Best Readings

K. K. Wong, K. F. Tong, Y. Zhang, and Z. Zheng, “Fluid antenna system for 6G: When Bruce Lee inspires wireless communications,” IET Electronics Letters, vol. 56, no. 24, pp. 1288-1290, November 2020.

This is the first paper that introduces fluid antenna system (FAS), presenting the vision of shape and position flexibility of antennas for wireless communications.

K. K. Wong, K. F. Tong, Y. Shen, Y. Chen, and Y. Zhang, “Bruce Lee-Inspired Fluid Antenna System: Six Research Topics and the Potentials for 6G,” Frontiers in Communications and Networks, section Wireless Communications, vol. 3, no. 853416, March 2022.

This is one of the earliest papers that provides an overview of fluid antenna system (FAS) and fluid antenna multiple access (FAMA). It presents novel 1D and 2D fluid antenna architectures and outlines several interesting research topics such as reconfigurable intelligent surfaces (RIS), multiple-input multiple-output (MIMO), port selection, physical layer security, etc.

W. K. New, K. K. Wong, H. Xu, C. Wang, F. R. Ghadi, J. Zhang, J. Rao, R. Murch, et al., “A Tutorial on Fluid Antenna System for 6G Networks: Encompassing Communication Theory, Optimization Methods and Hardware Designs,” IEEE Communications Surveys & Tutorials, early access, DOI: 10.1109/COMST.2024.3498855.

This is the first tutorial paper on FAS that comprehensively covers channel modelling, signal processing and estimation techniques, information-theoretic insights, new multiple access techniques, and hardware designs. It also outlines the challenges of FAS and explores the potential of using FAS to improve the performance of other current technologies. By providing insights and guidance, this tutorial aims to inspire researchers to explore new horizons and fully exploit the potential of FAS.

A. Shojaeifard, K. K. Wong, K. F. Tong, Z. Chu, A. Mourad, A. Haghighat, I. Hemadeh, N. T. Nguyen, V. Tapio, and M. Juntti, “MIMO Evolution Beyond 5G Through Reconfigurable Intelligent Surfaces and Fluid Antenna Systems,” Proceedings of IEEE, vol. 110, no. 9, pp. 1244-1265, September 2022.

This is one of the earliest overview papers that jointly considers MIMO, RIS, and FAS. It highlights the key characteristics of these technologies and explores the promising potential of integrating MIMO, FAS, and RIS.

K. K. Wong, W. K. New, X. Hao, K. F. Tong, and C. B. Chae, “Fluid Antenna System-Part I: Preliminaries,” IEEE Communications Letters, vol. 27, no. 8, pp. 1919-1923, August 2023.

This letter reviews recent advances in FAS channel models and examines the fundamental performance of FAS, including the diversity and multiplexing gains in multiple-input multiple-output FAS (MIMO-FAS), as well as the generalized degrees of freedom in fluid antenna multiple access (FAMA).

K. K. Wong, K. F. Tong, and C. B. Chae, “Fluid Antenna System-Part II: Research Opportunities,” IEEE Communications Letters, vol. 27, no. 8, pp. 1924-1928, August 2023. 

This letter discusses open problems in FAS and highlights research opportunities, including approximating FAS as a continuous aperture MIMO system, utilizing MIMO-FAS for massive connectivity, and exploring FAS for wireless power transfer and physical layer security.

K. K. Wong, K. F. Tong, and C. B. Chae, “Fluid Antenna System-Part III: A New Paradigm of Distributed Artificial Scattering Surfaces for Massive Connectivity,” IEEE Communications Letters, vol. 27, no. 8, pp. 1929-1933, August 2023.

This letter presents a novel approach that leverages RIS as distributed artificial scattering surfaces to create rich scattering environments. This method simplifies the optimization process while improving the performance of fluid antenna multiple access (FAMA).

J. Zheng, J. Zhang, H. Du, D. Niyato, S. Sun, B. Ai, and K. B. Letaief, “Flexible-Position MIMO for Wireless Communications: Fundamentals, Challenges, and Future Directions,” IEEE Wireless Communications, early access, DOI: 10.1109/MWC.011.2300428.

This magazine paper comprehensively summarizes and analyzes the fundamentals, unique characteristics, advantages, potential applications, and future directions of the flexible-position multiple-input multiple-output (FLP-MIMO) technique. It shows that due to the switchable or moveable features of the antennas, FLP-MIMO makes it possible to match the channel hardening achieved by a large number of fixed antennas in the traditional systems, using fewer antennas, and can thus achieve similar or better spectral-energy efficiency at lower cost.

K. K. Wong, A. Shojaeifard, K. F. Tong, and Y. Zhang, “Fluid Antenna Systems,” IEEE Transactions on Wireless Communications, vol. 20, no. 3, pp. 1950-1962, March 2021.

This is one of the first papers to systematically study the performance of FAS. First, a novel model is proposed to characterize the Rayleigh fading channel and spatial correlation among ports, from which exact and approximate closed-form expressions for the outage probability are derived. Then, by deriving an upper bound on the outage probability, it is shown that if the number of ports is large enough, even within a tiny space, a single-antenna FAS can outperform the traditional system that has multiple fixed antennas and uses a maximum ratio combining (MRC) receiver.

K. K. Wong, A. Shojaeifard, K. F. Tong, and Y. Zhang, “Performance Limits of Fluid Antenna Systems,” IEEE Communications Letters, vol. 24, no. 11, pp. 2469-2472, November 2020.

This paper investigates the performance limits of a FAS-assisted point-to-point channel by analyzing the level crossing rate (LCR), average fade duration (AFD), and ergodic capacity. Additionally, a lower bound on the capacity is derived. The results demonstrate that FAS offers promising performance, not only in terms of outage probability but also across various metrics such as LCR, capacity, and more.

K. K. Wong, and K. F. Tong, “Fluid Antenna Multiple Access,” IEEE Transactions on Wireless Communications, vol. 21, no. 7, pp. 4801-4815, July 2022.

This is the first paper that proposes to support multiple access in mobile systems using FAS. This technique, called fluid antenna multiple access (FAMA), exploits spatial moments of deep fading suffered by the interference to achieve a favorable channel condition for the desired signal, and does not require sophisticated signal processing. The results show that it is possible for FAMA to support hundreds of users using only one fluid antenna of a few wavelengths of space at each user, resulting in a significant improvement in the average network outage rate.

K. K. Wong, D. Morales-Jimenez, K. F. Tong, and C. B. Chae, “Slow Fluid Antenna Multiple Access,” IEEE Transactions on Communications, vol. 71, no. 5, pp. 2831-2846, May 2023.

This paper considers a more practical scenario where the fluid antennas at the users only update their ports if the fading channels change. This approach is referred to as slow FAMA. This paper investigates the interference immunity of slow FAMA by analyzing the outage probability. The results indicate that while the performance of slow FAMA is inferior to symbol-based fast FAMA, a significant multiplexing gain can still be achieved if the users’ fluid antennas have large numbers of ports.

M. Khammassi, A. Kammoun, and M.-S. Alouini, “A New Analytical Approximation of the Fluid Antenna System Channel,” IEEE Transactions on Wireless Communications, vol. 22, no. 12, pp. 8843-8858, December 2023. 

This paper is the first to study how to accurately capture the spatial correlation among FAS ports, as modeled by Jake’s model, while maintaining analytical tractability. An accurate model is first built by incorporating more parameters to better characterize the FAS channel distribution. Then, a two-stage approximation of the channel model is proposed, simplifying the subsequent outage probability analysis and making it significantly more manageable. This work paves the way for future research on FAS.

H. Xu, K. K. Wong, W. K. New, K. F. Tong, Y. Zhang and C. B. Chae, “Revisiting Outage Probability Analysis for Two-User Fluid Antenna Multiple Access System,” IEEE Transactions on Wireless Communications, vol. 23, no. 8, pp. 9534-9548, August 2024.

This paper investigates the outage performance of a FAMA network using an eigenvalue-based channel model, which accurately captures the spatial correlation among the ports. While accurate, this model is highly complex for further analysis. To address this issue, a two-stage approximation method leveraging the properties of Hermitian Toeplitz matrices is proposed, significantly simplifying the outage probability analysis. The results validate the effectiveness of the approximations and demonstrate the promising performance of FAS.

F. R. Ghadi, K. K. Wong, F. Javier López-Martínez, and K. F. Tong, “Copula-Based Performance Analysis for Fluid Antenna Systems Under Arbitrary Fading Channels,” IEEE Communications Letters, vol. 27, no. 11, pp. 3068-3072, November 2023. 

This paper proposes a novel copula-based technique to describe the spatial correlation across the ports for point-to-point FAS-aided communications. Specifically, it uses the popular Archimedean copulas to construct the distribution of the maximum of the correlated fading channel coefficients, in a closed-form expression. The proposed analytical cumulative distribution function is valid for any chosen fading distribution and captures both negative and positive correlations, as well as linear and non-linear dependence structures. Furthermore, the outage probability of the system is analyzed under correlated Nakagami-m fading channels.

P. Ramírez-Espinosa, D. Morales-Jimenez, and K. K. Wong, “A New Spatial Block-Correlation Model for Fluid Antenna Systems,” IEEE Transactions on Wireless Communications, early access, DOI: 10.1109/TWC.2024.3434509.

This paper addresses the problem that classical channel models of FAS, such as Jake's, are too complex for practical analysis, while state-of-the-art approximations are often too simplistic and lack sufficient accuracy. It proposes a general framework for approximating the spatial correlation of FAS by block-diagonal matrices. The proposed block correlation model can closely approximate the results obtained with realistic models (Jake’s and Clarke’s), and makes the subsequent outage probability analysis of 1D and 2D FAMA systems much more tractable.

H. Xu, G. Zhou, K.-K. Wong, W. K. New, C. Wang, C.-B. Chae, R. Murch, S. Jin, and Y. Zhang, “Channel Estimation for FAS-Assisted Multiuser mmWave Systems,” IEEE Communications Letters, vol. 28, no. 3, pp. 632–636, March 2024.

This paper proposes a novel channel estimation approach for a multi-user mmWave TDD system where the base station uses a fixed multi-antenna uniform linear array, while each mobile user is equipped with a linear FAS. With the proposed approach, the antennas of all users only need to switch and transmit orthogonal pilots at a few estimation locations, and accurate channel state information can be reconstructed at all ports.

W. Ma, L. Zhu, and R. Zhang, “Compressed Sensing based Channel Estimation for Movable Antenna Communications,” IEEE Communications Letters, vol. 27, no. 10, pp. 2747-2751, October 2023. 

This paper is among the first to address channel estimation for wireless communications with movable antennas at both the transmitter and receiver. It focuses on reconstructing the channel response at any location using a geometric channel model with multi-path components. By selecting a limited number of channel measurements within a given region, this approach effectively reduces pilot overhead and computational complexity.

S. Ji, C. Psomas and J. Thompson, “Correlation-Based Machine Learning Techniques for Channel Estimation with Fluid Antennas,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Seoul, Korea, Republic of, March 2024, pp. 8891-8895.

This paper proposes an effective channel estimation method in fluid antenna systems by leveraging the spatial correlation matrix. To address varying correlation due to environmental changes, it introduces dedicated sub-networks for different conditions used in machine learning, and also proposes a hard selection approach to dynamically adjust the number of ports required for estimation.

H. Zhang, J. Wang, C. Wang, C. C. Wang, K. K. Wong, B. Wang, and C. Chae, “Learning-Induced Channel Extrapolation for Fluid Antenna Systems Using Asymmetric Graph Masked Autoencoder,” IEEE Wireless Communications Letters, vol. 13, no. 6, pp. 1665-1669, June 2024.

This paper investigates channel estimation based on channel extrapolation for high-resolution fluid antenna systems. It introduces an asymmetric graph masked autoencoder (AGMAE) designed to handle spatial channel extrapolation with reduced complexity and improved generalization.

Z. Xiao, S. Cao, L. Zhu, Y. Liu, B. Ning, X.-G. Xia, and R. Zhang, “Channel Estimation for Movable Antenna Communication Systems: A Framework based on Compressed Sensing,” IEEE Transactions on Wireless Communications, vol. 23, no. 9, pp. 11814-11830, September 2024. 

This paper presents a channel estimation framework for movable antenna communication systems, exploiting the multi-path field response channel structure and using compressed sensing. In particular, the paper proposes two criteria for selecting optimal movable antenna measurement positions to ensure successful multi-path component recovery.

C. Psomas, P. J. Smith, H. A. Suraweera, and I. Krikidis, “Continuous fluid antenna systems: Modeling and analysis,” IEEE Communications Letters, vol. 27, pp. 3370–3374, December 2023.

This paper focuses on a continuous fluid antenna system (CFAS) and presents a general framework for its design and analytical evaluation. Closed-form analytical expressions are derived for the level crossing rate (LCR) and the average fade duration of the continuous signal-to-interference ratio (SIR) process over the FA’s length. By leveraging the LCR expression, the system’s outage performance is characterized with a bound on the cumulative distribution function of the SIR’s supremum.

C. Skouroumounis and I. Krikidis, “Fluid antenna systems with linear MMSE channel estimation for large-scale cellular networks,” IEEE Transactions on Communications, vol. 71, pp. 1112–1125, February 2023.

This paper develops a framework to analyze the outage performance of fluid antenna-enabled communications, considering channel estimation errors in limited coherence intervals. It introduces a novel estimation method and low-complexity port-selection technique to improve performance. The study reveals a trade-off between network performance and channel estimation quality.

W. K. New, K. -K. Wong, H. Xu, F. R. Ghadi, R. Murch and C. -B. Chae, "Channel Estimation and Reconstruction in Fluid Antenna System: Oversampling is Essential," in IEEE Transactions on Wireless Communications, early access, DOI: 10.1109/TWC.2024.3491507.

This paper investigates an electromagnetic-compliant channel model and identifies a fundamental tradeoff between the accuracy of channel reconstruction and the number of estimated channels. The findings reveal that half-wavelength sampling is inadequate for perfect reconstruction, underscoring the necessity of oversampling to enhance accuracy. Additionally, the study demonstrates that fluid antenna systems (FAS) with imperfect channel state information (CSI) can outperform traditional antenna selection (TAS) with perfect CSI. Furthermore, it is shown that there exists an optimal fluid antenna size that maximizes the achievable rate when accounting for the overheads required for full CSI acquisition. 

H. Yang, H. Xu, K. K. Wong, C. Chae, R. Murch, S. Jin, and Y. Zhang, “Position Index Modulation for Fluid Antenna System,” IEEE Transactions on Wireless Communications, early access, DOI: 10.1109/TWC.2024.3446658.

This paper presents a novel index modulation scheme, called Position Index Modulation (PIM), which utilizes FAS ports as indices for information transmission. A transceiver architecture is designed that integrates PIM with traditional amplitude-phase modulation schemes, such as QAM and PSK. Bit error rate and data rate analysis, along with simulation results, validate the advantages of the proposed modulation scheme and the use of FAS.

C. Psomas, G. M. Kraidy, K. K. Wong, and I. Krikidis, “On the Diversity and Coded Modulation Design of Fluid Antenna Systems,” IEEE Transactions on Wireless Communications, vol. 23, no. 3, pp. 2082-2096, March 2024.

This paper presents an innovative investigation into the performance of three distinct structural fluid antennas (FA): linear, circular, and wheel-shaped, each exhibiting different correlation patterns between their ports. By considering errors due to post-scheduling delays, an analytical framework for deriving the outage probability with outdated channel estimates is developed. In addition, space-time coded modulations that combine space-time rotations with code diversity are designed for FA port combining to achieve optimal performance over FA-correlated block-fading channels.

J. Zhu, G. Chen, P. Gao, P. Xiao, Z. Lin and A. Quddus, “Index Modulation for Fluid Antenna-Assisted MIMO Communications: System Design and Performance Analysis,” IEEE Transactions on Wireless Communications, vol. 23, no. 8, pp. 9701-9713, August 2024.

This paper incorporates the principle of index modulation (IM) into fluid antenna assisted system, and proposes a modulation scheme, namely FA-IM, to improve the spectral efficiency. To realize this technique, a novel FA position pattern codebook and a low-complexity detector are designed by exploiting the inherent sparsity of the transmitted FA-IM signal vectors. Then, the average bit error probability is analyzed, which validates the promising performance of the proposed modulation scheme and the FA technique.

Y. Chen and T. Xu, “Fluid Antenna Index Modulation Communications,” IEEE Wireless Communications Letters, vol. 13, no. 4, pp. 1203-1207, April 2024.

This paper explores the integration of fluid antenna (FA) and index modulation (IM) techniques, and proposes a novel modulation scheme, namely FAIM, to enhance the adaptability and performance of wireless communication systems. To achieves fast classification of index patterns using limited training data, a wavelet scattering neural network (WSNN) is implemented in the proposed FAIM framework.

E. Faddoul, G. M. Kraidy, C. Psomas and I. Krikidis, "Advanced Channel Coding Designs for Index-Modulated Fluid Antenna Systems," IEEE Transactions on Communications, early access, DOI: 10.1109/TCOMM.2024.3446616.

This paper explores the integration of index-modulated (IM) transmissions within fluid antenna (FA) systems, by leveraging FA's reconfigurability and single RF chain setup. A closed-form bit error rate expression is derived, demonstrating superior performance over conventional fixed-antenna IM systems, particularly in spatially correlated environments. Moreover, channel coding techniques such as set partition coding (SPC) and turbo-coded modulation are applied to further improve performance, revealing significant gains in highly correlated scenarios.

N. Vashakidze, J. J. Boutros, G. M. Kraidy and I. Krikidis, "Effective Diversity and Coding Gain over Fluid Antenna Channels," in Proc. 2024 IEEE 25th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Lucca, Italy, 2024, pp. 291-295.

This paper examines fluid antenna channels, highlighting that maximum diversity is determined by dominant eigenvalues from the channel correlation matrix. Reed-Solomon-coded QAM constellations are proposed to achieve the channel’s diversity limits. Key findings show that diversity and coding gain quickly saturate with more antennas and that transmitting over all ports performs better than selecting a subset when CSI is unavailable.

I. Krikidis, C. Psomas, A. K. Singh, and K. Jamieson, “Optimizing Configuration Selection in Reconfigurable-Antenna MIMO Systems: Physics-Inspired Heuristic Solvers,” IEEE Transactions on Communications, early access, DOI: 10.1109/TCOMM.2024.3420768.

This paper addresses the flexible/reconfigurable antenna configuration selection for point-to-point MIMO systems, proposing a novel approach using physics-inspired heuristics. It introduces coherent Ising machines (CIMs) and quantum annealing (QA) to optimize antenna configurations for maximizing signal-to-noise ratio (SNR), providing near-optimal performance with polynomial complexity. Additionally, a simulated annealing (SA) heuristic with parallel tempering is proposed, showing superior performance in maximizing end-to-end Shannon capacity compared to conventional methods.

C. N. Efrem and I. Krikidis, “Transmit and Receive Antenna Port Selection for Channel Capacity Maximization in Fluid-MIMO Systems,” IEEE Wireless Communications Letters, early access, DOI: 10.1109/LWC.2024.3458417.

This letter studies a discrete optimization problem, namely, the maximization of channel capacity in fluid multiple-input multiple-output (fluid-MIMO) systems through the selection of antenna ports/positions at both the transmitter and the receiver. First, they present a new joint convex relaxation (JCR) problem by using an upper bound on the channel capacity and exploiting the binary nature of optimization variables. Then, they develop and analyze two optimization algorithms with different performance-complexity tradeoffs. The first algorithm is based on JCR and reduced exhaustive search (JCR&RES), while the second on JCR and alternating optimization (JCR&AO).

L. Zhu, W. Ma,  B. Ning, and R. Zhang, “Movable-Antenna Enhanced Multiuser Communication via Antenna Position Optimization,” IEEE Transactions on Wireless Communications, vol. 23, no. 7, pp. 7214-7229, July 2024.

This paper investigates fluid antenna multiple access (FAMA) for uplink transmission from multiple users with movable antennas to a base station with a fixed antenna array. It ueses a field-response based channel model and develops two effective algorithms based on zero-forcing (ZF) and minimum mean square error (MMSE) methods to solve the non-convex optimization problem involving coupled variables, delivering promising performance.

H. Qin, W. Chen, Z. Li, Q. Wu, N. Cheng, and F. Chen, “Antenna Positioning and Beamforming Design for Fluid Antenna-Assisted Multi-User Downlink Communications,” IEEE Wireless Communications Letters, vol. 13, no. 4, pp. 1073-1077, April 2024.

This work investigates a multiuser communication system where multiple users are equipped with fluid antennas (FAs). By adopting a field-response based channel model, it effectively characterizes the downlink channel in relation to FA positions. The paper minimizes total transmit power by adjusting FA positions and the beamforming matrix, achieving improved performance.

Z. Cheng, N. Li, J. Zhu, X. She, C. Ouyang, and P. Chen, “Sum-Rate Maximization for Fluid Antenna Enabled Multiuser Communications,” IEEE Communications Letters, vol. 28, no. 5, pp. 1206-1210, May 2024. 

This work investigates a novel multiuser communication system where the base station is equipped with an array of multiple fluid antennas (FAs) to serve multiple single-antenna users. It aims to maximize the downlink sum-rate through the joint optimization of the transmit beamforming vector and the positions of the transmit FAs.

W. Ma, L. Zhu, and R. Zhang, “MIMO Capacity Characterization for Movable Antenna Systems,” IEEE Transactions on Wireless Communications, vol. 23, no. 4, pp. 3392-3407, September 2023. 

This paper proposes an innovative MIMO communication system, where both the transmitter and receiver are equipped with movable antennas to enhance capacity. This paper maximizes capacity by effectively optimizing both antenna positions and transmit signal covariance, demonstrating significant performance improvements.

G. Hu, Q. Wu, K. Xu, J. Ouyang, J. Si, Y. Cai, and N. A. Dhahir, “Fluid Antennas-Enabled Multiuser Uplink: A Low-Complexity Gradient Descent for Total Transmit Power Minimization,” IEEE Communications Letters, vol. 28, no. 3, pp. 602-606, March 2024.

This paper investigates multiuser uplink communications from single-antenna users to a base station equipped with multiple fluid antennas, aiming to optimize antenna positions to minimize total transmit power while satisfying rate requirements. It innovatively proposes an equivalent problem of minimizing the sum of eigenvalue reciprocals of a matrix dependent on antenna positions, offering an effective solution for enhanced performance.

L. Zhang, H. Yang, Y. Zhao, and J. Hu, “Joint Port Selection and Beamforming Design for Fluid Antenna Assisted Integrated Data and Energy Transfer,” IEEE Wireless Communications Letters, vol. 13, no. 7, pp. 1833-1837, July 2024.

This work investigates a multiuser fluid antenna (FA) assisted integrated data and energy transfer (IDET) system, marking a first attempt for tiny-size low-power devices. It introduces an efficient algorithm to maximize the weighted energy harvesting power at energy receivers (ERs) by jointly optimizing port selection and transmit beamforming, even under imperfect CSI.

W. Mei, X. Wei, B. Ning, Z. Chen, and R. Zhang, “Movable-Antenna Position Optimization: A Graph-Based Approach,” IEEE Wireless Communications Letters, vol. 13, no. 7, pp. 1853-1857, July 2024. 

This paper proposes a novel approach to optimize movable antenna positions by discretizing the transmit region into sampling points, transforming a continuous optimization problem into a discrete one. By ingeniously recasting it as a fixed-hop shortest path problem in graph theory, this paper introduces an effective algorithm that solves the problem optimally in polynomial time, offering improved performance over continuous methods.

G. Hu, Q. Wu, J. Ouyang, K. Xu, Y. Cai, and N. Al-Dhahir, “Movable-Antenna-Array-Enabled Communications with CoMP Reception,” IEEE Communications Letters, vol. 28, no. 4, pp. 947-951, April 2024.

This paper explores movable-antenna-array-enabled wireless communication with coordinated multi-point (CoMP) reception, aiming to maximize the effective received SNR. The study reveals that optimizing the principal eigenvalue of a Hermitian channel matrix, dependent on MA array positions, is key to addressing the original non-convex problem. The optimal beamforming solution is derived in closed form, and theoretical performance bounds are analyzed.

Y. Chen, S. Li, Y. Hou, and X. Tao, “Energy-Efficiency Optimization for Slow Fluid Antenna Multiple Access Using Mean-Field Game,” IEEE Wireless Communications Letters, vol. 13, no. 4, pp. 915-918, April 2024.

This paper investigates the downlink energy efficiency (EE) of slow fluid antenna multiple access (s-FAMA), in which fluid antennas switch their positions when channel states change. It effectively models the optimal control of base station transmit power and user antenna positions as a mean-field game (MFG), providing a unique approach to improving system performance.

J. Yao, T. Wu, X. Lai, M. Jin, C. Pan, M. Elkashlan, and K. K. Wong, “Proactive Monitoring via Jamming in Fluid Antenna Systems,” IEEE Communications Letters, vol. 28, no. 7, pp. 1698-1702, July 2024.

This paper investigates the use of fluid antenna systems to enhance monitoring performance in suspicious communications by minimizing outage probability through antenna position switching. It effectively maximizes the average monitoring rate, leveraging a non-convex optimization problem reformulated with an upper bound. The unique optimal solution is efficiently achieved using the bisection search method, demonstrating improved monitoring performance.

W. K. New, K. K. Wong, X. Hao, K. F. Tong, and C. B. Chae, “Fluid Antenna System: New Insights on Outage Probability and Diversity Gain,” IEEE Transactions on Wireless Communications, vol. 23, no. 1, pp. 128-140, January 2024. 

The paper analyzes the performance limits of FAS in a point-to-point scenario, presenting closed-form expressions for outage probability and diversity gain despite using a fully correlated channel model. It demonstrates that increasing the number of ports beyond a certain threshold for a fixed antenna length results in linearly dependent channels, limiting performance gains.

W. K. New, K. K. Wong, H. Xu, K. F. Tong, and C. B. Chae, “An Information-Theoretic Characterization of MIMO-FAS: Optimization, Diversity-Multiplexing Tradeoff and q-Outage Capacity,” IEEE Transactions on Wireless Communications, vol. 23, no. 6, pp. 5541–5556, June 2024.

This paper proves that the diversity-multiplexing tradeoff of a multiple-input multiple-output fluid antenna system (MIMO-FAS) serves as an outer bound to that of traditional MIMO. It also jointly optimizes antenna positions (or active ports), beamforming, and power allocation at high signal-to-noise ratio (SNR), while examining the mutual coupling effect.

F. R. Ghadi, K. K. Wong, F. Javier López-Martínez, C. B. Chae, K. F. Tong and Y. Zhang, “A Gaussian Copula Approach to the Performance Analysis of Fluid Antenna Systems,” IEEE Transactions on Wireless Communications, early access, DOI: 10.1109/TWC.2024.3454558.

This work proposes a flexible copula-based approach to accurately model spatial correlation in a planar FAS. This method simplifies the process while validating arbitrary fading distributions and maintaining analytical tractability. Analytical expressions for the cumulative distribution function and probability density function of the equivalent channel with arbitrary fading distribution are derived. In addition, the outage probability and delay outage rate are derived to analyze the system performance. The results reveal the accuracy of the proposed method for the considered FAS.

L. Zhu, W. Ma, and R. Zhang, “Modeling and Performance Analysis for Movable Antenna Enabled Wireless Communications,” IEEE Transactions on Wireless Communications, vol. 23, no. 6, pp. 6234-6250, June 2024.

This work is the first to apply fluid antennas to both the transmitter and receiver, analyzing the maximum channel gain achievable by a single receive movable antenna based on a geometric multi-path channel model. It reveals the periodic behavior of multi-path channel gain in deterministic channels and derives an upper bound for the expected maximum channel gain in stochastic channels for different path numbers. The study provides valuable insights into maximizing channel gain in fluid antenna systems.

C. Skouroumounis and I. Krikidis, “Fluid Antenna-Aided Full Duplex Communications: A Macroscopic Point-of-View,” IEEE Journal on Selected Areas in Communications, vol. 41, no. 9, pp. 2879-2892, September 2023.

This paper explores the integration of the fluid antenna (FA) and full-duplex (FD) technologies in a homogeneous cellular network characterized by stochastic geometry. A novel channel estimation method is first proposed. Based on the obtained imperfect channel state information, the outage probability and the average sum rate of the system are analyzed. The results highlight the beneficial synergy of FA and FD, which allows to increase the achieved average sum-rate performance by about 45% over the conventional static FD communication.

J. D. Vega-Sánchez, A. E. López-Ramírez, L. Urquiza-Aguiar, and D. P. M. Osorio, “Novel Expressions for the Outage Probability and Diversity Gains in Fluid Antenna System,”  IEEE Wireless Communications Letters, vol. 13, no. 2, pp. 372-376, February 2024. 

This paper analyzes the outage probability (OP) performance of fluid antenna system (FAS) receivers in spatially correlated Nakagami-m fading channels. It introduces a novel asymptotic matching method to derive a simple yet accurate closed-form approximation for the OP of a maximum-gain combining FAS (MGC-FAS). The method effectively approximates the cumulative density function (CDF) of each branch and the overall system, providing closed-form expressions for both the OP and asymptotic OP.

J. D. Vega-Sánchez, L. Urquiza-Aguiar, H. R. C. Mora, N. V. O. Garzón and D. P. M. Osorio, “Fluid Antenna System: Secrecy Outage Probability Analysis,” IEEE Transactions on Vehicular Technology, vol. 73, no. 8, pp. 11458-11469, August 2024.

This paper explores the potential of FAS to improve secrecy performance in wiretap channels. In this scenario, both the transmitter and the eavesdropper utilize traditional fixed-position antennas, whereas the legitimate receiver employs FAS technology. The study investigates two FAS approaches: non-diversity FAS and maximum gain combining FAS (MGC-FAS). The secrecy outage probabilities for both implementations are analyzed and compared to understand their effectiveness in enhancing secure communications.

P. D. Alvim, H. S. Silva, F. O. Barcelos, P. R. de Moura, U. S. Dias, O. S. Badarneh, and R. A. A. de Souza, “On the Performance of Fluid Antennas Systems Under α-μ Fading Channels,” IEEE Wireless Communications Letters, vol. 13, no. 1, pp. 108-112, January 2024.

This paper is the first to analyze the impact of channel non-linearity in fluid antenna systems, deriving first and second-order statistics under α-μ fading channels. Novel expressions for outage probability (OP) and ergodic channel capacity are introduced to evaluate system performance. Additionally, an exact reduction in OP for an N-th port relative to an (N−1)-port is presented, contributing significantly to existing research.

C. Skouroumounis and I. Krikidis, “SWIPT in FA-enabled cellular networks: A stochastic geometry Copula-based approach,” in Proc. IEEE International Conference on Communications (ICC), Denver, USA, June 2024.

This paper evaluates fluid antenna-enabled user equipment in simultaneous wireless information and power transfer (SWIPT) networks using stochastic geometry and copula theory. By employing the Frank copula, a closed-form expression for the cumulative distribution function of the signal-to-interference ratio (SIR) is derived under correlated Nakagami-µ fading channels. Numerical results reveal a 30% improvement in information decoding with a 6% reduction in energy harvesting compared to conventional antennas.

C. Psomas and I. Krikidis, “Switched combining for reconfigurable fluid antenna systems,” in Proc. IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Lucca, Italy, September 2024.

This paper studies the performance of a switched combining scheme for fluid antenna systems, which aims to decrease the channel estimation overhead. A novel analytical approximation is derived for the statistical distribution of the signal-to-interference-plus-noise ratio (SINR). A theoretical framework is provided for the outage probability, the average number of port SINR estimations, and the probability of switching to a different port between consecutive time slots.

K. K. Wong, K. F. Tong, Y. Chen, and Y. Zhang, “Fast Fluid Antenna Multiple Access Enabling Massive Connectivity,” IEEE Communications Letters, vol. 27, no. 2, pp. 711-715, February 2023.

This paper introduces a novel approach, termed fast FAMA, for massive connectivity. Each user selects the port that maximizes the symbol-level signal energy to the interference energy. An algorithm that estimates the best port for reception at every symbol instance is proposed to realize this technique. The results illustrate that fast FAMA can achieve massive connectivity without precoding nor multiuser detection and confirm the importance of using distributed BS antennas.

W. K. New, K. K. Wong, H. Xu, K. F. Tong, C. B. Chae, and Y. Zhang, “Fluid Antenna System Enhancing Orthogonal and Non-Orthogonal Multiple Access,” IEEE Communications Letters, vol. 28, no. 1, pp. 218-222, January 2024. 

This paper investigates the applications of FAS for enhancing both orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) schemes in the downlink. It shows that FAS can enhance the performance of these multiple access schemes. Moreover, the paper reveals that OMA without channel state information at the transmitter in FAS can even outperform NOMA in the traditional antenna systems.

K. K. Wong, C. B. Chae, and K. F. Tong, “Compact Ultra Massive Antenna Array: A Simple Open-Loop Massive Connectivity Scheme.” IEEE Transactions on Wireless Communications, vol. 23, no. 6, pp. 6279-6294, June 2024. 

This paper presents a highly innovative multiple access scheme called compact ultra massive antenna array (CUMA), which eliminates the need for precoding and power control at the base station and interference cancellation at the user side, making it an efficient solution for massive connectivity. Each user activates many ports to receive the signal, where the ports are chosen to ensure that the in-phase and quadrature components of the desired signal are constructively added at the ports, while the interference signals are randomly superimposed. The signal-to-interference ratio and data rate of the users are analyzed, confirming the super advantages of CUMA in supporting massive connectivity.

K. K. Wong, K. F. Tong, Y. Chen, Y. Zhang, and C. B. Chae, “Opportunistic Fluid Antenna Multiple Access,” IEEE Transactions on Wireless Communications, vol. 22, no. 11, pp. 7819-7833, November 2023. 

This paper explores the integration of opportunistic scheduling with fluid antenna multiple access (FAMA) and provides an in-depth analysis of the performance gains achieved by this combination. By leveraging the strengths of both techniques, the paper quantifies the improvements in multiplexing gain, the required number of users, or the required outage probability at each user to achieve a given multiplexing gain, and so on. The results highlight the synergistic benefits of this novel approach.

J. Zheng, T. Wu, X. Lai, C. Pan, M. Elkashlan, and K. K. Wong, “FAS-Assisted NOMA Short-Packet Communication Systems,” IEEE Transactions on Vehicular Technology, vol. 73, no. 7, pp. 10732-10737, July 2024.

This paper investigates the application of non-orthogonal multiple access (NOMA) in FAS, with a focus on short packet communications. It derives the average block error rate and demonstrates that the diversity order for each user in FAS can be increased to match the number of preset locations (or ports). Moreover, the results suggest that the performance gain attributable to the FAS can exceed that achieved by NOMA.

N. Li, P. Wu, B. Ning, and L. Zhu, “Sum Rate Maximization for Movable Antenna Enabled Uplink NOMA,” IEEE Wireless Communications Letters, vol. 13, no. 8, pp. 2140-2144, August 2024.

This paper similarly investigates orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) schemes in a movable antenna system but focuses on the uplink direction. The results similarly show that the movable antenna system with NOMA achieves the best performance, followed by the movable antenna system with OMA, then the traditional antenna system with NOMA, and finally the traditional antenna system with OMA.

C. Wang, Z. Li, K. K., Wong, R. Murch, C. B. Chae, and S. Jin, “AI-Empowered Fluid Antenna Systems: Opportunities, Challenges, and Future Directions,” IEEE Wireless Communications, early access, DOI: 10.1109/MWC.017.2300527.

This paper explores a vision of how AI can enable FAS to overcome challenges such as precoding, antenna position optimization, etc., using a learning-based approach to enable FAS to excel. Furthermore, the emerging integrated sensing and communication (ISAC) scenarios are used as a case study to illustrate the potential of enhancing FAS with AI capabilities. 

N. Waqar, K. K. Wong, C. B. Chae, R. Murch, S. Jin, and A. Sharples, “Opportunistic Fluid Antenna Multiple Access via Team-Inspired Reinforcement Learning,” IEEE Transactions on Wireless Communications, vol. 23, no. 9, pp. 12068-12083, September 2024.

This paper introduces an opportunistic fluid antenna multiple access (oFAMA) system for better spectrum sharing in multiuser settings. It proposes using decentralized reinforcement learning (RL) to select robust users and antenna ports, improving network performance. The authors also propose a novel team-theoretic derivative network implementation that further improves the performance of multi-agent RL.

Z. Chai, K. K. Wong, K. F. Tong, Y. Chen, and Y. Zhang, “Port Selection for Fluid Antenna Systems,” IEEE Communications Letters, vol. 26, no. 5, pp. 1180-1184, May 2022.

This is one of the earliest papers on FAS that combines analytical approximations with machine learning techniques, such as long short-term memory (LSTM) and the smart predict-and-optimize framework, to solve channel estimation and optimal antenna position (or port selection) problems.

N. Waqar, K. K. Wong, K. F. Tong, A. Sharples, and Y. Zhang, “Deep Learning Enabled Slow Fluid Antenna Multiple Access,” IEEE Communications Letters, vol. 27, no. 3, pp. 861-865, March 2023. 

This paper discusses fluid antenna multiple access (FAMA), where antennas switch to locations with deep fades of interference to improve signal reception. To simplify port selection, it proposes a low-complexity deep learning approach based on LSTMs that estimates signal quality using very limited observations. Simulations show that this method can greatly reduce outages and improve multiplexing.

J. Zou, S. Sun, and C. Wang, “Online Learning-Induced Port Selection for Fluid Antenna in Dynamic Channel Environment,” IEEE Wireless Communications Letters, vol. 13, no. 2, pp. 313-317, February 2024.

This is one of the earliest papers to apply the multi-armed bandit approach to address channel estimation and optimal antenna positioning in slow fluid antenna multiple access (FAMA). The paper derives the lower bound of the expected reward and the upper bound of the expected regret, and demonstrates that the multi-armed bandit can achieve performance near that of perfect channel state information with a well-chosen balance between exploitation and exploration.

C. Weng, Y. Chen, L. Zhu, and Y. Wang, “Learning-Based Joint Beamforming and Antenna Movement Design for Movable Antenna Systems,” IEEE Wireless Communications Letters, vol. 13, no. 8, pp. 2120-2124, August 2024.

This paper utilizes deep reinforcement learning to jointly optimize the transmit beamforming vector and antenna positions in a movable antenna system at both the transmitter and receivers, even with imperfect channel state information. The results show that using a single agent to learn the antenna movement policy at both the transmitter and receiver can yield a higher sum-rate than using independent agents.

D. Zhang, S. Ye, M. Xiao, K. Wang, M. Di Renzo and M. Skoglund, "Fluid Antenna Array Enhanced Over-the-Air Computation," IEEE Wireless Communications Letters, vol. 13, no. 6, pp. 1541-1545, June 2024.

This paper investigates enhancing over-the-air computation (AirComp) using fluid antenna (FA) arrays to improve wireless data aggregation. The authors propose a joint optimization of transceiver design and antenna positioning to minimize mean squared error (MSE). Their method significantly outperforms traditional fixed antenna arrays in terms of MSE, showing potential for applications in wireless federated learning systems​​.

C. Wang, G. Li, H. Zhang, K. K. Wong, Z. Li, D. W. K. Ng, and C. B. Chae, “Fluid Antenna System Liberating Multiuser MIMO for ISAC via Deep Reinforcement Learning,” IEEE Transactions on Wireless Communications, vol. 23, no. 9, pp. 10879-10894, September 2024.

This is one of the earliest papers to develop an end-to-end deep reinforcement learning framework for estimating the channel and optimizing the transmit beamforming vector as well as antenna positions (or port selections) in FAS. Additionally, this paper proposes a constraint-aware deep learning network to address integrated sensing and communication (ISAC) precoder design, as well as a masked autoencoder to extrapolate channel state information.

J. Zou, H. Xu, C. Wang, L. Xu, S. Sun, K. Meng, C. Masouros, and K. K. Wong, “Shifting the ISAC Trade-Off with Fluid Antenna Systems,” IEEE Wireless Communications Letters, early access, DOI: 10.1109/LWC.2024.3473991.

This paper proposes a signal model for FAS-enabled ISAC and aims to explore the enhancements in the ISAC trade-off that the FAS capability offers. The transmit power of the system is minimized by jointly optimizing the transmit beamforming and port selection of FAS, while satisfying both communication and sensing requirements. The results demonstrate the great potential advantage of FAS for improving the flexibility of balancing the sensing and communication performance.

L. Zhou, J. Yao, M. Jin, T. Wu, and K. K. Wong, “Fluid Antenna-Assisted ISAC Systems,” IEEE Wireless Communications Letters, early access, DOI: 10.1109/LWC.2024.3476148.

This paper proposes a FAS-assisted ISAC system where both the BS and the user have FAS, and the BS communicates with the user and simultaneously senses a target. The downlink communication rate is maximized by jointly optimizing the transmit beamforming and the locations of the fluid antennas at the BS and the CU, while the sensing beampattern gain requirement is guaranteed. The results have verified the great performance of FAS in supporting ISAC.

G. Hu, Q. Wu, D. Xu, K. Xu, J. Si, Y. Cai, and N. A. Dhahir, “Intelligent Reflecting Surface-Aided Wireless Communication with Movable Elements,” IEEE Wireless Communications Letters, vol. 13, no. 4, pp. 1173-1177, April 2024.

This paper explores the applications of intelligent reflecting surfaces (IRS) where the position of each IRS element can be flexibly reconfigured. In scenarios where the IRS phase shifters are discrete, it is shown that reconfiguring the positions of individual IRS elements can effectively eliminate phase offsets, enabling performance close to that of IRS with continuous phase shifters.

F. R. Ghadi, K. K. Wong, W. K. New, H. Xu, R. Murch and Y. Zhang, “On Performance of RIS-Aided Fluid Antenna Systems,” IEEE Wireless Communications Letters, vol. 13, no. 8, pp. 2175-2179, August 2024.

This paper evaluates the impact of planar FAS on reconfigurable intelligent surface (RIS)-assisted communications. It is assumed that a fixed-position base station aims to send a message to an FAS-equipped mobile user with the assistance of an RIS. After deriving the distribution of the equivalent channel at the user, the system outage probability and delay outage rate are evaluated, demonstrating how considering an FAS with only one activated port improves the RIS- assisted system.

J. Chen, Y. Xiao, J. Zhu, Z. Peng, X. Lei, and P. Xiao, “Low-Complexity Beamforming Design for RIS-Assisted Fluid Antenna Systems,” in Proc. International Conference on Communications (ICC) Workshops, Denver, CO, USA, 2024, pp. 1377-1382.

This paper investigates a low-complexity joint beamforming scheme for RIS-assisted FAS systems using statistical channel state information. It is shown that the positions of the antennas can be adjusted to simultaneously steer the main-lobe towards the RIS and the grating-lobe towards the user. This scheme is especially beneficial in line-of-sight dominant scenarios.

J. Zhang, J. Rao, Z. Li, Z. Ming, C. Y. Chiu, K. K. Wong, K. F. Tong, and R. Much, “A Novel Pixel-Based Reconfigurable Antenna Applied in Fluid Antenna Systems with High Switching Speed,” IEEE Open Journal of Antennas and Propagation, early access, DOI: 10.1109/OJAP.2024.3489215.

This paper begins by demonstrating that mechanical movement or liquid manipulation in FAS is equivalent to radiation pattern reconfiguration. Building on this observation, a pixel-based reconfigurable antenna design for FAS (PRA-FAS) is proposed, which enables microsecond-level FAS port switching to support packet-by-packet adaptability. Simulation and experimental results from a PRA-FAS prototype operating at 2.5 GHz show that the design meets FAS requirements, including port correlation and impedance matching.

C. B. Fortuny, K. F. Tong, A. Al-Armaghany, and K. K. Wong, “A Low-Cost Fluid Switch for Frequency-Reconfigurable Vivaldi Antenna,” IEEE Antennas and Wireless Propagation Letters, vol. 16, pp. 3151-3154, November 2017.

This paper presents a liquid-based fluid antenna that enables tuning between two frequency bands: 3.2 GHz and 4.5 GHz. Specifically, the length of the antenna’s slot can be adjusted to reconfigure the frequency, and the measured gain is approximately 11 dBi in both bands, with an isolation of 15 dB between them. Simulations and real-world tests confirmed that the antenna maintained stable performance across both frequency bands.

C. B. Fortuny, L. Cai, K. F. Tong, and K. K. Wong, “Low Cost 3D-Printed Coupling-Fed Frequency Agile Fluidic Monopole Antenna System,” IEEE Access, vol. 7, pp. 95058-95064, July 2019. 

The paper designs a liquid-based fluid antenna that utilizes ionized fluids to achieve frequency tuning between 3.2 GHz and 5 GHz. By adjusting the ionized fluid height in the antenna's slot, the frequency can be controlled. The system exhibits a peak efficiency of approximately 80% and a maximum gain of 3.8 dBi at 4.5 GHz. Simulations and experiments show that the antenna provides stable radiation patterns and performance across the frequency range.

Y. Huang, L. Xing, C. Song, S. Wang, and F. Elhouni, “Liquid Antennas: Past, Present and Future,” IEEE Open Journal of Antennas and Propagation, vol. 2, pp. 473–487, 2021.

This paper provides a comprehensive review of liquid antennas, discussing their potential applications in areas such as 5G, IoT, and wearable technology. It also addresses the technical challenges, including fabrication and reconfigurability, that must be overcome for these antennas to be viable for commercial use.

S. Dash, C. Psomas, and I. Krikidis. “Selection of Metallic Liquid in Sub-6 GHz Antenna Design for 6G Networks,” Scientific Reports, vol. 13, no. 1, pp. 20551, 2023.

This paper compares the performance of three metallic liquids - Mercury, EGaIn (gallium indium alloy), and Graphene. These reconfigurable fluid antennas are evaluated at a frequency of 5.6 GHz using microfluidic channels over a liquid crystal polymer substrate. Among the three, Graphene metallic liquid demonstrated superior performance, with a gain of 5.0 dBi and a radiation efficiency of 81%, outperforming Mercury and EGaIn. This makes Graphene the most suitable choice for sub-6 GHz antenna designs.

L. Jing, M. Li, and R. Murch, “Compact Pattern Reconfigurable Pixel Antenna with Diagonal Pixel Connections,” IEEE Transactions on Antennas and Propagation, vol. 70, no. 10, pp. 8951–8961, October 2022.

This paper introduces a novel design approach for pattern reconfigurable pixel antennas. The key innovation is the use of diagonal connections between pixel elements, providing additional degrees of freedom for beam steering and enhancing radiation performance.

R. Deng, Y. Zhang, H. Zhang, B. Di, H. Zhang, H. V. Poor, and L. Song, “Reconfigurable Holographic Surfaces for Ultra-Massive MIMO in 6G: Practical Design, Optimization and Implementation,” IEEE Journal on Selected Areas in Communications, vol. 41, no. 8, pp. 2367–2379, August 2023.

This paper introduces a novel approach to reconfigurable holographic surfaces (RHS), which use densely packed metamaterial elements. The design integrates feeds with the metasurface, converting input signals into reference waves that propagate across RHS elements. When the reference and object waves are in phase, the element radiates energy; otherwise, it radiates minimally. This process resembles a FAS, where only a subset of elements is used for transmission rather than the entire surface.