Sixth-generation (6G) networks are rapidly becoming a focal point of global technological innovation, driven by the need to support hyper-reliable, low-latency, and intelligent connectivity for applications such as immersive extended reality, autonomous systems, and ubiquitous sensing. While 6G promises transformative advancements in wireless communication, achieving its ambitious goals poses significant fundamental challenges. One natural direction is to scale multiple-input multiple-output (MIMO) technology to unprecedented levels; however, doing so introduces substantial hardware complexity and power consumption. To overcome these limitations, recent research has explored antenna reconfigurability as a novel degree of freedom (DoF) at the physical (PHY) layer. Among these efforts, the fluid antenna system (FAS) has emerged as a compelling concept, offering reconfigurability in both spatial positioning and physical structure. This idea has inspired related innovations, including movable antennas, flexible-position MIMO, reconfigurable MIMO architectures, and adaptive antenna arrays, collectively referred to as next-generation reconfigurable antenna (NGRA) systems. While prior work has primarily focused on spatial flexibility, this article introduces a generalized model of FAS that incorporates both structural and morphological fluidity, enabling the vision of “shapeless and formless” antennas in future wireless systems. We analyze FAS’s potential to enhance key performance metrics such as coverage, energy efficiency, reliability, and spectral capacity. In addition, we outline implementation challenges and explore synergies with key 6G enablers, including reconfigurable intelligent surfaces (RIS), non-terrestrial networks (NTN), integrated sensing and communication (ISAC), and artificial intelligence (AI). This survey provides a comprehensive overview of NGRA systems and identifies promising directions for future research in reconfigurable wireless technologies.

This paper examines the impact of emerging fluid antenna systems (FAS) on reconfigurable intelligent surface (RIS)-aided secure communications. Specifically, we consider a classic wiretap channel, where a fixed-antenna transmitter sends confidential information to an FAS-equipped legitimate user with the help of an RIS, while an FAS-equipped eavesdropper attempts to decode the message. To evaluate the proposed wireless scenario, we first introduce the cumulative distribution function (CDF) and probability density function (PDF) of the signal-to-noise ratio (SNR) at each node, using the central limit theorem and the Gaussian copula function. We then derive a compact analytical expression for the secrecy outage probability (SOP). Our numerical results reveal how the incorporation of FAS and RIS can significantly enhance the performance of secure communications.

Surface wave communication (SWC) is an emerging technology garnering significant interest for its diverse potential applications in communications. However, accurately computing electromagnetic field strength, which is related to the path loss, in reconfigurable surface structures, particularly for long-distance transmission, presents an ongoing challenge. To address this, we introduce a novel analytical model employing surface wave ray tracing. Unlike conventional simulations, our analytical approach enables precise computation of the electromagnetic field strength attenuation in both short and long-distance transmissions, providing invaluable insights for practical SWC implementations. Our proposed model takes into account key system parameters such as surface material, thickness, cavity porosity, and other variables influencing propagation performance. This facilitates analysis of optimal reconfigurable structures. Simulation results validate the model’s accuracy in short-distance transmission, thereby endorsing its effectiveness in studying surface wave path loss over longer distances. Furthermore, our study demonstrates the SWC superiority over traditional coaxial cable and space-wave communication in mitigating path loss. Additionally, we explore the impacts of various factors such as different dielectric layers, wall materials, leakage, and pathway width on SWC performance, providing deeper insights into designing optimal reconfigurable structures for SWC applications.

This paper investigates a multiuser millimeter-wave (mmWave) uplink system in which each user is equipped with a multi-antenna fluid antenna system (FAS) while the base station (BS) has multiple fixed-position antennas. Our primary objective is to maximize the system capacity by optimizing the transmit covariance matrices and the antenna position vectors of the users jointly. To gain insights, we start by deriving upper bounds and approximations for the capacity. Then we delve into the capacity maximization problem. Beginning with the simple scenario of a single user equipped with a single-antenna FAS, we demonstrate that a closed-form optimal solution exists when there are only two propagation paths between the user and the BS. In the case where multiple propagation paths are present, a near-optimal solution can also be obtained through a one-dimensional search method. Expanding our focus to multiuser cases, in which users are equipped with either single- or multi-antenna FAS, we show that the original capacity maximization problems can be reformulated into distinct rank-one programmings. Then, we propose alternating optimization algorithms to deal with the transformed problems. Simulation results indicate that FAS can improve the capacity of the multiple access channel (MAC) greatly, and the proposed algorithms outperform all the benchmarks.

Fluid antenna system (FAS) is gaining attention as an innovative technology for boosting diversity and multiplexing gains. As a key innovation, it presents the possibility to overcome interference by position reconfigurability on one radio frequency (RF) chain, giving rise to the concept of fluid antenna multiple access (FAMA). While FAMA is originally designed to deal with interference mainly by position change and treat interference as noise, this is not rate optimal, especially when suffering from a strong interference channel (IC) where all positions have strong interference. To tackle this, this paper considers a two-user strong IC where FAMA is used in conjunction with simultaneous non-unique decoding (SND). Specifically, we analyze the key statistics for the signal-to-noise ratio (SNR) and interference-to-noise ratio (INR) for a canonical two-user IC setup, and subsequently derive the delay outage rate (DOR), outage probability (OP) and ergodic capacity (EC) of the FAMA-IC. Our numerical results illustrate huge benefits of FAMA with SND over traditional fixed-position antenna systems (TAS) with SND in the fading IC.

Fluid antenna system (FAS) has recently surfaced as a promising technology for the upcoming sixth generation (6G) wireless networks. Unlike traditional antenna system (TAS) with fixed antenna location, FAS introduces a flexible component in which the radiating element can switch its position within a predefined space. This capability allows FAS to achieve additional diversity and multiplexing gains. Nevertheless, to fully reap the benefits of FAS, obtaining channel state information (CSI) over the predefined space is crucial. In this paper, we study the system with a transmitter equipped with a traditional fixed antenna and a receiver with a fluid antenna by considering an electromagnetic-compliant channel model. We address the challenges of channel estimation and reconstruction using Nyquist sampling and maximum likelihood estimation (MLE) methods. Our analysis reveals a fundamental tradeoff between the accuracy of the reconstructed channel and the number of estimated channels, indicating that half-wavelength sampling is insufficient for perfect reconstruction and that oversampling is essential to enhance accuracy. Despite its advantages, oversampling can introduce practical challenges. Consequently, we propose a suboptimal sampling distance that facilitates efficient channel reconstruction. In addition, we employ the MLE method to bound the channel estimation error by$\epsilon $, with a specific confidence interval (CI). Our findings enable us to determine the minimum number of estimated channels and the total number of pilot symbols required for efficient channel reconstruction in a given space. Lastly, we investigate the rate performance of FAS and TAS and demonstrate that FAS with imperfect CSI can outperform TAS with perfect CSI. In contrast to existing works, we also show that there is an optimal fluid antenna size that maximizes the achievable rate when considering the energy and bandwidth overheads for full CSI acquisition.

This paper investigates the capacity performance of an uplink fluid antenna system (FAS), in which the base station (BS) is equipped with multiple fluid antennas and each user has a single fluid antenna. We aim to maximize the capacity of the system by optimizing the transmit power, and the user and BS antenna positions. Beginning with simple cases where the number of paths or the number of BS antennas is small, we reveal that the capacity is independent of the antenna positions. Then we give an upper bound on the capacity for the case where the BS has a single fluid antenna. After that, we show that in the optimal case, all users should transmit at the maximum power. Moreover, we propose an alternative algorithm to iteratively optimize the antenna positions at the BS and user sides. When keeping the user antenna positions fixed, the BS antenna positions are updated alternatively using a discrete exhaustive search in the single-user case. By transforming the capacity maximization problem into a difference-of-convex (DC) form, the majorization-minimization (MM) algorithm can also be applied to jointly optimize the BS antenna positions when there is a single user in the system. For the multiuser scenario, the antenna positions at the BS side are optimized utilizing the gradient descent method. We show that the user antenna positions can also be optimized using the discrete exhaustive search or the MM algorithm. Simulation results show that FAS can greatly improve the system capacity compared to traditional fixed-position antenna systems.

This paper investigates the performance of physical layer security (PLS) in fluid antenna-aided communication systems under arbitrary correlated fading channels. In particular, it is considered that a single fixed-antenna transmitter aims to send confidential information to a legitimate receiver equipped with a planar fluid antenna system (FAS), while an eavesdropper, also taking advantage of a planar FAS, attempts to decode the desired message. For this scenario, we first present analytical expressions of the equivalent channel distributions at the legitimate user and eavesdropper by using copula, so that the obtained analytical results are valid for any arbitrarily correlated fading distributions. Then, with the help of Gauss-Laguerre quadrature, we derive compact analytical expressions for the average secrecy capacity (ASC), the secrecy outage probability (SOP), and the secrecy energy efficiency (SEE) for the FAS wiretap channel. Moreover, for exemplary purposes, we also obtain the compact expression of ASC, SOP, and SEE by utilizing the Gaussian copula under correlated Rayleigh fading channels as a special case. Eventually, numerical results indicate that applying the fluid antenna with only one activated port to PLS can guarantee more secure and reliable transmission, when compared to traditional antenna systems (TAS) exploiting maximal ratio combining (MRC) and antenna selection (AS) under selection combining (SC).

To enable innovative applications and services, both industry and academia are exploring new technologies for sixth generation (6G) communications. One of the promising candidates is fluid antenna system (FAS). Unlike existing systems, FAS is a novel communication technology where its antenna can freely change its position and shape within a given space. Compared to the traditional systems, this unique capability has the potential of providing higher diversity and interference-free communications. Nevertheless, the performance limits of FAS remain unclear as its system properties are difficult to analyze. To address this, we approximate the outage probability and diversity gain of FAS in closed-form expressions. We then propose a suboptimal FAS with$N^{\ast}$ports, where a significant gain can be obtained over FAS with$N^{\ast}-1$ports whilst FAS with$N^{\ast}+1$ports only yields marginal improvement over the proposed suboptimal FAS. In this paper, we also provide analytical and simulation results to unfold the key factors that affect the performance of FAS. Limited to systems with one active radio frequency (RF)-chain, we show that the proposed suboptimal FAS outperforms single-antenna (SISO) system and selection combining (SC) system in terms of outage probability. Interestingly, when the given space is$\frac {\lambda }{2}$, the outage probability of the proposed suboptimal FAS with one active RF-chain achieves near to that of the maximal ratio combining (MRC) system with multiple active RF-chains.

Multiple-input multiple-output (MIMO) system has been the defining mobile communications technology in recent generations. With the ever-increasing demands looming towards the sixth generation (6G), we are in need of additional degrees of freedom that deliver further gains beyond MIMO. To this goal, fluid antenna system (FAS) has emerged as a new way to obtain spatial diversity using reconfigurable position-switchable antennas. Considering the case with more than one ports activated on a 2D fluid antenna surface at both ends, we take the information-theoretic approach to study the achievable performance limits of the MIMO-FAS. First of all, we propose a suboptimal scheme, referred to as QR MIMO-FAS, to maximize the rate at high signal-to-noise ratio (SNR) via joint port selection, transmit and receive beamforming and power allocation. We then derive the optimal diversity and multiplexing tradeoff (DMT) of MIMO-FAS. From the DMT, we highlight that MIMO-FAS outperforms traditional MIMO antenna systems. Further, we introduce a new metric, namelyq-outage capacity, which can jointly consider rate and outage probability. Through this metric, our results indicate that MIMO-FAS surpasses traditional MIMO greatly.

Fluid antenna system (FAS) is a new flexible antenna technology that offers a new approach to multiple access, referred to as fluid antenna multiple access (FAMA). The performance of FAMA has been investigated but previous results were based on simplified spatial correlation models. In this paper, we will revisit FAMA for the two-user case and study the outage probability by characterizing the joint spatial correlation among the ports. We first derive a closed-form lower bound on the outage probability and reveal that in the absence of spatial correlation, the outage probability of the system decreases exponentially as the number of ports increases. We then show that the channel model can be greatly simplified by focusing upon a limited number of channel variables, allowing us to derive the outage probability using the approximate model. To gain insight, we further approximate the channel model and provide another approximation of the outage probability that is easier to compute. Simulation results validate the approximations and demonstrate that the outage probability decreases with the number of ports but has an error floor unless the antenna size is increased. Also, when the number of ports is fixed, the outage probability initially decreases exponentially with the size but eventually approaches the lower bound.

Fluid antenna system (FAS) is an emerging flexible antenna technology that provides a new way for multiple access. In fluid antenna multiple access (FAMA), each user switches its fluid antenna to the location (i.e., port) in which the interfering users suffer from a deep fade for interference-free communication. Previous work has attempted to understand the interference immunity of FAMA but the results are limited to simplified spatial correlation models. In this paper, we revisit the FAMA system with only two users by characterizing the joint spatial correlation amongst all the ports. Using this model, however, the number of variables determining each channel coefficient scales with that of ports, hence making the analysis intractable. To tackle this, we first show that the channel model could be considerably simplified by taking into account only a few variables, and then derive the outage probability for the considered FAMA system by using the approximated model. Simulation results show that the simplified channel model can quickly approach the exact one and that the outage probability decreases with the number of ports but has an error floor unless the size of fluid antenna is increased.

Unmanned aerial vehicle (UAV) is increasingly becoming a promising tool in communication system. On one hand, UAV has been used as an aerial communication platform to provide wireless communications. On the other hand, UAV has been employed as an aerial user equipment to perform arbitrary missions. Both of these applications are promising in terms of cost, flexibility, and automation. Nevertheless, existing works usually consider these applications separately. This limits the role of UAV in future communication system. Thus, in this paper, we relax this limitation and further visualize what UAV might provide us in the future. Specifically, we present a vision of UAV in future communication system along with new applications and capabilities. This vision provides a blueprint on how engineers can design the future communication system with UAVs playing prominent roles. We then review the current advances of UAV communications to understand the gap between vision and reality. Lastly, we outline the critical issues, challenges, and future directions toward this vision.

Abstract Unmanned aerial vehicles (UAVs) have gained considerable interests in numerous civil applications. To push forward its potentials, cellular-connected UAVs have been introduced. Nevertheless, cellular networks face several bottlenecks such as spectrum scarcity and limited concurrent connectivity. To address these issues, non-orthogonal multiple access (NOMA) can be adopted. NOMA provides several opportunities for cellular-connected UAVs such as larger rate region, balanced performance between system throughput and fairness, and reduced delay. In this paper, we review important findings of the related studies, and outline new opportunities and challenges in NOMA for cellular-connected UAVs. Monte-Carlo simulations are then performed to analyze the new aerial user’s (AU)’s signal characteristics and evaluate the NOMA performance for co-existence of AU and terrestrial user (TU). Our preliminary results show that NOMA is a promising strategy for cellular-connected UAVs.

Efficiency of cellular-connected UAVs is challenged by spectrum inefficiency, limited number of concurrent connectivity, and strong interference. To overcome these issues, in this paper, we study the performance of downlink non-orthogonal multiple access for cellular-connected UAVs. We develop a novel framework based on stochastic geometry for the co-existence of aerial users (AUs) and terrestrial users (TUs), where the spatial distribution of the base stations (BSs) follows a Poisson Point Process. In our analysis, two user association policies and two types of receive antennas are considered while an inter-cell interference coordination (ICIC) technique is also in place. As the main performance measures, we then analytically derive the coverage probability and average rate of AUs and TUs. These derivations are then used to provide quantitative insights on the impact of different system parameters and settings including AU's altitude, TU's distance from the BS, power allocation, successive interference cancellation (SIC) constraints, user association policy, antenna beamwidth, and the number of coordinated BSs. Based on our analysis we then propose an interference-aware scheme based on maximum-SINR user association, directional antenna, and ICIC. A benchmark scheme based on minimum-distance user association, omni-directional antenna, and without ICIC is considered. Compared to the benchmark scheme, our proposed scheme improves the AU's coverage probability by threefold and TU's average rate by six-fold. Compared to the orthogonal multiple access, our proposed scheme trades off a slight reduction in the AU's coverage probability (~1%) with a significant increase in the achieved rate of the TUs (603Kbps/resource block).

Connecting aerial users (AU)s to cellular networks expands the potential of many applications. Nevertheless, existing cellular networks are not designed to efficiently serve AUs and terrestrial users (TU)s. Limited spectrum and high interference make the task even more challenging. Thus, in this paper, we examine the feasibility of fixed-power non-orthogonal multiple access (NOMA) scheme in meeting the demands of AUs and TUs by leveraging stochastic geometry. To this end, we propose a tractable framework for evaluating the coverage of AU and TU in cellular networks, where BSs are distributed using Poisson Point Process (PPP). We then derive the coverage probability of AU and TU. Using these analytical expressions, we further analyze the impact of different network parameters such as AU's altitude, TU's distance, and power allocation, and obtain key insights for designing an efficient NOMA scheme. Our results show that i) increasing AU's altitude does not always degrade the signal-to-interference ratio (SIR), ii) fixed-power NOMA scheme cannot be employed solely to serve AU's control and command (C& C) link, iii) pairing a near TU with a typical AU may prevent significant performance degradation, and iv) mitigating the interference of TU could improve the performance of AU.

Scarcity of the radio spectrum and high inter-cell interference (ICI) are major impediments to efficient connectivity in cellular-connected unmanned aerial vehicles (UAV)s. To address these issues, we propose aerial-terrestrial network non-orthogonal multiple access (ATN-NOMA). In the proposed scheme, we pair the aerial user (AU) and terrestrial user (TU) in a NOMA setting to leverage their asymmetric channel gains and rate demands in downlink communication. The high ICI issue at the AU receiver is further managed by equipping the AU with an adjustable beamwidth directional antenna and forming a distributed beamforming among the coordinated terrestrial base stations (BSs). The proposed ATN-NOMA scheme obtains the optimal beamwidth and power allocation to maximize the TUs' sum-rate subject to the AU's Quality-of-Service (QoS) requirement. The corresponding optimization is a non-convex optimization problem for which we exploit the structure of the problem to obtain a local optimal solution. We further compare TUs' sum-rate and AU's outage probability of the proposed scheme with multiple schemes. Simulation results show that our proposed scheme significantly outperforms the existing schemes and further demonstrate a robust performance against UAV altitude variations.

In this paper, we consider a downlink wireless communication system with the co-existence of ground user (GU) and mobile aerial user (AU). Existing solutions rely on orthogonal multiple access (OMA) to support these users, however, OMA is unable to provide the best rate and outage performance because its spectral efficiency is limited by the users' channel conditions and rate requirements. Thus, we propose an aerial-ground non-orthogonal multiple access (AG-NOMA) scheme that pairs the GU and AU for data and control links, respectively. Unlike terrestrial non-orthgonal multiple access (NOMA), the key idea of AG-NOMA is to exploit the asymmetric features of the channels and rate demands of the GU and AU in the downlink communication. Based on these opportunities, we investigate the maximum achievable GU rate over a time-varying wireless channel while satisfying the AU Quality-of-Service (QoS) requirement with perfect and partial channel state information (CSI). For perfect CSI, we derive the optimal successive interference cancellation (SIC) policy, power allocation, GU rate, and feasibility conditions in closed-form expressions. For partial CSI, we also derive the suboptimal SIC policy and power allocation in closed-form expressions, and further discussed a tradeoff between the achievable rate and reliability. This tradeoff depends on the system parameters, and thus we have suggested some appropriate parameters based on theoretical support and standard requirements to strike a balance between rate and reliability. Our simulation results show that AG-NOMA scheme with perfect and partial CSI can achieve up to +99% GU rate-improvement as compared to OMA and provide a more sustainable rate-improvement and/or lower outage probability than terrestrial NOMA scheme.