined channel strength based user ordering is proposed in [29]. ...
... Therefore, authors in [29]- [32] considered the Rician distribution to correctly model the BS-RIS-user link. To improve the rate performance, the combined channel strength based user ordering is proposed in [29]. MISO two-user PD-NOMA system in combination with RIS is proposed in [30], which is more energy efficient than the RIS-assisted OMA system. ...
Preprint
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Reconfigurable intelligent surface (RIS)-assisted power-domain non-orthogonal multiple access (PD-NOMA) system has emerged as a revolutionary technology to enhance the spectrum efficiency for future wireless networks. This work introduces two novel RIS systems, namely, RIS partition-assisted (RISP) PD-NOMA (RISP-PD-NOMA) and RISP-Quadrature NOMA (RISP-Q-NOMA), to improve the signal quality of all users by dedicating fixed RIS units to each user for phase cancellation. The closed-form expressions of average sum-rate, outage probability, and diversity order of both systems are evaluated under the Rician fading channel for perfect and imperfect successive interference cancellation (SIC). Further, the performance of both systems is compared with RIS-division PDNOMA (RISD-PD-NOMA) system where the RIS subsurface assigned to one user is not exposed to another user. It is noticed from the analysis that under perfect SIC, RISP-PD-NOMA outperforms RISP-Q-NOMA and RISD-PD-NOMA. However, under imperfect SIC, RISP-Q-NOMA demonstrates superior performance than other systems, due to lesser number of SIC operations that lead to less stringent constraints on the power allocation, reduce detection delay, and improve SIC stability. Furthermore, the analytical expression for bit error rate (BER) of RISP-PD-NOMA and RISP-Q-NOMA is derived, and then closed-form expression of average-BER is evaluated. Numerical results demonstrate that RISP-Q-NOMA is superior to RISPPD-NOMA and RISD-PD-NOMA.
... However, both the BS and RISs are part of the infrastructure, and the RISs are typically positioned for exploiting the line-of-sight (LoS) path with respect to the fixed BS in NG networks for increasing the received signal power. Hence, the impact of fading environments on RIS networks has also attracted attention [33]. A fairness-oriented algorithm was proposed in a RIS-aided NOMA network [33], where Rician fading channels were used for modelling the channel gains. ...
... Hence, the impact of fading environments on RIS networks has also attracted attention [33]. A fairness-oriented algorithm was proposed in a RIS-aided NOMA network [33], where Rician fading channels were used for modelling the channel gains. Note that when the Nakagami and Rice fading parameter obey the following constraint m = (K+1) 2 2K+1 , these fading channels are identical [34, eq. ...
Article
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Reconfigurable intelligent surfaces (RISs) constitute a promising performance enhancement for next-generation (NG) wireless networks in terms of enhancing both their spectral efficiency (SE) and energy efficiency (EE). We conceive a system for serving paired power-domain non-orthogonal multiple access (NOMA) users by designing the passive beamforming weights at the RISs. In an effort to evaluate the network performance, we first derive the best-case and worst-case of new channel statistics for characterizing the effective channel gains. Then, we derive the best-case and worst-case of our closed-form expressions derived both for the outage probability and for the ergodic rate of the prioritized user. For gleaning further insights, we investigate both the diversity orders of the outage probability and the high-signal- to-noise (SNR) slopes of the ergodic rate. We also derive both the SE and EE of the proposed network. Our analytical results demonstrate that the base station (BS)-user links have almost no impact on the diversity orders attained when the number of RISs is high enough. Numerical results are provided for confirming that: i) the high-SNR slope of the RIS-aided network is one; ii) the proposed RIS-aided NOMA network has superior network performance compared to its orthogonal counterpart.
... However, most existing works assume that RISs can only reflect the incident signal [6][7][8][9][10][11][12][13][14][15][16]. As a result, the communication system can not exploit the benefits of the RIS when the transmitter and receiver are not located on the same side of the RIS. ...
... Thus, it is essential to consider the initialization method as well as the swap-operation scheme carefully. Note that invoking AO, the objective values of problem (10) and (14) are non-decreasing during each iteration [7]. Moreover, since the system throughput is upper-bounded by a finite value, the proposed AO algorithm is guaranteed to converge. ...
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Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is a promising technology to achieve full-space coverage. This paper investigates the resource allocation problem in a STAR-RIS-assisted multi-carrier communication network. To maximize the system sum-rate, a joint optimization problem for orthogonal multiple access (OMA) is first formulated, which is a mixed-integer non-linear programming problem. To solve this challenging problem, we first propose a channel assignment scheme utilizing matching theory and then invoke the alternating optimization-based method to optimize the resource allocation policy and beamforming vectors iteratively. Furthermore, the sum-rate maximization problem for non-orthogonal multiple access (NOMA) is investigated. To efficiently solve it, we first propose a location-based matching algorithm to determine the sub-channel assignment, where a transmitted user and a reflected user are grouped on a sub-channel. Then, a three-step approach is proposed, where the decoding orders, beamforming-coefficient vectors, and power allocation are optimized by employing semidefinite programming, convex upper bound approximation, and geometry programming, respectively. Numerical results unveil that: 1) For OMA, a general design that includes same-side user-pairing for channel assignment is preferable, while for NOMA, the proposed transmission-and-reflection scheme can achieve near-optimal performance. 2) The STAR-RIS-aided NOMA network significantly outperforms the networks employing conventional RISs and OMA.
... Since the RIS can reconfigure the users' channels, the optimal NOMA decoding order in [108] was obtained via exhaustive search, which may lead to an unacceptable computational complexity. To address this issue, some low-complexity user ordering schemes were proposed in [109], [110], and were shown to be able to achieve a similar performance as the exhaustive search. Inspired by the observation that for multi-user MISO transmission, NOMA can achieve the same performance as DPC when the quasi-degradation condition is satisfied [25], the authors of [111] exploited this condition in RISaided communication systems. ...
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Due to the explosive growth in the number of wireless devices and diverse wireless services, such as virtual/augmented reality and Internet-of-Everything, next generation wireless networks face unprecedented challenges caused by heterogeneous data traffic, massive connectivity, and ultra-high bandwidth efficiency and ultra-low latency requirements. To address these challenges, advanced multiple access schemes are expected to be developed, namely next generation multiple access (NGMA), which are capable of supporting massive numbers of users in a more resource- and complexity-efficient manner than existing multiple access schemes. As the research on NGMA is in a very early stage, in this paper, we explore the evolution of NGMA with a particular focus on non-orthogonal multiple access (NOMA), i.e., the transition from NOMA to NGMA. In particular, we first review the fundamental capacity limits of NOMA, elaborate the new requirements for NGMA, and discuss several possible candidate techniques. Moreover, given the high compatibility and flexibility of NOMA, we provide an overview of current research efforts on multi-antenna techniques for NOMA, promising future application scenarios of NOMA, and the interplay between NOMA and other emerging physical layer techniques. Furthermore, we discuss advanced mathematical tools for facilitating the design of NOMA communication systems, including conventional optimization approaches and new machine learning techniques. Next, we propose a unified framework for NGMA based on multiple antennas and NOMA, where both downlink and uplink transmission are considered, thus setting the foundation for this emerging research area. Finally, several practical implementation challenges for NGMA are highlighted as motivation for future work.
... • Reconfigurable intelligent surface: The emerging technology of reconfigurable intelligent surface (RIS) for wireless communications can be employed to effectively control the radio propagation environment by adjusting the signal's waveform parameters, for instance, phase and amplitude, based on configuring some low-cost reflecting elements [229]. The applications of the RIS technique in PD-NOMA systems was recently introduced in [230], [231]. On the other hand, its application in CD-NOMA systems is not yet examined except to one research work in which the authors proposed RIS-assisted SCMA system [232]. ...
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The massive connectivity is among other unprecedented requirements which are expected to be satisfied in order to follow the perpetual increase of connected devices in the era of internet of things. In contrast to the family of conventional orthogonal multiple access schemes, the key distinguishing feature of non-orthogonal multiple access (NOMA) is its capacity to support the massive connectivity. Sparse code multiple access (SCMA) is one of the powerful schemes of code-domain NOMA (CD-NOMA) and is among the promising candidates of multiple access techniques to be employed in future generations of wireless communication systems thanks to the sparsity pattern of its codebooks. This technique has been actively investigated in recent years. In this paper, we provide a comprehensive survey of the state-of-the-art of SCMA. First, we will pinpoint SCMA place in the NOMA landscape including power-domain NOMA and CD-NOMA with the aim of justifying why SCMA is prominent. Then, its system architecture is highlighted and its basic principles are presented, afterwards a review of exiting codebook designs and available SCMA detectors is provided, before showing how resources are expected to be assigned, and how SCMA can be combined with other existing and emerging technologies. Finally, we present a range of future research trends and challenging open issues that should be addressed to optimize SCMA performance.
... Fortunately, non-orthogonal multiple access (NOMA) with a new degree of freedom, namely the power domain, has been established as a promising technique for the solution of this problem [2]. Some latest work investigating NOMA from different aspects can be found in [3] [4] [5]. Although grant-based (GB) has been widely studied, it fails to provide sufficient access to IoT users with short packets, since multiple M. Fayaz handshakes are required before the transmission. ...
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Grant-free non-orthogonal multiple access (GFNOMA) is a potential multiple access framework for short-packet internet-of-things (IoT) networks to enhance connectivity. However, the resource allocation problem in GF-NOMA is challenging due to the absence of closed-loop power control. We design a prototype of transmit power pool (PP) to provide open-loop power control. IoT users acquire their transmit power in advance from this prototype PP solely according to their communication distances. Firstly, a multi-agent deep Q-network (DQN) aided GF-NOMA algorithm is proposed to determine the optimal transmit power levels for the prototype PP. More specifically, each IoT user acts as an agent and learns a policy by interacting with the wireless environment that guides them to select optimal actions. Secondly, to prevent the Q-learning model overestimation problem, double DQN (DDQN) based GF-NOMA algorithm is proposed. Numerical results confirm that the DDQN based algorithm finds out the optimal transmit power levels that form the PP. Comparing with the conventional online learning approach, the proposed algorithm with the prototype PP converges faster under changing environments due to limiting the action space based on previous learning. The considered GF-NOMA system outperforms the networks with fixed transmission power, namely all the users have the same transmit power and the traditional GF with orthogonal multiple access techniques, in terms of throughput.
... Recently, intelligent reflecting surface (IRS) as a passive and low-energy technology has been attracted great attention from academia and industry to improve the efficiency of energy harvesting and system performance of WPCNs. Specifically, each element of the IRS can independently adjust the phase shifts of the incident signal in real time, meanwhile, the IRS can reflect the signals to the desired users in a lowpower way [12], [13]. Moreover, in [14], a resource allocation problem with the sum-rate maximization of users was studied for an IRS-assisted WPCN. ...
... where ∆θ = 2π T and ∆β = 1 T . The effective phase shifts and amplitude coefficients matrix Φ need to be selected from the above two sets in (29) and (30). Based on the proposed design at RISs, the interference residue at user k in cluster m can be transformed into ...
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Reconfigurable intelligent surface (RIS) technique stands as a promising signal enhancement or signal cancellation technique for next generation networks. We design a novel passive beamforming weight at RISs in a multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) network for simultaneously serving paired users, where a signal cancellation based (SCB) design is employed. In order to implement the proposed SCB design, we first evaluate the minimal required number of RISs in both the diffuse scattering and anomalous reflector scenarios. Then, new channel statistics are derived for characterizing the effective channel gains. In order to evaluate the network's performance, we derive the closed-form expressions both for the outage probability (OP) and for the ergodic rate (ER). The diversity orders as well as the high-signal-to-noise (SNR) slopes are derived for engineering insights. The network's performance of a finite resolution design has been evaluated. Our analytical results demonstrate that: i) the inter-cluster interference can be eliminated with the aid of large number of RIS elements; ii) the line-of-sight of the BS-RIS and RIS-user links are required for the diffuse scattering scenario, whereas the LoS links are not compulsory for the anomalous reflector scenario.
... The fourth aspect is reconfigurable intelligent surfaces (RIS) for NOMA. As an example, [20] investigates the performance of RIS-assisted downlink NOMA network, which contains a multiple end-users. Moreover, [20] utilizes the optimization techniques to enhance the aforementioned system performance. ...
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This paper investigates the fundamental performance limits of downlink power-domain non-orthogonal multiple access network in the presence of co-channel interference. The investigation considers that base-station makes multiple information exchanges with a designated mobile terminal via the power-domain non-orthogonal multiple access technique. The investigation also considers that the designated mobile terminal is under the effect of a limited number of co-channel interference, which are independent and identical distributed random variables subject to Rayleigh fading. Analytical, asymptotic, and Monte-Carlo based intensive computer simulation results reveal that co-channel interference causes coding gain losses on free-interference case, utilized as a benchmark, in high signal-to-noise-ratios. Outage probability, error probability, ergodic rate, and throughput performance metrics are considered for the performance analysis. Optimization techniques and also asymptotic analysis are provided.
... Of course, applying SIC among all users entails some challenges especially in large-scale multicell BCs with multiple-antenna BSs and many arXiv:2206.03795v1 [cs.IT] 8 Jun 2022 This paper [49] [24], [25] [31], [46] [9]- [12], [14]- [22] [39]- [42] [43]- [45] [37] ...
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In this paper, we study the performance of reconfigurable intelligent surfaces (RISs) in a multicell broadcast channel (BC) that employs improper Gaussian signaling (IGS) jointly with non-orthogonal multiple access (NOMA) to optimize either the minimum-weighted rate or the energy efficiency (EE) of the network. We show that although the RIS can significantly improve the system performance, it cannot mitigate interference completely, so we have to employ other interference-management techniques to further improve performance. We show that the proposed NOMA-based IGS scheme can substantially outperform proper Gaussian signaling (PGS) and IGS schemes that treat interference as noise (TIN) in particular when the number of users per cell is larger than the number of base station (BS) antennas (referred to as overloaded networks). In other words, IGS and NOMA complement to each other as interference management techniques in multicell RIS-assisted BCs. Furthermore, we consider three different feasibility sets for the RIS components showing that even a RIS with a small number of elements provides considerable gains for all the feasibility sets.
... (1) Perfect instantaneous CSI: Most of the existing works have considered transmission design based on the assumption that the instantaneous CSI is perfectly available. Based on this assumption, the performance gains provided by introducing an RIS in various wireless applications have been investigated, such as mmWave/terahertz systems [84], [89], [142]- [145], multicell systems [101], [146], [147], physical layer security systems [83], [87], [88], [97], [98], [148], [149], simultaneous wireless information and power transfer (SWIPT) [99], [108], [113], [150]- [154], mobile edge computing networks [74], [111], [155]- [160], multicast networks [96], [161], cognitive radio networks [138], [162], [163], non-orthogonal multiple access [90], [92], [110], [112], [164]- [169], two-way communications [85], [100], and full-duplex (FD) communication [170]. In these works, the AO method was adopted to alternately optimize the beamforming vectors at the BS and the phase shifts at the RIS, and the phase shift optimization problem was addressed using the algorithms summarized in Subsection III-A. ...
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In the past as well as present wireless communication systems, the wireless propagation environment is regarded as an uncontrollable black box that impairs the received signal quality, and its negative impacts are compensated for by relying on the design of various sophisticated transmission/reception schemes. However, the improvements through applying such schemes operating at two endpoints (i.e., transmitter and receiver) only are limited even after five generations of wireless systems. Reconfigurable intelligent surface (RIS) or intelligent reflecting surface (IRS) have emerged as a new and revolutionary technology that can configure the wireless environment in a favorable manner by properly tuning the phase shifts of a large number of passive and low-cost reflecting elements, thus standing out as a promising candidate technology for the next-/sixth-generation (6G) wireless system. However, to reap the performance benefits promised by RIS/IRS, efficient signal processing techniques are crucial, for a variety of purposes such as channel estimation, transmission design, radio localization, and so on. In this paper, we provide a comprehensive overview of recent advances on RIS/IRS-aided wireless systems from the signal processing perspective. We also highlight promising research directions that are worthy of investigation in the future.
... From the literature study, we conclude that RIS, CoMP, SWIPT, and NOMA are very successful technologies and, the merits of their combinations were proved time and again. For example, it is proved that RIS-NOMA outperforms its orthogonal counterpart, RIS-OMA [31][32][33] in terms of spectral efficiency. The achievable rates and outage performances of RIS-CoMP, 34 CoMP-NOMA, 17,35 CoMP-SWIPT-NOMA, 36 and RIS-CoMP-NOMA 29 are also explored in the literature. ...
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We consider a reconfigurable intelligent surface (RIS) aided coordinated multipoint (CoMP) transmission to enhance the achievable rates of the cell edge users (CEUs) in a downlink non-orthogonal multiple access (NOMA) network enabled with simultaneous wireless information and power transfer (SWIPT). The RIS-enabled SWIPT-CoMP-NOMA network improves the spectral efficiency and counterbalances the rate trade-off introduced by SWIPT. The distribution model of the effective channel gain plays a vital role in ergodic capacity and outage probability analysis. In this article, we approximate the effective channel gains of the CEUs using the inverse Gaussian (IG) function and validate its accuracy using the well-known Kolmogorov–Smirnov distance (KSD) test. Using the proposed channel gain model, we derive closed-form expressions for the ergodic rates and outage probabilities of the CEUs. Using Monte-Carlo simulations, we confirm the accuracy of the derived analytical expressions and show that the proposed IG function is more accurate than the Gamma function in estimating the outage probability. We also found that the proposed system always offers a higher ergodic rate than the SWIPT-CoMP-NOMA system.
... Different from the existing technologies, RIS has the potential to realize the diverse applications of 6G at low hardware cost and energy consumption. As illustrated in Fig. 1, RIS can also be integrated with the emerging communication technologies, such as non-orthogonal multiple access (NOMA), thus improving connectivity and coverage range [14][15][16][17]. RIS can mitigate the signal blockage in mmWave and terahertz (THz) communications [18][19][20][21][22]. ...
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Reconfigurable intelligent surfaces (RISs) are envisioned to transform the propagation space into a smart radio environment (SRE) to realize the diverse applications of sixth-generation (6G) wireless communication. By smartly tuning the massive number of elements via controller, an RIS can passively phase-shift the electromagnetic (EM) waves to enhance the system performance. The absence of radio-frequency (RF) chains makes RIS an energy-efficient and cost-effective solution for future wireless networks. In this paper, we explore the state-of-the-art research on different aspects of RIS-assisted communication. Specifically, we first introduce the fundamentals of RIS, including the RIS's structure, operating principle, and deployment strategies. We then comprehensively discuss the emerging applications of RISs for 6G wireless networks. In addition, we elaborate on the crucial challenges for RIS-assisted networks, namely, RIS channel state information (CSI) acquisition and passive beamforming optimization. Furthermore, we present the recent research contributions leveraging the artificial intelligence (AI) based techniques for channel estimation, phase-shift optimization, and resource allocation in RIS-assisted networks. Finally, to provide effective guidance for future research, we highlight important research directions for realizing RIS-assisted network.
... In [20], a combined-channel-strength based user-ordering scheme for NOMA decoding was first proposed to optimize the rate performance in the RIS-assisted downlink NOMA system. Some authors focused on the multiple-input single-output (MISO) scenario with the assistance of RIS and NOMA techniques. ...
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In this paper, the queue-aware simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) assisted non-orthogonal multiple access (NOMA) communication system is investigated to ensure the system stability, where the long-term stability-oriented problem is reformulated to maximize the per-slot queue-weighted sum rate (QWSR) of users based on the Lyapunov drift theory. By jointly optimizing the NOMA decoding order, the active beamforming coefficients at the BS, and the passive transmission and reflection coefficients at the STAR-RIS, three STAR-RIS operating protocols are considered, namely energy splitting (ES), mode switching (MS), and time switching (TS). For ES, the blocked coordinate descent and the successive convex approximation methods are invoked to handle the highly-coupled and non-convex problem. For MS, the proposed algorithm is further extended to a penalty-based two-loop algorithm to solve the binary amplitude constrained problem. For TS, the formulated problem is decomposed into two subproblems, each of which can be solved in a similar manner to ES. Simulation results show that: i) our proposed STAR-RIS assisted NOMA communication achieves better performance than the conventional schemes; ii) the reformulated QWSR maximization problem confirms the system stability; and iii) TS achieves superior performance with respect to both the QWSR and the average queue length.
... Focusing on RIS channel modeling and optimization algorithms, the authors in [127] considered RISs to deal with multiple blocked users. In [128] and [129], the authors proposed a NOMA solution with RIS partitioning to enhance the spectrum efficiency. ...
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The demanding objectives for the future sixth generation (6G) of wireless communication networks have spurred recent research efforts on novel materials and radio-frequency front-end architectures for wireless connectivity, as well as revolutionary communication and computing paradigms. Among the pioneering candidate technologies for 6G belong the reconfigurable intelligent surfaces (RISs), which are artificial planar structures with integrated electronic circuits that can be programmed to manipulate the incoming electromagnetic field in a wide variety of functionalities. Incorporating RISs in wireless networks has been recently advocated as a revolutionary means to transform any wireless signal propagation environment to a dynamically programmable one, intended for various networking objectives, such as coverage extension and capacity boosting, spatiotemporal focusing with benefits in energy efficiency and secrecy, and low electromagnetic field exposure. Motivated by the recent increasing interests in the field of RISs and the consequent pioneering concept of the RIS-enabled smart wireless environments, in this paper, we overview and taxonomize the latest advances in RIS hardware architectures as well as the most recent developments in the modeling of RIS unit elements and RIS-empowered wireless signal propagation. We also present a thorough overview of the channel estimation approaches for RIS-empowered communications systems, which constitute a prerequisite step for the optimized incorporation of RISs in future wireless networks. Finally, we discuss the relevance of the RIS technology in the latest wireless communication standards, and highlight the current and future standardization activities for the RIS technology and the consequent RIS-empowered wireless networking approaches.
... In [25], the authors consider the NOMA-strong user acting as decode-and-forward relays to assist NOMA-weak user. For the aspect of multi-user scenarios, [26] proposed a joint optimization scheme of user-ordering and phaseshifts at RIS to maximize the minimum SINR of all users. In [27], the authors minimized the total transmit power by DC based alternating optimization method. ...
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Computation offloading in mobile edge computing (MEC) systems emerges as a novel paradigm of supporting various resource-intensive applications. However, the potential capabilities of MEC cannot be fully unleashed when the communication links are blocked by obstacles. This paper investigates a double-reconfigurable-intelligent-surfaces (RISs) assisted MEC system. To efficiently utilize the limited frequency resource, the users can partially offload their computational tasks to the MEC server deployed at base station (BS) by adopting non-orthogonal multiple access (NOMA) protocol. We aim to minimize the energy consumption of users with limited resource by jointly optimizing the transmit power of users, the offloading fraction of users and the phase-shifts of RISs. Since the problem is non-convex with highly coupled variables, the block coordinate descent (BCD) method is leveraged to alternatively optimize the decomposed four subproblems. Specifically, we invoke successive convex approximation for low complexity (SCALE) and Dinkelbach technique to tackle the fractional programming of power optimization. Then the offloading fraction is obtained by closed-form solution. Further, we leverage semidefinite relaxation (SDR) and bisection method to address the phase-shifts design of double RISs. Finally, numerical results illustrate that the proposed double-RIS assisted NOMA scheme is capable of efficiently reducing the energy consumption and achieves significant performance gain over the benchmark schemes.
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This paper considers an intelligent reflecting surface (IRS) assisted multi-input multi-output (MIMO) power splitting (PS) based simultaneous wireless information and power transfer (SWIPT) system with multiple PS receivers (PSRs). The objective is to maximize the achievable data rate of the system by jointly optimizing the PS ratios at the PSRs, the active transmit beamforming (ATB) at the access point (AP), and the passive reflective beamforming (PRB) at the IRS, while the constraints on maximum transmission power at the AP, the reflective phase shift of each element at the IRS, the individual minimum harvested energy requirement of each PSR, and the domain of PS ratio of each PSR are all satisfied. For this unsolved problem, however, since the optimization variables are intricately coupled and the constraints are conflicting, the formulated problem is non-convex, and cannot be addressed by employing exist approaches directly. To this end, we propose a joint optimization framework to solve this problem. Particularly, we reformulate it as an equivalent form by employing the Lagrangian dual transform and the fractional programming transform, and decompose the transformed problem into several sub-problems. Then, we propose an alternate optimization algorithm by capitalizing on the dual sub-gradient method, the successive convex approximation method, and the penalty-based majorization-minimization approach, to solve the sub-problems iteratively, and obtain the optimal solutions in nearly closed-forms. Numerical simulation results verify the effectiveness of the IRS in SWIPT system and indicate that the proposed algorithm offers a substantial performance gain.
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The reconfigurable intelligent surface (RIS), which consists of a large number of passive and low-cost reflecting elements, has been recognized as a revolutionary technology to enhance the performance of future wireless networks. This paper considers an RIS assisted multicast transmission, where a base station (BS) with multiple-antenna multicasts common message to multiple single-antenna mobile users (MUs) under the assistance of an RIS. An equivalent channel model for the considered multicast transmission is analyzed, and then an optimization problem for the corresponding channel capacity is formulated to obtain the optimal covariance matrix and phase shifts. In order to solve the above non-convex and non-differentiable problem, this paper first exploits the gradient descent method and alternating optimization, to approach the locally optimal solution for any number of MUs. Then, this paper considers a special case, which can obtain the global optimal solution, and shows the sufficient and necessary condition for this special case. Finally, the order growth of the maximal capacity is obtained when the numbers of the reflecting elements, the BS antennas, and the MUs go to infinity.
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This paper considers a symbiotic radio (SR) system enhanced by a reconfigurable intelligent surface (RIS). The active transmit beamforming of the base station (BS) and the passive beamforming of the RIS are jointly designed to minimize the BS’s transmission power, subject to the signal-to-interference-plus-noise ratio constraints for decoding backscatter signal, and the rate constraint for primary communication. To solve the non-convex problem, we propose an efficient iterative algorithm based on alternating optimization and semi-definite relaxation. The algorithm’s convergence and complexity are analyzed. Numerical results show that the RIS-enhanced SR achieves lower transmission power than SR without RIS.
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Cognitive radio (CR) is an effective solution to improve the spectral efficiency (SE) of wireless communications by allowing the secondary users (SUs) to share spectrum with primary users (PUs). Meanwhile, intelligent reflecting surface (IRS) has been recently proposed as a promising approach to enhance SE and energy efficiency (EE) of wireless communication systems through intelligently reconfiguring the channel environment. In this paper, we consider an IRS-assisted downlink CR system, in which a secondary access point (SAP) communicates with multiple SUs without affecting multiple PUs in the primary network and all nodes are equipped with multiple antennas. Our design objective is to maximize the achievable weighted sum rate (WSR) of SUs subject to the total transmit power constraint at the SAP and the interference constraints at PUs, by jointly optimizing the transmit precoding at the SAP and the reflecting coefficients at the IRS. To deal with the complex objective function, the problem are reformulated by employing the well-known weighted minimum mean-square error (WMMSE) method and an alternating optimization (AO)-based algorithm is proposed. Furthermore, a special scenario with only one PU is considered and an AO-based algorithm with lower complexity is proposed. Finally, some numerical simulations have been done to demonstrate that the proposed algorithm outperforms other benchmark schemes.
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In next-generation wireless communication systems, non-orthogonal multiple access (NOMA) has been recognized as essential technology for improving the spectrum efficiency. NOMA allows multiple users transmit data using the same resource block simultaneously with proper user pairing. Most of the pairing schemes, however, require prior information, such as location information of the users, leading to difficulties in realizing prompt user pairing. To realize real-time operations without prior information in NOMA, a bandit algorithm using chaotically oscillating time series, which we refer to as the laser chaos decision-maker, was demonstrated. However, this scheme did not consider the detailed communication processes, e.g., modulation, error correction code, etc. In this study, in order to adapt the laser chaos decision-maker to real communication systems, we propose a user pairing scheme based on acknowledgment (ACK) and negative acknowledgment (NACK) information considering detailed communication channels. Furthermore, based on the insights gained by the analysis of parameter dependencies, we introduce an adaptive pairing method to minimize the bit error rate of the NOMA system under study. The numerical results show that the proposed method achieves superior performances than the traditional using pairing schemes, i.e., Conventional-NOMA pairing scheme (C-NOMA) and Unified Channel Gain Difference pairing scheme (UCGD-NOMA), and ϵ-greedy-based user pairing scheme. As the cell radius of the NOMA system gets smaller, the superior on the BER of our proposed scheme gets bigger. Specifically, our proposed scheme can decrease the BER from 10−1 to 10−5 compared to the conventional schemes when the cell radius is 400 m.
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In this paper, we propose intelligent reflecting surfaces (IRS) assisted secure wireless communications with multi-input and multi-output antennas (IRS-MIMOME). The considered scenario is an access point (AP) equipped with multiple antennas communicates with a multi-antenna enabled legitimate user in the downlink at the present of an eavesdropper configured with multiple antennas. Particularly, the joint optimization of the transmit covariance matrix at the AP and the reflecting coefficients at the IRS to maximize the secrecy rate for the IRS-MIMOME system is investigated, with two different assumptions on the phase shifting capabilities at the IRS, i.e., the IRS has the continuous reflecting coefficients and the IRS has the discrete reflecting coefficients. For the former case, due to the non-convexity of the formulated problem, an alternating optimization (AO)-based algorithm is proposed, i.e., for given the reflecting coefficients at the IRS, the successive convex approximation (SCA)-based algorithm is used to solve the transmit covariance matrix optimization, while given the transmit covariance matrix at the AP, alternative optimization is used again in individually optimizing of each reflecting coefficient at the IRS with other fixed reflecting coefficients. For the individual reflecting coefficient optimization, the close-form or an interval of the optimal solution is provided. Then, the proposed algorithm is extended to the discrete reflecting coefficient model at the IRS. Finally, some numerical simulations have been done to demonstrate that the proposed algorithm outperforms other benchmark schemes.
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Recently, intelligent reflecting surfaces (IRSs) have drawn huge attention as a promising solution for 6G networks to enhance diverse performance metrics in a cost-effective way. For massive connectivity toward a higher spectral efficiency, we address an intelligent reflecting surface (IRS) to an uplink nonorthogonal multiple access (NOMA) network supported by a multiantenna receiver. We maximize the sum rate of the IRS-aided NOMA network by optimizing the IRS reflection pattern under unit modulus and practical reflection. For a moderate-sized IRS, we obtain an upper bound on the optimal sum rate by solving a determinant maximization (max-det) problem after rank relaxation, which also leads to a feasible solution through Gaussian randomization. For a large number of IRS elements, we apply the iterative algorithms relying on the gradient, such as Broyden–Fletcher–Goldfarb–Shanno (BFGS) and limited-memory BFGS algorithms for which the gradient of the sum rate is derived in a computationally efficient form. The results show that the max-det approach provides a near-optimal performance under unit modulus reflection, while the gradient-based iterative algorithms exhibit merits in performance and complexity for a large-sized IRS with practical reflection.
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Employing large intelligent surfaces (LISs) is a promising solution for improving the coverage and rate of future wireless systems. These surfaces comprise massive numbers of nearly-passive elements that interact with the incident signals, for example by reflecting them, in a smart way that improves the wireless system performance. Prior work focused on the design of the LIS reflection matrices assuming full channel knowledge. Estimating these channels at the LIS, however, is a key challenging problem. With the massive number of LIS elements, channel estimation or reflection beam training will be associated with (i) huge training overhead if all the LIS elements are passive (not connected to a baseband) or with (ii) prohibitive hardware complexity and power consumption if all the elements are connected to the baseband through a fully-digital or hybrid analog/digital architecture. This paper proposes efficient solutions for these problems by leveraging tools from compressive sensing and deep learning. First, a novel LIS architecture based on sparse channel sensors is proposed. In this architecture, all the LIS elements are passive except for a few elements that are active (connected to the baseband). We then develop two solutions that design the LIS reflection matrices with negligible training overhead. In the first approach, we leverage compressive sensing tools to construct the channels at all the LIS elements from the channels seen only at the active elements. In the second approach, we develop a deep-learning based solution where the LIS learns how to interact with the incident signal given the channels at the active elements, which represent the state of the environment and transmitter/receiver locations. We show that the achievable rates of the proposed solutions approach the upper bound, which assumes perfect channel knowledge, with negligible training overhead and with only a few active elements, making them promising for future LIS systems.
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In this paper we explore non-orthogonal multiple access (NOMA) in millimeter-wave (mmWave) communications (mmWave-NOMA). In particular, we consider a typical problem, i.e., maximization of the sum rate of a 2-user mmWave-NOMA system. In this problem, we need to find the beamforming vector to steer towards the two users simultaneously subject to an analog beamforming structure, while allocating appropriate power to them. As the problem is non-convex and may not be converted to a convex problem with simple manipulations, we propose a suboptimal solution to this problem. The basic idea is to decompose the original joint beamforming and power allocation problem into two sub-problems which are relatively easy to solve: one is a power and beam gain allocation problem, and the other is a beamforming problem under a constant-modulus constraint. Extension of the proposed solution from 2-user mmWave-NOMA to more-user mmWave-NOMA is also discussed. Extensive performance evaluations are conducted to verify the rational of the proposed solution, and the results also show that the proposed sub-optimal solution achieve close-to-bound sum-rate performance, which is significantly better than that of time-division multiple access (TDMA).
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Non-orthogonal multiple access (NOMA) is a promising technique for the fifth generation mobile communication due to its high spectral efficiency. By applying superposition coding and successive interference cancellation techniques at the receiver, multiple users can be multiplexed on the same subchannel in NOMA systems. Previous works focus on subchannel assignment and power allocation to achieve the maximization of sum rate; however, the energy-efficient resource allocation problem has not been well studied for NOMA systems. In this paper, we aim to optimize subchannel assignment and power allocation to maximize the energy efficiency for the downlink NOMA network. Assuming perfect knowledge of the channel state information at base station, we propose a low-complexity suboptimal algorithm, which includes energy-efficient subchannel assignment and power proportional factors determination for subchannel multiplexed users. We also propose a novel power allocation across subchannels to further maximize energy efficiency. Since both optimization problems are non-convex, difference of convex programming is used to transform and approximate the original non-convex problems to convex optimization problems. Solutions to the resulting optimization problems can be obtained by solving the convex sub-problems iteratively. Simulation results show that the NOMA system equipped with the proposed algorithms yields much better sum rate and energy efficiency performance than the conventional orthogonal frequency division multiple access scheme.
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In this paper, we investigate resource allocation algorithm design for multicarrier non-orthogonal multiple access (MC-NOMA) systems employing a full-duplex (FD) base station (BS) for serving multiple half-duplex (HD) downlink (DL) and uplink (UL) users simultaneously. The proposed algorithm is obtained from the solution of a non-convex optimization problem for the maximization of the weighted sum system throughput. We apply monotonic optimization to develop an optimal joint power and subcarrier allocation policy. The optimal resource allocation policy serves as a system performance benchmark due to its high computational complexity. Furthermore, a suboptimal iterative scheme based on successive convex approximation is proposed to strike a balance between computational complexity and optimality. Our simulation results reveal that the proposed suboptimal algorithm achieves a close-to-optimal performance. Besides, FD MC-NOMA systems employing the proposed resource allocation algorithms provide a substantial system throughput improvement compared to conventional HD multicarrier orthogonal multiple access (MC-OMA) systems and other baseline schemes. Also, our results unveil that the proposed FD MC-NOMA systems achieve a fairer resource allocation compared to traditional HD MC-OMA systems.
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In this letter, a downlink multiple-input-multiple-output non-orthogonal multiple access scenario is considered. We investigate a dynamic user clustering problem from a fairness perspective. In order to solve this optimization problem, three sub-optimal algorithms, namely, top-down A, top-down B, and bottom up, are proposed to realize the different tradeoffs of complexity and throughput of the worst user. In addition, for each given user clustering case, we optimize the power allocation coefficients for the users in each cluster by adopting a bisection search-based algorithm. Numerical results show that the proposed algorithms can lower the complexity with an acceptable degradation on the throughput compared with the exhaustive search method. It is worth noting that the top-down B algorithm can achieve a good tradeoff between the complexity and the throughput among the three proposed algorithms.
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Non-orthogonal multiple access (NOMA) is expected to be a promising multiple access technique for 5G networks due to its superior spectral efficiency. In this letter, the ergodic capacity maximization problem is first studied for the Rayleigh fading multiple-input multiple-output (MIMO) NOMA systems with statistical channel state information at the transmitter (CSIT). We propose both optimal and low complexity suboptimal power allocation schemes to maximize the ergodic capacity of MIMO NOMA system with total transmit power constraint and minimum rate constraint of the weak user. Numerical results show that the proposed NOMA schemes significantly outperform the traditional orthogonal multiple access scheme.
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In non-orthogonal multiple access (NOMA) downlink, multiple data flows are superimposed in the power domain and user decoding is based on successive interference cancellation. NOMA's performance highly depends on the power split among the data flows and the associated power allocation (PA) problem. In this letter, we study NOMA from a fairness standpoint and we investigate PA techniques that ensure fairness for the downlink users under i) instantaneous channel state information (CSI) at the transmitter, and ii) average CSI. Although the formulated problems are non-convex, we have developed low-complexity polynomial algorithms that yield the optimal solution in both cases considered.
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Convex optimization problems arise frequently in many different fields. A comprehensive introduction to the subject, this book shows in detail how such problems can be solved numerically with great efficiency. The focus is on recognizing convex optimization problems and then finding the most appropriate technique for solving them. The text contains many worked examples and homework exercises and will appeal to students, researchers and practitioners in fields such as engineering, computer science, mathematics, statistics, finance, and economics.
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We study the convergence properties of a (block) coordinate descent method applied to minimize a nondifferentiable (nonconvex) function f(x 1, . . . , x N ) with certain separability and regularity properties. Assuming that f is continuous on a compact level set, the subsequence convergence of the iterates to a stationary point is shown when either f is pseudoconvex in every pair of coordinate blocks from among N-1 coordinate blocks or f has at most one minimum in each of N-2 coordinate blocks. If f is quasiconvex and hemivariate in every coordinate block, then the assumptions of continuity of f and compactness of the level set may be relaxed further. These results are applied to derive new (and old) convergence results for the proximal minimization algorithm, an algorithm of Arimoto and Blahut, and an algorithm of Han. They are applied also to a problem of blind source separation.
Large intelligent surface/antennas (LISA): Making reflective radios smart
  • Y.-C Liang
  • R Long
  • Q Zhang
  • J Chen
  • H V Cheng
  • H Guo
Y.-C. Liang, R. Long, Q. Zhang, J. Chen, H. V. Cheng, and H. Guo, "Large intelligent surface/antennas (LISA): Making reflective radios smart," J. Commun. Inf. Netw., vol. 4, no. 2, pp. 40-50, Jun. 2019.